2016-2017
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2017 |
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Al Bitar, A., Mialon, A., Kerr, Y. H., Cabot, F., Richaume, P., Jacquette, E., et al. (2017). The global SMOS Level 3 daily soil moisture and brightness temperature maps. Earth System Science Data, 9(1), 293–315.
Abstract: The objective of this paper is to present the multi-orbit (MO) surface soil moisture (SM) and angle-binned brightness temperature (TB) products for the SMOS (Soil Moisture and Ocean Salinity) mission based on a new multi-orbit algorithm. The Level 3 algorithm at CATDS (Centre Aval de Traitement des Donnees SMOS) makes use of MO retrieval to enhance the robustness and quality of SM retrievals. The motivation of the approach is to make use of the longer temporal autocorrelation length of the vegetation optical depth (VOD) compared to the corresponding SM autocorrelation in order to enhance the retrievals when an acquisition occurs at the border of the swath. The retrieval algorithm is implemented in a unique operational processor delivering multiple parameters (e.g. SM and VOD) using multi-angular dual-polarisation TB from MO. A subsidiary angle-binned TB product is provided. In this study the Level 3 TB V310 product is showcased and compared to SMAP (Soil Moisture Active Passive) TB. The Level 3 SM V300 product is compared to the single-orbit (SO) retrievals from the Level 2 SM processor from ESA with aligned configuration. The advantages and drawbacks of the Level 3 SM product (L3SM) are discussed. The comparison is done on a global scale between the two datasets and on the local scale with respect to in situ data from AMMA-CATCH and USDA ARS Watershed networks. The results obtained from the global analysis show that the MO implementation enhances the number of retrievals: up to 9% over certain areas. The comparison with the in situ data shows that the increase in the number of retrievals does not come with a decrease in quality, but rather at the expense of an increased time lag in product availability from 6 h to 3.5 days, which can be a limiting factor for applications like flood forecast but reasonable for drought monitoring and climate change studies. The SMOS L3 soil moisture and L3 brightness temperature products are delivered using an open licence and free of charge using a web application(https://www.catds.fr/sipad/).TheRE04 products, versions 300 and 310, used in this paper are also available at ftp://ext-catds-cpdc:catds2010@ftp.ifremer.fr/Land_products/GRIDDED/L3SM/RE04/.
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Amory, C., Gallee, H., Naaim-Bouvet, F., Favier, V., Vignon, E., Picard, G., et al. (2017). Seasonal Variations in Drag Coefficient over a Sastrugi-Covered Snowfield in Coastal East Antarctica. Boundary-Layer Meteorology, 164(1), 107–133.
Abstract: The surface of windy Antarctic snowfields is subject to drifting snow, which leads to the formation of sastrugi. In turn, sastrugi contribute to the drag exerted by the snowsurface on the atmosphere and hence influence drifting snow. Although the surface drag over rough sastrugi fields has been estimated for individual locations in Antarctica, its variation over time and with respect to drifting snow has received little attention. Using year-round data from a meteorological mast, seasonal variations in the neutral drag coefficient at a height of 10m (C-DN10) in coastal Adelie Land are presented and discussed in light of the formation and behaviour of sastrugi based on observed aeolian erosion patterns. The measurements revealed high C-DN10 values (>= 2 x 10(-3)) and limited drifting snow (35% of the time) in summer (December-February) versus lower C-DN10 values (approximate to 1.5 x 10(-3)) associated with more frequent drifting snow (70% of the time) in winter (March-November). Without the seasonal distinction, there was no clear dependence of C-DN10 on friction velocity or wind direction, but observations revealed a general increase in C-DN10 with rising air temperature. Themain hypothesis defended here is that higher temperatures increase snowcohesion and the development of sastrugi just after snow deposition while inhibiting the sastrugi streamlining process by raising the erosion threshold. This increases the contribution of the sastrugi form drag to the total surface drag in summer when winds are lighter and more variable. The analysis also showed that, in the absence of erosion, single snowfall events can reduce C-DN10 to 1 x 10(-3) due to the burying of pre-existing microrelief under newly deposited snow. The results suggest that polar atmospheric models should account for spatial and temporal variations in snow surface roughness through a dynamic representation of the sastrugi form drag.
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Archundia, D., Duwig, C., Lehembre, F., Chiron, S., Morel, M. C., Prado, B., et al. (2017). Antibiotic pollution in the Katari subcatchment of the Titicaca Lake: Major transformation products and occurrence of resistance genes. Science Of The Total Environment, 576, 671–682.
Abstract: An increasing number of studies pointed out the ubiquitous presence of medical residues in surface and ground water as well as in soil compartments. Not only antibiotics can be found in the environment but also their transformation products about which little information is generally available. The development of bacterial resistance to antibiotics is particularly worrying as it can lead to sanitary and health problems. Studies about the dissemination of antibiotics and associated resistances in the Bolivian Altiplano are scarce. We provide baseline information on the occurrence of Sulfamethoxazole (SMX) and Trimethoprim (TMP) antibiotics as well as on the most common human SMX transformation products (TP) and on the occurrence of sulfonamide resistance genes. The studied water and soil compartments presented high levels of antibiotic pollution. This situation was shown to be mainly linked with uncontrolled discharges of treated and untreated wastewaters, resulting on the presence of antibiotics in the Titicaca Lake. SMXTPs were detected in surface waters and on soil sampled next to the wastewater treatment plant (WWTP). SMX resistance genes sull and sulll were widely detected in the basin hydrological network, even in areas unpolluted with antibiotics. Mechanisms of co-selection of antibiotic-and metal-resistance may be involved in the prevalence of ARG's in pristine areas with no anthropogenic activity and free of antibiotic pollution. (C) 2016 Elsevier B.V. All rights reserved.
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Archundia, D., Duwig, C., Spadini, L., Uzu, G., Guedron, S., Morel, M. C., et al. (2017). How Uncontrolled Urban Expansion Increases the Contamination of the Titicaca Lake Basin (El Alto, La Paz, Bolivia). Water Air And Soil Pollution, 228(1).
Abstract: Cities in developing countries encounter rapid waves of social transformation and economic development where the environment is mostly a neglected aspect. The Katari watershed encompasses mining areas, El Alto city (one of the fastest growing urban areas in South America and the biggest in the Altiplano) as well as agricultural areas. Its outlet is Cohana Bay, one of the most polluted areas of Lake Titicaca. Here we propose an integrative approach (hydrological, physicochemical, chemical and bacterial data) to understand the pollution problem of this developing area, in which a variety of anthropogenic activities takes place. Both mining and urban areas appear to be sources of metal pollution. Nutrient and bacterial contaminations are mainly related to urban and industrial discharges. These situations have impacts in the basin from the mining area down to Cohana Bay of Lake Titicaca. Pollutant concentration patterns are highly influenced by seasonal hydrology variations. The poor quality of surface waters in the basin represents a risk for human and animal populations, as well as for the quality of aquifers located underneath El Alto city.
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Asay-Davis, X. S., Jourdain, N. C., & Nakayama, Y. (2017). Developments in Simulating and Parameterizing Interactions Between the Southern Ocean and the Antarctic Ice Sheet. Current Climate Change Reports, 3(4), 316–329.
Abstract: Recent advances in both ocean modeling and melt parameterization in ice-sheet models point the way toward coupled ice sheet�ocean modeling, which is needed to quantify Antarctic mass loss and the resulting sea-level rise. The latest Antarctic ocean modeling shows that complex interactions between the atmosphere, sea ice, icebergs, bathymetric features, and ocean circulation on many scales determine which water masses reach ice-shelf cavities and how much heat is available to melt ice. Meanwhile, parameterizations of basal melting in standalone ice-sheet models have evolved from simplified, depth-dependent functions to more sophisticated models, accounting for ice-shelf basal topography, and the evolution of the sub-ice-shelf buoyant flow. The focus of recent work has been on better understanding processes or adding new model capabilities, but a broader community effort is needed in validating models against observations and producing melt-rate projections. Given time, community efforts in coupled ice sheet�ocean modeling, already underway, will tackle the considerable challenges involved in building, initializing, constraining, and performing projections with coupled models, leading to reduced uncertainties in Antarctica�s contribution to future sea-level rise.
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Attard, G., Rossier, Y., & Eisenlohr, L. (2017). Underground structures increasing the intrinsic vulnerability of urban groundwater: Sensitivity analysis and development of an empirical law based on a groundwater age modelling approach. Journal Of Hydrology, 552, 460–473.
Abstract: In a previous paper published in Journal of Hydrology, it was shown that underground structures are responsible for a mixing process between shallow and deep groundwater that can favour the spreading of urban contamination. In this paper, the impact of underground structures on the intrinsic vulnerability of urban aquifers was investigated. A sensitivity analysis was performed using a 2D deterministic modelling approach based on the reservoir theory generalized to hydrodispersive systems to better understand this mixing phenomenon and the mixing affected zone (MAZ) caused by underground structures. It was shown that the maximal extent of the MAZ caused by an underground structure is reached approximately 20 years after construction. Consequently, underground structures represent a long-term threat for deep aquifer reservoirs. Regarding the construction process, draining operations have a major impact and favour large-scale mixing between shallow and deep groundwater. Consequently, dewatering should be reduced and enclosed as much as possible. The role played by underground structures' dimensions was assessed. The obstruction of the first aquifer layer caused by construction has the greatest influence on the MAZ. The cumulative impact of several underground structures was assessed. It was shown that the total MAZ area increases linearly with underground structures' density. The role played by materials' properties and hydraulic gradient were assessed. Hydraulic conductivity, anisotropy and porosity have the strongest influence on the development of MAZ. Finally, an empirical law was derived to estimate the MAZ caused by an underground structure in a bi-layered aquifer under unconfined conditions. This empirical law, based on the results of the sensitivity analysis developed in this paper, allows for the estimation of MAZ dimensions under known material properties and underground structure dimensions. This empirical law can help urban planners assess the area of influence of underground structures and protect urban strategic reservoirs. (C) 2017 Elsevier B.V. All rights reserved.
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Attard, G., Rossier, Y., Winiarski, T., & Eisenlohr, L. (2017). Urban underground development confronted by the challenges of groundwater resources: Guidelines dedicated to the construction of underground structures in urban aquifers. Land Use Policy, 64, 461–469.
Abstract: Urban subsoil provides space, groundwater, geothermal energy and geomaterials, and the importance underlying the management of this strategic space is well-known. In the context of vertical urban growth, groundwater management is a crucial issue. In particular, operational tools dedicated to urban planners are needed in urban areas prone to conflicts over use in order to aid the decision-making process. The aim of this paper is to propose guidelines dedicated to urban underground development in view to incorporating groundwater resource management. These guidelines are supported by analyses of the scientific literature dealing with: (1) the impacts caused by underground structures on urban groundwater, and (2) the quantitative overview of urban groundwater resources at the operational level in terms of quantity, quality and temperature. Finally, perspectives are discussed regarding the research work needed to ensure better management of urban groundwater resources confronted by underground development. (C) 2017 Elsevier Ltd. All rights reserved.
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Awessou, K. G. B., Peugeot, C., Rocheteau, A., Seguis, L., Do, F. C., Galle, S., et al. (2017). Differences in transpiration between a forest and an agroforestry tree species in the Sudanian belt. Agroforestry Systems, 91(3), 403–413.
Abstract: Average population growth in the African Sudanian belt is 3 % per year. This leads to a significant increase in cultivated areas at the expense of fallows and forests. For centuries, rural populations have been practicing agroforestry dominated by Vitellaria paradoxa parklands. We wanted to know whether agroforestry can improve local rainfall recycling as well as forest. We compared transpiration and its seasonal variations between Vitellaria paradoxa, the dominant species in fallows, and Isoberlinia doka, the dominant species in dry forests in the Sudanian belt. The fallow and dry forest we studied are located in northwestern Benin, where average annual rainfall is 1200 mm. Sap flow density (SFD) was measured by transient thermal dissipation, from which tree transpiration was deduced. Transpiration of five trees per species was estimated by taking into account the radial profile of SFD. The effect of the species and of the season on transpiration was tested with a generalized linear mixed model. Over the three-year study period, daily transpiration of the agroforestry trees, V. paradoxa (diameters 8-38 cm) ranged between 4.4 and 26.8 L day(-1) while that of the forest trees, I. doka, (diameters 20-38 cm) ranged from 9.8 to 92.6 L day(-1). Daily transpiration of V. paradoxa was significantly lower (15 %) in the dry season than in the rainy season, whereas daily transpiration by I. doka was significantly higher (13 %) in the dry season than in the rainy season. Our results indicate that the woody cover of agroforestry systems is less efficient in recycling local rainfall than forest cover, not only due to lower tree density but also to species composition.
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Barbante, C., Spolaor, A., Cairns, W. R. L., & Boutron, C. (2017). Man's footprint on the Arctic environment as revealed by analysis of ice and snow. Earth-Science Reviews, 168, 218–231.
Abstract: The date of the definitive start to the Anthropocene is still under debate, and although a lot of progress has been made, currently available data is not precise enough to define the start of the human-dominated geological epoch. We know that during and after the industrial revolution, humans started having a much greater impact on the Earth's environment. Increases in population have led to increases in resource and fossil fuel use, leaving a marked impact on our planet. This impact, in the Northern hemisphere, is effectively recorded in the snow and ice of the Arctic. Human activity has changed various biogeochemical cycles to such an extent, that the climate has started to change. This has disturbed the biosphere, pushing it to adapt in response to these changes through evolutionary pressure. The Arctic is a particularly vulnerable environment and mankind is having a profound impact on its fragile equilibrium. Higher concentrations of heavy metals, organic compounds and radionuclides have been detected in ice cores as well as snow. Although climatic changes are evident on a global scale, in the Arctic these changes have been amplified. Advances in laboratory analysis methods have been applied to ice cores and surface snow samples to help us understand the mechanisms governing this fragile environment and to evaluate the impact and amplitude of human activity. Despite these advances, the fluxes and distributions over time of anthropogenic organic compounds is largely unknown. Hopefully advances in analytical methods will mean that this is not the case in the future. (C) 2017 Elsevier B.V. All rights reserved.
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Barraza, F., Schreck, E., Leveque, T., Uzu, G., Lopez, F., Ruales, J., et al. (2017). Cadmium bioaccumulation and gastric bioaccessibility in cacao: A field study in areas impacted by oil activities in Ecuador. Environmental Pollution, 229, 950–963.
Abstract: Cacao from South America is especially used to produce premium quality chocolate. Although the European Food Safety Authority has not established a limit for cadmium (Cd) in chocolate raw material, recent studies demonstrate that Cd concentrations in cacao beans can reach levels higher than the legal limits for dark chocolate (0.8 mg kg(-1), effective January 1st, 2019). Despite the fact that the presence of Cd in agricultural soils is related to contamination by fertilizers, other potential sources must be considered in Ecuador. This field study was conducted to investigate Cd content in soils and cacao cultivated on Ecuadorian farms in areas impacted by oil activities. Soils, cacao leaves, and pod husks were collected from 31 farms in the northern Amazon and Pacific coastal regions exposed to oil production and refining and compared to two control areas. Human gastric bioaccessibility was determined in raw cacao beans and cacao liquor samples in order to assess potential health risks involved. Our results show that topsoils (0-20 cm) have higher Cd concentrations than deeper layers, exceeding the Ecuadorian legislation limit in 39% of the sampling sites. Cacao leaves accumulate more Cd than pod husks or beans but, nevertheless, 50% of the sampled beans have Cd contents above 0.8 mg kg-1. Root-to-cacao transfer seems to be the main pathway of Cd uptake, which is not only regulated by physico-chemical soil properties but also agricultural practices. Additionally, natural Cd enrichment by volcanic inputs must not be neglected. Finally, Cd in cacao trees cannot be considered as a tracer of oil activities. Assuming that total Cd content and its bioaccessible fraction (up to 90%) in cacao beans and liquor is directly linked to those in chocolate, the health risk associated with Cd exposure varies from low to moderate. (C) 2017 Elsevier Ltd. All rights reserved.
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Barrere, M., Domine, F., Decharme, B., Morin, S., Vionnet, V., & Lafaysse, M. (2017). Evaluating the performance of coupled snow-soil models in SURFEXv8 to simulate the permafrost thermal regime at a high Arctic site. Geoscientific Model Development, 10(9).
Abstract: Climate change projections still suffer from a limited representation of the permafrost-carbon feedback. Predicting the response of permafrost temperature to climate change requires accurate simulations of Arctic snow and soil properties. This study assesses the capacity of the coupled land surface and snow models ISBA-Crocus and ISBA-ES to simulate snow and soil properties at Bylot Island, a high Arctic site. Field measurements complemented with ERA-Interim reanalyses were used to drive the models and to evaluate simulation outputs. Snow height, density, temperature, thermal conductivity and thermal insulance are examined to determine the critical variables involved in the soil and snow thermal regime. Simulated soil properties are compared to measurements of thermal conductivity, temperature and water content. The simulated snow density profiles are unrealistic, which is most likely caused by the lack of representation in snow models of the upward water vapor fluxes generated by the strong temperature gradients within the snowpack. The resulting vertical profiles of thermal conductivity are inverted compared to observations, with high simulated values at the bottom of the snowpack. Still, ISBA-Crocus manages to successfully simulate the soil temperature in winter. Results are satisfactory in summer, but the temperature of the top soil could be better reproduced by adequately representing surface organic layers, i.e., mosses and litter, and in particular their water retention capacity. Transition periods (soil freezing and thawing) are the least well reproduced because the high basal snow thermal conductivity induces an excessively rapid heat transfer between the soil and the snow in simulations. Hence, global climate models should carefully consider Arctic snow thermal properties, and especially the thermal conductivity of the basal snow layer, to perform accurate predictions of the permafrost evolution under climate change.
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Bellier, J., Bontron, G., & Zin, I. (2017). Using Meteorological Analogues for Reordering Postprocessed Precipitation Ensembles in Hydrological Forecasting. Water Resources Research, 53(12), 10085–10107.
Abstract: Meteorological ensemble forecasts are nowadays widely used as input of hydrological models for probabilistic streamflow forecasting. These forcings are frequently biased and have to be statistically postprocessed, using most of the time univariate techniques that apply independently to individual locations, lead times and weather variables. Postprocessed ensemble forecasts therefore need to be reordered so as to reconstruct suitable multivariate dependence structures. The Schaake shuffle and ensemble copula coupling are the two most popular methods for this purpose. This paper proposes two adaptations of them that make use of meteorological analogues for reconstructing spatiotemporal dependence structures of precipitation forecasts. Performances of the original and adapted techniques are compared through a multistep verification experiment using real forecasts from the European Centre for Medium-Range Weather Forecasts. This experiment evaluates not only multivariate precipitation forecasts but also the corresponding streamflow forecasts that derive from hydrological modeling. Results show that the relative performances of the different reordering methods vary depending on the verification step. In particular, the standard Schaake shuffle is found to perform poorly when evaluated on streamflow. This emphasizes the crucial role of the precipitation spatiotemporal dependence structure in hydrological ensemble forecasting.
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Bellier, J., Zin, I., & Bontron, G. (2017). Sample Stratification in Verification of Ensemble Forecasts of Continuous Scalar Variables: Potential Benefits and Pitfalls. Monthly Weather Review, 145(9), 3529–3544.
Abstract: In the verification field, stratification is the process of dividing the sample of forecast-observation pairs into quasi-homogeneous subsets, in order to learn more on how forecasts behave under specific conditions. A general framework for stratification is presented for the case of ensemble forecasts of continuous scalar variables. Distinction is made between forecast-based, observation-based, and external-based stratification, depending on the criterion on which the sample is stratified. The formalism is applied to two widely used verification measures: the continuous ranked probability score (CRPS) and the rank histogram. For both, new graphical representations that synthesize the added information are proposed. Based on the definition of calibration, it is shown that the rank histogram should be used within a forecast-based stratification, while an observation-based stratification leads to significantly nonflat histograms for calibrated forecasts. Nevertheless, as previous studies have warned, statistical artifacts created by a forecast-based stratification may still occur, thus a graphical test to detect them is suggested. To illustrate potential insights about forecast behavior that can be gained from stratification, a numerical example with two different datasets of mean areal precipitation forecasts is presented.
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Bessieres, L., Leroux, S., Brankart, J. M., Molines, J. M., Moine, M. P., Bouttier, P. A., et al. (2017). Development of a probabilistic ocean modelling system based on NEMO 3.5: application at eddying resolution. Geoscientific Model Development, 10(3), 1091–1106.
Abstract: This paper presents the technical implementation of a new, probabilistic version of the NEMO ocean-sea-ice modelling system. Ensemble simulations with N members running simultaneously within a single executable, and interacting mutually if needed, are made possible through an enhanced message-passing interface (MPI) strategy including a double parallelization in the spatial and ensemble dimensions. An example application is then given to illustrate the implementation, performances, and potential use of this novel probabilistic modelling tool. A large ensemble of 50 global ocean-sea-ice hindcasts has been performed over the period 1960-2015 at eddy-permitting resolution (1/4 degrees) for the OCCIPUT (oceanic chaos – impacts, structure, predictability) project. This application aims to simultaneously simulate the intrinsic/chaotic and the atmospherically forced contributions to the ocean variability, from mesoscale turbulence to interannual-to-multidecadal timescales. Such an ensemble indeed provides a unique way to disentangle and study both contributions, as the forced variability may be estimated through the ensemble mean, and the intrinsic chaotic variability may be estimated through the ensemble spread.
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Bievre, G., Lacroix, P., Oxarango, L., Goutaland, D., Monnot, G., & Fargier, Y. (2017). Integration of geotechnical and geophysical techniques for the characterization of a small earth-filled canal dyke and the localization of water leakage. Journal Of Applied Geophysics, 139, 1–15.
Abstract: This paper investigates the combined use of extensive geotechnical, hydrogeological and geophysical techniques to assess a small earth dyke with a permanent hydraulic head, namely a canal embankment. The experimental site was chosen because of known issues regarding internal erosion and piping phenomena. Two leakages were visually located following the emptying of the canal prior to remediation works. The results showed a good agreement between the geophysical imaging techniques (Electrical Resistivity Tomography, P- and SH-waves Tomography) and the geotechnical data to detect the depth to the bedrock and its lateral variations. It appeared that surface waves might not be fully adapted for dyke investigation because of the particular geometry of the studied dyke, non-respectful of the 10 assumption, and which induced depth and velocity discrepancies retrieved from Rayleigh and Love waves inversion. The use of these classical prospecting techniques however did not allow to directly locate the two leakages within the studied earth dyke. The analysis of ambient vibration time series with a modified beam-forming algorithm allowed to localize the most energetic water flow prior to remediation works. It was not possible to detect the leakage after remediation works, suggesting that they efficiently contributed to significantly reduce the water flow. The second leakage was not detected probably because of a non-turbulent water flow, generating few energetic vibrations. (C) 2017 Elsevier B.V. All rights reserved.
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Blanchet, J., & Creutin, J. D. (2017). Co-Occurrence of Extreme Daily Rainfall in the French Mediterranean Region. Water Resources Research, 53(11), 9330–9349.
Abstract: We propose in this article a statistical framework to study local disparities in the co-occurrence of extreme rainfall in the French Mediterranean region. We employ a region-of-influence approach by studying the likelihood of the 3% largest daily rainfall to occur simultaneously at less than 50 km distance, when moving across the region. Our model uses an anisotropic max-stable process allowing us to properly represent the co-occurrence of daily extremes and including the possibility of a preferred direction of co-occurrence. We use this framework on a dense network composed of almost 900 daily stations spread over a 100,000 km(2) region of southern France under a Mediterranean influence, with data back to 1948. This density allows us to study the spatial patterns in the co-occurrence of extreme rainfall at fine scale and by so to characterize the main precipitation systems leading to extremes in the region. We show in particular that concomitant extremes are the most likely along the crest line of the Massif Central, which is also the area where the magnitude of extremes is among the largest. This may be of concern for flood risk management.
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Bock, M., Schmitt, J., Beck, J., Seth, B., Chappellaz, J., & Fischer, H. (2017). Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH4 ice core records. Proceedings Of The National Academy Of Sciences Of The United States Of America, 114(29), E5778–E5786.
Abstract: Atmospheric methane (CH4) records reconstructed from polar ice cores represent an integrated view on processes predominantly taking place in the terrestrial biogeosphere. Here, we present dual stable isotopic methane records [delta(CH4)-C-13 and delta D(CH4)] from four Antarctic ice cores, which provide improved constraints on past changes in natural methane sources. Our isotope data show that tropical wetlands and seasonally inundated floodplains are most likely the controlling sources of atmospheric methane variations for the current and two older interglacials and their preceding glacial maxima. The changes in these sources are steered by variations in temperature, precipitation, and the water table as modulated by insolation, (local) sea level, and monsoon intensity. Based on our delta D(CH4) constraint, it seems that geologic emissions of methane may play a steady but only minor role in atmospheric CH4 changes and that the glacial budget is not dominated by these sources. Superimposed on the glacial/interglacial variations is a marked difference in both isotope records, with systematically higher values during the last 25,000 y compared with older time periods. This shift cannot be explained by climatic changes. Rather, our isotopic methane budget points to a marked increase in fire activity, possibly caused by biome changes and accumulation of fuel related to the late Pleistocene megafauna extinction, which took place in the course of the last glacial.
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Bozzetti, C., El Haddad, I., Salameh, D., Daellenbach, K. R., Fermo, P., Gonzalez, R., et al. (2017). Organic aerosol source apportionment by offline-AMS over a full year in Marseille. Atmospheric Chemistry And Physics, 17(13), 8247–8268.
Abstract: We investigated the seasonal trends of OA sources affecting the air quality of Marseille (France), which is the largest harbor of the Mediterranean Sea. This was achieved by measurements of nebulized filter extracts using an aerosol mass spectrometer (offline-AMS). In total 216 PM2.5 (particulate matter with an aerodynamic diameter < 2.5 μm) filter samples were collected over 1 year from August 2011 to July 2012. These filters were used to create 54 composite samples which were analyzed by offline-AMS. The same samples were also analyzed for major water-soluble ions, metals, elemental and organic carbon (EC / OC), and organic markers, including n-alkanes, hopanes, polycyclic aromatic hydrocarbons (PAHs), lignin and cellulose pyrolysis products, and nitrocatechols. The application of positive matrix factorization (PMF) to the water-soluble AMS spectra enabled the extraction of five factors, related to hydrocarbonlike OA (HOA), cooking OA (COA), biomass burning OA (BBOA), oxygenated OA (OOA), and an industry-related OA (INDOA). Seasonal trends and relative contributions of OA sources were compared with the source apportionment of OA spectra collected from the AMS field deployment at the same station but in different years and for shorter monitoring periods (February 2011 and July 2008). Online-and offline-AMS source apportionment revealed comparable seasonal contribution of the different OA sources. Results revealed that BBOA was the dominant source during winter, representing on average 48% of the OA, while during summer the main OA component was OOA (63% of OA mass on average). HOA related to traffic emissions contributed on a yearly average 17% to the OA mass, while COA was a minor source contributing 4 %. The contribution of INDOA was enhanced during winter (17% during winter and 11% during summer), consistent with an increased contribution from light alkanes, light PAHs (fluoranthene, pyrene, phenanthrene), and selenium, which is commonly considered as a unique coal combustion and coke production marker. Online-and offline-AMS source apportionments revealed evolving levoglucosan : BBOA ratios, which were higher during late autumn and March. A similar seasonality was observed in the ratios of cellulose combustion markers to lignin combustion markers, highlighting the contribution from cellulose-rich biomass combustion, possibly related to agricultural activities.
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Bozzetti, C., Sosedova, Y., Xiao, M., Daellenbach, K. R., Ulevicius, V., Dudoitis, V., et al. (2017). Argon offline-AMS source apportionment of organic aerosol over yearly cycles for an urban, rural, and marine site in northern Europe. Atmospheric Chemistry And Physics, 17(1), 117–141.
Abstract: The widespread use of Aerodyne aerosol mass spectrometers (AMS) has greatly improved real-time organic aerosol (OA) monitoring, providing mass spectra that contain sufficient information for source apportionment. However, AMS field deployments remain expensive and demanding, limiting the acquisition of long-term datasets at many sampling sites. The offline application of aerosol mass spectrometry entailing the analysis of nebulized water extracted filter samples (offline-AMS) increases the spatial coverage accessible to AMS measurements, being filters routinely collected at many stations worldwide. PM1 (particulate matter with an aerodynamic diameter <1 μm) filter samples were collected during an entire year in Lithuania at three different locations representative of three typical environments of the southeast Baltic region: Vilnius (urban background), Rugsteliskis (rural terrestrial), and Preila (rural coastal). Aqueous filter extracts were nebulized in Ar, yielding the first AMS measurements of water-soluble atmospheric organic aerosol (WSOA) without interference from air fragments. This enables direct measurement of the CO+ fragment contribution, whose intensity is typically assumed to be equal to that of CO2+. Offline-AMS spectra reveal that the water-soluble CO2+:CO+ ratio not only shows values systematically >1 but is also dependent on season, with lower values in winter than in summer. AMS WSOA spectra were analyzed using positive matrix factorization (PMF), which yielded four factors. These factors included biomass burning OA (BBOA), local OA (LOA) contributing significantly only in Vilnius, and two oxygenated OA (OOA) factors, summer OOA (S-OOA) and background OOA (B-OOA), distinguished by their seasonal variability. The contribution of traffic exhaust OA (TEOA) was not resolved by PMF due to both low concentrations and low water solubility. Therefore, the TEOA concentration was estimated using a chemical mass balance approach, based on the concentrations of hopanes, specific markers of traffic emissions. AMS-PMF source apportionment results were consistent with those obtained from PMF applied to marker concentrations (i.e., major inorganic ions, OC/EC, and organic markers including polycyclic aromatic hydrocarbons and their derivatives, hopanes, long-chain alkanes, monosaccharides, anhydrous sugars, and lignin fragmentation products). OA was the largest fraction of PM1 and was dominated by BBOA during winter with an average concentration of 2 μg m(-3) (53% of OM), while S-OOA, probably related to biogenic emissions, was the prevalent OA component during summer with an average concentration of 1.2 μg m(-3) (45% of OM). PMF ascribed a large part of the CO+ explained variability ( 97 %) to the OOA and BBOA factors. Accordingly, we discuss a new CO+ parameterization as a function of CO2+ and C2H4O2+ fragments, which were selected to describe the variability of the OOA and BBOA factors.
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Braud, I., Desprats, J. F., Ayral, P. A., Bouvier, C., & Vandervaere, J. P. (2017). Mapping topsoil field-saturated hydraulic conductivity from point measurements using different methods. Journal Of Hydrology And Hydromechanics, 65(3), 264–275.
Abstract: Topsoil field-saturated hydraulic conductivity, Kf(s), is a parameter that controls the partition of rainfall between infiltration and runoff and is a key parameter in most distributed hydrological models. There is a mismatch between the scale of local in situ Kf(s) measurements and the scale at which the parameter is required in models for regional mapping. Therefore methods for extrapolating local Kf(s) values to larger mapping units are required. The paper explores the feasibility of mapping Kf(s) in the Cevennes-Vivarais region, in south-east France, using more easily available GIS data concerning geology and land cover. Our analysis makes uses of a data set from infiltration measurements performed in the area and its vicinity for more than ten years. The data set is composed of Kf(s) derived from infiltration measurements performed using various methods: Guelph permeameters, double ring and single ring infiltrotrometers and tension infiltrometers. The different methods resulted in a large variation in Kf(s) up to several orders of magnitude. A method is proposed to pool the data from the different infiltration methods to create an equivalent set of Kf(s). Statistical tests showed significant differences in Kf(s) distributions in function of different geological formations and land cover. Thus the mapping of Kf(s) at regional scale was based on geological formations and land cover. This map was compared to a map based on the Rawls and Brakensiek (RB) pedotransfer function (mainly based on texture) and the two maps showed very different patterns. The RB values did not fit observed equivalent Kf(s) at the local scale, highlighting that soil texture alone is not a good predictor of Kf(s).
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Breant, C., Martinerie, P., Orsi, A., Arnaud, L., & Landais, A. (2017). Modelling firn thickness evolution during the last deglaciation: constraints on sensitivity to temperature and impurities. Climate Of The Past, 13(7), 833–853.
Abstract: The transformation of snow into ice is a complex phenomenon that is difficult to model. Depending on surface temperature and accumulation rate, it may take several decades to millennia for air to be entrapped in ice. The air is thus always younger than the surrounding ice. The resulting gas-ice age difference is essential to documenting the phasing between CO2 and temperature changes, especially during deglaciations. The air trapping depth can be inferred in the past using a firn densification model, or using delta N-15 of air measured in ice cores. All firn densification models applied to deglaciations show a large disagreement with delta N-15 measurements at several sites in East Antarctica, predicting larger firn thickness during the Last Glacial Maximum, whereas delta N-15 suggests a reduced firn thickness compared to the Holocene. Here we present modifications of the LGGE firn densification model, which significantly reduce the model-data mismatch for the gas trapping depth evolution over the last deglaciation at the coldest sites in East Antarctica (Vostok, Dome C), while preserving the good agreement between measured and modelled modern firn density profiles. In particular, we introduce a dependency of the creep factor on temperature and impurities in the firn densification rate calculation. The temperature influence intends to reflect the dominance of different mechanisms for firn compaction at different temperatures. We show that both the new temperature parameterization and the influence of impurities contribute to the increased agreement between modelled and measured delta N-15 evolution during the last deglaciation at sites with low temperature and low accumulation rate, such as Dome C or Vostok. We find that a very low sensitivity of the densification rate to temperature has to be used in the coldest conditions. The inclusion of im-purity effects improves the agreement between modelled and measured delta N-15 at cold East Antarctic sites during the last deglaciation, but deteriorates the agreement between modelled and measured delta N-15 evolution at Greenland and Antarctic sites with high accumulation unless threshold effects are taken into account. We thus do not provide a definite solution to the firnification at very cold Antarctic sites but propose potential pathways for future studies.
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Brienza, M., Duwig, C., Perez, S., & Chiron, S. (2017). 4-nitroso-sulfamethoxazole generation in soil under denitrifying conditions: Field observations versus laboratory results. Journal Of Hazardous Materials, 334, 185–192.
Abstract: The formation of 4-nitroso-sulfamethoxazole and 4-nitro-SMX, two transformation products (TPs) of sulfamethoxazole (SMX) was investigated under batch soil slurry experiments and in a field study. Due to their low occurrence levels (ng/L) in environmental waters, a suitable analytical method based on liquid chromatography – high resolution – mass spectrometry was developed. Consequently, field observations revealed, for the first time, the occurrence of 4-nitroso-SMX in groundwater at concentrations as high as 18 ng/L.Nitric oxide (NO center dot) steady-state concentrations were determined in soil slurry experiments because this reactive specie accounted for the formation of 4-nitroso-SMX and 4-nitro-SMX. Measurements revealed that environmental SMX concentrations (0.2-2 μg/L) at neutral pH induced the accumulation of nitric oxide. Under acidic conditions (pH< 6), nitrous acid (HONO) was the major source of nitric oxide while under neutral/basic conditions nitric oxide release was related to the inhibition of denitrification processes. Under laboratory experiments, SMX nitration reaction appeared to be an irreversible transformation pathway, while 4-nitroso-SMX was slowly transformed over time. The occurrence of 4-nitroso-SMX conditions was therefore unexpected in the field study but could be due to its continuous input from soil and/or its relative persistence under anoxic conditions. A mechanism for 4-nitroso-SMX formation was proposed involving a nitrosative desamination pathway through a phenyl radical. (C) 2017 Elsevier B.V. All rights reserved.
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Brodeau, L., Barnier, B., Gulev, S. K., & Woods, C. (2017). Climatologically Significant Effects of Some Approximations in the Bulk Parameterizations of Turbulent Air-Sea Fluxes. Journal Of Physical Oceanography, 47(1), 5–28.
Abstract: This paper quantifies the impacts of approximations and assumptions in the parameterization of bulk formulas on the exchange of momentum, heat, and freshwater computed between the ocean and atmosphere. An ensemble of sensitivity experiments is examined. Climatologies of wind stress, turbulent heat flux, and evaporation for the period 1982-2014 are computed using SST and surface meteorological state variables from ERA-Interim. Each experiment differs from the defined control experiment in only one aspect of the parameterization of the bulk formulas. The wind stress is most sensitive to the closure used to relate the neutral drag coefficient to the wind speed in the bulk algorithm, which mainly involves the value of the Charnock parameter. The disagreement between the state-of-the-art algorithms examined is typically on the order of 10%. The largest uncertainties in turbulent heat flux and evaporation are also related to the choice of the algorithm (typically 15%) but also emerge in experiments examining approximations related to the surface temperature and saturation humidity. Thus, approximations for the skin temperature and the saltrelated reduction of saturation humidity have a substantial impact on the heat flux and evaporation (typically 10%). Approximations such as the use of a fixed air density, sea level pressure, or simplified formula for the saturation humidity lead to errors no larger than 4% when tested individually. The impacts of these approximations combine linearly when implemented together, yielding errors up to 20% over mid-and subpolar latitudes.
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Brondex, J., Gagliardini, O., Gillet-Chaulet, F., & Durand, G. (2017). Sensitivity of grounding line dynamics to the choice of the friction law. Journal Of Glaciology, 63(241), 854–866.
Abstract: Basal slip accounts for a large part of the flow of ice streams draining ice from Antarctica and Greenland into the ocean. Therefore, an appropriate representation of basal slip in ice flow models is a prerequisite for accurate sea level rise projections. Various friction laws have been proposed to describe basal slip in models. Here, we compare the influence on grounding line (GL) dynamics of four friction laws: the traditional Weertman law and three effective pressure-dependent laws, namely the Schoof, Tsai and Budd laws. It turns out that, even when they are tuned to a common initial reference state, the Weertman, Budd and Schoof laws lead to thoroughly different steady-state positions, although the Schoof and Tsai laws lead to much the same result. In particular, under certain circumstances, it is possible to obtain a steady GL located on a reverse slope area using the Weertman law. Furthermore, the predicted transient evolution of the GL as well as the projected contributions to sea level rise over a 100-year time horizon vary significantly depending on the friction law. We conclude on the importance of choosing an appropriate law for reliable sea level rise projections and emphasise the need for a coupling between ice flow models and physically based subglacial hydrological models.
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Brun, F., Berthier, E., Wagnon, P., Kaab, A., & Treichler, D. (2017). A spatially resolved estimate of High Mountain Asia glacier mass balances from 2000 to 2016. Nature Geoscience, 10(9), 668–+.
Abstract: High Mountain Asia hosts the largest glacier concentration outside the polar regions. These glaciers are important contributors to streamflow in one of the most populated areas of the world. Past studies have used methods that can provide only regionally averaged glacier mass balances to assess the glacier contribution to rivers and sea level rise. Here we compute the mass balance for about 92% of the glacierized area of High Mountain Asia using time series of digital elevation models derived from satellite stereo-imagery. We calculate a total mass change of -16.3 +/- 3.5 Gt yr(-1) (-0.18 +/- 0.04 m w.e. yr(-1)) between 2000 and 2016, which is less negative than most previous estimates. Region-wide mass balances vary from 4.0 +/- 1.5 Gt yr(-1) (-0.62 +/- 0.23 m w.e. yr(-1)) in Nyainqentanglha to +1.4 +/- 0.8 Gt yr(-1) (+0.14 +/- 0.08 m w.e. yr(-1)) in Kunlun, with large intra-regional variability of individual glacier mass balances (standard deviation within a region similar to 0.20m w.e. yr(-1)). Specifically, our results shed light on the Nyainqentanglha and Pamir glacier mass changes, for which contradictory estimates exist in the literature. They provide crucial information for the calibration of the models used for projecting glacier response to climatic change, as these models do not capture the pattern, magnitude and intra-regional variability of glacier changes at present.
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Brunner, M. I., Viviroli, D., Sikorska, A. E., Vannier, O., Favre, A. - C., & Seibert, J. (2017). Flood type specific construction of synthetic design hydrographs. Water Resources Research, 53(2), 1390–1406.
Abstract: Accurate estimates of flood peaks, corresponding volumes, and hydrographs are required to design safe and cost-effective hydraulic structures. In this paper, we propose a statistical approach for the estimation of the design variables peak and volume by constructing synthetic design hydrographs for different flood types such as flash-floods, short-rain floods, long-rain floods, and rain-on-snow floods. Our approach relies on the fitting of probability density functions to observed flood hydrographs of a certain flood type and accounts for the dependence between peak discharge and flood volume. It makes use of the statistical information contained in the data and retains the process information of the flood type. The method was tested based on data from 39 mesoscale catchments in Switzerland and provides catchment specific and flood type specific synthetic design hydrographs for all of these catchments. We demonstrate that flood type specific synthetic design hydrographs are meaningful in flood-risk management when combined with knowledge on the seasonality and the frequency of different flood types.
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Bull, C. Y. S., Kiss, A. E., Jourdain, N. C., England, M. H., & van Sebille, E. (2017). Wind Forced Variability in Eddy Formation, Eddy Shedding, and the Separation of the East Australian Current. Journal Of Geophysical Research-Oceans, 122(12), 9980–9998.
Abstract: The East Australian Current (EAC), like many other subtropical western boundary currents, is believed to be penetrating further poleward in recent decades. Previous observational and model studies have used steady state dynamics to relate changes in the westerly winds to changes in the separation behavior of the EAC. As yet, little work has been undertaken on the impact of forcing variability on the EAC and Tasman Sea circulation. Here using an eddy-permitting regional ocean model, we present a suite of simulations forced by the same time-mean fields, but with different atmospheric and remote ocean variability. These eddy-permitting results demonstrate the nonlinear response of the EAC to variable, nonstationary inhomogeneous forcing. These simulations show an EAC with high intrinsic variability and stochastic eddy shedding. We show that wind stress variability on time scales shorter than 56 days leads to increases in eddy shedding rates and southward eddy propagation, producing an increased transport and southward reach of the mean EAC extension. We adopt an energetics framework that shows the EAC extension changes to be coincident with an increase in offshore, upstream eddy variance (via increased barotropic instability) and increase in subsurface mean kinetic energy along the length of the EAC. The response of EAC separation to regional variable wind stress has important implications for both past and future climate change studies.
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Burke, E. J., Ekici, A., Huang, Y., Chadburn, S. E., Huntingford, C., Ciais, P., et al. (2017). Quantifying uncertainties of permafrost carbon-climate feedbacks. Biogeosciences, 14(12), 3051–3066.
Abstract: The land surface models JULES (Joint UK Land Environment Simulator, two versions) and ORCHIDEE-MICT (Organizing Carbon and Hydrology in Dynamic Ecosystems), each with a revised representation of permafrost carbon, were coupled to the Integrated Model Of Global Effects of climatic aNomalies (IMOGEN) intermediate-complexity climate and ocean carbon uptake model. IMOGEN calculates atmospheric carbon dioxide (CO2) and local monthly surface climate for a given emission scenario with the land-atmosphere CO2 flux exchange from either JULES or ORCHIDEE-MICT. These simulations include feedbacks associated with permafrost carbon changes in a warming world. Both IMOGEN-JULES and IMOGEN-ORCHIDEE-MICT were forced by historical and three alternative future-CO2-emission scenarios. Those simulations were performed for different climate sensitivities and regional climate change patterns based on 22 different Earth system models (ESMs) used for CMIP3 (phase 3 of the Coupled Model Intercomparison Project), allowing us to explore climate uncertainties in the context of permafrost carbon-climate feedbacks. Three future emission scenarios consistent with three representative concentration pathways were used: RCP2.6, RCP4.5 and RCP8.5. Paired simulations with and without frozen carbon processes were required to quantify the impact of the permafrost carbon feedback on climate change. The additional warming from the permafrost carbon feedback is between 0.2 and 12% of the change in the global mean temperature (Delta T) by the year 2100 and 0.5 and 17% of Delta T by 2300, with these ranges reflecting differences in land surface models, climate models and emissions pathway. As a percentage of Delta T, the permafrost carbon feedback has a greater impact on the low-emissions scenario (RCP2.6) than on the higher-emissions scenarios, suggesting that permafrost carbon should be taken into account when evaluating scenarios of heavy mitigation and stabilization. Structural differences between the land surface models (particularly the representation of the soil carbon decomposition) are found to be a larger source of uncertainties than differences in the climate response. Inertia in the permafrost carbon system means that the permafrost carbon response depends on the temporal trajectory of warming as well as the absolute amount of warming. We propose a new policy-relevant metric – the frozen carbon residence time (FCRt) in years – that can be derived from these complex land surface models and used to quantify the permafrost carbon response given any pathway of global temperature change.
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Burr, A., Noel, W., Trecourt, P., Bourcier, M., Gillet-Chaulet, F., Philip, A., et al. (2017). The anisotropic contact response of viscoplastic monocrystalline ice particles. Acta Materialia, 132, 576–585.
Abstract: The contact between two particles of ice is studied experimentally and modeled. Experiments on the gradual indentation of a single or on two monocrystalline cylinders of ice demonstrate the effect of the relative orientation of their basal planes with the loading axis. Using polarized light and Digital Image Correlation, the main deformation mechanisms are documented. Experimental observations show the strong anisotropy of deformation, with strain localization into shear bands, corresponding to basal gliding of dislocations. When the rotation of cylinders is hindered by a large initial contact size, a simple model is proposed. It is based on indentation theory and takes into account the preferential viscoplastic deformation on the basal plane. The model form is validated by experimental data and by finite element simulations that incorporate the transverse isotropic law of an ice crystal. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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Calas, A., Uzu, G., Martins, J. M. F., Voisin, D., Spadini, L., Lacroix, T., et al. (2017). The importance of simulated lung fluid (SLF) extractions for a more relevant evaluation of the oxidative potential of particulate matter. Scientific Reports, 7.
Abstract: Particulate matter (PM) induces oxidative stress in vivo, leading to adverse health effects. Oxidative potential (OP) of PM is increasingly studied as a relevant metric for health impact (instead of PM mass concentration) as much of the ambient particle mass do not contribute to PM toxicity. Several assays have been developed to quantify PM oxidative potential and a widely used one is the acellular dithiothreitol (DTT) assay. However in such assays, particles are usually extracted with methanol or Milli-Q water which is unrepresentative of physiological conditions. For this purpose, OPDTT measurements after simulated lung fluids (SLF) extraction, in order to look at the impact of simulated lung fluid constituents, were compared to Milli-Q water extraction measurements. Our major finding is a significant decrease of the OPDTT when the artificial lysosomal fluid (ALF) solution was used. Indeed, ligand compounds are present in the SLF solutions and some induce a decrease of the OP when compared to water extraction. Our results suggest that the effect of ligands and complexation in lining fluids towards PM contaminants probably has been underestimated and should be investigated further.
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Calonne, N., Montagnat, M., Matzl, M., & Schneebeli, M. (2017). The layered evolution of fabric and microstructure of snow at Point Barnola, Central East Antarctica. Earth And Planetary Science Letters, 460, 293–301.
Abstract: Snow fabric, defined as the distribution of the c-axis orientations of the ice crystals in snow, is poorly known. So far, only one study exits that measured snow fabric based on a statistically representative technique. This recent study has revealed the impact of temperature gradient metamorphism on the evolution of fabric in natural snow, based on cold laboratory experiments. On polar ice sheets, snow properties are currently investigated regarding their strong variability in time and space, notably because of their potential influence on firn processes and consequently on ice core analysis. Here, we present measurements of fabric and microstructure of snow from Point Barnola, East Antarctica (close to Dome C). We analyzed a snow profile from 0 to 3 m depth, where temperature gradients occur. The main contributions of the paper are (1) a detailed characterization of snow in the upper meters of the ice sheet, especially by providing data on snow fabric, and (2) the study of a fundamental snow process, never observed up to now in a natural snowpack, namely the role of temperature gradient metamorphism on the evolution of the snow fabric. Snow samples were scanned by micro-tomography to measure continuous profiles of microstructural properties (density, specific surface area and pore thickness). Fabric analysis was performed using an automatic ice texture analyzer on 77 representative thin sections cut out from the samples. Different types of snow fabric could be identified and persist at depth. Snow fabric is significantly correlated with snow microstructure, pointing to the simultaneous influence of temperature gradient metamorphism on both properties. We propose a mechanism based on preferential grain growth to explain the fabric evolution under temperature gradients. Our work opens the question of how such a layered profile of fabric and microstructure evolves at depth and further influences the physical and mechanical properties of snow and firn. More generally, it opens the way to further studies on the influence of the snow fabric in snow processes related to anisotropic properties of ice such as grain growth, mechanical response, electromagnetic behavior. (C) 2016 The Author(s). Published by Elsevier B.V.
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Chadburn, S. E., Krinner, G., Porada, P., Bartsch, A., Beer, C., Marchesini, L. B., et al. (2017). Carbon stocks and fluxes in the high latitudes: using site-level data to evaluate Earth system models. Biogeosciences, 14(22), 5143–5169.
Abstract: It is important that climate models can accurately simulate the terrestrial carbon cycle in the Arctic due to the large and potentially labile carbon stocks found in permafrost-affected environments, which can lead to a positive climate feedback, along with the possibility of future carbon sinks from northward expansion of vegetation under climate warming. Here we evaluate the simulation of tundra carbon stocks and fluxes in three land surface schemes that each form part of major Earth system models (JSBACH, Germany; JULES, UK; ORCHIDEE, France). We use a site-level approach in which comprehensive, high-frequency datasets allow us to disentangle the importance of different processes. The models have improved physical permafrost processes and there is a reasonable correspondence between the simulated and measured physical variables, including soil temperature, soil moisture and snow. We show that if the models simulate the correct leaf area index (LAI), the standard C3 photosynthesis schemes produce the correct order of magnitude of carbon fluxes. Therefore, simulating the correct LAI is one of the first priorities. LAI depends quite strongly on climatic variables alone, as we see by the fact that the dynamic vegetation model can simulate most of the differences in LAI between sites, based almost entirely on climate inputs. However, we also identify an influence from nutrient limitation as the LAI becomes too large at some of the more nutrient-limited sites. We conclude that including moss as well as vascular plants is of primary importance to the carbon budget, as moss contributes a large fraction to the seasonal CO2 flux in nutrient-limited conditions. Moss photosynthetic activity can be strongly influenced by the moisture content of moss, and the carbon uptake can be significantly different from vascular plants with a similar LAI. The soil carbon stocks depend strongly on the rate of input of carbon from the vegetation to the soil, and our analysis suggests that an improved simulation of photosynthesis would also lead to an improved simulation of soil carbon stocks. However, the stocks are also influenced by soil carbon burial (e.g. through cryoturbation) and the rate of heterotrophic respiration, which depends on the soil physical state. More detailed below-ground measurements are needed to fully evaluate biological and physical soil processes. Furthermore, even if these processes are well modelled, the soil carbon profiles cannot resemble peat layers as peat accumulation processes are not represented in the models. Thus, we identify three priority areas for model development: (1) dynamic vegetation including (a) climate and (b) nutrient limitation effects; (2) adding moss as a plant functional type; and an (3) improved vertical profile of soil carbon including peat processes.
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Chauve, T., Montagnat, M., Barou, F., Hidas, K., Tommasi, A., & Mainprice, D. (2017). Investigation of nucleation processes during dynamic recrystallization of ice using cryo-EBSD. Philosophical Transactions Of The Royal Society A-Mathematical Physical And Engineering Sciences, 375(2086).
Abstract: Nucleation mechanisms occurring during dynamic recrystallization play a crucial role in the evolution of microstructures and textures during high temperature deformation. In polycrystalline ice, the strong viscoplastic anisotropy induces high strain heterogeneities between grains which control the recrystallization mechanisms. Here, we study the nucleation mechanisms occurring during creep tests performed on polycrystalline columnar ice at high temperature and stress (T=-5 degrees C; sigma = 0.5 MPa) by post-mortem analyses of deformation microstructures using cryogenic electron backscatter diffraction. The columnar geometry of the samples enables discrimination of the nuclei from the initial grains. Various nucleation mechanisms are deduced from the analysis of the nuclei relations with the dislocation sub-structures within grains and at grain boundaries. Tilt sub-grain boundaries and kink bands are the main structures responsible for development of polygonization and mosaic sub-structures. Nucleation by bulging at serrated grain boundaries is also an efficient nucleation mechanism near the grain boundaries where strain incompatibilities are high. Observation of nuclei with orientations not related to the 'parent' ones suggests the possibility of 'spontaneous' nucleation driven by the relaxation of the dislocation-related internal stress field. The complexity of the nucleation mechanisms observed here emphasizes the impact of stress and strain heterogeneities on dynamic recrystallization mechanisms. This article is part of the themed issue 'Microdynamics of ice'.
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Chauve, T., Montagnat, M., Lachaud, C., Georges, D., & Vacher, P. (2017). Strain field evolution at the ductile-to-brittle transition: a case study on ice. Solid Earth, 8(5).
Abstract: This paper presents, for the first time, the evolution of the local heterogeneous strain field around intra-granular cracking in polycrystalline ice, at the onset of tertiary creep. Owing to the high homologous temperature conditions and relatively low compressive stress applied, stress concentration at the crack tips is relaxed by plastic mechanisms associated with dynamic recrystallization. Strain field evolution followed by digital image correlation (DIC) directly shows the redistribution of strain during crack opening, but also the redistribution driven by crack tip plasticity mechanisms and recrystallization. Associated local changes in microstructure induce modifications of the local stress field evidenced by crack closure during deformation. At the ductile-to-brittle transition in ice, micro-cracking and dynamic recrystallization mechanisms can co-exist and interact, the later being efficient to relax stress concentration at the crack tips.
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Chauve, T., Montagnat, M., Piazolo, S., Journaux, B., Wheeler, J., Barou, F., et al. (2017). Non-basal dislocations should be accounted for in simulating ice mass flow. Earth And Planetary Science Letters, 473, 247–255.
Abstract: Prediction of ice mass flow and associated dynamics is pivotal at a time of climate change. Ice flow is dominantly accommodated by the motion of crystal defects – the dislocations. In the specific case of ice, their observation is not always accessible by means of the classical tools such as X-ray diffraction or transmission electron microscopy (TEM). Part of the dislocation population, the geometrically necessary dislocations (GNDs) can nevertheless be constrained using crystal orientation measurements via electron backscattering diffraction (EBSD) associated with appropriate analyses based on the Nye (1950) approach. The present study uses the Weighted Burgers Vectors, a reduced formulation of the Nye theory that enables the characterization of GNDs. Applied to ice, this method documents, for the first time, the presence of dislocations with non-basal [c] or < c + a > Burgers vectors. These [c] or (c + a) dislocations represent up to 35% of the GNDs observed in laboratory-deformed ice samples. Our findings offer a more complex and comprehensive picture of the key plasticity processes responsible for polycrystalline ice creep and provide better constraints on the constitutive mechanical laws implemented in ice sheet flow models used to predict the response of Earth ice masses to climate change. (C) 2017 Elsevier B.V. All rights reserved.
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Colliander, A., Jackson, T. J., Bindlish, R., Chan, S., Das, N., Kim, S. B., et al. (2017). Validation of SMAP surface soil moisture products with core validation sites. Remote Sensing Of Environment, 191, 215–231.
Abstract: The NASA Soil Moisture Active Passive (SMAP) mission has utilized a set of core validation sites as the primary methodology in assessing the soil moisture retrieval algorithm performance. Those sites provide well calibrated in situ soil moisture measurements within SMAP product grid pixels for diverse conditions and locations. The estimation of the average soil moisture within the SMAP product grid pixels based on in situ measurements is more reliable when location specific calibration of the sensors has been performed and there is adequate replication over the spatial domain, with an up-scaling function based on analysis using independent estimates of the soil moisture distribution. SMAP fulfilled these requirements through a collaborative Cal/Val Partner program. This paper presents the results from 34 candidate core validation sites for the first eleven months of the SMAP mission. As a result of the screening of the sites prior to the availability of SMAP data, out of the 34 candidate sites 18 sites fulfilled all the requirements at one of the resolution scales (at least). The rest of the sites are used as secondary information in algorithm evaluation. The results indicate that the SMAP radiometer-based soil moisture data product meets its expected performance of 0.04 m(3)/m(3) volumetric soil moisture (unbiased root mean square error); the combined radar-radiometer product is close to its expected performance of 0.04 m(3)/m(3), and the radar-based product meets its target accuracy of 0.06 m(3)/m(3) (the lengths of the combined and radar-based products are truncated to about 10 weeks because of the SMAP radar failure). Upon completing the intensive Cal/Val phase of the mission the SMAP project will continue to enhance the products in the primary and extended geographic domains, in co-operation with the Cal/Val Partners, by continuing the comparisons over the existing core validation sites and inclusion of candidate sites that can address shortcomings. (C) 2017 Elsevier Inc. All rights reserved.
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D'Amboise, C. J. L., Muller, K., Oxarango, L., Morin, S., & Schuler, T. V. (2017). Implementation of a physically based water percolation routine in the Crocus/SURFEX (V7.3) snowpack model. Geoscientific Model Development, 10(9), 3547–3566.
Abstract: We present a new water percolation routine added to the one-dimensional snowpack model Crocus as an alternative to the empirical bucket routine. This routine solves the Richards equation, which describes flow of water through unsaturated porous snow governed by capillary suction, gravity and hydraulic conductivity of the snow layers. We tested the Richards routine on two data sets, one recorded from an automatic weather station over the winter of 2013-2014 at Filefjell, Norway, and the other an idealized synthetic data set. Model results using the Richards routine generally lead to higher water contents in the snow layers. Snow layers often reached a point at which the ice crystals' surface area is completely covered by a thin film of water (the transition between pendular and funicular regimes), at which feedback from the snow metamorphism and compaction routines are expected to be nonlinear. With the synthetic simulation 18% of snow layers obtained a saturation of > 10% and 0.57% of layers reached saturation of > 15 %. The Richards routine had a maximum liquid water content of 173.6 kg m(-3) whereas the bucket routine had a maximum of 42.1 kg m(-3). We found that wet-snow processes, such as wet-snow metamorphism and wet-snow compaction rates, are not accurately represented at higher water contents. These routines feed back on the Richards routines, which rely heavily on grain size and snow density. The parameter sets for the water retention curve and hydraulic conductivity of snow layers, which are used in the Richards routine, do not represent all the snow types that can be found in a natural snowpack. We show that the new routine has been implemented in the Crocus model, but due to feedback amplification and parameter uncertainties, meaningful applicability is limited. Updating or adapting other routines in Crocus, specifically the snow compaction routine and the grain metamorphism routine, is needed before Crocus can accurately simulate the snowpack using the Richards routine.
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Daellenbach, K. R., Stefenelli, G., Bozzetti, C., Vlachou, A., Fermo, P., Gonzalez, R., et al. (2017). Long-term chemical analysis and organic aerosol source apportionment at nine sites in central Europe: source identification and uncertainty assessment. Atmospheric Chemistry And Physics, 17(21), 13265–13282.
Abstract: Long-term monitoring of organic aerosol is important for epidemiological studies, validation of atmospheric models, and air quality management. In this study, we apply a recently developed filter-based offline methodology using an aerosol mass spectrometer (AMS) to investigate the regional and seasonal differences of contributing organic aerosol sources. We present offline AMS measurements for particulate matter smaller than 10 μm at nine stations in central Europe with different exposure characteristics for the entire year of 2013 (819 samples). The focus of this study is a detailed source apportionment analysis (using positive matrix factorization, PMF) including in-depth assessment of the related uncertainties. Primary organic aerosol (POA) is separated in three components: hydrocarbon-like OA related to traffic emissions (HOA), cooking OA (COA), and biomass burning OA (BBOA). We observe enhanced production of secondary organic aerosol (SOA) in summer, following the increase in biogenic emissions with temperature (summer oxygenated OA, SOOA). In addition, a SOA component was extracted that correlated with an anthropogenic secondary inorganic species that is dominant in winter (winter oxy-genated OA, WOOA). A factor (sulfur-containing organic, SC-OA) explaining sulfur-containing fragments (CH3SO2+),which has an event-driven temporal behaviour, was also identified. The relative yearly average factor contributions range from 4 to 14% for HOA, from 3 to 11% for COA, from 11 to 59% for BBOA, from 5 to 23% for SC-OA, from 14 to 27% forWOOA, and from 15 to 38% for SOOA. The uncertainty of the relative average factor contribution lies between 2 and 12% of OA. At the sites north of the alpine crest, the sum of HOA, COA, and BBOA (POA) contributes less to OA (POA/OA = 0.3) than at the southern alpine valley sites (0.6). BBOA is the main contributor to POA with 87% in alpine valleys and 42% north of the alpine crest. Furthermore, the influence of primary biological particles (PBOAs), not resolved by PMF, is estimated and could contribute significantly to OA in PM10.
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Dangles, O., Rabatel, A., Kraemer, M., Zeballos, G., Soruco, A., Jacobsen, D., et al. (2017). Ecosystem sentinels for climate change? Evidence of wetland cover changes over the last 30 years in the tropical Andes. Plos One, 12(5).
Abstract: While the impacts of climate change on individual species and communities have been well documented there is little evidence on climate-mediated changes for entire ecosystems. Pristine alpine environments can provide unique insights into natural, physical and ecological response to climate change yet broad scale and long-term studies on these potential 'ecosystem sentinels' are scarce. We addressed this issue by examining cover changes of 1689 high-elevation wetlands (temporarily or perennial water-saturated grounds) in the Bolivian Cordillera Real, a region that has experienced significant warming and glacier melting over the last 30 years. We combined high spatial resolution satellite images from PLEIADES with the long-term images archive from LANDSAT to 1) examine environmental factors (e.g., glacier cover, wetland and watershed size) that affected wetland cover changes, and 2) identify wetlands' features that affect their vulnerability (using habitat drying as a proxy) in the face of climate change. Over the (1984-2011) period, our data showed an increasing trend in the mean wetland total area and number, mainly related to the appearance of wet grassland patches during the wetter years. Wetland cover also showed high inter-annual variability and their area for a given year was positively correlated to precipitation intensities in the three months prior to the image date. Also, round wetlands located in highly glacierized catchments were less prone to drying, while relatively small wetlands with irregularly shaped contours suffered the highest rates of drying over the last three decades. High Andean wetlands can therefore be considered as ecosystem sentinels for climate change, as they seem sensitive to glacier melting. Beyond the specific focus of this study, our work illustrates how satellite-based monitoring of ecosystem sentinels can help filling the lack of information on the ecological consequences of current and changing climate conditions, a common and crucial issue especially in less-developed countries.
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De Simone, F., Artaxo, P., Bencardino, M., Cinnirella, S., Carbone, F., D'Amore, F., et al. (2017). Particulate-phase mercury emissions from biomass burning and impact on resulting deposition: a modelling assessment. Atmospheric Chemistry And Physics, 17(3), 1881–1899.
Abstract: Mercury (Hg) emissions from biomass burning (BB) are an important source of atmospheric Hg and a major factor driving the interannual variation of Hg concentrations in the troposphere. The greatest fraction of Hg from BB is released in the form of elemental Hg (Hg-(g)(0)). However, little is known about the fraction of Hg bound to particulate matter (Hg-P) released from BB, and the factors controlling this fraction are also uncertain. In light of the aims of the Minamata Convention to reduce intentional Hg use and emissions from anthropogenic activities, the relative importance of Hg emissions from BB will have an increasing impact on Hg deposition fluxes. Hg speciation is one of the most important factors determining the redistribution of Hg in the atmosphere and the geographical distribution of Hg deposition. Using the latest version of the Global Fire Emissions Database (GFEDv4.1s) and the global Hg chemistry transport model, ECHMERIT, the impact of Hg speciation in BB emissions, and the factors which influence speciation, on Hg deposition have been investigated for the year 2013. The role of other uncertainties related to physical and chemical atmospheric processes involving Hg and the influence of model parametrisations were also investigated, since their interactions with Hg speciation are complex. The comparison with atmospheric Hg-P concentrations observed at two remote sites, Amsterdam Island (AMD) and Manaus (MAN), in the Amazon showed a significant improvement when considering a fraction of Hg-P from BB. The set of sensitivity runs also showed how the quantity and geographical distribution of Hg-P emitted from BB has a limited impact on a global scale, although the inclusion of increasing fractions Hg-P does limit Hg-(g)(0) availability to the global atmospheric pool. This reduces the fraction of Hg from BB which deposits to the world's oceans from 71 to 62 %. The impact locally is, however, significant on northern boreal and tropical forests, where fires are frequent, uncontrolled and lead to notable Hg inputs to local ecosystems. In the light of ongoing climatic changes this effect could be potentially be exacerbated in the future.
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Deal, E., Favre, A. C., & Braun, J. (2017). Rainfall variability in the Himalayan orogen and its relevance to erosion processes. Water Resources Research, 53(5), 4004–4021.
Abstract: Rainfall is an important driver of erosion processes. The mean rainfall rate is often used to account for the erosive impact of a particular climate. However, for some erosion processes, erosion rate is a nonlinear function of rainfall, e.g., due to a threshold for erosion. When this is the case, it is important to take into account the full distribution of rainfall, instead of just the mean. In light of this, we have characterized the variability of daily rainfall over the Himalayan orogen using high spatial and temporal resolution rainfall data sets. We find significant variations in rainfall variability over the Himalayan orogen, with increasing rainfall variability to the west and north of the orogen. By taking into account variability of rainfall in addition to mean rainfall rate, we find a pattern of rainfall that, from a geomorphological perspective, is significantly different from mean rainfall rate alone. Using these findings, we argue that short-term rainfall variability may help explain observed short and long-term erosion rates in the Himalayan orogen. Plain Language Summary An important topic in earth science is understanding how climate and tectonic forces interact to shape the surface of the Earth. One of the main influences that climate has on the Earth's surface is to cause erosion by delivering water to landscapes as rain and snow. Wetter climates should, in general, cause more erosion than drier climates, which is why the mean annual rainfall has traditionally been used as a measure of the erosive strength of climate. However, field evidence seems to suggest that the effect of climate is more sophisticated than can be captured with the mean annual rainfall. There is a considerable body of theory demonstrating that the intensity of rainfall, or storminess, is an equally important aspect of how climate causes erosion. In light of this, we have characterized the storminess in the Himalayan mountain range, a place with high erosion rates driven by rainfall. We show that there is large variation in storminess from place to place in the Himalayas, and that this can help explain observed rates of erosion, which do not match mean annual rainfall rates.
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Delmonte, B., Paleari, C. I., Ando, S., Garzanti, E., Andersson, P. S., Petit, J. R., et al. (2017). Causes of dust size variability in central East Antarctica (Dome B): Atmospheric transport from expanded South American sources during Marine Isotope Stage 2. Quaternary Science Reviews, 168, 55–68.
Abstract: We here investigate the spatial and temporal variability of eolian dust particle sorting recorded in the Dome B (77 degrees 05' S, 94 55' E) ice core, central East Antarctica, during Marine Isotope Stage (MIS) 2. We address the question whether such changes reflect variable transport pathways from a unique source area or rather a variable apportionment from diverse Southern Hemisphere sources transported at different elevation in the troposphere. The Sr-Nd radiogenic isotope composition of glacial dust samples as well as single-particle Raman mineralogy support the hypothesis of a single dust provenance both for coarse and fine mode dust events at Dome B. The southern South American provenance of glacial dust in Antarctica deduced from these results indicate a dust composition coherent with a mixture of volcanic material and minerals derived from metamorphic and plutonic rocks. Additionally, Dome B glacial samples contain aragonite particles along with diatom valves of marine benthic/epiphytic species and freshwater species living today in the northern Antarctic Peninsula and southern South America. These data suggest contribution from the exposed Patagonian continental shelf and glacial outwash plains of southern Patagonia at the time when sea level reached its minimum. Our results confirm that dust sorting is controlled by the relative intensity of the two main patterns of tropospheric dust transport onto the inner Plateau, i.e. fast low-level advection and long-range high-altitude transport including air subsidence over Antarctica. (C) 2017 Elsevier Ltd. All rights reserved.
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Destro, E., Marra, F., Nikolopoulos, E. I., Zoccatelli, D., Creutin, J. D., & Borga, M. (2017). Spatial estimation of debris flows-triggering rainfall and its dependence on rainfall return period. Geomorphology, 278, 269–279.
Abstract: Forecasting the occurrence of debris flows is fundamental for issuing hazard warnings, and often focuses on rainfall as a triggering agent and on the use of empirical rainfall thresholds based on rain gauge observations. A recognized component of the uncertainty associated with the use of rainfall thresholds is related to the sampling of strongly varying rainfall variability with sparse rain gauge networks. In this work we examine the spatial distribution of rainfall depth in areas up to 10 km from the debris flow initiation points as a function of return period, and we exploit this information to analyze the errors expected in the estimation of debris flow triggering rainfall when rain gauge data are used. In particular, we investigate the impact of rain gauge density and of the use of different interpolation methods. High-resolution, adjusted radar rainfall estimates, representing the best available spatially-distributed-rainfall estimates at the debris flows initiation point and in the surrounding area, are sampled by stochastically generated rain gauge networks characterized by varying densities. Debris flow triggering rainfall is estimated by means of three rainfall interpolation methods: nearest neighbor, inverse distance weighting and ordinary kriging. On average, triggering rainfall shows a local peak corresponding to the debris flow initiation point, with a decay of rainfall with distance which increases with the return period of the triggering rainfall. Interpolation of the stochastically generated rain gauge measurements leads to an underestimation of the triggering rainfall that, irrespective of the interpolation methods, increases with the return period and decreases with the rain gauge density. For small return period events and high rain gauge density, the differences among the methods are minor. With increasing the return period and decreasing the rain gauge density, the nearest neighbor method is less biased, because it makes use only of the closest rain gauge to the debris flow initiation point. On the contrary, inverse distance weighting and ordinary kriging, which are using rain gauges located farther from the debris flows in addition to the closest one, exhibit negative biases that increase with return period. The standard deviation of the interpolated values is larger when the nearest neighbor is used with respect to inverse distance weighting and ordinary kriging. For large return period and low rain gauge density, the differences among the methods are minor. (C) 2016 Elsevier B.V. All ri
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Dickson, N., Comte, J. - C., Koussoube, Y., Ofterdinger, U., & Vouillamoz, J. - M. (2017). Analysis and numerical modelling of large-scale controls on aquifer structure and hydrogeological properties in the African basement (Benin, West Africa). Geological Society Special Publications, . |
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Dieng, A. L., Sall, S. M., Eymard, L., Leduc-Leballeur, M., & Lazar, A. (2017). Trains of African Easterly Waves and Their Relationship to Tropical Cyclone Genesis in the Eastern Atlantic. Monthly Weather Review, 145(2), 599–616.
Abstract: In this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July-September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from the West African coast to the central Atlantic. The tracked systems are separated into four groups: (i) systems originating from the north zone of the midtropospheric African easterly jet (AEJ), (ii) those coming from the south part of AEJ, (iii) systems that are associated with a downstream trough located around 2000 km westward (termed DUO systems), and (iv) those that are not associated with such a close downstream trough (termed SOLO systems). By monitoring the embedded 700-hPa-filtered relative vorticity and 850-hPa wind convergence anomaly associated with these families along their trajectories, it is shown that the DUO generally have stronger dynamical structure and statistically have a longer lifetime than the SOLO ones. It is suggested that the differences between them may be due to the presence of the previous intense downstream trough in DUO cases, enhancing the low-level convergence behind them. Moreover, a study of the relationship between system trajectories and tropical depressions occurring between the West African coast and 40 degrees W showed that 90% of tropical depressions are identifiable from the West African coast in tracked systems, mostly in the DUO cases originating from the south zone of the AEJ.
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Divya, P. V., Viswanadham, B. V. S., & Gourc, J. P. (2017). Centrifuge model study on the performance of fiber reinforced clay-based landfill covers subjected to flexural distress. Applied Clay Science, 142, 173–184.
Abstract: The influence of discrete and randomly distributed polyester (PET) fibers in improving the crack resistance and water-tightness of clay barriers was studied by conducting a series of centrifuge model tests at 40 gravities using a large beam centrifuge available at Indian Institute of Technology Bombay. Model clay barriers with and without fibers were subjected to flexural distress by inducing differential settlements using a settlement simulator. Two types of clay barriers were prepared by using two different types of soil, namely bentonite amended silty soil (Soil A) and kaolin clay-sand mixture (Soil B). Digital image cross-correlation (DIC) technique was used for analyzing the images captured by a digital photo camera and a charged couple device (CCD) camera, mounted along with the model. Unreinforced soil barriers (URSB) were found to lose their water-tightness and integrity at relatively lower distortion levels compared to identical fiber reinforced soil barriers (FRSB). Also, the performance of URSB and FRSB was found to be superior for Soil A compared to Soil B. The capability of PET fibers on preservation of existing unreinforced soil barriers was also demonstrated. The strain at crack initiation, epsilon(c), for FRSB is 2.90 and 2.36 times higher than identical URSB for Soil A and Soil B, respectively. Similarly, the strain at the onset of water breakthrough, epsilon(b), for FRSB is 2.14 and 2.79 times higher than identical URSB for Soil A and Soil B, respectively. There is a significant influence of fiber inclusion in retarding the crack initiation and water breakthrough at the onset of flexural distress for both the soil types. Thus, FRSB can withstand more distortion and strain while maintaining the integrity of the clay barrier. This observed behaviour of restraining cracks and improved performance of FRSB is primarily due to the reinforcement effect achieved due to soil-fiber interactions. (C) 2016 Elsevier B.V. All rights reserved.
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Divya, P. V., Viswanadham, B. V. S., & Gourc, J. P. (2017). Centrifuge Modeling and Digital Image Cross-Correlation Analysis of Geofiber-Reinforced Clay-Based Landfill Covers. Journal Of Geotechnical And Geoenvironmental Engineering, 143(1).
Abstract: The influence of discrete and randomly distributed fibers (DRDF) on the improvement of the resistance to cracking of clay-based landfill covers was studied by conducting a series of centrifuge model tests. A motor-based differential settlement simulator (MDSS) was used to induce differential settlements to the soil layers during centrifuge tests performed at 40g using the 4.5 m radius beam centrifuge at Indian Institute of Technology Bombay. A digital image cross-correlation (DIC) technique was used to capture deformations of the model barriers and to obtain strain field distributions. Thickness of the unreinforced barriers (URSB) and fiber-reinforced barriers (FRSB) were varied as 15 mm (similar to 0.6 m in the field) and 25 mm (1.0 m). Fiber lengths were varied as 30, 60, and 90 mm. The strain at water breakthrough of 15 mm (0.6 m) and 25 mm (1 m) FRSB were respectively 1.9 and 2.2 times higher compared to identical URSBs. A relatively crack-free surface was observed for FRSB with longer fibers even after subjected to differential settlement of 1 m. (C) 2016 American Society of Civil Engineers.
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Donat-Magnin, M., Jourdain, N. C., Spence, P., Le Sommer, J., Gallee, H., & Durand, G. (2017). Ice-Shelf Melt Response to Changing Winds and Glacier Dynamics in the Amundsen Sea Sector, Antarctica. Journal Of Geophysical Research-Oceans, 122(12), 10206–10224.
Abstract: It has been suggested that the coastal Southern Ocean subsurface may warm over the 21st century in response to strengthening and poleward shifting winds, with potential adverse effects on West Antarctic glaciers. However, using a 1/12 degrees ocean regional model that includes ice-shelf cavities, we find a more complex response to changing winds in the Amundsen Sea. Simulated offshore subsurface waters get colder under strengthened and poleward shifted winds representative of the SAM projected trend. The buoyancy-driven circulation induced by ice-shelf melt transports this cold offshore anomaly onto the continental shelf, leading to cooling and decreased melt below 450 m. In the vicinity of ice-shelf fronts, Ekman pumping contributes to raise the isotherms in response to changing winds. This effect overwhelms the horizontal transport of colder offshore waters at intermediate depths (between 200 and 450 m), and therefore increases melt rates in the upper part of the ice-shelf cavities, which reinforces the buoyancy-driven circulation and further contributes to raise the isotherms. Then, prescribing an extreme grounding line retreat projected for 2100, the total melt rates simulated underneath Thwaites and Pine Island are multiplied by 2.5. Such increase is explained by a larger ocean/ice interface exposed to CDW, which is then amplified by a stronger melt-induced circulation along the ice draft. Our main conclusions are that (1) outputs from ocean models that do not represent ice shelf cavities (e.g., CMIP5 models) should not be directly used to predict the thermal forcing of future ice shelf cavities; (2) coupled ocean/ice sheet models with a velocity-dependent melt formulation are needed for future projections of glaciers experiencing a significant grounding line retreat.
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Druel, A., Peylin, P., Krinner, G., Ciais, P., Viovy, N., Peregon, A., et al. (2017). Towards a more detailed representation of high-latitude vegetation in the global land surface model ORCHIDEE (ORC-HL-VEGv1.0). Geoscientific Model Development, 10(12), 4693–4722.
Abstract: Simulation of vegetation-climate feedbacks in high latitudes in the ORCHIDEE land surface model was improved by the addition of three new circumpolar plant functional types (PFTs), namely non-vascular plants representing bryophytes and lichens, Arctic shrubs and Arctic C-3 grasses. Non-vascular plants are assigned no stomatal conductance, very shallow roots, and can desiccate during dry episodes and become active again during wet periods, which gives them a larger phenological plasticity (i.e. adaptability and resilience to severe climatic constraints) compared to grasses and shrubs. Shrubs have a specific carbon allocation scheme, and differ from trees by their larger survival rates in winter, due to protection by snow. Arctic C3 grasses have the same equations as in the original ORCHIDEE version, but different parameter values, optimised from in situ observations of biomass and net primary productivity (NPP) in Siberia. In situ observations of living biomass and productivity from Siberia were used to calibrate the parameters of the new PFTs using a Bayesian optimisation procedure. With the new PFTs, we obtain a lower NPP by 31% (from 55 degrees N), as well as a lower roughness length (-41%), transpiration (-33%) and a higher winter albedo (by +3.6%) due to increased snow cover. A simulation of the water balance and runoff and drainage in the high northern latitudes using the new PFTs results in an increase of fresh water discharge in the Arctic ocean by 11% (+140 km(3) yr(-1)), owing to less evapotranspiration. Future developments should focus on the competition between these three PFTs and boreal tree PFTs, in order to simulate their area changes in response to climate change, and the effect of carbon-nitrogen interactions.
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Ducousso, N., Le Sommer, J., Molines, J. M., & Bell, M. (2017). Impact of the “Symmetric Instability of the Computational Kind” at mesoscale- and submesoscale-permitting resolutions. Ocean Modelling, 120, 18–26.
Abstract: The energy-and enstrophy-conserving momentum advection scheme (EEN) used over the last 10 years in NEMO is subject to a spurious numerical instability. This instability, referred to as the Symmetric Instability of the Computational Kind (SICK), arises from a discrete imbalance between the two components of the vector-invariant form of momentum advection. The properties and the method for removing this instability have been documented by Hollingsworth et al. (1983), but the extent to which the SICK may interfere with processes of interest at mesoscale-and submesoscale-permitting resolutions is still unkown. In this paper, the impact of the SICK in realistic ocean model simulations is assessed by comparing model integrations with different versions of the EEN momentum advection scheme. Investigations are undertaken with a global mesoscale-permitting resolution (1/4 degrees) configuration and with a regional North Atlantic Ocean submesoscale-permitting resolution (1/60 degrees) configuration. At both resolutions, the instability is found to alter primarily the most energetic current systems, such as equatorial jets, western boundary currents and coherent vortices. The impact of the SICK is found to increase with model resolution with a noticeable impact at mesoscale-permitting resolution and a dramatic impact at submesoscale-permitting resolution. The SICK is shown to distort the normal functioning of current systems, by redirecting the slow energy transfer between balanced motions to a spurious energy transfer to internal inertia-gravity waves and to dissipation. Our results indicate that the SICK is likely to have significantly corrupted NEMO solutions (when run with the EEN scheme) at mesocale-permitting and finer resolutions over the last 10 years.
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Dumont, M., Arnaud, L., Picard, G., Libois, Q., Lejeune, Y., Nabat, P., et al. (2017). In situ continuous visible and near-infrared spectroscopy of an alpine snowpack. Cryosphere, 11(3), 1091–1110.
Abstract: Snow spectral albedo in the visible/near-infrared range has been continuously measured during a winter season at Col de Porte alpine site (French Alps; 45.30(circle) N, 5.77(circle) E; 1325ma.s.l.). The evolution of such alpine snowpack is complex due to intensive precipitation, rapid melt events and Saharan dust deposition outbreaks. This study highlights that the resulting intricate variations of spectral albedo can be successfully explained by variations of the following snow surface variables: specific surface area (SSA) of snow, effective light-absorbing impurities content, presence of liquid water and slope. The methodology developed in this study disentangles the effect of these variables on snow spectral albedo. The presence of liquid water at the snow surface results in a spectral shift of the albedo from which melt events can be identified with an occurrence of false detection rate lower than 3.5 %. Snow SSA mostly impacts spectral albedo in the near-infrared range. Impurity deposition mostly impacts the albedo in the visible range but this impact is very dependent on snow SSA and surface slope. Our work thus demonstrates that the SSA estimation from spectral albedo is affected by large uncertainties for a tilted snow surface and medium to high impurity contents and that the estimation of impurity content is also affected by large uncertainties, especially for low values below 50 ng g(-1) black carbon equivalent. The proposed methodology opens routes for retrieval of SSA, impurity content, melt events and surface slope from spectral albedo. However, an exhaustive accuracy assessment of the snow black properties retrieval would require more independent in situ measurements and is beyond the scope of the present study. This time series of snow spectral albedo nevertheless already provides a new insight into our understanding of the evolution of snow surface properties.
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Dupire, S., Curt, T., & Bigot, S. (2017). Spatio-temporal trends in fire weather in the French Alps. Science Of The Total Environment, 595, 801–817.
Abstract: The Alpine area is particularly sensitive to climatic and environmental changes that might impact socioecosystems and modify the regime of natural hazards. Among them, wildfire is of major importance as it threatens both ecosystems and human lives and infrastructures. Wildfires result from complex interactions between available vegetation fuels, climate and weather, and humans who decide of the land use and are the main source of fire ignitions. The changes in fire weather during the past decades are rather unknown in the French Alps especially due to their complex topography. Moreover, local institutions and managers wonder if the ongoing climate changes might increase fire risk and affect the environmental quality and the different ecosystem services provided by the mountain forests. In this context, we used the national forest fires database together with daily meteorological observations from 1959 to 2015 to investigate the changes in wildfire danger in the French Alps. We analysed the spatial and temporal variations in terms of intensity, frequency, seasonality and window of opportunity of two fire weather indices: the fine fuel moisture code (FFMC) and the fire weather index (FWI) that measure the daily water content of vegetation and the potential intensity of fires, respectively. Our results showed a major contrast between Southern Alps with a high fire weather danger on average and a significant increase in the past decades, and Northern Alps with low to moderate danger on average that increased only at low elevations. This study contributes to the understanding of the consequences of ongoings climate changes on wildfires in the French Alps. It produced high resolution results that account for the topographic and climatic variability of the area. Finally, the maps of the different fire weather components have practical implications for fire management and modelling and for preventing indirect effects of fires on ecosystems and human assets. (C) 2017 Elsevier B.V. All rights reserved.
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Durán Moro, M., Brankart, J. - M., Brasseur, P., & Verron, J. (2017). Exploring image data assimilation in the prospect of high-resolution satellite oceanic observations. Ocean Dynamics, 67(7), 875–895.
Abstract: Satellite sensors increasingly provide high-resolution (HR) observations of the ocean. They supply observations of sea surface height (SSH) and of tracers of the dynamics such as sea surface salinity (SSS) and sea surface temperature (SST). In particular, the Surface Water Ocean Topography (SWOT) mission will provide measurements of the surface ocean topography at very high-resolution (HR) delivering unprecedented information on the meso-scale and submeso-scale dynamics. This study investigates the feasibility to use these measurements to reconstruct meso-scale features simulated by numerical models, in particular on the vertical dimension. A methodology to reconstruct three-dimensional (3D) multivariate meso-scale scenes is developed by using a HR numerical model of the Solomon Sea region. An inverse problem is defined in the framework of a twin experiment where synthetic observations are used. A true state is chosen among the 3D multivariate states which is considered as a reference state. In order to correct a first guess of this true state, a two-step analysis is carried out. A probability distribution of the first guess is defined and updated at each step of the analysis: (i) the first step applies the analysis scheme of a reduced-order Kalman filter to update the first guess probability distribution using SSH observation; (ii) the second step minimizes a cost function using observations of HR image structure and a new probability distribution is estimated. The analysis is extended to the vertical dimension using 3D multivariate empirical orthogonal functions (EOFs) and the probabilistic approach allows the update of the probability distribution through the two-step analysis. Experiments show that the proposed technique succeeds in correcting a multivariate state using meso-scale and submeso-scale information contained in HR SSH and image structure observations. It also demonstrates how the surface information can be used to reconstruct the ocean state below the surface.
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Engeland, K., Borga, M., Creutin, J. D., Francois, B., Ramos, M. H., & Vidal, J. P. (2017). Space-time variability of climate variables and intermittent renewable electricity production – A review. Renewable & Sustainable Energy Reviews, 79, 600–617.
Abstract: A major part of renewable electricity production is characterized by a large degree of intermittency driven by the natural variability of climate factors such as air temperature, wind velocity, solar radiation, precipitation, evaporation, and river runoff. The main strategies to handle this intermittency include energy-storage, -transport, -diversity and-information. The three first strategies smooth out the variability of production in time and space, whereas the last one aims a better balance between production and demand. This study presents a literature review on the space-time variability of climate variables driving the intermittency of wind-, solar and hydropower productions and their joint management in electricity systems. A vast body of studies pertains to this question bringing results covering the full spectrum of resolutions and extents, using a variety of data sources, but mostly dealing with a single source. Our synthesis highlights the consistency of these works, and, besides astronomic forcing, we identify three broad climatic regimes governing the variability of renewable production and load. At sub-daily time scales, the three considered renewables have drastically different pattern sizes in response to small scale atmospheric processes. At regional scales, large perturbation weather patterns consistently control wind and solar production, hydropower having a clearly distinct type of pattern. At continental scales, all renewable sources and load seem to display patterns of constant space characteristics and no indication of marked temporal trends.
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Ezersky, M. G., Legchenko, A., Eppelbaum, L., & Al-Zoubi, A. (2017). Overview of the geophysical studies in the Dead Sea coastal area related to evaporite karst and recent sinkhole development. International Journal Of Speleology, 46(2), 277–302.
Abstract: Since the early 80s, a progressively increasing number of sinkholes appeared along the Dead Sea coastal line. It has been found that their appearance is strongly correlating with the lowering of the Dead Sea level taking place with the rate of approximately 1 m/yr. Location of areas affected by sinkhole development corresponds to location of the salt formation deposited during the latest Pleistocene, when the Lake Lisan receded to later become the Dead Sea. Water flowing to the Dead Sea from adjacent and underlying aquifers dissolves salt and creates caverns that cause ground subsidence and consequent formation of sinkholes. Before subsidence, these caverns are not visible on the surface but can be investigated with surface geophysical methods. For that, we applied Surface Nuclear Magnetic Resonance (SNMR), Transient Electromagnetic (TEM) Seismic refraction and reflection, Multichannel Analysis of Surface waves (MASW), microgravity and magnetic surveys and their combinations. Our geophysical results allowed us to locate the salt formation and to detect caverns in salt thus contributing to better understanding sinkhole development mechanisms. Comparison of sinkhole appearance along the western DS shore derived from the recent database (2017) shows that predictions made on the base of geophysical data (2005-2008) are now confirmed thus demonstrating efficiency of our study. In this paper, we briefly present a summary of up to date knowledge of the geology and hydrogeology of Dead Sea basin, of the physical properties of the salt rock and the most popular models explaining mechanisms of sinkhole development. We also share our experience gained during geophysical studies carried out in the framework of national and international research projects in this area for the last 20 years.
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Farinotti, D., Brinkerhoff, D. J., Clarke, G. K. C., Fuerst, J. J., Frey, H., Gantayat, P., et al. (2017). How accurate are estimates of glacier ice thickness? Results from ITMIX, the Ice Thickness Models Intercomparison eXperiment. Cryosphere, 11(2), 949–970.
Abstract: Knowledge of the ice thickness distribution of glaciers and ice caps is an important prerequisite for many glaciological and hydrological investigations. A wealth of approaches has recently been presented for inferring ice thickness from characteristics of the surface. With the Ice Thickness Models Intercomparison eXperiment (ITMIX) we performed the first coordinated assessment quantifying individual model performance. A set of 17 different models showed that individual ice thickness estimates can differ considerably – locally by a spread comparable to the observed thickness. Averaging the results of multiple models, however, significantly improved the results: on average over the 21 considered test cases, comparison against direct ice thickness measurements revealed deviations on the order of 10 +/- 24% of the mean ice thickness (1 sigma estimate). Models relying on multiple data sets – such as surface ice velocity fields, surface mass balance, or rates of ice thickness change -showed high sensitivity to input data quality. Together with the requirement of being able to handle large regions in an automated fashion, the capacity of better accounting for uncertainties in the input data will be a key for an improved next generation of ice thickness estimation approaches.
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Favier, V., Krinner, G., Amory, C., Gallee, H., Beaumet, J., & Agosta, C. (2017). Antarctica-Regional Climate and Surface Mass Budget. Current Climate Change Reports, 3(4), 303–315.
Abstract: We review recent literature on atmospheric, surface ocean and sea-ice observations and modeling results in the Antarctic sector and relate the observed climatic trends with the potential changes in the surface mass balance (SMB) of the ice sheet since 1900. Estimates of regional scale SMB distribution and trends remain subject to large uncertainties. Approaches combining and comparing multiple satellite and model-based assessments of ice sheet mass balance aim at reducing these knowledge gaps. During the last decades, significant changes in atmospheric circulation occurred around Antarctica, due to the exceptional positive trend in the Southern Annular Mode and to the climate variability observed in the tropical Pacific at the end of the twentieth century. Even though climate over the East Antarctic Ice-Sheet remained quite stable, a warming and precipitation increase was observed over the West Antarctic Ice-Sheet and over the West Antarctic Peninsula (AP) during the twentieth century. However, the high regional climate variability overwhelms climate changes associated to human drivers of global temperature changes, as reflected by a slight recent decadal cooling trend over the AP. Climate models still fail to accurately reproduce the multi-decadal SMB trends at a regional scale, and progress has to be achieved in reproducing atmospheric circulation changes related to complex ocean/ice/atmosphere interactions. Complex processes are also still insufficiently considered, such as (1) specific polar atmospheric processes (clouds, drifting snow, and stable boundary layer physics), (2) surface firn physics involved in the surface drag variations, or in firn air depletion and albedo feedbacks. Finally, progress in reducing the uncertainties relative to projections of the future SMB of Antarctica will largely depend on climate model capability to correctly consider teleconnections with low and mid-latitudes, and on the ability to correct them for biases, taking into account the coupling between ocean, ice, and atmosphere in high southern latitudes.
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Fettweis, X., Box, J. E., Agosta, C., Amory, C., Kittel, C., Lang, C., et al. (2017). Reconstructions of the 1900-2015 Greenland ice sheet surface mass balance using the regional climate MAR model. Cryosphere, 11(2), 1015–1033.
Abstract: With the aim of studying the recent Greenland ice sheet (GrIS) surface mass balance (SMB) decrease relative to the last century, we have forced the regional climate MAR (ModSle Atmospherique Regional; version 3.5.2) model with the ERA-Interim (ECMWF Interim Re-Analysis; 1979-2015), ERA-40 (1958-2001), NCEP-NCARv1 (National Centers for Environmental Prediction-National Center for Atmospheric Research Reanalysis version 1; 19482015), NCEP-NCARv2 (1979-2015), JRA-55 (Japanese 55year Reanalysis; 1958-2014), 20CRv2(c) (Twentieth Century Reanalysis version 2; 1900-2014) and ERA-20C (19002010) reanalyses. While all these forcing products are reanalyses that are assumed to represent the same climate, they produce significant differences in the MAR-simulated SMB over their common period. A temperature adjustment of C 1 ffi C (respectively 1 degrees C) was, for example, needed at the MAR boundaries with ERA-20C (20CRv2) reanalysis, given that ERA-20C (20CRv2) is similar to 1 degrees C colder (warmer) than ERAInterim over Greenland during the period 1980-2010. Comparisons with daily PROMICE (Programme for Monitoring of the Greenland Ice Sheet) near-surface observations support these adjustments. Comparisons with SMB measurements, ice cores and satellite-derived melt extent reveal the most accurate forcing datasets for the simulation of the GrIS SMB to be ERA-Interim and NCEP-NCARv1. However, some biases remain in MAR, suggesting that some improvements are still needed in its cloudiness and radiative schemes as well as in the representation of the bare ice albedo. Results from all MAR simulations indicate that (i) the period 1961-1990, commonly chosen as a stable reference period for Greenland SMB and ice dynamics, is actually a period of anomalously positive SMB (C 40 Gt yr 1) compared to 1900-2010; (ii) SMB has decreased significantly after this reference period due to increasing and unprecedented melt reaching the highest rates in the 120-year common period; (iii) before 1960, both ERA-20C and 20CRv2forced MAR simulations suggest a significant precipitation increase over 1900-1950, but this increase could be the result of an artefact in the reanalyses that are not well-enough constrained by observations during this period and (iv) since the 1980s, snowfall is quite stable after having reached a maximum in the 1970s. These MAR-based SMB and accumulation reconstructions are, however, quite similar to those from Box (2013) after 1930 and confirm that SMB was quite stable from the 1940s to the 1990s. Finally, only the ERA-2
0Cforced simulation suggests that SMB during the 1920-1930 warm period over Greenland was comparable to the SMB of the 2000s, due to both higher melt and lower precipitation than normal. |
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Fichez, R., Archundia, D., Grenz, C., Douillet, P., Mendieta, F. G., Moreno, M. O., et al. (2017). Global climate change and local watershed management as potential drivers of salinity variation in a tropical coastal lagoon (Laguna de Terminos, Mexico). Aquatic Sciences, 79(2), 219–230.
Abstract: The wide range of ecological goods and services provided by tropical coastal lagoons and wetlands are under considerable pressure due to the synergistic effects of local anthropogenic impact and global climate change. In transitional waters, salinity is a key driver of ecological processes mostly depending on the balance between marine and river inputs, a balance that can be significantly modified by climate change and by anthropogenic alteration of the watershed. Mesoamerica being considered as a climate change hot-spot and as an ecoregion strongly vulnerable to global change, our study aimed at analyzing the relationship between salinity, river runoff, and rainfall variability in a tropical coastal lagoon and to assess the respective influence of climate change and watershed management. The study focusing on the large and shallow coastal lagoon of Laguna de Terminos in south eastern Mexico established: (1) the variability in salinity distribution along the yearly cycle and the occurrence of a high salinity anomaly period during the wet season of 2009; (2) the relationship between lagoon waters salinity and river inputs further underlying the anomalous situation encountered in 2009; (3) a long term increase in river discharge during the past 60 years, indicating potential salinity decrease in the lagoon during that same period; (4) an absence of any change in rainfall linking the increase in runoff to watershed management rather than long term trend in climate change. Additionally, the specific context of the 2009-2010 Central-Equatorial Pacific El Nio is underlined and the potential relationship between river discharge and ENSO is discussed. Those results should be of significant practical value to decision-makers who are often keen to point the finger at global climate change when local environmental management is also and sometime most significantly responsible.
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Fourteau, K., Fain, X., Martinerie, P., Landais, A., Ekaykin, A. A., Lipenkov, V. Y., et al. (2017). Analytical constraints on layered gas trapping and smoothing of atmospheric variability in ice under low-accumulation conditions. Climate Of The Past, 13(12), 1815–1830.
Abstract: We investigate for the first time the loss and alteration of past atmospheric information from air trapping mechanisms under low-accumulation conditions through continuous CH4 (and CO) measurements. Methane concentration changes were measured over the Dansgaard-Oeschger event 17 (DO-17, similar to 60000 yrBP) in the Antarctic Vostok 4G-2 ice core. Measurements were performed using continuous-flow analysis combined with laser spectroscopy. The results highlight many anomalous layers at the centimeter scale that are unevenly distributed along the ice core. The anomalous methane mixing ratios differ from those in the immediate surrounding layers by up to 50 ppbv. This phenomenon can be theoretically reproduced by a simple layered trapping model, creating very localized gas age scale inversions. We propose a method for cleaning the record of anomalous values that aims at minimizing the bias in the overall signal. Once the layered-trapping-induced anomalies are removed from the record, DO-17 appears to be smoother than its equivalent record from the high-accumulation WAIS Divide ice core. This is expected due to the slower sinking and densification speeds of firn layers at lower accumulation. However, the degree of smoothing appears surprisingly similar between modern and DO-17 conditions at Vostok. This suggests that glacial records of trace gases from low-accumulation sites in the East Antarctic plateau can provide a better time resolution of past atmospheric composition changes than previously expected. We also developed a numerical method to extract the gas age distributions in ice layers after the removal of the anomalous layers based on comparison with a weakly smoothed record. It is particularly adapted for the conditions of the East Antarctic plateau, as it helps to characterize smoothing for a large range of very low-temperature and low-accumulation conditions.
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Francois, B., Martino, S., Tofte, L. S., Hingray, B., Mo, B., & Creutin, J. D. (2017). Effects of Increased Wind Power Generation on Mid-Norway's Energy Balance under Climate Change: A Market Based Approach. Energies, 10(2).
Abstract: Thanks to its huge water storage capacity, Norway has an excess of energy generation at annual scale, although significant regional disparity exists. On average, the Mid-Norway region has an energy deficit and needs to import more electricity than it exports. We show that this energy deficit can be reduced with an increase in wind generation and transmission line capacity, even in future climate scenarios where both mean annual temperature and precipitation are changed. For the considered scenarios, the deficit observed in winter disappears, i.e., when electricity consumption and prices are high. At the annual scale, the deficit behaviour depends more on future changes in precipitation. Another consequence of changes in wind production and transmission capacity is the modification of electricity exchanges with neighbouring regions which are also modified both in terms of average, variability and seasonality.
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Froidurot, S., & Diedhiou, A. (2017). Characteristics of wet and dry spells in the West African monsoon system. Atmospheric Science Letters, 18(3), 125–131.
Abstract: Using 17 years (1998-2014) of daily TRMM 3B42 rainfall data, we provide a climatological characterization of wet and dry spells in West Africa, which should serve to assess the ability of climate model to simulate these high impact events. The study focuses on four subregions (Western and Central Sahel, Sudanian zone and Guinea Coast). Defining wet (dry) spells as sequences of consecutive days with precipitation higher (lower) than 1 mm, we describe the space-time variability of wet and dry spell occurrence. This climatology stresses the influence of the relief on the number and duration of these spells. The spatio-temporal variability of the wet and dry spells also appears to be closely related to the spatio-temporal variability of the West African monsoon. The number of wet spells of all durations and of 2-3 day dry spells have similar features with a maximum occurrence during the local rainy seasons and a spatial pattern similar to the mean annual rainfall with a north-south gradient. In contrast, dry spells lasting more than four days show some singularities such as a low occurrence over the Sahelian band or high occurrence along the Guinea Coast mainly from Ivory Coast to Benin. Moreover, the seasonal cycle of these longer dry spells presents higher occurrences at the beginning and the end of the rainy seasons.
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Furst, J. J., Gillet-Chaulet, F., Benham, T. J., Dowdeswell, J. A., Grabiec, M., Navarro, F., et al. (2017). Application of a two-step approach for mapping ice thickness to various glacier types on Svalbard. Cryosphere, 11(5), 2003–2032.
Abstract: The basal topography is largely unknown beneath most glaciers and ice caps, and many attempts have been made to estimate a thickness field from other more accessible information at the surface. Here, we present a two-step reconstruction approach for ice thickness that solves mass conservation over single or several connected drainage basins. The approach is applied to a variety of test geometries with abundant thickness measurements including marine-and landterminating glaciers as well as a 2400 km(2) ice cap on Svalbard. The input requirements are kept to a minimum for the first step. In this step, a geometrically controlled, non-local flux solution is converted into thickness values relying on the shallow ice approximation (SIA). In a second step, the thickness field is updated along fast-flowing glacier trunks on the basis of velocity observations. Both steps account for available thickness measurements. Each thickness field is presented together with an error-estimate map based on a formal propagation of input uncertainties. These error estimates point out that the thickness field is least constrained near ice divides or in other stagnant areas. Withholding a share of the thickness measurements, error estimates tend to overestimate mismatch values in a median sense. We also have to accept an aggregate uncertainty of at least 25% in the reconstructed thickness field for glaciers with very sparse or no observations. For Vestfonna ice cap (VIC), a previous ice volume estimate based on the same measurement record as used here has to be corrected upward by 22 %. We also find that a 13% area fraction of the ice cap is in fact grounded below sea level. The former 5% estimate from a direct measurement interpolation exceeds an aggregate maximum range of 6-23% as inferred from the error estimates here.
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Geay, T., Belleudy, P., Gervaise, C., Habersack, H., Aigner, J., Kreisler, A., et al. (2017). Passive acoustic monitoring of bed load discharge in a large gravel bed river. Journal Of Geophysical Research-Earth Surface, 122(2), 528–545.
Abstract: Surrogate technologies to monitor bed load discharge have been developed to supplement and ultimately take over traditional direct methods. Our research deals with passive acoustic monitoring of bed load flux using a hydrophone continuously deployed near a river bed. This passive acoustic technology senses any acoustic waves propagated in the river environment and particularly the sound due to interparticle collisions emitted during bed load movement. A data set has been acquired in the large Alpine gravel-bedded Drau River. Analysis of the short-term frequency response of acoustic signals allows us to determine the origin of recorded noises and to consider their frequency variations. Results are compared with ancillary field data of water depth and bed load transport inferred from the signals of a geophone array. Hydrophone and geophone signals are well correlated. Thanks to the large network of deployed geophones, analysis of the spatial resolution of hydrophone measurements shows that the sensor is sensitive to bed load motion not only locally but over distances of 5-10m (10-20% of river width). Our results are promising in terms of the potential use of hydrophones for monitoring bed load transport in large gravel bed rivers: acoustic signals represent a large river bed area, rather than being local; hydrophones can be installed in large floods; they can be deployed at a low cost and provide continuous monitoring at high temporal resolution.
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Geay, T., Belleudy, P., Laronne, J. B., Camenen, B., & Gervaise, C. (2017). Spectral variations of underwater river sounds. Earth Surface Processes And Landforms, 42(14), 2447–2456.
Abstract: Passive acoustic monitoring of the self-generated noise of particle impacts has been shown to be correlated to bedload flux and bedload size. However, few studies have concentrated on the role of acoustic wave propagation in a river. For the first time, the river environment is modeled as a Pekeris waveguide, where a wave number integration technique is used to predict the transformation of sounds through their propagation paths. Focusing on the distance of a hydrophone from the channel bed and cutting off the low frequencies produced by impacts between gravel particles, we demonstrate that acoustic propagation modifies the spectral content of bedload-generated sound. Acoustic signals analyzed with the proposed model are interpreted by comparison to Helley-Smith bedload data obtained during flood conditions on the large gravel-bedded Arc-en-Maurienne River, France. This study shows that careful attention to acoustic propagation effects is required when estimating bedload grain size distribution with hydrophones in rivers, especially for rivers with slopes higher than 1%. Bedload monitoring with a hydrophone is particularly appropriate for large gravel-bed rivers – especially so during large floods, when in situ sampling is difficult or impractical and the impact of acoustic propagation is weaker relative to the self-generated noise of bedload impacts. Copyright (c) 2017 John Wiley & Sons, Ltd.
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Geng, L., Murray, L. T., Mickley, L. J., Lin, P., Fu, Q., Schauer, A. J., et al. (2017). Isotopic evidence of multiple controls on atmospheric oxidants over climate transitions. Nature, 546(7656), 133–+.
Abstract: The abundance of tropospheric oxidants, such as ozone (O-3) and hydroxyl (OH) and peroxy radicals (HO2 + RO2), determines the lifetimes of reduced trace gases such as methane and the production of particulate matter important for climate and human health. The response of tropospheric oxidants to climate change is poorly constrained owing to large uncertainties in the degree to which processes that influence oxidants may change with climate(1) and owing to a lack of palaeo-records with which to constrain levels of atmospheric oxidants during past climate transitions(2). At present, it is thought that temperature-dependent emissions of tropospheric O-3 precursors and water vapour abundance determine the climate response of oxidants, resulting in lower tropospheric O-3 in cold climates while HOx (= OH + HO2 + RO2) remains relatively buffered(3). Here we report observations of oxygen-17 excess of nitrate (a proxy for the relative abundance of atmospheric O-3 and HOx) from a Greenland ice core over the most recent glacial-interglacial cycle and for two Dansgaard-Oeschger events. We find that tropospheric oxidants are sensitive to climate change with an increase in the O-3/HOx ratio in cold climates, the opposite of current expectations. We hypothesize that the observed increase in O-3/HOx in cold climates is driven by enhanced stratosphere-to-troposphere transport of O-3, and that reactive halogen chemistry is also enhanced in cold climates. Reactive halogens influence the oxidative capacity of the troposphere directly as oxidants themselves and indirectly(4) via their influence on O-3 and HOx. The strength of stratosphere-to-troposphere transport is largely controlled by the Brewer-Dobson circulation(5), which may be enhanced in colder climates owing to a stronger meridional gradient of sea surface temperatures(6), with implications for the response of tropospheric oxidants(7) and stratospheric thermal and mass balance(8). These two processes may represent important, yet relatively unexplored, climate feedback mechanisms during major climate transitions.
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Genthon, C., Piard, L., Vignon, E., Madeleine, J. B., Casado, M., & Gallee, H. (2017). Atmospheric moisture supersaturation in the near-surface atmosphere at Dome C, Antarctic Plateau. Atmospheric Chemistry And Physics, 17(1), 691–704.
Abstract: Supersaturation often occurs at the top of the troposphere where cirrus clouds form, but is comparatively unusual near the surface where the air is generally warmer and laden with liquid and/or ice condensation nuclei. One exception is the surface of the high Antarctic Plateau. One year of atmospheric moisture measurement at the surface of Dome C on the East Antarctic Plateau is presented. The measurements are obtained using commercial hygrometry sensors modified to allow air sampling without affecting the moisture content, even in the case of supersaturation. Supersaturation is found to be very frequent. Common unadapted hygrometry sensors generally fail to report supersaturation, and most reports of atmospheric moisture on the Antarctic Plateau are thus likely biased low. The measurements are compared with results from two models implementing cold microphysics parameterizations: the European Center for Medium-range Weather Forecasts through its operational analyses, and the Model Atmospherique Regional. As in the observations, supersaturation is frequent in the models but the statistical distribution differs both between models and observations and between the two models, leaving much room for model improvement. This is unlikely to strongly affect estimations of surface sublimation because supersaturation is more frequent as temperature is lower, and moisture quantities and thus water fluxes are small anyway. Ignoring supersaturation may be a more serious issue when considering water isotopes, a tracer of phase change and temperature, largely used to reconstruct past climates and environments from ice cores. Because observations are easier in the surface atmosphere, longer and more continuous in situ observation series of atmospheric supersaturation can be obtained than higher in the atmosphere to test parameterizations of cold microphysics, such as those used in the formation of high-altitude cirrus clouds in meteorological and climate models.
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Getirana, A., Boone, A., Peugeot, C., & ALMIP2, W. G. (2017). Streamflows over a West African Basin from the ALMIP2 Model Ensemble. Journal of Hydrometeorology, 18(7), 1831–1845.
Abstract: Comparing streamflow simulations against observations has become a straightforward way to evaluate a land surface model�s (LSM) ability in simulating water budget within a catchment. Using a mesoscale river routing scheme (RRS), this study evaluates simulated streamflows over the upper Ouémé River basin resulting from 14 LSMs within the framework of phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2). The ALMIP2 RRS (ARTS) has been used to route LSM outputs. ARTS is based on the nonlinear Muskingum�Cunge method and a simple deep water infiltration formulation representing water-table recharge as previously observed in that region. Simulations are performed for the 2005�08 period during which ground observations are largely available. Experiments are designed using different ground-based rainfall datasets derived from two interpolation methods: the Thiessen technique and a combined kriging�Lagrangian methodology. LSM-based total runoff (TR) averages vary from 0.07 to 1.97 mm day−1, while optimal TR was estimated as ~0.65 mm day−1. This highly affected the RRS parameterization and streamflow simulations. Optimal Nash�Sutcliffe coefficients for LSM-averaged streamflows varied from 0.66 to 0.92, depending on the gauge station. However, individual LSM performances show a wider range. A more detailed rainfall distribution provided by the kriging�Lagrangian methodology resulted in overall better streamflow simulations. The early runoff generation related to reduced infiltration rates during early rainfall events features as one of the main reasons for poor LSM performances.
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Giorio, C., Monod, A., Bregonzio-Rozier, L., DeWitt, H. L., Cazaunau, M., Temime-Roussel, B., et al. (2017). Cloud Processing of Secondary Organic Aerosol from Isoprene and Methacrolein Photooxidation. Journal Of Physical Chemistry A, 121(40), 7641–7654.
Abstract: Aerosol-cloud interaction contributes to the largest uncertainties in the estimation and interpretation of the Earth's changing energy budget. The present study explores experimentally the impacts of water condensation-evaporation events, mimicking processes occurring, in atmospheric clouds, on the molecular composition of secondary organic aerosol (SOA) from the photooxidation of methacrolein. A range of on and off-line mass spectrometry techniques were used to obtain a detailed chemical characterization of SOA formed in control experiments in dry conditions, in triphasic experiments simulating gas-particle-cloud droplet interactions (starting from dry conditions and from 60% relative humidity (RH)), and in bulk aqueous-phase experiments. We observed that cloud events trigger fast SOA formation accompanied by evaporative losses. These evaporative losses decreased SOA concentration in the simulation chamber by 25-32% upon RH increase, while aqueous SOA was found to be metastable and slowly evaporated after cloud dissipation. In the simulation chamber, SOA composition measured with a high-resolution time-of flight aerosol mass spectrometer, did not change during cloud events compared with high RH conditions (RH > 80%). In all experiments, off-line mass spectrometry techniques emphasize the critical role of 2-methylglyceric acid as a major product of isoprene chemistry, as an important contributor to the total SOA mass (15-20%) and as a key building block of oligomers found in the particulate phase. Interestingly, the comparison between the series of oligomers obtained from experiments performed under different conditions show a markedly different reactivity. In particular, long reaction times at high RH seem to create the conditions for aqueous-phase processing to occur in a more efficient manner than during two relatively short cloud events.
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Gladstone, R. M., Warner, R. C., Galton-Fenzi, B. K., Gagliardini, O., Zwinger, T., & Greve, R. (2017). Marine ice sheet model performance depends on basal sliding physics and sub-shelf melting. Cryosphere, 11(1), 319–329.
Abstract: Computer models are necessary for understanding and predicting marine ice sheet behaviour. However, there is uncertainty over implementation of physical processes at the ice base, both for grounded and floating glacial ice. Here we implement several sliding relations in a marine ice sheet flow-line model accounting for all stress components and demonstrate that model resolution requirements are strongly dependent on both the choice of basal sliding relation and the spatial distribution of ice shelf basal melting. Sliding relations that reduce the magnitude of the step change in basal drag from grounded ice to floating ice (where basal drag is set to zero) show reduced dependence on resolution compared to a commonly used relation, in which basal drag is purely a power law function of basal ice velocity. Sliding relations in which basal drag goes smoothly to zero as the grounding line is approached from inland (due to a physically motivated incorporation of effective pressure at the bed) provide further reduction in resolution dependence. A similar issue is found with the imposition of basal melt under the floating part of the ice shelf: melt parameterisations that reduce the abruptness of change in basal melting from grounded ice (where basal melt is set to zero) to floating ice provide improved convergence with resolution compared to parameterisations in which high melt occurs adjacent to the grounding line. Thus physical processes, such as sub-glacial outflow (which could cause high melt near the grounding line), impact on capability to simulate marine ice sheets. If there exists an abrupt change across the grounding line in either basal drag or basal melting, then high resolution will be required to solve the problem. However, the plausible combination of a physical dependency of basal drag on effective pressure, and the possibility of low ice shelf basal melt rates next to the grounding line, may mean that some marine ice sheet systems can be reliably simulated at a coarser resolution than currently thought necessary.
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Gourdeau, L., Djath, B., Ganachaud, A., Nino, F., Birol, F., Verron, J., et al. (2017). Altimetry in a Regional Tropical Sea. Ieee Geoscience And Remote Sensing Magazine, 5(3), 44–52. |
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Gourdeau, L., Verron, J., Chaigneau, A., Cravatte, S., & Kessler, W. (2017). Complementary Use of Glider Data, Altimetry, and Model for Exploring Mesoscale Eddies in the Tropical Pacific Solomon Sea. Journal Of Geophysical Research-Oceans, 122(11), 9209–9229.
Abstract: Mesoscale activity is an important component of the Solomon Sea circulation that interacts with the energetic low-latitude western boundary currents of the South Tropical Pacific Ocean carrying waters of subtropical origin before joining the equatorial Pacific. Mixing associated with mesoscale activity could explain water mass transformation observed in the Solomon Sea that likely impacts El Nino Southern Oscillation dynamics. This study makes synergetic use of glider data, altimetry, and high-resolution model for exploring mesoscale eddies, especially their vertical structures, and their role on the Solomon Sea circulation. The description of individual eddies observed by altimetry and gliders provides the first elements to characterize the 3-D structure of these tropical eddies, and confirms the usefulness of the model to access a more universal view of such eddies. Mesoscale eddies appear to have a vertical extension limited to the Surface Waters (SW) and the Upper Thermocline Water (UTW), i.e., the first 140-150 m depth. Most of the eddies are nonlinear, meaning that eddies can trap and transport water properties. But they weakly interact with the deep New Guinea Coastal Undercurrent that is a key piece of the equatorial circulation. Anticyclonic eddies are particularly efficient to advect salty and warm SW coming from the intrusion of equatorial Pacific waters at Solomon Strait, and to impact the characteristics of the New Guinea Coastal Current. Cyclonic eddies are particularly efficient to transport South Pacific Tropical Water (SPTW) anomalies from the North Vanuatu Jet and to erode by diapycnal mixing the high SPTW salinity.
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Goursaud, S., Masson-Delmotte, V., Favier, V., Preunkert, S., Fily, M., Gallee, H., et al. (2017). A 60-year ice-core record of regional climate from Adelie Land, coastal Antarctica. Cryosphere, 11(1), 343–362.
Abstract: A 22.4 m-long shallow firn core was extracted during the 2006/2007 field season from coastal Adelie Land. Annual layer counting based on subannual analyses of delta O-18 and major chemical components was combined with 5 reference years associated with nuclear tests and non-retreat of summer sea ice to build the initial ice-core chronology (19462006), stressing uncertain counting for 8 years. We focus here on the resulting delta O-18 and accumulation records. With an average value of 21.8 +/- 6.9 cmw. e. yr(-1), local accumulation shows multi-decadal variations peaking in the 1980s, but no long-term trend. Similar results are obtained for delta O-18, also characterised by a remarkably low and variable amplitude of the seasonal cycle. The ice-core records are compared with regional records of temperature, stake area accumulation measurements and variations in sea-ice extent, and outputs from two models nudged to ERA (European Re-analysis) atmospheric reanalyses: the high-resolution atmospheric general circulation model (AGCM), including stable water isotopes ECHAM5-wiso (European Centre Hamburg model), and the regional atmospheric model Modele Atmospherique Regional (AR). A significant linear correlation is identified between decadal variations in delta O-18 and regional temperature. No significant relationship appears with regional sea-ice extent. A weak and significant correlation appears with Dumont d'Urville wind speed, increasing after 1979. The model-data comparison highlights the inadequacy of ECHAM5-wiso simulations prior to 1979, possibly due to the lack of data assimilation to constrain atmospheric reanalyses. Systematic biases are identified in the ECHAM5-wiso simulation, such as an overestimation of the mean accumulation rate and its interannual variability, a strong cold bias and an underestimation of the mean delta O-18 value and its interannual variability. As a result, relationships between simulated delta O-18 and temperature are weaker than observed. Such systematic precipitation and temperature biases are not displayed by MAR, suggesting that the model resolution plays a key role along the Antarctic ice sheet coastal topography. Interannual variations in ECHAM5-wiso temperature and precipitation accurately capture signals from meteorological data and stake observations and are used to refine the initial ice-core chronology within 2 years. After this adjustment, remarkable positive (negative) delta O-18 anomalies are identified in the ice-core record and the ECHAM5-wiso simulation in 1986 and 20
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Gratiot, N., Bildstein, A., Anh, T. T., Thoss, H., Denis, H., Michallet, H., et al. (2017). Sediment flocculation in the Mekong River estuary, Vietnam, an important driver of geomorphological changes. Comptes Rendus Geoscience, 349(6-7), 260–268.
Abstract: Over the past several decades, major hydro-sedimentary changes have occurred in both continental and coastal regions of the Mekong Delta, and this has severely impacted coastal erosion. A good characterization of floc properties and of their changes over time is necessary to gain comprehensive understanding and modelling of hydrodynamics and of the associated geomorphological changes. This paper quantifies the influence of sediment concentration, turbulence and differential particle settling on flocculation through field and laboratory investigations of the Mekong estuary. For concentrations lower than 200 mg.L-1, the particles do not exhibit a dynamic response to their environment, while for higher concentrations (up to 3-4 g.L-1), particle size and settling velocity increased by more than one order of magnitude. Flocculation by differential settling has not been sufficiently investigated yet, but this study reveals it as a predominant factor for siltation in quiescent environments such as the ones existing in the inner mangrove fringe. Such results are important to provide realistic simulations of the coastal evolution. (C) 2017 Academie des sciences. Published by Elsevier Masson SAS.
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Grazioli, J., Genthon, C., Boudevillain, B., Duran-Alarcon, C., Del Guasta, M., Madeleine, J. B., et al. (2017). Measurements of precipitation in Dumont d'Urville, Adelie Land, East Antarctica. Cryosphere, 11(4), 1797–1811.
Abstract: The first results of a campaign of intensive observation of precipitation in Dumont d'Urville, Antarctica, are presented. Several instruments collected data from November 2015 to February 2016 or longer, including a polarimetric radar (MXPol), a Micro Rain Radar (MRR), a weighing gauge (Pluvio(2)), and a Multi-Angle Snowflake Camera (MASC). These instruments collected the first ground-based measurements of precipitation in the region of Adelie Land (Terre Adelie), including precipitation microphysics. Microphysical observations during the austral summer 2015/2016 showed that, close to the ground level, aggregates are the dominant hydrometeor type, together with small ice particles (mostly originating from blowing snow), and that riming is a recurring process. Eleven percent of the measured particles were fully developed graupel, and aggregates had a mean riming degree of about 30 %. Spurious precipitation in the Pluvio2 measurements in windy conditions, leading to phantom accumulations, is observed and partly removed through synergistic use of MRR data. The yearly accumulated precipitation of snow (300 m above ground), obtained by means of a local conversion relation of MRR data, trained on the Pluvio2 measurement of the summer period, is estimated to be 815 mm of water equivalent, with a confidence interval ranging between 739.5 and 989 mm. Data obtained in previous research from satellite-borne radars, and the ERA-Interim reanalysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) provide lower yearly totals: 655 mm for ERA-Interim and 679 mm for the climatological data over DDU. ERA-Interim overestimates the occurrence of low-intensity precipitation events especially in summer, but it compensates for them by underestimating the snowfall amounts carried by the most intense events. Overall, this paper provides insightful examples of the added values of precipitation monitoring in Antarctica with a synergistic use of in situ and remote sensing measurements.
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Grazioli, J., Madeleine, J. B., Gallee, H., Forbes, R. M., Genthon, C., Krinner, G., et al. (2017). Katabatic winds diminish precipitation contribution to the Antarctic ice mass balance. Proceedings Of The National Academy Of Sciences Of The United States Of America, 114(41), 10858–10863.
Abstract: Snowfall in Antarctica is a key term of the ice sheet mass budget that influences the sea level at global scale. Over the continental margins, persistent katabatic winds blow all year long and supply the lower troposphere with unsaturated air. We show that this dry air leads to significant low-level sublimation of snowfall. We found using unprecedented data collected over 1 year on the coast of Adelie Land and simulations from different atmospheric models that low-level sublimation accounts for a 17% reduction of total snowfall over the continent and up to 35% on the margins of East Antarctica, significantly affecting satellite-based estimations close to the ground. Our findings suggest that, as climate warming progresses, this process will be enhanced and will limit expected precipitation increases at the ground level.
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Grippa, M., Kergoat, L., Boone, A., Peugeot, C., Demarty, J., Cappelaere, B., et al. (2017). Modeling Surface Runoff and Water Fluxes over Contrasted Soils in the Pastoral Sahel: Evaluation of the ALMIP2 Land Surface Models over the Gourma Region in Mali. Journal of Hydrometeorology, 18(7), 1847–1866.
Abstract: Land surface processes play an important role in the West African monsoon variability. In addition, the evolution of hydrological systems in this region, and particularly the increase of surface water and runoff coefficients observed since the 1950s, has had a strong impact on water resources and on the occurrence of floods events. This study addresses results from phase 2 of the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Model Intercomparison Project (ALMIP2), carried out to evaluate the capability of different state-of-the-art land surface models to reproduce surface processes at the mesoscale. Evaluation of runoff and water fluxes over the Mali site is carried out through comparison with runoff estimations over endorheic watersheds as well as evapotranspiration (ET) measurements. Three remote-sensing-based ET products [ALEXI, MODIS, and Global Land Evaporation Amsterdam Model (GLEAM)] are also analyzed. It is found that, over deep sandy soils, surface runoff is generally overestimated, but the ALMIP2 multimodel mean reproduces in situ measurements of ET and water stress events rather well. However, ALMIP2 models are generally unable to distinguish among the two contrasted hydrological systems typical of the study area. Employing as input a soil map that explicitly represents shallow soils improves the representation of water fluxes for the models that can account for their representation. Shallow soils are shown to be also quite challenging for remote-sensing-based ET products, even if their effect on evaporative loss was captured by the diagnostic thermal-based ALEXI. A better representation of these soils, in soil databases, model parameterizations, and remote sensing algorithms, is fundamental to improve the estimation of water fluxes in this part of the Sahel.
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Groisman, P., Shugart, H., Kicklighter, D., Henebry, G., Tchebakova, N., Maksyutov, S., et al. (2017). Northern Eurasia Future Initiative (NEFI): facing the challenges and pathways of global change in the twenty-first century. Progress In Earth And Planetary Science, 4.
Abstract: During the past several decades, the Earth system has changed significantly, especially across Northern Eurasia. Changes in the socio-economic conditions of the larger countries in the region have also resulted in a variety of regional environmental changes that can have global consequences. The Northern Eurasia Future Initiative (NEFI) has been designed as an essential continuation of the Northern Eurasia Earth Science Partnership Initiative (NEESPI), which was launched in 2004. NEESPI sought to elucidate all aspects of ongoing environmental change, to inform societies and, thus, to better prepare societies for future developments. A key principle of NEFI is that these developments must now be secured through science-based strategies co-designed with regional decision-makers to lead their societies to prosperity in the face of environmental and institutional challenges. NEESPI scientific research, data, and models have created a solid knowledge base to support the NEFI program. This paper presents the NEFI research vision consensus based on that knowledge. It provides the reader with samples of recent accomplishments in regional studies and formulates new NEFI science questions. To address these questions, nine research foci are identified and their selections are briefly justified. These foci include warming of the Arctic; changing frequency, pattern, and intensity of extreme and inclement environmental conditions; retreat of the cryosphere; changes in terrestrial water cycles; changes in the biosphere; pressures on land use; changes in infrastructure; societal actions in response to environmental change; and quantification of Northern Eurasia's role in the global Earth system. Powerful feedbacks between the Earth and human systems in Northern Eurasia (e.g., mega-fires, droughts, depletion of the cryosphere essential for water supply, retreat of sea ice) result from past and current human activities (e.g., large-scale water withdrawals, land use, and governance change) and potentially restrict or provide new opportunities for future human activities. Therefore, we propose that integrated assessment models are needed as the final stage of global change assessment. The overarching goal of this NEFI modeling effort will enable evaluation of economic decisions in response to changing environmental conditions and justification of mitigation and adaptation efforts.
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Guedron, S., Point, D., Acha, D., Bouchet, S., Baya, P. A., Tessier, E., et al. (2017). Mercury contamination level and speciation inventory in Lakes Titicaca & Uru-Uru (Bolivia): Current status and future trends. Environmental Pollution, 231, 262–270.
Abstract: Aquatic ecosystems of the Bolivian Altiplano (similar to 3800 m a.s.l.) are characterized by extreme hydro-climatic constrains (e.g., high UV-radiations and low oxygen) and are under the pressure of increasing anthropogenic activities, unregulated mining, agricultural and urban development. We report here a complete inventory of mercury (Hg) levels and speciation in the water column, atmosphere, sediment and key sentinel organisms (i.e., plankton, fish and birds) of two endorheic Lakes of the same watershed differing with respect to their size, eutrophication and contamination levels. Total Hg (THg) and mono-methylmercury (MMHg) concentrations in filtered water and sediment of Lake Titicaca are in the lowest range of reported levels in other large lakes worldwide. Downstream, Hg levels are 3-10 times higher in the shallow eutrophic Lake Uru-Uru than in Lake Titicaca due to high Hg inputs from the surrounding mining region. High percentages of MMHg were found in the filtered and unfiltered water rising up from <1 to similar to 50% THg from the oligoihetero-trophic Lake Titicaca to the eutrophic Lake Uru-Uru. Such high % MMHg is explained by a high in situ MMHg production in relation to the sulfate rich substrate, the low oxygen levels of the water column, and the stabilization of MMHg due to abundant ligands present in these alkaline waters. Differences in MMHg concentrations in water and sediments compartments between Lake Titicaca and Uru-Uru were found to mirror the offset in MMHg levels that also exist in their respective food webs. This suggests that in situ MMHg baseline production is likely the main factor controlling MMHg levels in fish species consumed by the local population. Finally, the increase of anthropogenic pressure in Lake Titicaca may probably enhance eutrophication processes which favor MMHg production and thus accumulation in water and biota. (C) 2017 Elsevier Ltd. All rights reserved.
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Hachani, S., Boudevillain, B., Delrieu, G., & Bargaoui, Z. (2017). Drop Size Distribution Climatology in Cevennes-Vivarais Region, France. Atmosphere, 8(12).
Abstract: Mediterranean regions are prone to heavy rainfall, flash floods, and erosion issues. Drop size distribution (DSD) is a key element for studying these phenomena through the hydrological variables which can be derived from it (rainfall rates and totals, kinetic energy fluxes). This paper proposes a five-year (2012-2016) DSD climatology, summarized by scaling parameters for concentration, size, and shape. The DSD network is composed of two longitudinal transects of three OTT Parsivel optical disdrometers each, across the Mediterranean Cevennes-Vivarais region. The influence of several factors are analysed: location (distance from the sea, orographic environment), season, daily synoptic weather situation (derived from geopotential heights, at 700 and 1000 hPa), rainfall type (analysed from 5 min radar data), as well as some combinations of these factors. It was found and/or confirmed that the orographic environment, season, weather patterns associated with the exposure to low level atmospheric flows, and rainfall types influenced the microphysical processes, leading to rainfall, measured at the ground. Consequently, the DSD characteristics, as well as the relationships between the rainfall rate and reflectivity factor, are influenced by these factors.
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Hattori, S., Savarino, J., Kamezaki, K., Ishino, S., Dyckmans, J., Fujinawa, T., et al. (2017). Automated system measuring triple oxygen and nitrogen isotope ratios in nitrate using the bacterial method and N2O decomposition by microwave discharge (vol 30, pg 2635, 2016). Rapid Communications In Mass Spectrometry, 31(4), 396. |
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Hidas, K., Tommasi, A., Mainprice, D., Chauve, T., Barou, F., & Montagnat, M. (2017). Microstructural evolution during thermal annealing of ice-I-h. Journal Of Structural Geology, 99, 31–44.
Abstract: We studied the evolution of the microstructure of ice-I-h during static recrystallization by stepwise annealing experiments. We alternated thermal annealing and electron backscatter diffraction (EBSD) analyses on polycrystalline columnar ice pre-deformed in uniaxial compression at temperature of -7 degrees C to macroscopic strains of 3.0-5.2. Annealing experiments were carried out at -5 degrees C and -2 degrees C up to a maximum of 3.25 days, typically in 5-6 steps. EBSD crystal orientation maps obtained after each annealing step permit the description of microstructural changes. Decrease in average intragranular misorientation at the sample scale and modification of the misorientation across subgrain boundaries provide evidence for recovery from the earliest stages of annealing. This initial evolution is similar for all studied samples irrespective of their initial strain or annealing temperature. After an incubation period >= 1.5 h, recovery is accompanied by recrystallization (nucleation and grain boundary migration). Grain growth proceeds at the expense of domains with high intragranular misorientations, consuming first the most misorientated parts of primary grains. Grain growth kinetics fits the parabolic growth law with grain growth exponents in the range of 2.4-4.0. Deformation-induced tilt boundaries and kink bands may slow down grain boundary migration. They are stable features during early stages of static recrystallization, only erased by normal growth, which starts after > 24 h of annealing. (C) 2017 Elsevier Ltd. All rights reserved.
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Horton, P., Jaboyedoff, M., & Obled, C. (2017). Global Optimization of an Analog Method by Means of Genetic Algorithms. Monthly Weather Review, 145(4), 1275–1294.
Abstract: Analog methods are based on a statistical relationship between synoptic meteorological variables (predictors) and local weather (predictand, to be predicted). This relationship is defined by several parameters, which are often calibrated by means of a semiautomatic sequential procedure. This calibration approach is fast, but has strong limitations. It proceeds through successive steps, and thus cannot handle all parameter dependencies. Furthermore, it cannot automatically optimize some parameters, such as the selection of pressure levels and temporal windows (hours of the day) at which the predictors are compared. To overcome these limitations, the global optimization technique of genetic algorithms is considered, which can jointly optimize all parameters of the method, and get closer to a global optimum, by taking into account the dependencies of the parameters. Moreover, it can objectively calibrate parameters that were previously assessed manually and can take into account new degrees of freedom. However, genetic algorithms must be tailored to the problem under consideration. Multiple combinations of algorithms were assessed, and new algorithms were developed (e.g., the chromosome of adaptive search radius, which is found to be very robust), in order to provide recommendations regarding the use of genetic algorithms for optimizing several variants of analog methods. A global optimization approach provides new perspectives for the improvement of analog methods, and for their application to new regions or new predictands.
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Horton, P., Obled, C., & Jaboyedoff, M. (2017). The analogue method for precipitation prediction: finding better analogue situations at a sub-daily time step. Hydrology And Earth System Sciences, 21(7), 3307–3323.
Abstract: Analogue methods (AMs) predict local weather variables (predictands) such as precipitation by means of a statistical relationship with predictors at a synoptic scale. The analogy is generally assessed on gradients of geopotential heights first to sample days with a similar atmospheric circulation. Other predictors such as moisture variables can also be added in a successive level of analogy. The search for candidate situations similar to a given target day is usually undertaken by comparing the state of the atmosphere at fixed hours of the day for both the target day and the candidate analogues. This is a consequence of using standard daily precipitation time series, which are available over longer periods than sub-daily data. However, it is unlikely for the best analogy to occur at the exact same hour for the target and candidate situations. A better analogue situation may be found with a time shift of several hours since a better fit can occur at different times of the day. In order to assess the potential for finding better analogues at a different hour, a moving time window (MTW) has been introduced. The MTW resulted in a better analogy in terms of the atmospheric circulation and showed improved values of the analogy criterion on the entire distribution of the extracted analogue dates. The improvement was found to increase with the analogue rank due to an accumulation of better analogues in the selection. A seasonal effect has also been identified, with larger improvements shown in winter than in summer. This may be attributed to stronger diurnal cycles in summer that favour predictors taken at the same hour for the target and analogue days. The impact of the MTW on the precipitation prediction skill has been assessed by means of a sub-daily precipitation series transformed into moving 24 h totals at 12, 6, and 3 h time steps. The prediction skill was improved by the MTW, as was the reliability of the prediction. Moreover, the improvements were greater for days with heavy precipitation, which are generally related to more dynamic atmospheric situations in which the timing is more specific and for which fewer records are available in the meteorological archive. The improvements of the analogy criterion and the performance scores on precipitation were both found to be higher for MTWs with a smaller time step of 3 h. A 3 h MTW provides 8 times more candidate situations even though they are not fully independent. Since the MTW provides additional situations to the pool of possible analogues, it can be considered as an inflation of the meteorological archive. Because this technique is simple and easily applicable, it should be considered for several applications in different contexts, such as operational forecasting or climate-related studies.
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Ioana-Toroimac, G., Zaharia, L., Minea, G., & Morosanu, G. A. (2017). Using a multi-criteria analysis to identify rivers with hydromorphological restoration priority: Braided rivers in the south-eastern Subcarpathians (Romania). Science Of The Total Environment, 599, 700–709.
Abstract: In order to systematically plan river restoration actions at a regional scale, this paper develops a multi-criteria analysis that classifies rivers, based on their priority for hydromorphological restoration. This priority is defined by severe human pressures within the erodible corridor of the river, drastic alteration of the stream channel, and low intensity of river pattern functioning. Based on relevant indicators for three groups of features (human pressures, channel changes, and river functionality), a Hydromorphological Restoration Priority Index (HRPI) was designed. The high values (>66%) of HRPI reflect an urgent need for hydromorphological restoration while low values (<33%) reveal a less immediate necessity for restoration. The proposed methodology was applied on braided sectors of rivers crossing the south-eastern (Curvature) Subcarpathians (Romania). The values of the total HRPI ranged between 21% (Zabrdut River) and almost 44% (Prahova River). According to our results, most of the analyzed sectors have a low need for hydromorphological restoration of the braided pattern, while some have a moderate necessity for restoration. Whereas the Prahova River has the highest HRPI, it should be given priority for restoration at a regional scale, which corresponds to the objectives of River Basin Management Plans for the interval beyond 2021. (C) 2017 Elsevier B.V. All rights reserved.
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Ishino, S., Hattori, S., Savarino, J., Jourdain, B., Preunkert, S., Legrand, M., et al. (2017). Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica. Atmospheric Chemistry And Physics, 17(5), 3713–3727.
Abstract: Triple oxygen isotopic compositions (Delta O-17 = delta O-17-0.52 x delta O-18) of atmospheric sulfate (SO42-) and nitrate (NO3-) in the atmosphere reflect the relative contribution of oxidation pathways involved in their formation processes, which potentially provides information to reveal missing reactions in atmospheric chemistry models. However, there remain many theoretical assumptions for the controlling factors of Delta O-17(SO42-) and Delta O-17(NO3-) values in those model estimations. To test one of those assumption that Delta O-17 values of ozone (O-3) have a flat value and do not influence the seasonality of Delta O-17(SO42-) and Delta O-17(NO3-) values, we performed the first simultaneous measurement of Delta O-17 values of atmospheric sulfate, nitrate, and ozone collected at Dumont d'Urville (DDU) Station (66 degrees 40 ' S, 140 degrees 01 ' E) throughout 2011. Delta O-17 values of sulfate and nitrate exhibited seasonal variation characterized by minima in the austral summer and maxima in winter, within the ranges of 0.9-3.4 and 23.0-41.9 %, respectively. In contrast, Delta O-17 values of ozone showed no significant seasonal variation, with values of 26 +/- 1% throughout the year. These contrasting seasonal trends suggest that seasonality in Delta O-17(SO42-) and Delta O-17(NO3-) values is not the result of changes in Delta O-17(O-3), but of the changes in oxidation chemistry. The trends with summer minima and are caused by sunlight-driven changes in the relative contribution of O3 oxidation to the oxidation by HOx, ROx, and H2O2. In addition to that general trend, by comparing Delta O-17(SO42-) and Delta O-17(NO3-) values to ozone mixing ratios, we found that Delta O-17(SO42-) values observed in spring (September to November) were lower than in fall (March to May), while there was no significant spring and fall difference in Delta O-17(NO3-) values. The relatively lower sensitivity of Delta O-17(SO42-) values to the ozone mixing ratio in spring compared to fall is possibly explained by (i) the increased contribution of SO2 oxidations by OH and H2O2 caused by NOx emission from snowpack and/ or (ii) SO2 oxidation by hypohalous acids (HOX = HOCl + HOBr) in the aqueous phase.
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Jomelli, V., Mokadem, F., Schimmelpfennig, I., Chapron, E., Rinterknecht, V., Favier, V., et al. (2017). Sub-Antarctic glacier extensions in the Kerguelen region (49 degrees S, Indian Ocean) over the past 24,000 years constrained by Cl-36 moraine dating. Quaternary Science Reviews, 162, 128–144.
Abstract: Similar to many other regions in the world, glaciers in the southern sub-polar regions are currently retreating. In the Kerguelen Islands (49 degrees S, 69 degrees E), the mass balance of the Cook Ice Cap (CIC), the largest ice cap in this region, experienced dramatic shrinking between 1960 and 2013 with retreat rates among the highest in the world. This observation needs to be evaluated in a long-term context. However, data on the past glacier extents are sparse in the sub-Antarctic regions. To investigate the deglaciation pattern since the Last Glacial Maximum (LGM) period, we present the first 13 cosmogenic Cl-36 surface exposure ages from four sites in the Kerguelen Islands. The Cl-36 ages from erratic and moraine boulders span from 24.4 +/- 2.7 ka to 03 +/- 0.1 ka. We combined these ages with existing glacio-marine radiocarbon ages and bathymetric data to document the temporal and spatial changes of the island's glacial history. Ice began to retreat on the main island before 24.4 +/- 2.7 ka until around the time of the Antarctic Cold Reversal (ACR) period (-14.5-12.9 ka), during which the Bontemps moraine was formed by the advance of a CIC outlet glacier. Deglaciation continued during the Holocene probably until 3 ka with evidence of minor advances during the last millennium. This chronology is in pace with major changes in delta O-18 in a recent West Antarctica ice core record, showing that Kerguelen Islands glaciers are particularly sensitive and relevant to document climate change in the southern polar regions. (C) 2017 Elsevier Ltd. All rights reserved.
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Jourdain, C., Belleudy, P., Tal, M., & Malavoi, J. R. (2017). The role of hydrology on vegetation removal in a heavily managed gravel bed river: the Isere, Combe de Savoie, France. Geomorphologie-Relief Processus Environnement, 23(3), 203–217.
Abstract: The impacts of frequent or moderate floods (return interval <1 year to 10 years) on vegetated macroform turnover in river beds are interesting from a management perspective: artificial floods are an option often considered for managing vegetation in rivers. However their effects are not well documented. In this context, our study was aimed at characterizing the impact of high flow events of different magnitudes on sediment and vegetation dynamics in a heavily managed gravel bed river, from aerial photos and hydrological time series. Specifically, our aims were: (i) to characterize the evolution of vegetation surfaces at the reach scale and over decadal time scales, (ii) to analyze the link between different descriptors of the hydrological time series and vegetation removal, and (iii) to outline the main mechanisms of vegetation removal at these scales. Our study was conducted on the Isere River, a heavily managed gravel bed river flowing in the French Alps for the period 1996-2015. The hydrological parameters which best explained vegetation removal on the Isere bed were the volumes of water flowing between 350 m(3).s(-1) (return interval <1 year) and 600 m(3).s(-1) (return interval of approximately 5 years). Most vegetation removal was due to lateral erosion of the upstream end of the bars. The areas of vegetation removal were modest compared to those of newly established vegetation: 3.4% of vegetated area on average was removed annually, while vegetation cover increased by 6.2%. These results suggest that on this reach, artificial floods could potentially invigorate sediment dynamics and vegetation renewal, but are unlikely to be sufficient to maintain the bed free of vegetation.
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Jourdain, N. C., Mathiot, P., Merino, N., Durand, G., Le Sommer, J., Spence, P., et al. (2017). Ocean circulation and sea-ice thinning induced by melting ice shelves in the Amundsen Sea. Journal Of Geophysical Research-Oceans, 122(3), 2550–2573.
Abstract: A 1/128 ocean model configuration of the Amundsen Sea sector is developed to better understand the circulation induced by ice-shelf melt and the impacts on the surrounding ocean and sea ice. Eighteen sensitivity experiments to drag and heat exchange coefficients at the ice shelf/ocean interface are performed. The total melt rate simulated in each cavity is function of the thermal Stanton number, and for a given thermal Stanton number, melt is slightly higher for lower values of the drag coefficient. Sub-ice-shelf melt induces a thermohaline circulation that pumps warm circumpolar deep water into the cavity. The related volume flux into a cavity is 100-500 times stronger than the melt volume flux itself. Ice-shelf melt also induces a coastal barotropic current that contributes 45612% of the total simulated coastal transport. Due to the presence of warm circumpolar deep waters, the melt-induced inflow typically brings 4-20 times more heat into the cavities than the latent heat required for melt. For currently observed melt rates, approximately 6-31% of the heat that enters a cavity with melting potential is actually used to melt ice shelves. For increasing sub-ice-shelf melt rates, the transport in the cavity becomes stronger, and more heat is pumped from the deep layers to the upper part of the cavity then advected toward the ocean surface in front of the ice shelf. Therefore, more ice-shelf melt induces less sea-ice volume near the ice sheet margins. Plain Language Summary The ice-shelf cavities of the Amundsen Sea, Antarctica, act as very powerful pumps that create strong inflows of warm water under the ice-shelves, as well as significant circulation changes in the entire region. Such warm inflows bring more heat than required to melt ice, so that a large part of that heat exits ice-shelf cavities without being used. Due to mixing between warm deep waters and melt freshwater, melt-induced flows are warm and buoyant when they leave cavities. Therefore, they reach the ocean surface near ice-shelf fronts and can melt significant amounts of sea ice. It is thus suggested that climatic trends in sub ice-shelf melt could partly explain sea ice trends near the ice-sheet margins in the Amundsen Sea region.
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Kawamura, K., Abe-Ouchi, A., Motoyama, H., Ageta, Y., Aoki, S., Azuma, N., et al. (2017). State dependence of climatic instability over the past 720,000 years from Antarctic ice cores and climate modeling. Science Advances, 3(2).
Abstract: Climatic variabilities on millennial and longer time scales with a bipolar seesaw pattern have been documented in paleoclimatic records, but their frequencies, relationships with mean climatic state, and mechanisms remain unclear. Understanding the processes and sensitivities that underlie these changes will underpin better understanding of the climate system and projections of its future change. We investigate the long-term characteristics of climatic variability using a new ice-core record from Dome Fuji, East Antarctica, combined with an existing long record from the Dome C ice core. Antarctic warming events over the past 720,000 years are most frequent when the Antarctic temperature is slightly below average on orbital time scales, equivalent to an intermediate climate during glacial periods, whereas interglacial and fully glaciated climates are unfavourable for a millennial-scale bipolar seesaw. Numerical experiments using a fully coupled atmosphere-ocean general circulation model with freshwater hosing in the northern North Atlantic showed that climate becomes most unstable in intermediate glacial conditions associated with large changes in sea ice and the Atlantic Meridional Overturning Circulation. Model sensitivity experiments suggest that the prerequisite for the most frequent climate instabilitywith bipolar seesaw pattern during the late Pleistocene era is associated with reduced atmospheric CO2 concentration via global cooling and sea ice formation in the North Atlantic, in addition to extended Northern Hemisphere ice sheets.
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Koïta, M., Yonli, H., Soro, D., Dara, A., & Vouillamoz, J. - M. (2017). Taking into Account the Role of the Weathering Profile in Determining Hydrodynamic Properties of Hard Rock Aquifers. Geosciences, 7(3), 89. |
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Kozachek, A., Mikhalenko, V., Masson-Delmotte, V., Ekaykin, A., Ginot, P., Kutuzov, S., et al. (2017). Large-scale drivers of Caucasus climate variability in meteorological records and Mt El'brus ice cores. Climate Of The Past, 13(5), 473–489.
Abstract: A 181.8 m ice core was recovered from a borehole drilled into bedrock on the western plateau of Mt El'brus (43 degrees 20'53.9 '' N, 42 degrees 25'36.0 '' E; 5115m a.s.l.) in the Caucasus, Russia, in 2009 (Mikhalenko et al., 2015). Here, we report on the results of the water stable isotope composition from this ice core with additional data from the shallow cores. The distinct seasonal cycle of the isotopic composition allows dating by annual layer counting. Dating has been performed for the upper 126 m of the deep core combined with 20 m from the shallow cores. The whole record covers 100 years, from 2013 back to 1914. Due to the high accumulation rate (1380 mm w.e. year(-1)) and limited melting, we obtained isotopic composition and accumulation rate records with seasonal resolution. These values were compared with available meteorological data from 13 weather stations in the region and also with atmosphere circulation indices, back-trajectory calculations, and Global Network of Isotopes in Precipitation (GNIP) data in order to decipher the drivers of accumulation and ice core isotopic composition in the Caucasus region. In the warm season (May-October) the isotopic composition depends on local temperatures, but the correlation is not persistent over time, while in the cold season (November-April), atmospheric circulation is the predominant driver of the ice core's isotopic composition. The snow accumulation rate correlates well with the precipitation rate in the region all year round, which made it possible to reconstruct and expand the precipitation record at the Caucasus highlands from 1914 until 1966, when reliable meteorological observations of precipitation at high elevation began.
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Lafore, J. - P., Beucher, F., Peyrillé, P., Diongue-Niang, A., Chapelon, N., Bouniol, D., et al. (2017). A multi-scale analysis of the extreme rain event of Ouagadougou in 2009: High-impact weather system, West Africa, African Easterly Waves. Quarterly Journal Of The Royal Meteorological Society, . |
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Lafore, J. P., Beucher, F., Peyrille, P., Diongue-Niang, A., Chapelon, N., Bouniol, D., et al. (2017). A multi-scale analysis of the extreme rain event of Ouagadougou in 2009. Quarterly Journal Of The Royal Meteorological Society, 143(709), 3094–3109.
Abstract: This study presents a multi-scale analysis of an extreme rain event that occurred in Burkina Faso on 1 September 2009 with an absolute record of 263 mm rainfall observed at Ouagadougou. This high-impact weather system results from the combination of several favourable ingredients at different scales. The sea-surface temperature anomaly patterns in July-August 2009 of both the Atlantic cold tongue, the Tropical Atlantic Dipole and the Mediterranean Sea are favourable factors for the northward penetration of the West African monsoon. The intense convective activity of the last 10-day period in August is associated with the crossing of a convectively coupled Kelvin wave increasing the African easterly wave (AEW) activity, and of an equatorial Rossby wave. At the synoptic scale this event corresponds to the passage of a train of three AEWs with increasing magnitude. Behind the first AEW trough axis, an intense and deep southerly monsoon burst develops. It contributes to the amplification of the second AEW and its breaking is associated with the formation of an intense meso-vortex on the southern flank of the African easterly jet. Compared to the fast-moving squall line, the dominant type of precipitating weather system over the Sahel, the Ouagadougou precipitating system appears to be a moist vortex propagating slowly, allowing rainfall accumulation, without wind gusts or convective cold pools observed at the surface. The main precipitation area is located about 2 degrees longitude downshear (westward due to the African easterly jet) of the centre of this strong meso-vortex.
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Landais, A., Casado, M., Prie, F., Magand, O., Arnaud, L., Ekaykin, A., et al. (2017). Surface studies of water isotopes in Antarctica for quantitative interpretation of deep ice core data. Comptes Rendus Geoscience, 349(4), 139–150.
Abstract: Polar ice cores are unique climate archives. Indeed, most of them have a continuous stratigraphy and present high temporal resolution of many climate variables in a single archive. While water isotopic records (delta D or delta O-18) in ice cores are often taken as references for past atmospheric temperature variations, their relationship to temperature is associated with a large uncertainty. Several reasons are invoked to explain the limitation of such an approach; in particular, post-deposition effects are important in East Antarctica because of the low accumulation rates. The strong influence of post-deposition processes highlights the need for surface polar research programs in addition to deep drilling programs. We present here new results on water isotopes from several recent surface programs, mostly over East Antarctica. Together with previously published data, the new data presented in this study have several implications for the climatic reconstructions based on ice core isotopic data: (1) The spatial relationship between surface mean temperature and mean snow isotopic composition over the first meters in depth can be explained quite straightforwardly using simple isotopic models tuned to d-excess vs. delta O-18 evolution in transects on the East Antarctic sector. The observed spatial slopes are significantly higher (similar to 0.7-0.8 parts per thousand. degrees C (1) for delta O-18 vs. temperature) than seasonal slopes inferred from precipitation data at Vostok and Dome C (0.35 to 0.46 parts per thousand. degrees C (1)). We explain these differences by changes in condensation versus surface temperature between summer and winter in the central East Antarctic plateau, where the inversion layer vanishes in summer. (2) Post-deposition effects linked to exchanges between the snow surface and the atmospheric water vapor lead to an evolution of delta O-18 in the surface snow, even in the absence of any precipitation event. This evolution preserves the positive correlation between the delta O-18 of snow and surface temperature, but is associated with a much slower delta O-18-vs-temperature slope than the slope observed in the seasonal precipitation. (3) Post-deposition effects clearly limit the archiving of high-resolution (seasonal) climatic variability in the polar snow, but we suggest that sites with an accumulation rate of the order of 40 kg.m (2).yr (1) may record a seasonal cycle at shallow depths. (C) 2017 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
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Leauthaud, C., Cappelaere, B., Demarty, J., Guichard, F., Velluet, C., Kergoat, L., et al. (2017). A 60-year reconstructed high-resolution local meteorological data set in Central Sahel (1950-2009): evaluation, analysis and application to land surface modelling. International Journal Of Climatology, 37(5), 2699–2718.
Abstract: The Sahel has experienced strong climate variability in the past decades. Understanding its implications for natural and cultivated ecosystems is pivotal in a context of high population growth and mainly agriculture-based livelihoods. However, efforts to model processes at the land-atmosphere interface are hindered, particularly when the multi-decadal timescale is targeted, as climatic data are scarce, largely incomplete and often unreliable. This study presents the generation of a long-term, high-temporal resolution, multivariate local climatic data set for Niamey, Central Sahel. The continuous series spans the period 1950-2009 at a 30-min timescale and includes ground station-based meteorological variables (precipitation, air temperature, relative and specific humidity, air pressure, wind speed, downwelling long- and short-wave radiation) as well as process-modelled surface fluxes (upwelling long- and short-wave radiation, latent, sensible and soil heat fluxes and surface temperature). A combination of complementary techniques (linear/spline regressions, a multivariate analogue method, artificial neural networks and recursive gap filling) was used to reconstruct missing meteorological data. The complete surface energy budget was then obtained for two dominant land cover types, fallow bush and millet, by applying the meteorological forcing data set to a finely field-calibrated land surface model. Uncertainty in reconstructed data was expressed by means of a stochastic ensemble of plausible historical time series. Climatological statistics were computed at sub-daily to decadal timescales and compared with local, regional and global data sets such as CRU and ERA-Interim. The reconstructed precipitation statistics, approximate to 1 degrees C increase in mean annual temperature from 1950 to 2009, and mean diurnal and annual cycles for all variables were in good agreement with previous studies. The new data set, denoted NAD (Niamey Airport-derived set) and publicly available, can be used to investigate the water and energy cycles in Central Sahel, while the methodology can be applied to reconstruct series at other stations.
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Leduc-Leballeur, M., Picard, G., Macelloni, G., Arnaud, L., Brogioni, M., Mialon, A., et al. (2017). Influence of snow surface properties on L-band brightness temperature at Dome C, Antarctica. Remote Sensing Of Environment, 199, 427–436.
Abstract: L-band radiometer measurements collected over the Dome similar to C area from 2010 to 2015 indicated that the brightness temperature (T-B) was relatively stable at vertical (V) polarization (standard deviation lower than 1 K at annual scale), while it was slightly more variable at horizontal (H) polarization. During the 2014-2015 austral summer, an exceptional situation was recorded by both the DOMEX ground radiometer and the European Space Agency (ESA)'s Soil Moisture and Ocean Salinity (SMOS) satellite. From November 2014 to March 2015, T-B H showed a progressive and significant increase until 20 March 2015 when it sharply decreased by about 5 K (at 52.5 incidence angle) within a few days. In parallel to the increase in T-B H, glaciological and meteorological in situ measurements showed a wind speed that was lower than usual and a low-density snow layer being progressively set up on the surface. This was consistent with the exceptional hoar event observed, as well as with snow accumulation on the surface. On the other hand, the decrease in T-B H was related to the passing over Dome C of a storm that removed or compacted the layer of light snow on the surface. The WALOMIS (Wave Approach for LOw-frequency Microwave emission in Snow) snow-emission model was used with in situ measurements of the snowpack as inputs for evaluating the effect of changes observed on the snow surface in T-B H. The simulations indicated that the surface snow density variations were sufficient for predicting the increasing and decreasing trends of the T-B H. However, the thickness variations of the superficial layer were essential so as to obtain a better agreement with the SMOS observations. This result confirmed that the L-band T-B H was affected by the snow properties of the top centimeters of the snowpack, in spite of the large penetration depth (hundreds of meters). Both the surface snow density and the thickness of the superficial layer were relevant, due to coherent interference effects.
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Legchenko, A., Comte, J. C., Ofterdinger, U., Vouillamoz, J. M., Lawson, F. M. A., & Walsh, J. (2017). Joint use of singular value decomposition and Monte-Carlo simulation for estimating uncertainty in surface NMR inversion. Journal Of Applied Geophysics, 144, 28–36.
Abstract: We propose a simple and robust approach for investigating uncertainty in the results of inversion in geophysics. We apply this approach to inversion of Surface Nuclear Magnetic Resonance (SNMR) data, which is also known as Magnetic Resonance Sounding (MRS). Solution of this inverse problem is known to be non-unique. We inverse MRS data using the well-known Tikhonov regularization method, which provides an optimal solution as a trade-off between the stability and accuracy. Then, we perturb this model by random values and compute the fitting error for the perturbed models. The magnitude of these perturbations is limited by the uncertainty estimated with the singular value decomposition (SVD) and taking into account experimental errors. We use 106 perturbed models and show that the large majority of these models, which have all the water content within the variations given by the SVD estimate, do not fit data with an acceptable accuracy. Thus, we may limit the solution space by only the equivalent inverse models that fit data with the accuracy close to that of the initial inverse model. For representing inversion results, we use three equivalent solutions instead of the only one: the “best” solution given by the regularization or other inversion technic and the extreme variations of this solution corresponding to the equivalent models with the minimum and the maximum volume of water. For demonstrating our approach, we use synthetic data sets and experimental data acquired in the framework of investigation of a hard rock aquifer in the Ireland (County Donegal). (C) 2017 Elsevier B.V. All rights reserved.
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Legrand, M., Preunkert, S., Weller, R., Zipf, L., Elsasser, C., Merchel, S., et al. (2017). Year-round record of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) – Part 2: Biogenic sulfur (sulfate and methanesulfonate) aerosol. Atmospheric Chemistry And Physics, 17(22), 14055–14073.
Abstract: Multiple year-round (2006-2015) records of the bulk and size-segregated composition of aerosol were obtained at the inland site of Concordia located in East Antarctica. The well-marked maximum of non-sea salt sulfate (nssSO(4)) in January (100 +/- 28 ng m(-3) versus 4.4 +/- 2.3 ng m(-3) in July) is consistent with observations made at the coast (280 +/- 78 ng M-3 in January versus 16 +/- 9 ng m(-3) in July at Dumont d'Urville, for instance). In contrast, the well-marked maximum of MSA at the coast in January (60 +/- 23 ng M-3 at Dumont d'Urville) is not observed at Concordia (5.2 +/- 2.0 ng M-3 in January). Instead, the MSA level at Concordia peaks in October (5.6 +/- 1.9 ng m(-3)) and March (14.9 +/- 5.7 ng m(-3)). As a result, a surprisingly low MSA-to-nssSO(4) ratio (RMSA) is observed at Concordia in mid-summer (0.0 +/- 5 0.02 in January versus 0.25 +/- 0.09 in March). We find that the low value of RMSA in mid-summer at Concordia is mainly driven by a drop of MSA levels that takes place in submicron aerosol (0.3 pm diameter). The drop of MSA coincides with periods of high photochemical activity as indicated by high ozone levels, strongly suggesting the occurrence of an efficient chemical destruction of MSA over the Antarctic plateau in mid-summer. The relationship between MSA and nssSO(4) levels is examined separately for each season and indicates that concentration of non-biogenic sulfate over the Antarctic plateau does not exceed 1 ng M-3 in fall and winter and remains close to 5 ng M-3 in spring. This weak non-biogenic sulfate level is discussed in the light of radionuclides (Pb-210, Be-10, and Be-7) also measured on bulk aerosol samples collected at Concordia. The findings highlight the complexity in using MSA in deep ice cores extracted from inland Antarctica as a proxy of past dimethyl sulfide emissions from the Southern Ocean.
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Legrand, M., Preunkert, S., Wolff, E., Weller, R., Jourdain, B., & Wagenbach, D. (2017). Year-round records of bulk and size-segregated aerosol composition in central Antarctica (Concordia site) Part 1: Fractionation of sea-salt particles. Atmospheric Chemistry And Physics, 17(22), 14039–14054.
Abstract: Multiple year-round records of bulk and size segregated composition of aerosol were obtained at the inland site of Concordia located at Dome C in East Antarctica. In parallel, sampling of acidic gases on denuder tubes was carried out to quantify the concentrations of HCl and HNO3 present in the gas phase. These time series are used to examine aerosol present over central Antarctica in terms of chloride depletion relative to sodium with respect to freshly emitted sea-salt aerosol as well as depletion of sulfate relative to sodium with respect to the composition of seawater. A depletion of chloride relative to sodium is observed over most of the year, reaching a maximum of similar to 20 ngm(-3) in spring when there are still large sea-salt amounts and acidic components start to recover. The role of acidic sulfur aerosol and nitric acid in replacing chloride from sea-salt particles is here discussed. HCl is found to be around twice more abundant than the amount of chloride lost by sea-salt aerosol, suggesting that either HCl is more efficiently transported to Concordia than sea-salt aerosol or re-emission from the snow pack over the Antarctic plateau represents an additional significant HCl source. The size-segregated composition of aerosol collected in winter (from 2006 to 2011) indicates a mean sulfate to sodium ratio of sea-salt aerosol present over central Antarctica of 0.16 +/- 0.05, suggesting that, on average, the sea-ice and open-ocean emissions equally contribute to sea-salt aerosol load of the inland Antarctic atmosphere. The temporal variability of the sulfate depletion relative to sodium was examined at the light of air mass backward trajectories, showing an overall decreasing trend of the ratio (i.e., a stronger sulfate depletion relative to sodium) when air masses arriving at Dome C had traveled a longer time over sea ice than over open ocean. The findings are shown to be useful to discuss sea-salt ice records extracted at deep drilling sites located inland Antarctica.
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Li, Y., Jourdain, N. C., Taschetto, A. S., Sen Gupta, A., Argueso, D., Masson, S., et al. (2017). Resolution dependence of the simulated precipitation and diurnal cycle over the Maritime Continent. Climate Dynamics, 48(11), 4009–4028.
Abstract: The Maritime Continent is a region of intense rainfall characterised by a strong diurnal cycle. This study investigates the sensitivity of rainfall characteristics to resolution in a coupled regional climate model configuration, in particular focusing on processes that modulate the diurnal cycle. Model biases are resolution dependent. Increasing resolution from 3/4A degrees to 1/4A degrees improves the mean state sea surface temperature and precipitation biases. However, at higher resolutions (1/12A degrees) rainfall becomes too strong in most areas. Daily maximum rainfall is simulated about 2-4 h earlier than in observations over both the land and the ocean, with only small improvements over high topography at higher resolution. We develop a technique to examine cross-coastal processes associated with the rainfall diurnal cycle along all coastlines. This is used to investigate the sensitivity of the diurnal cycle to resolution and to the direction of the prevailing wind. During offshore prevailing winds, most land rainfall is confined near the coastline and associated with a shallow land-sea breeze circulation at all resolution (though rainfall partly develops directly inland at 1/12A degrees). During onshore prevailing winds, rainfall propagates from the coastline to the island interior at 1/4A degrees and 1/12A degrees, whereas it appears directly over the island interior at 3/4A degrees, and this is associated with a deep convective cell across the coastline for all resolutions. Oceanic rainfall propagates far offshore during offshore prevailing winds at all resolutions, whereas it tends to remain confined near the coastline under onshore prevailing winds condition, particularly at higher resolution.
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Lim, S., Fain, X., Ginot, P., Mikhalenko, V., Kutuzov, S., Paris, J. D., et al. (2017). Black carbon variability since preindustrial times in the eastern part of Europe reconstructed from Mt. Elbrus, Caucasus, ice cores. Atmospheric Chemistry And Physics, 17(5), 3489–3505.
Abstract: Black carbon (BC), emitted by fossil fuel combustion and biomass burning, is the second largest manmade contributor to global warming after carbon dioxide (Bond et al., 2013). However, limited information exists on its past emissions and atmospheric variability. In this study, we present the first high-resolution record of refractory BC (rBC, including mass concentration and size) reconstructed from ice cores drilled at a high-altitude eastern European site in Mt. Elbrus (ELB), Caucasus (5115ma. s.l.). The ELB ice core record, covering the period 1825-2013, reflects the atmospheric load of rBC particles at the ELB site transported from the European continent with a larger rBC input from sources located in the eastern part of Europe. In the first half of the 20th century, European anthropogenic emissions resulted in a 1.5-fold increase in the ice core rBC mass concentrations with respect to its level in the preindustrial era (before 1850). The summer (winter) rBC mass concentrations increased 5-fold (3.3-fold) in 1960-1980, followed by a decrease until similar to 2000. Over the last decade, the rBC signal for summertime slightly increased. We have compared the signal with the atmospheric BC load simulated using past BC emissions (ACCMIP and MACCity inventories) and taken into account the contribution of different geographical regions to rBC distribution and deposition at the ELB site. Interestingly, the observed rBC variability in the ELB ice core record since the 1960s is not in perfect agreement with the simulated atmospheric BC load. Similar features between the ice core rBC record and the best scenarios for the atmospheric BC load support anthropogenic BC increase in the 20th century being reflected in the ELB ice core record. However, the peak in BC mass concentration observed in similar to 1970 in the ice core is estimated to occur a decade later from past inventories. BC emission inventories for the period 1960s-1970s may be underestimating European anthropogenic emissions. Furthermore, for summertime snow layers of the 2000s, the slightly increasing trend of rBC deposition likely reflects recent changes in anthropogenic and biomass burning BC emissions in the eastern part of Europe. Our study highlights that the past changes in BC emissions of eastern Europe need to be considered in assessing ongoing air quality regulation.
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Litt, M., Sicart, J. E., Six, D., Wagnon, P., & Helgason, W. D. (2017). Surface-layer turbulence, energy balance and links to atmospheric circulations over a mountain glacier in the French Alps. Cryosphere, 11(2), 971–987.
Abstract: Over Saint-Sorlin Glacier in the French Alps (45 degrees N, 6.1 degrees E; similar to 3 km(2)) in summer, we study the atmospheric surface-layer dynamics, turbulent fluxes, their uncertainties and their impact on surface energy balance (SEB) melt estimates. Results are classified with regard to largescale forcing. We use high-frequency eddy-covariance data and mean air-temperature and wind-speed vertical profiles, collected in 2006 and 2009 in the glacier's atmospheric surface layer. We evaluate the turbulent fluxes with the eddycovariance (sonic) and the profile method, and random errors and parametric uncertainties are evaluated by including different stability corrections and assuming different values for surface roughness lengths. For weak synoptic forcing, local thermal effects dominate the wind circulation. On the glacier, weak katabatic flows with a wind-speed maximum at low height (2-3 m) are detected 71% of the time and are generally associated with small turbulent kinetic energy (TKE) and small net turbulent fluxes. Radiative fluxes dominate the SEB. When the large-scale forcing is strong, the wind in the valley aligns with the glacier flow, intense downslope flows are observed, no wind-speed maximum is visible below 5 m, and TKE and net turbulent fluxes are often intense. The net turbulent fluxes contribute significantly to the SEB. The surface-layer turbulence production is probably not at equilibrium with dissipation because of interactions of largescale orographic disturbances with the flow when the forcing is strong or low-frequency oscillations of the katabatic flow when the forcing is weak. In weak forcing when TKE is low, all turbulent fluxes calculation methods provide similar fluxes. In strong forcing when TKE is large, the choice of roughness lengths impacts strongly the net turbulent fluxes from the profile method fluxes and their uncertainties. However, the uncertainty on the total SEB remains too high with regard to the net observed melt to be able to recommend one turbulent flux calculation method over another.
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Loizeau, S., Rossier, Y., Gaudet, J. P., Refloch, A., Besnard, K., Angulo-Jaramillo, R., et al. (2017). Water infiltration in an aquifer recharge basin affected by temperature and air entrapment. Journal Of Hydrology And Hydromechanics, 65(3), 222–233.
Abstract: Artificial basins are used to recharge groundwater and protect water pumping fields. In these basins, infiltration rates are monitored to detect any decrease in water infiltration in relation with clogging. However, miss-estimations of infiltration rate may result from neglecting the effects of water temperature change and air-entrapment. This study aims to investigate the effect of temperature and air entrapment on water infiltration at the basin scale by conducting successive infiltration cycles in an experimental basin of 11869 m(2) in a pumping field at Crepieux-Charmy (Lyon, France). A first experiment, conducted in summer 2011, showed a strong increase in infiltration rate; which was linked to a potential increase in ground water temperature or a potential dissolution of air entrapped at the beginning of the infiltration. A second experiment was conducted in summer, to inject cold water instead of warm water, and also revealed an increase in infiltration rate. This increase was linked to air dissolution in the soil. A final experiment was conducted in spring with no temperature contrast and no entrapped air (soil initially water-saturated), revealing a constant infiltration rate. Modeling and analysis of experiments revealed that air entrapment and cold water temperature in the soil could substantially reduce infiltration rate over the first infiltration cycles, with respective effects of similar magnitude. Clearly, both water temperature change and air entrapment must be considered for an accurate assessment of the infiltration rate in basins.
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Lopez-Moreno, J. I., Valero-Garces, B., Mark, B., Condom, T., Revuelto, J., Azorin-Molina, C., et al. (2017). Hydrological and depositional processes associated with recent glacier recession in Yanamarey catchment, Cordillera Blanca (Peru). Science Of The Total Environment, 579, 272–282.
Abstract: In this study, we investigate changes in the glaciated surface and the formation of lakes in the headwater of the Querococha watershed in Cordillera Blanca (Peru) using 24 Landsat images from 1975 to 2014. Information of glacier retreat was integrated with available climate data, the first survey of recent depositional dynamics in proglacial Yanamarey Lake (4600 m a.s.l.), and a relatively short hydrological record (2002-2014) at the outlet of Yanamarey Lake. A statistically significant temperature warming (0.21 degrees C decade(-1) for mean annual temperature) has been detected in the region, and it caused a reduction of the glacierized area since 1975 from 3.5 to 1.4 km(-2). New small lakes formed in the deglaciated areas, increasing the flooded area from 1.8 ha in 1976 to 2.8 ha in 2014. A positive correlation between annual rates of glacier recession and runoff was found. Sediment cores revealed a high sedimentation rate (>1 cm yr(-1)) and two contrasted facies, suggesting a shift toward a reduction of meltwater inputs and higher hydrological variability likely due to an increasing role of precipitation on runoff during the last decades. Despite the age control uncertainties, the main transition likely occurred around 19982000, correlating with the end of the phase with maximum warming rates and glacier retreat during the 1980s and 1990s, and the slowing down of expansion of surface lake-covered surface. With this hydrological – paleolimnological approach we have documented the association between recent climate variability and glacier recession and the rapid transfer of hydroclimate signal to depositional and geochemical processes in high elevation Andean environments. This, study also alerts about water quality risks as proglacial lakes act as secondary reservoirs that trap trace and minor elements in high altitude basins. (C) 2016 Elsevier B.V. All rights reserved.
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Marra, F., Destro, E., Nikolopoulos, E. I., Zoccatelli, D., Creutin, J. D., Guzzetti, F., et al. (2017). Impact of rainfall spatial aggregation on the identification of debris flow occurrence thresholds. Hydrology And Earth System Sciences, 21(9), 4525–4532.
Abstract: The systematic underestimation observed in debris flow early warning thresholds has been associated with the use of sparse rain gauge networks to represent highly non-stationary rainfall fields. Remote sensing products permit concurrent estimates of debris-flow-triggering rainfall for areas poorly covered by rain gauges, but the impact of using coarse spatial resolutions to represent such rainfall fields is still to be assessed. This study uses fine-resolution radar data for similar to 100 debris flows in the eastern Italian Alps to (i) quantify the effect of spatial aggregation (1-20 km grid size) on the estimation of debris-flow-triggering rainfall and on the identification of early warning thresholds and (ii) compare thresholds derived from aggregated estimates and rain gauge networks of different densities. The impact of spatial aggregation is influenced by the spatial organization of rainfall and by its dependence on the severity of the triggering rainfall. Thresholds from aggregated estimates show 8-21% variation in the parameters whereas 10-25% systematic variation results from the use of rain gauge networks, even for densities as high as 1/10 km(-2).
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Messager, E., Nomade, S., Wilhelm, B., Joannin, S., Scao, V., Von Grafenstein, U., et al. (2017). New pollen evidence from Nariani (Georgia) for delayed postglacial forest expansion in the South Caucasus. Quaternary Research, 87(1), 121–132.
Abstract: The nature and timing of environmental changes throughout the last glacial-interglacial transition in the South Caucasus, and more widely in eastern Europe, are still not fully understood. According to certain pollen records, forest expansion occurred in many areas several millennia after what is considered worldwide as the onset of the Holocene. The current problem we face is that the time lag in forest expansion varies from one sequence to another, sometimes with no delay at all. Moreover, the potential forcing/controlling factors behind this complex pattern, contrary to the almost synchronous global Holocene warming, are still a matter for debate. Accordingly, we revisit the issue of forest expansion through vegetation history obtained in the South Caucasus using a new pollen record, retrieved from the Nariani paleolake (South Georgia). These data attest to a steppic phase, initially dominated by Amaranthaceae-Chenopodiaceae (12,700-10,500 cal yr BP), then by Poaceae (10,500-9000 cal yr BP), culminating with a more forested phase (9000-5000 cal yr BP). Although some palaeoclimatic regional reconstructions show a wet early Holocene, we interpret the delay in forest expansion recorded in Nariani (2500 years) as the result of reduced spring precipitation, which would have limited forest development at that time.
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Miles, E. S., Steiner, J. F., & Brun, F. (2017). Highly variable aerodynamic roughness length (z(0)) for a hummocky debris-covered glacier. Journal Of Geophysical Research-Atmospheres, 122(16), 8447–8466.
Abstract: The aerodynamic roughness length (z(0)) is an essential parameter in surface energy balance studies, but few literature values exist for debris-covered glaciers. We use microtopographic and aerodynamic methods to assess the spatial variability of z(0) for Lirung Glacier, Nepal. We apply structure from motion to produce digital elevation models for three nested domains: five 1 m(2) plots, a 21,300 m(2) surface depression, and the lower 550,000 m(2) of the debris-mantled tongue. Wind and temperature sensor towers were installed in the vicinity of the plots within the surface depression in October 2014. We calculate z(0) according to a variety of transect-based microtopographic parameterizations for each plot, then develop a grid version of the algorithms by aggregating data from all transects. This grid approach is applied to the surface depression digital elevation model to characterize z(0) spatial variability. The algorithms reproduce the same variability among transects and plots, but z(0) estimates vary by an order of magnitude between algorithms. Across the study depression, results from different algorithms are strongly correlated. Using Monin-Obukov similarity theory, we derive z(0) values from the meteorological data. Using different stability criteria, we derive median values of z(0) between 0.03 m and 0.05 m, but with considerable uncertainty due to the glacier's complex topography. Considering estimates from these algorithms, results suggest that z(0) varies across Lirung Glacier between similar to 0.005 m (gravels) to similar to 0.5 m (boulders). Future efforts should assess the importance of such variable z(0) values in a distributed energy balance model.
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Miller, O. L., Solomon, D. K., Miège, C., Koenig, L. S., Forster, R. R., Montgomery, L., et al. (2017). Hydraulic Conductivity of a Firn Aquifer in Southeast Greenland. Frontiers in Earth Science, , 5–38. |
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Molg, N., Ceballos, J. L., Huggel, C., Micheletti, N., Rabatel, A., & Zemp, M. (2017). Ten years of monthly mass balance of Conejeras glacier, Colombia, and their evaluation using different interpolation methods. Geografiska Annaler Series A-Physical Geography, 99(2), 155–176.
Abstract: Understanding global climate change and its impacts on glaciers in the inner tropics is challenged by an absent climate seasonality that requires glacier monitoring at increased frequencies. Conejeras glacier in Colombia has been monitored monthly for 10 years, contributing to the limited knowledge of glacier mass development in this region. We acquired a terrestrial Lidar digital elevation model (DEM) and performed a full homogenization of the time series. Applying a number of interpolation methods, we calculated glacier-wide balances and deduced respective uncertainties. All interpolation methods revealed comparable variations in monthly surface mass balance, but the profile method failed in certain cases. We recommend using the Index-site method for monthly and annual and the Contour-line method for annual surface mass balances. Even when strongly reducing the stake network, the Index-site method and geostatistical interpolations (Kriging and Topo to Raster) showed robust and reliable results. Conejeras glacier is strongly down-wasting with a mass loss of 29 400mm w.e. and an area shrinkage of 20% within 10 years. Surface mass balance variations were strongest from November to February and depend largely on the intensity of El Nino Southern Oscillation. With a repeat DEM in the near future the glaciological time series could be validated with the geodetic mass balance. We recommend continuing the monthly monitoring programme, but complementing it with an energy balance study using additional meteorological data to better explain the glacier-climate interactions. However, to track the glacier's mass variations, a monitoring network with lower measurement frequency and stake density would be sufficient.
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Montgomery, L. N., Schmerr, N., Burdick, S., RR., F., Koenig, L. S., Legchenko, A., et al. (2017). Investigation of Firn Aquifer Structure in Southeastern Greenland Using Active Source Seismology. Frontiers in Earth Science, , 5–10. |
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Moreau, L., Lachaud, C., Thery, R., Predoi, M. V., Marsan, D., Larose, E., et al. (2017). Monitoring ice thickness and elastic properties from the measurement of leaky guided waves: A laboratory experiment. Journal Of The Acoustical Society Of America, 142(5), 2873–2880.
Abstract: The decline of Arctic sea ice extent is one of the most spectacular signatures of global warming, and studies converge to show that this decline has been accelerating over the last four decades, with a rate that is not reproduced by climate models. To improve these models, relying on comprehensive and accurate field data is essential. While sea ice extent and concentration are accurately monitored from microwave imagery, an accurate measure of its thickness is still lacking. Moreover, measuring observables related to the mechanical behavior of the ice (such as Young's modulus, Poisson's ratio, etc.) could provide better insights in the understanding of sea ice decline, by completing current knowledge so far acquired mostly from radar and sonar data. This paper aims at demonstrating on the laboratory scale that these can all be estimated simultaneously by measuring seismic waves guided in the ice layer. The experiment consisted of leaving a water tank in a cold room in order to grow an ice layer at its surface. While its thickness was increasing, ultrasonic guided waves were generated with a piezoelectric source, and measurements were subsequently inverted to infer the thickness and mechanical properties of the ice with very good accuracy. (C) 2017 Acoustical Society of America.
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Morera, S. B., Condom, T., Crave, A., Steer, P., & Guyot, J. L. (2017). The impact of extreme El Ni(n)over-tildeo events on modern sediment transport along the western Peruvian Andes (1968-2012). Scientific Reports, 7.
Abstract: Climate change is considered as one of the main factors controlling sediment fluxes in mountain belts. However, the effect of El Nino, which represents the primary cause of inter-annual climate variability in the South Pacific, on river erosion and sediment transport in the Western Andes remains unclear. Using an unpublished dataset of Suspended Sediment Yield (SSY) in Peru (1968-2012), we show that the annual SSY increases by 3-60 times during Extreme El Nino Events (EENE) compared to normal years. During EENE, 82% to 97% of the annual SSY occurs from January to April. We explain this effect by a sharp increase in river water discharge due to high precipitation rates and transport capacity during EENE. Indeed, sediments accumulate in the mountain and piedmont areas during dry normal years, and are then rapidly mobilized during EENE years. The effect of EENE on SSY depends on the topography, as it is maximum for catchments located in the North of Peru (3-7 degrees S), exhibiting a concave up hypsometric curve, and minimum for catchments in the South (7-18 degrees S), with a concave down hypsometric curve. These findings highlight how the sediment transport of different topographies can respond in very different ways to large climate variability.
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Moro, M. D., Brankart, J. M., Brasseur, P., & Verron, J. (2017). Exploring image data assimilation in the prospect of high-resolution satellite oceanic observations. Ocean Dynamics, 67(7), 875–895.
Abstract: Satellite sensors increasingly provide high-resolution (HR) observations of the ocean. They supply observations of sea surface height (SSH) and of tracers of the dynamics such as sea surface salinity (SSS) and sea surface temperature (SST). In particular, the Surface Water Ocean Topography (SWOT) mission will provide measurements of the surface ocean topography at very high-resolution (HR) delivering unprecedented information on the meso-scale and submeso-scale dynamics. This study investigates the feasibility to use these measurements to reconstruct meso-scale features simulated by numerical models, in particular on the vertical dimension. A methodology to reconstruct three-dimensional (3D) multivariate meso-scale scenes is developed by using a HR numerical model of the Solomon Sea region. An inverse problem is defined in the framework of a twin experiment where synthetic observations are used. A true state is chosen among the 3D multivariate states which is considered as a reference state. In order to correct a first guess of this true state, a two-step analysis is carried out. A probability distribution of the first guess is defined and updated at each step of the analysis: (i) the first step applies the analysis scheme of a reduced-order Kalman filter to update the first guess probability distribution using SSH observation; (ii) the second step minimizes a cost function using observations of HR image structure and a new probability distribution is estimated. The analysis is extended to the vertical dimension using 3D multivariate empirical orthogonal functions (EOFs) and the probabilistic approach allows the update of the probability distribution through the two-step analysis. Experiments show that the proposed technique succeeds in correcting a multivariate state using meso-scale and submeso-scale information contained in HR SSH and image structure observations. It also demonstrates how the surface information can be used to reconstruct the ocean state below the surface.
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Narcisi, B., Petit, J. R., & Langone, A. (2017). Last glacial tephra layers in the Talos Dome ice core (peripheral East Antarctic Plateau), with implications for chronostratigraphic correlations and regional volcanic history. Quaternary Science Reviews, 165, 111–126.
Abstract: Tephra isochrons offer considerable potential for correlating diverse palaeoarchives and highlighting regional climatic differences. They are especially useful when applied to polar ice records encompassing the last glacial, as these clearly portray the pronounced millennial-scale climate variability that characterised this period. Here we present the continuous record of primary fallout tephra layers in the East Antarctic Talos Dome ice core (72 degrees 49 ' S, 159 degrees 11 ' E), developed upon examination of the core sections spanning the glacial period 16.5 to 71 ka. A total of ca. 45 discrete tephra deposits precisely positioned stratigraphically relative to the temperature record for the core and dated using the AICC2012 timescale, were identified. Quantitative grain size, particle morphology, major and trace element composition using Coulter Counter, SEM, EPMA-WDS, and LA-ICP-MS analytical methods were studied as diagnostic features for tephra characterisation. The tephrostratigraphic framework provides a reference for future precise comparison between ice and sediment sequences across the Antarctic continent. Indeed, several potential markers characterised by distinct volcanic glass geochemistry and/or particular stratigraphic location (e.g., a 17.6-ka ash layer deposited during the well-known major acidity event) are now available for the direct linkage of palaeoclimatic archives. The Talos Dome tephra sequence, dominated by mid distal pyroclastic products from the nearby Northern Victoria Land volcanoes, also represents the most comprehensive and best time-constrained record of regional Antarctic volcanism yet developed. It documents nearly continuous sustained explosive activity during the considered time interval and, combined with previous ice-core tephra results for the last and the current interglacial periods, suggests progressive compositional shift through time. (C) 2017 Elsevier Ltd. All rights reserved.
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Narcisi, B., Petit, J. R., & Langone, A. (2017). Last glacial tephra layers in the Talos Dome ice core (peripheral East Antarctic Plateau), with implications for chronostratigraphic correlations and regional volcanic history (vol 165, pg 111, 2017). Quaternary Science Reviews, 172, 142–143. |
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Newland, M. J., Martinerie, P., Witrant, E., Helmig, D., Worton, D. R., Hogan, C., et al. (2017). Changes to the chemical state of the Northern Hemisphere atmosphere during the second half of the twentieth century. Atmospheric Chemistry And Physics, 17(13), 8269–8283.
Abstract: The NOx (NO and NO2) and HOx (OH and HO2) budgets of the atmosphere exert a major influence on atmospheric composition, controlling removal of primary pollutants and formation of a wide range of secondary products, including ozone, that can influence human health and climate. However, there remain large uncertainties in the changes to these budgets over recent decades. Due to their short atmospheric lifetimes, NOx and HOx are highly variable in space and time, and so the measurements of these species are of limited value for examining long-term, largescale changes to their budgets. Here, we take an alternative approach by examining long-term atmospheric trends of alkyl nitrates, the production efficiency of which is dependent on the atmospheric [NO] / [HO2] ratio. We derive long-term trends in the alkyl nitrates from measurements in firn air from the NEEM site, Greenland. Their mixing ratios increased by a factor of 3-5 between the 1970s and 1990s. This was followed by a steep decline to the sampling date of 2008. Moreover, we examine how the trends in the alkyl nitrates compare to similarly derived trends in their parent alkanes (i.e. the alkanes which, when oxidised in the presence of NOx, lead to the formation of the alkyl nitrates). The ratios of the alkyl nitrates to their parent alkanes increased from around 1970 to the late 1990s. This is consistent with large changes to the [NO] / [HO2] ratio in the Northern Hemisphere atmosphere during this period. Alternatively, they could represent changes to concentrations of the hydroxyl radical, OH, or to the transport time of the air masses from source regions to the Arctic.
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Nicolet, G., Eckert, N., Morin, S., & Blanchet, J. (2017). A multi-criteria leave-two-out cross-validation procedure for max-stable process selection. Spatial Statistics, 22, 107–128.
Abstract: Max-stable processes are the extension of the univariate extreme value theory to the spatial case. Contrary to the univariate case, there is no unique parametric form for the limiting distribution in the spatial case, and several max-stable processes can be found in the literature. Selecting the best of them for the data under study is still an open question. This paper proposes a procedure for discriminating max-stable processes by focusing on their spatial dependence structure. Specifically, it combines a leave-two-out cross-validation scheme and a large panel of adapted criteria. We compare five of the most commonly used max-stable processes, using as a case study a large data set of winter maxima of 3-day precipitation amounts in the French Alps (90 stations from 1958 to 2012). All the introduced criteria show that the extremal-t, geometric Gaussian and Brown-Resnick processes are equally able to represent the structure of dependence of the data, regardless of the number of stations or years. Although these results have to be confirmed by replicating the study in other contexts, they may be valid for a wide range of environmental applications. (C) 2017 Elsevier B.V. All rights reserved.
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Nord, G., Boudevillain, B., Berne, A., Branger, F., Braud, I., Dramais, G., et al. (2017). A high space-time resolution dataset linking meteorological forcing and hydro-sedimentary response in a mesoscale Mediterranean catchment (Auzon) of the Ardeche region, France. Earth System Science Data, 9(1).
Abstract: A comprehensive hydrometeorological dataset is presented spanning the period 1 January 201131 December 2014 to improve the understanding of the hydrological processes leading to flash floods and the relation between rainfall, runoff, erosion and sediment transport in a mesoscale catchment (Auzon, 116 km(2)) of the Mediterranean region. Badlands are present in the Auzon catchment and well connected to high-gradient channels of bedrock rivers which promotes the transfer of suspended solids downstream. The number of observed variables, the various sensors involved (both in situ and remote) and the space-time resolution (similar to km(2), similar to min) of this comprehensive dataset make it a unique contribution to research communities focused on hydrometeorology, surface hydrology and erosion. Given that rainfall is highly variable in space and time in this region, the observation system enables assessment of the hydrological response to rainfall fields. Indeed, (i) rainfall data are provided by rain gauges (both a research network of 21 rain gauges with a 5 min time step and an operational network of 10 rain gauges with a 5 min or 1 h time step), S-band Doppler dual-polarization radars (1 km(2), 5 min resolution), disdrometers (16 sensors working at 30 s or 1 min time step) and Micro Rain Radars (5 sensors, 100m height resolution). Additionally, during the special observation period (SOP-1) of the HyMeX (Hydrological Cycle in the Mediterranean Experiment) project, two X-band radars provided precipitation measurements at very fine spatial and temporal scales (1 ha, 5 min). (ii) Other meteorological data are taken from the operational surface weather observation stations of Meteo-France (including 2m air temperature, atmospheric pressure, 2 m relative humidity, 10m wind speed and direction, global radiation) at the hourly time resolution (six stations in the region of interest). (iii) The monitoring of surface hydrology and suspended sediment is multi-scale and based on nested catchments. Three hydrometric stations estimate water discharge at a 2-10 min time resolution. Two of these stations also measure additional physico-chemical variables (turbidity, temperature, conductivity) and water samples are collected automatically during floods, allowing further geochemical characterization of water and suspended solids. Two experimental plots monitor overland flow and erosion at 1 min time resolution on a hillslope with vineyard. A network of 11 sensors installed in the intermittent hydrographic network continuously me
asures water level and water temperature in headwater subcatchments (from 0.17 to 116 km(2)) at a time resolution of 2-5 min. A network of soil moisture sensors enables the continuous measurement of soil volumetric water content at 20 min time resolution at 9 sites. Additionally, concomitant observations (soil moisture measurements and stream gauging) were performed during floods between 2012 and 2014. Finally, this dataset is considered appropriate for understanding the rainfall variability in time and space at fine scales, improving areal rainfall estimations and progressing in distributed hydrological and erosion modelling. |
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Olivera-Guerra, L., Mattar, C., Merlin, O., Duran-Alarcon, C., Santamaria-Artigas, A., & Fuster, R. (2017). An operational method for the disaggregation of land surface temperature to estimate actual evapotranspiration in the arid region of Chile. Isprs Journal Of Photogrammetry And Remote Sensing, 128, 170–181.
Abstract: Monitoring evapotranspiration in arid and semi-arid environments plays a key role in water irrigation scheduling for water use efficiency. This work presents an operational method for evapotranspiration retrievals based on disaggregated Land Surface Temperature (LST). The retrieved LSTs from Landsat-8 and MODIS data were merged in order to provide an 8-day composite LST product at 100 x 100 m resolution. The method was tested in the arid region of Copiapo, Chile using data from years 2013-2014 and validated using data from years 2015-2016. In-situ measurements from agrometeorological stations such as air temperature and potential evapotranspiration (ETU) estimated at the location were used in the ET estimation method. The disaggregation method was developed by taking into account (1) the spatial relationship between Landsat-8 and MODIS LST, (2) the spatial relationship between LST and the Normalized Difference Vegetation Index (NDVI) at high spatial resolution (Landsat-8), and (3) the temporal variations along the year of both relationships aforementioned. The comparison between disaggregated LST at 100 m resolution and in situ LST measurements presents a coefficient of determination (r(2)), in average, equal to 0.70 and a RMSE equal to 3.6 K. The disaggregated LST was used in an operational model to estimate the actual evapotranspiration (ETa). The ETa shows good results in terms of seasonal variations and in comparison to the evapotranspiration estimated by using crop coefficients (kc). The comparison between remotely sensed and in situ ETa presents an overall r(2) close to 0.67 and a RMSE equal to 0.6 mm day(-1) for both crops. These results are important for further improvements in water use sustain ability in the Copiapo valley, which is currently affected by high water demand. (C) 2017 International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS). Published by Elsevier B.V. All rights reserved.
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Oueslati, B., Pohl, B., Moron, V., Rome, S., & Janicot, S. (2017). Characterization of Heat Waves in the Sahel and Associated Physical Mechanisms. Journal Of Climate, 30(9), 3095–3115.
Abstract: Great effort is made to address heat waves (HWs) in developed countries because of their devastating impacts on society, economy, and environment. However, HWs are still understudied over developing countries. This is particularly true in West Africa, and especially in the Sahel, where temperatures recurrently reach critical values, such as during the 2010 HW event in the western Sahel. This work aims at characterizing the Sahelian HWs during boreal spring seasons (April-May-June) and understanding the mechanisms associated with such extreme events. Over the last three decades, Sahelian HWs have been becoming more frequent, lasting longer, covering larger areas, and reaching higher intensities. The physical mechanisms associated with HWs are examined to assess the respective roles of atmospheric dynamics and radiative and turbulent fluxes by analyzing the surface energy budget. Results suggest that the greenhouse effect of water vapor is the main driver of HWs in the western Sahel, increasing minimum temperatures by enhanced downward longwave radiation. Atmospheric circulation plays an important role in sustaining these warm anomalies by advecting moisture from the Atlantic Ocean and the Guinean coasts into the Sahel. Maximum temperature anomalies are mostly explained by increased downward shortwave radiation due to a reduction in cloud cover. Interannual variability of HWs is affected by the delayed impact of El Nio-Southern Oscillation (ENSO), with anomalous temperature warming following warm ENSO events, resulting from an amplified water vapor feedback.
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Palerme, C., Claud, C., Dufour, A., Genthon, C., Wood, N. B., & L'Ecuyer, T. (2017). Evaluation of Antarctic snowfall in global meteorological reanalyses. Atmospheric Research, 190, 104–112.
Abstract: Recent precipitation observations provided by CloudSat are used to evaluate the ability of various meteorological analyses and reanalyses to reproduce Antarctic snowfall. The performance of the ECMWF Interim Re-Analysis (ERA Interim), the Climate Forecast System Reanalysis (CFSR), the Japanese 55-year Reanalysis (JRA55), the Modern Era Retrospective-Analysis for Research and Application (MERRA), and the Modern Era Retrospective-Analysis for Research and Application 2 (MERRA-2), as well as ECMWF operational analyses are compared over the 2007-2010 period. The mean snowfall rate over Antarctica north of 82 degrees S simulated by the reanalyses between 2007 and 2010 ranges from 165 to 225 mm per year, while CloudSat observations indicate a value of 172 mm per year. ERA Interim produces the closest match to the observed snowfall rate, but all the reanalyses reproduce well the seasonal and interannual variability of Antarctic snowfall reported in CloudSat observations. (C) 2017 The Authors. Published by Elsevier B.V.
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Palerme, C., Genthon, C., Claud, C., Kay, J. E., Wood, N. B., & L'Ecuyer, T. (2017). Evaluation of current and projected Antarctic precipitation in CMIP5 models. Climate Dynamics, 48(1-2), 225–239.
Abstract: On average, the models in the Fifth Climate Model Intercomparison Project archive predict an increase in Antarctic precipitation from 5.5 to 24.5 % between 1986-2005 and 2080-2099, depending on greenhouse gas emissions scenarios. This translates into a moderation of future sea level rise ranging from -19 to -71 mm between 2006 and 2099. However, comparison with CloudSat and ERA-Interim data show that almost all the models overestimate current Antarctic precipitation, some by more than 100 %. If only the models that agree with CloudSat data within 20 % of error are considered, larger precipitation changes (from 7.4 to 29.3 %) and impact on sea level (from -25 to -85 mm) are predicted. A common practice of averaging all models to evaluate climate projections thus leads to a significant underestimation of the contribution of Antarctic precipitation to future sea level. Models simulate, on average, a 7.4 %/A degrees C precipitation change with surface temperature warming. The models in better agreement with CloudSat observations for Antarctic snowfall predict, on average, larger temperature and Antarctic sea ice cover changes, which could explain the larger changes in Antarctic precipitation simulated by these models. The agreement between the models, CloudSat data and ERA-Interim is generally less in the interior of Antarctica than at the peripheries, but the interior is also where climate change will induce the smallest absolute change in precipitation. About three-quarters of the impact on sea level will result from precipitation change over the half most peripheral and lowest elevation part of the surface of Antarctica.
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Papagiannaki, K., Kotroni, V., Lagouvardos, K., Ruin, I., & Bezes, A. (2017). Urban Area Response to Flash Flood-Triggering Rainfall, Featuring Human Behavioral Factors: The Case of 22 October 2015 in Attica, Greece. Weather Climate And Society, 9(3), 621–638.
Abstract: Over the past several decades, flash floods that occurred in Attica, Greece, caused serious property and infrastructure damages, disruptions in economic and social activities, and human fatalities. This paper investigated the link between rainfall and flash flood impact during the catastrophic event that affected Attica on 22 October 2015, while also addressing human risk perception and behavior as a response to flash floods. The methodology included the analysis of the space-time correlation of rainfall with the citizens' calls to the emergency fire services for help, and the statistical analysis of people's responses to an online behavioral survey. The results designated critical rainfall thresholds associated with flash flood impact in the four most affected subareas of the Attica region. The impact magnitude was found to be associated with the localized accumulated rainfall. Vulnerability factors, namely, population density, geographical, and environmental features, may have contributed to the differences in the impact magnitudes between the examined subareas. The analysis of the survey's behavioral responses provided insights into peoples' risk perception and coping responses relative to the space-time distribution of rainfall. The findings of this study were in agreement with the hypothesis that the more severe the rainfall, the higher peoples' severity assessment and the intensity of emotional response. Deeper feelings of fear and worry were found to be related to more adjustments to the scheduled activities and travels. Additionally, being alert to the upcoming rainfall risk was found to be related to decreased worry and fear and to fewer changes in scheduled activities.
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Parrenin, F., Bazin, L., Capron, E., Landais, A., Lemieux-Dudon, B., & Masson-Delmotte, V. (2017). Icechrono1: A Probabilistic Model To Compute A Common And Optimized Chronology For Several Ice Cores. Quaternaire, 28(2), 179–184.
Abstract: Polar ice cores provide exceptional archives of past environmental conditions. The dating of ice cores and the estimation of the age-scale uncertainty are essential to interpret the climate and environmental records that they contain. It is, however, a complex problem which involves different methods. Here, we present IceChrono1, a new probabilistic model integrating various sources of chronological information to produce a common and optimized chronology for several ice cores, as well as its uncertainty. IceChrono1 is based on the inference of three quantities: the surface accumulation rate, the lock-in depth (LID) of air bubbles and the thinning function. The chronological information integrated into IceChrono1 are modeling scenarios of the sedimentation process (accumulation of snow, densification of snow into ice and air trapping, ice flow), ice- and air-dated horizons, ice and air depth intervals with known durations, Delta depth observations (depth shift between synchronous events recorded in the ice and in the air) and finally ice, air or mix stratigraphic links in between ice cores. the inference problem is formulated as a least squares optimisation, implying that all densities of probabilities are assumed to be Gaussian. It is numerically solved using the Levenberg-Marquardt algorithm (thus assuming that the model is almost linear in the vicinity of the solution) and a numerical evaluation of the model's Jacobian. IceChrono1 is freely available under the General Public License v3 open source license.
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Parrenin, F., Cavitte, M. G. P., Blankenship, D. D., Chappellaz, J., Fischer, H., Gagliardini, O., et al. (2017). Is there 1.5-million-year-old ice near Dome C, Antarctica? Cryosphere, 11(6), 2427–2437.
Abstract: Ice sheets provide exceptional archives of past changes in polar climate, regional environment and global atmospheric composition. The oldest dated deep ice core drilled in Antarctica has been retrieved at EPICA Dome C (EDC), reaching similar to 800 000 years. Obtaining an older paleoclimatic record from Antarctica is one of the greatest challenges of the ice core community. Here, we use internal isochrones, identified from airborne radar coupled to ice-flow modelling to estimate the age of basal ice along transects in the Dome C area. Three glaciological properties are inferred from isochrones: surface accumulation rate, geothermal flux and the exponent of the Lliboutry velocity profile. We find that old ice (> 1.5 Myr, 1.5 million years) likely exists in two regions: one similar to 40 km south-west of Dome C along the ice divide to Vostok, close to a secondary dome that we name “Little Dome C” (LDC), and a second region named “North Patch” (NP) located 10-30 km north-east of Dome C, in a region where the geothermal flux is apparently relatively low. Our work demonstrates the value of combining radar observations with ice flow modelling to accurately represent the true nature of ice flow, and understand the formation of ice-sheet architecture, in the centre of large ice sheets.
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Passalacqua, O., Ritz, C., Parrenin, F., Urbini, S., & Frezzotti, M. (2017). Geothermal flux and basal melt rate in the Dome C region inferred from radar reflectivity and heat modelling. Cryosphere, 11(5), 2231–2246.
Abstract: Basal melt rate is the most important physical quantity to be evaluated when looking for an old-ice drilling site, and it depends to a great extent on the geothermal flux (GF), which is poorly known under the East Antarctic ice sheet. Given that wet bedrock has higher reflectivity than dry bedrock, the wetness of the ice-bed interface can be assessed using radar echoes from the bedrock. But, since basal conditions depend on heat transfer forced by climate but lagged by the thick ice, the basal ice may currently be frozen whereas in the past it was generally melting. For that reason, the risk of bias between present and past conditions has to be evaluated. The objective of this study is to assess which locations in the Dome C area could have been protected from basal melting at any time in the past, which requires evaluating GF. We used an inverse approach to retrieve GF from radar-inferred distribution of wet and dry beds. A 1-D heat model is run over the last 800 ka to constrain the value of GF by assessing a critical ice thickness, i.e. the minimum ice thickness that would allow the present local distribution of basal melting. A regional map of the GF was then inferred over a 80 km x 130 km area, with a N-S gradient and with values ranging from 48 to 60m Wm(-2). The forward model was then emulated by a polynomial function to compute a time-averaged value of the spatially variable basal melt rate over the region. Three main subregions appear to be free of basal melting, two because of a thin overlying ice and one, north of Dome C, because of a low GF.
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Pattyn, F., Favier, L., Sun, S., & Durand, G. (2017). Progress in Numerical Modeling of Antarctic Ice-Sheet Dynamics. Current Climate Change Reports, 3(3), 174–184.
Abstract: Numerical modeling of the Antarctic ice sheet has gone through a paradigm shift over the last decade. While initially models focussed on long-time diffusive response to surface mass balance changes, processes occurring at the marine boundary of the ice sheet are progressively incorporated in newly developed state-of-the-art ice-sheet models. These models now exhibit fast, short-term volume changes, in line with current observations of mass loss. Coupling with ocean models is currently on its way and applied to key areas of the Antarctic ice sheet. New model intercomparisons have been launched, focusing on ice/ocean interaction (MISMIP+, MISOMIP) or ice-sheet model initialization and multi-ensemble projections (ISMIP6). Nevertheless, the inclusion of new processes pertaining to ice-shelf calving, evolution of basal friction, and other processes, also increase uncertainties in the contribution of the Antarctic ice sheet to future sea-level rise.
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Petitgirard, S., Malfait, W. J., Journaux, B., Collings, I. E., Jennings, E. S., Blanchard, I., et al. (2017). SiO2 Glass Density to Lower-Mantle Pressures. Physical Review Letters, 119(21).
Abstract: The convection or settling of matter in the deep Earth's interior is mostly constrained by density variations between the different reservoirs. Knowledge of the density contrast between solid and molten silicates is thus of prime importance to understand and model the dynamic behavior of the past and present Earth. SiO2 is the main constituent of Earth's mantle and is the reference model system for the behavior of silicate melts at high pressure. Here, we apply our recently developed x-ray absorption technique to the density of SiO2 glass up to 110 GPa, doubling the pressure range for such measurements. Our density data validate recent molecular dynamics simulations and are in good agreement with previous experimental studies conducted at lower pressure. Silica glass rapidly densifies up to 40 GPa, but the density trend then flattens to become asymptotic to the density of SiO2 minerals above 60 GPa. The density data present two discontinuities at similar to 17 and similar to 60 GPa that can be related to a silicon coordination increase from 4 to a mixed 5/6 coordination and from 5/6 to sixfold, respectively. SiO2 glass becomes denser than MgSiO3 glass at similar to 40 GPa, and its density becomes identical to that of MgSiO3 glass above 80 GPa. Our results on SiO2 glass may suggest that a variation of SiO2 content in a basaltic or pyrolitic melt with pressure has at most a minor effect on the final melt density, and iron partitioning between the melts and residual solids is the predominant factor that controls melt buoyancy in the lowermost mantle.
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Petrenko, V. V., Mith, A. M. S., Chaefer, H. S., Riedel, K., Brook, E., Baggenstos, D., et al. (2017). Minimal geological methane emissions during the Younger Dryas-Preboreal abrupt warming event. Nature, 548(7668), 443–446.
Abstract: Methane (CH4) is a powerful greenhouse gas and plays a key part in global atmospheric chemistry. Natural geological emissions (fossil methane vented naturally from marine and terrestrial seeps and mud volcanoes) are thought to contribute around 52 teragrams of methane per year to the global methane source, about 10 per cent of the total, but both bottom-up methods (measuring emissions)(1) and top-down approaches (measuring atmospheric mole fractions and isotopes)(2) for constraining these geological emissions have been associated with large uncertainties. Here we use ice core measurements to quantify the absolute amount of radiocarbon-containing methane ((CH4)-C-14) in the past atmosphere and show that geological methane emissions were no higher than 15.4 teragrams per year (95 per cent confidence), averaged over the abrupt warming event that occurred between the Younger Dryas and Preboreal intervals, approximately 11,600 years ago. Assuming that past geological methane emissions were no lower than today(3,4), our results indicate that current estimates of today's natural geological methane emissions (about 52 teragrams per year)(1,2) are too high and, by extension, that current estimates of anthropogenic fossil methane emissions(2) are too low. Our results also improve on and confirm earlier findings(5-7) that the rapid increase of about 50 per cent in mole fraction of atmospheric methane at the Younger Dryas-Preboreal event was driven by contemporaneous methane from sources such as wetlands; our findings constrain the contribution from old carbon reservoirs (marine methane hydrates(8), permafrost(9) and methane trapped under ice(10)) to 19 per cent or less (95 per cent confidence). To the extent that the characteristics of the most recent deglaciation and the Younger Dryas-Preboreal warming are comparable to those of the current anthropogenic warming, our measurements suggest that large future atmospheric releases of methane from old carbon sources are unlikely to occur.
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Pouget, J. C., Proano, D., Vera, A., Villacis, M., Condom, T., Escobar, M., et al. (2017). Glacio-hydrological modelling and water resources management in the Ecuadorian Andes: the example of Quito. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques, 62(3), 431–446.
Abstract: The strong socio-economic growth of Quito has led to significant projects of inter-basin transfers, intensifying high-altitude resources mobilization in environmentally sensitive areas and with accelerated melting of glaciers. In order to study various future scenarios, we propose modelling of the climate/glacier/hydrology/water resources management continuum, applied to Andean basins. Using the tool Water Evaluation and Planning (WEAP21), we developed: (1) semi-distributed hydro-climate modelling with monthly data using regional homogenization (vector method); (2) glacier modelling of water production and inter-annual evolution; (3) modelling in hydrological units depending on land cover; and (4) water management modelling distinguishing water rights, allocation and use. In this paper, we mainly present results of monthly hydrological calibrations (1963-2006), studying, in particular, the equifinality of various parameter settings. We show flexibility, robustness and limits of the proposed modelling, contributing to identification of uncertainties for evaluation of future scenarios.
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Prokopiou, M., Martinerie, P., Sapart, C. J., Witrant, E., Monteil, G., Ishijima, K., et al. (2017). Constraining N2O emissions since 1940 using firn air isotope measurements in both hemispheres. Atmospheric Chemistry And Physics, 17(7), 4539–4564.
Abstract: N2O is currently the third most important anthropogenic greenhouse gas in terms of radiative forcing and its atmospheric mole fraction is rising steadily. To quantify the growth rate and its causes over the past decades, we performed a multi-site reconstruction of the atmospheric N2O mole fraction and isotopic composition using new and previously published firn air data collected from Greenland and Antarctica in combination with a firn diffusion and densification model. The multi-site reconstruction showed that while the global mean N2O mole fraction increased from (290 +/- 1) nmol mol(-1) in 1940 to (322 +/- 1) nmol mol(-1) in 2008, the isotopic composition of atmospheric N2O decreased by (-2.2 +/- 0.2)parts per thousand for delta N-15(av), (-1.0 +/- 0.3)parts per thousand for delta O-18, (-1.3 +/- 0.6)parts per thousand for delta N-15(alpha), and (-2.8 +/- 0.6)parts per thousand for delta N-15(beta) over the same period. The detailed temporal evolution of the mole fraction and isotopic composition derived from the firn air model was then used in a two-box atmospheric model (comprising a stratospheric box and a tropospheric box) to infer changes in the isotopic source signature over time. The precise value of the source strength depends on the choice of the N2O lifetime, which we choose to fix at 123 years. The average isotopic composition over the investigated period is delta N-15(av) = (-7.6 +/- 0.8)parts per thousand (vs. air-N-2), delta O-18 D (32.2 +/- 0.2)parts per thousand (vs. Vienna Standard Mean Ocean Water – VSMOW) for delta O-18, delta N-15(alpha) = (-3.0 +/- 1.9)parts per thousand and delta N-15(beta) = (-11.7 +/- 2.3) parts per thousand. delta N-15(av), and delta N-15(beta) show some temporal variability, while for the other signatures the error bars of the reconstruction are too large to retrieve reliable temporal changes. Possible processes that may explain trends in N-15 are discussed. The N-15 site preference (= delta N-15 alpha – delta N-15(beta)) provides evidence of a shift in emissions from denitrification to nitrification, although the uncertainty envelopes are large.
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Rabatel, A., Sirguey, P., Drolon, V., Maisongrande, P., Arnaud, Y., Berthier, E., et al. (2017). Annual and Seasonal Glacier-Wide Surface Mass Balance Quantified from Changes in Glacier Surface State: A Review on Existing Methods Using Optical Satellite Imagery. Remote Sensing, 9(5).
Abstract: Glaciers are one of the terrestrial essential climate variables (ECVs) as they respond very sensitively to climate change. A key driver of their response is the glacier surface mass balance that is typically derived from field measurements. It deserves to be quantified over long time scales to better understand the accumulation and ablation processes at the glacier surface and their relationships with inter-annual changes in meteorological conditions and long-term climate changes. Glaciers with in situ monitoring of surface mass balance are scarce at the global scale, and satellite remote sensing provides a powerful tool to increase the number of monitored glaciers. In this study, we present a review of three optical remote sensing methods developed to quantify seasonal and annual glacier surface mass balances. These methodologies rely on the multitemporal monitoring of the end-of-summer snow line for the equilibrium-line altitude (ELA) method, the annual cycle of glacier surface albedo for the albedo method and the mapping of the regional snow cover at the seasonal scale for the snow-map method. Together with a presentation of each method, an application is illustrated. The ELA method shows promising results to quantify annual surface mass balance and to reconstruct multi-decadal time series. The other two methods currently need a calibration on the basis of existing in situ data; however, a generalization of these methods (without calibration) could be achieved. The two latter methods show satisfying results at the annual and seasonal scales, particularly for the summer surface mass balance in the case of the albedo method and for the winter surface mass balance in the case of the snow-map method. The limits of each method (e.g., cloud coverage, debris-covered glaciers, monsoon-regime and cold glaciers), their complementarities and the future challenges (e.g., automating of the satellite images processing, generalization of the methods needing calibration) are also discussed.
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Raisbeck, G. M., Cauquoin, A., Jouzel, J., Landais, A., Petit, J. R., Lipenkov, V. Y., et al. (2017). An improved north-south synchronization of ice core records around the 41 kyr Be-10 peak. Climate Of The Past, 13(3), 217–229.
Abstract: Using new high-resolution Be-10 measurements in the NGRIP, EDML and Vostok ice cores, together with previously published data from EDC, we present an improved synchronization between Greenland and Antarctic ice cores during the Laschamp geomagnetic excursion similar to 41 kyr ago. We estimate the precision of this synchronization to be +/- 20 years, an order of magnitude better than previous work. We discuss the implications of this new synchronization for making improved estimates of the depth difference between ice and enclosed gas of the same age (Delta depth), difference between age of ice and enclosed gas at the same depth (Delta age) in the EDC and EDML ice cores, spectral properties of the Be-10 profiles and phasing between Dansgaard-Oeschger-10 (in NGRIP) and AIM-10 (in EDML and EDC).
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Raynaud, D., Hingray, B., Zin, I., Anquetin, S., Debionne, S., & Vautard, R. (2017). Atmospheric analogues for physically consistent scenarios of surface weather in Europe and Maghreb. International Journal Of Climatology, 37(4), 2160–2176.
Abstract: This study compares the multivariate predictions of daily temperature, temperature range, precipitation, surface wind and solar radiation of a single-model analogue approach with an original multi-model analogy over 12 regions in Europe and Maghreb. Both approaches are based on two-level analogue models where atmospheric predictors are either dynamic or thermodynamic. In the multi-model approach, independent analogue models with predictand-specific predictors are used. In the single-model one, a unique analogue model and its associated set of predictors is applied to all predictands. Testing numerous large-scale predictors, we first identify the best predictor sets for each modelling strategy. Those obtained for the single-model approach are significantly different from those of the predictand-specific models. This is especially the case for local temperature and wind speed. Both methods perform similarly for precipitation, temperature range and radiation. We next assess the ability of both approaches to simulate physically coherent multivariate weather scenarios. With the single-model method, weather scenarios are obtained for each prediction day from observations sampled simultaneously on one analogue day. The physical consistency between variables is thus automatically fulfilled each day. This allows the single-model method to reproduce well the observed inter-predictand correlations, especially the significant correlations between radiation and precipitation and between radiation and temperature range. These results suggest a hybrid analogue model using a single-model for radiation, temperature range and precipitation, combined with a univariate approach for wind. Two options are proposed for temperature for which either the predictand-specific method or a single-model approach with an additional correction are conceivable. This hybrid approach leads to a possible compromise between reasonable univariate prediction skills and realistic inter-predictands correlations, both classically required for many impact studies.
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Refloch, A., Gaudet, J. P., Oxarango, L., & Rossier, Y. (2017). Estimation of saturated hydraulic conductivity from ring infiltrometer test taking into account the surface moisture stain extension. Journal Of Hydrology And Hydromechanics, 65(3), 321–324.
Abstract: A large single-ring infiltrometer test was performed in order to characterize the saturated hydraulic conductivity below an infiltration basin in the well field of Lyon (France). Two kinds of data are recorded during the experiment: the volume of water infiltrated over time and the extension of the moisture stain around the ring. Then numerical analysis was performed to determine the saturated hydraulic conductivity of the soil by calibration. Considering an isotropic hydraulic conductivity, the saturated hydraulic conductivity of the alluvial deposits is estimated at 3.8 10(-6) m s(-1). However, with this assumption, we are not able to represent accurately the extension of the moisture stain around the ring. When anisotropy of hydraulic conductivity is introduced, experimental data and simulation results are in good agreement, both for the volume of water infiltrated over time and the extension of the moisture stain. The vertical saturated hydraulic conductivity in the anisotropic configuration is 4.75 times smaller than in the isotropic configuration (8.0 10(-7) m s(-1)), and the horizontal saturated hydraulic conductivity is 125 times higher than the vertical saturated hydraulic conductivity (1.0 10(-4) m s(-1)).
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Reichle, R. H., De Lannoy, G. J. M., Liu, Q., Ardizzone, J. V., Colliander, A., Conaty, A., et al. (2017). Assessment of the SMAP Level-4 Surface and Root-Zone Soil Moisture Product Using In Situ Measurements. Journal Of Hydrometeorology, 18(10), 2621–2645.
Abstract: The Soil Moisture Active Passive (SMAP) mission Level-4 Surface and Root-Zone Soil Moisture (L4SM) data product is generated by assimilating SMAP L-band brightness temperature observations into the NASA Catchment land surface model. The L4SM product is available from 31 March 2015 to present (within 3 days from real time) and provides 3-hourly, global, 9-km resolution estimates of surface (0-5 cm) and root-zone (0-100 cm) soil moisture and land surface conditions. This study presents an overview of the L4SM algorithm, validation approach, and product assessment versus in situ measurements. Core validation sites provide spatially averaged surface (root zone) soil moisture measurements for 43 (17) "reference pixels'' at 9- and 36-km gridcell scales located in 17 (7) distinct watersheds. Sparse networks provide point-scale measurements of surface (root zone) soil moisture at 406 (311) locations. Core validation site results indicate that the L4SM product meets its soil moisture accuracy requirement, specified as an unbiased RMSE (ubRMSE, or standard deviation of the error) of 0.04 m(3) m(-3) or better. The ubRMSE for L4SM surface (root zone) soil moisture is 0.038 m(3) m(-3) (0.030 m(3) m(-3)) at the 9-km scale and 0.035 m(3) m(-3) (0.026 m(3) m(-3)) at the 36-km scale. The L4SM estimates improve (significantly at the 5% level for surface soil moisture) over model-only estimates, which do not benefit from the assimilation of SMAP brightness temperature observations and have a 9-km surface (root zone) ubRMSE of 0.042 m(3) m(-3) (0.032 m(3) m(-3)). Time series correlations exhibit similar relative performance. The sparse network results corroborate these findings over a greater variety of climate and land cover conditions.
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Reveillet, M., Vincent, C., Six, D., & Rabatel, A. (2017). Which empirical model is best suited to simulate glacier mass balances? Journal Of Glaciology, 63(237), 39–54.
Abstract: Based on an extensive dataset of surface mass balances (SMB) from four glaciers in the French Alps for the period 1995-2012 and in the framework of enhanced temperature-index models, we investigate the sensitivity of seasonal glacier SMB to temperature, solar radiation, precipitation and topographical variables. Our results reveal strong correlations between winter SMB and precipitation, although the precipitation gradient cannot explain the high-accumulation rates. Based on the available point measurements, we found no relevant relationship between winter SMB and topographical variables. Temperature was found to be the main driver of ice/snow ablation while solar radiation was found to strongly influence the spatial distribution of summer SMB. We compared the ability of several enhanced temperature-index models to accurately simulate point SMB and glacier-wide MB. Our analyses revealed that the uncertainties in the simulated annual SMB due to winter SMB uncertainties are larger than differences between models and prevented us from concluding, which model is the most suitable. In contrast with results of previous studies, including solar radiation in melt models did not improve the performances when modelling glacier-wide MB. We conclude that a classical degree-day model is sufficient to simulate the long-term glacier-wide MB if the underlying processes are not required to be resolved.
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Rhodes, R. H., Brook, E. J., McConnell, J. R., Blunier, T., Sime, L. C., Fain, X., et al. (2017). Atmospheric methane variability: Centennial-scale signals in the Last Glacial Period. Global Biogeochemical Cycles, 31(3), 575–590.
Abstract: In order to understand atmospheric methane (CH4) biogeochemistry now and in the future, we must apprehend its natural variability, without anthropogenic influence. Samples of ancient air trapped within ice cores provide the means to do this. Here we analyze the ultrahigh-resolution CH4 record of the West Antarctic Ice Sheet Divide ice core 67.2-9.8 ka and find novel, atmospheric CH4 variability at centennial time scales throughout the record. This signal is characterized by recurrence intervals within a broad 80-500 year range, but we find that age-scale uncertainties complicate the possible isolation of any periodic frequency. Lower signal amplitudes in the Last Glacial relative to the Holocene may be related to incongruent effects of firn-based signal smoothing processes. Within interstadial and stadial periods, the peak-to-peak signal amplitudes vary in proportion to the underlying millennial-scale oscillations in CH4 concentration-the relative amplitude change is constant. We propose that the centennial CH4 signal is related to tropical climate variability that influences predominantly low-latitude wetland CH4 emissions. Plain Language Summary Using a new method to measure methane concentrations of ancient air trapped in ice cores, we have detected variability in atmospheric methane concentration on centennial time scales in the Last Glacial Period for the first time. We know these signals represent past changes in atmospheric methane because they appear in several ice core records. We ropose that changes in methane emissions from tropical wetlands are responsible. How this new variability might be related to similar signals found in the late Holocene ice core records and the instrumental record of atmospheric methane is an open question.
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Richeton, T., Le, L. T., Chauve, T., Bernacki, M., Berbenni, S., & Montagnat, M. (2017). Modelling the transport of geometrically necessary dislocations on slip systems: application to single- and multi-crystals of ice. Modelling And Simulation In Materials Science And Engineering, 25(2).
Abstract: A model based on the elastic theory of continuously distributed dislocations, accounting for the transport of geometrically necessary dislocations (GND) on slip systems is developed. It allows keeping the crystallographic nature of glide by allocating velocities specific to slip systems to GND. At grain boundaries, the dislocation transport equation is resolved between a specific system in a grain and a specific system in the adjacent grain. It is used to simulate a compression creep test followed by unloading of a multiple slip deforming multi-crystal of ice during which kink band formation, grain boundary migration and localized grain nucleation are observed. The model predictions are compared to 2D strain fields obtained by digital image correlation and show a good agreement. Besides, the kink band position corresponds very well with an area of strong lattice misorientation predicted by the model and is also bounded by opposite densities of edge dislocations, in agreement with kink banding theory and characterization. Furthermore, the grain boundary migration is observed to happen from predicted low dislocation density area towards high dislocation ones-also in agreement with the theory. Lastly, the triple junctions where nucleation is observed are also characterized by high GND density and especially strong gradient of elastic energy density. These different features show the relevance of using a continuum theory of polarized dislocations per slip system to study the onset of relaxation mechanisms like kink banding, grain boundary migration and grain nucleation and possibly to propose nucleation and migration criteria.
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Roman-Cascon, C., Pellarin, T., Gibon, F., Brocca, L., Cosme, E., Crow, W., et al. (2017). Correcting satellite-based precipitation products through SMOS soil moisture data assimilation in two land-surface models of different complexity: API and SURFEX. Remote Sensing Of Environment, 200, 295–310.
Abstract: Global rainfall information is useful for many applications. However, real-time versions of satellite-based rainfall products are known to contain errors. Recent studies have demonstrated how the information about rainfall intrinsically contained in soil moisture data can be utilised for improving rainfall estimates. That is, soil moisture dynamics are impacted for several days by the accumulated amount of rainfall following within a particular event. In this context, soil moisture data from the Soil Moisture Ocean Salinity (SMOS) satellite is used in this study to correct rainfall accumulation estimates provided by satellite-based real-time precipitation products such as CMORPH, TRMM-3B42RT or PERSIANN. An algorithm based on the SMOS measurements data assimilation is tested in two land-surface models of different complexity: a simple hydrological model (Antecedent Precipitation Index (API)) and a more sophisticated state-of-the-art land-surface model (SURFEX (Surface Extemalisee)). We show how the assimilation technique, based on a particle filter method, generally leads to a significant improvement in rainfall estimates, with slightly better results for the simpler (and less computationally demanding) API model. This methodology has been evaluated for six years at ten sites around the world with different land use and climatological features. The results also show the limitations of the methodology in regions highly affected by mountainous terrain, forest or intense radio-frequency interference (RFI), which can notably affect the quality of the retrievals. The satisfactory results shown here invite the future operational application of the methodology in near-real time on a global scale.
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Rose, C., Sellegri, K., Moreno, I., Velarde, F., Ramonet, M., Weinhold, K., et al. (2017). CCN production by new particle formation in the free troposphere. Atmospheric Chemistry And Physics, 17(2), 1529–1541.
Abstract: Global models predict that new particle formation (NPF) is, in some environments, responsible for a substantial fraction of the total atmospheric particle number concentration and subsequently contributes significantly to cloud condensation nuclei (CCN) concentrations. NPF events were frequently observed at the highest atmospheric observatory in the world, on Chacaltaya (5240 m a.s.l.), Bolivia. The present study focuses on the impact of NPF on CCN population. Neutral cluster and Air Ion Spectrometer and mobility particle size spectrometer measurements were simultaneously used to follow the growth of particles from cluster sizes down to similar to 2 nm up to CCN threshold sizes set to 50, 80 and 100 nm. Using measurements performed between 1 January and 31 December 2012, we found that 61% of the 94 analysed events showed a clear particle growth and significant enhancement of the CCN-relevant particle number concentration. We evaluated the contribution of NPF, relative to the transport and growth of pre-existing particles, to CCN size. The averaged production of 50 nm particles during those events was 5072, and 1481 cm(-3) for 100 nm particles, with a larger contribution of NPF compared to transport, especially during the wet season. The data set was further segregated into boundary layer (BL) and free troposphere (FT) conditions at the site. The NPF frequency of occurrence was higher in the BL (48 %) compared to the FT (39 %). Particle condensational growth was more frequently observed for events initiated in the FT, but on average faster for those initiated in the BL, when the amount of condensable species was most probably larger. As a result, the potential to form new CCN was higher for events initiated in the BL (67% against 53% in the FT). In contrast, higher CCN number concentration increases were found when the NPF process initially occurred in the FT, under less polluted conditions. This work highlights the competition between particle growth and the removal of freshly nucleated particles by coagulation processes. The results support model predictions which suggest that NPF is an effective source of CCN in some environments, and thus may influence regional climate through cloud-related radiative processes.
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Roth, D. L., Finnegan, N. J., Brodsky, E. E., Rickenmann, D., Turowski, J. M., Badoux, A., et al. (2017). Bed load transport and boundary roughness changes as competing causes of hysteresis in the relationship between river discharge and seismic amplitude recorded near a steep mountain stream. Journal Of Geophysical Research-Earth Surface, 122(5), 1182–1200.
Abstract: Hysteresis in the relationship between bed load transport and river stage is a well-documented phenomenon with multiple known causes. Consequently, numerous studies have interpreted hysteresis in the relationship between seismic ground motion near rivers and some measure of flow strength (i.e., discharge or stage) as the signature of bed load transport. Here we test this hypothesis in the Erlenbach stream (Swiss Prealps) using a metric to quantitatively compare hysteresis in seismic data with hysteresis recorded by geophones attached beneath steel plates within the streambed, a well-calibrated proxy for direct sediment transport measurements. We find that while both the geophones and seismometers demonstrate hysteresis, the magnitude and direction of hysteresis are not significantly correlated between these data, indicating that the seismic signal at this site is primarily reflecting hysteresis in processes other than sediment transport. Seismic hysteresis also does not correlate significantly with the magnitude of sediment transport recorded by the geophones, contrary to previous studies' assumptions. We suggest that hydrologic sources and changes in water turbulence, for instance due to evolving boundary conditions at the bed, rather than changes in sediment transport rates, may sometimes contribute to or even dominate the hysteresis observed in seismic amplitudes near steep mountain rivers. Plain Language Summary An increasing number of studies have recently observed changes in the amount of seismic shaking (hysteresis) recorded near a river at a given discharge during floods. Most studies have assumed that this hysteresis was caused by changes in the amount of sediment being transported in the river and have therefore used the hysteresis to assess sediment transport rates and patterns. We examine concurrent seismic and sediment transport data from a steep mountain stream in the Swiss Prealps and find that changes in seismic shaking are unrelated and even opposed (increasing versus decreasing) to changes in sediment transport rates for four out of five transport events. Water turbulence, rather than sediment transport, appears to be the strongest source of seismic shaking, and changes in seismic shaking are most likely caused by changes in turbulence or how turbulence transmits energy through the river bed. These effects may be due to rearrangement of sediment around large boulders on the bed or slight shifting of the boulders themselves. Our results have significant implications for the growing field of fluvial seismology and the evaluation of seismic data near rivers, as previous interpretations of seismic hysteresis as evidence for sediment transport may not always be accurate.
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Royer, A., Roy, A., Montpetit, B., Saint-Jean-Rondeau, O., Picard, G., Brucker, L., et al. (2017). Comparison of commonly-used microwave radiative transfer models for snow remote sensing. Remote Sensing Of Environment, 190, 247–259.
Abstract: This paper reviews four commonly-used microwave radiative transfer models that take different electromagnetic approaches to simulate snow brightness temperature (T-B): the Dense Media Radiative Transfer – Multi-Layer model (DMRT-ML), the Dense Media Radiative Transfer – Quasi-Crystalline Approximation Mie scattering of Sticky spheres (DMRT-QMS), the Helsinki University of Technology n-Layers model (HUT-nlayers) and the Microwave Emission Model of Layered Snowpacks (MEMLS). Using the same extensively measured physical snow pack properties, we compared the simulated T-B at 11, 19 and 37 GHz from these four models. The analysis focuses on the impact of using different types of measured snow microstructure metrics in the simulations. In addition to density, snow microstructure is defined for each snow layer by grain optical diameter (Do) and stickiness for DMRT-ML and DMRT-QMS, mean grain geometrical maximum extent (Dmax) for HUT n-layers and the exponential correlation length for MEMLS. These metrics were derived from either in-situ measurements of snow specific surface area (SSA) or macrophotos of grain sizes (D-max), assuming non-sticky spheres for the DMRT models. Simulated T-B sensitivity analysis using the same inputs shows relatively consistent T-B behavior as a function of Do and density variations for the vertical polarization (maximum deviation of 18 K and 27 K, respectively), while some divergences appear in simulated variations for the polarization ratio (PR). Comparisons with ground based radiometric measurements show that the simulations based on snow SSA measurements have to be scaled with a model-specific factor of Do in order to minimize the root mean square error (RMSE) between measured and simulated T-B. Results using in-situ grain size measurements (SSA or D-max, depending on the model) give a mean T-B RMSE (19 and 37 GHz) of the order of 16-26 K, which is similar for all models when the snow microstructure metrics are scaled. However, the MEMLS model converges to better results when driven by the correlation length estimated from in-situ SSA measurements rather than D-max measurements. On a practical level, this paper shows that the SSA parameter, a snow property that is easy to retrieve in-situ, appears to be the most relevant parameter for characterizing snow microstructure, despite the need for a scaling factor. (C) 2017 Elsevier Inc. All rights reserved.
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Sabatier, P., Wilhelm, B., Ficetola Gentile, F., Moirons, F., Poulenard, J., Develle, A., et al. (2017). 6-kyr record of flood frequency and intensity in the western Mediterranean Alps – Interplay of solar and temperature forcing. Quaternary Science Reviews, 170, 121–135.
Abstract: The high-resolution sedimentological and geochemical analysis of a sediment sequence from Lake Savine (Western Mediterranean Alps, France) led to the identification of 220 event layers for the last 6000 years. 200 were triggered by flood events and 20 by underwater mass movements possibly related to earthquakes that occurred in 5 clusters of increase seismicity. Because human activity could influence the flood chronicle, the presence of pastures was reconstructed through ancient DNA, which suggested that the flood chronicle was mainly driven by hydroclimate variability. Weather reanalysis of historical floods allow to identify that mesoscale precipitation events called “East Return” events were the main triggers of floods recorded in Lake Savine. The first part of this palaeoflood record (6-4 kyr BP) was characterized by increases in flood frequency and intensity in phase with Northern Alpine palaeoflood records. By contrast, the second part of the record (i.e., since 4 kyr BP) was phased with Southern Alpine palaeoflood records. These results suggest a palaeohydrological transition at approximately 4 kyr BP, as has been previously described for the Mediterranean region. This may have resulted in a change of flood-prone hydro-meteorological processes, i.e., in the balance between occurrence and intensity of local convective climatic phenomena and their influence on Mediterranean mesoscale precipitation events in this part of the Alps. At a centennial timescale, increases in flood frequency and intensity corresponded to periods of solar minima, affecting climate through atmospheric changes in the Euro-Atlantic sector. (C) 2017 Elsevier Ltd. All rights reserved.
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Samake, A., Uzu, G., Martins, J. M. F., Calas, A., Vince, E., Parat, S., et al. (2017). The unexpected role of bioaerosols in the Oxidative Potential of PM. Scientific Reports, 7.
Abstract: Bioaerosols represent up to 15-25% of PM by mass, but there is currently no assessment of their impact on Oxidative Potential (OP), or capacity of particulate matter (PM) to produce damaging oxidative reactions in the human lungs. Here, the OP of selected bioaerosols (bacteria cells vs fungal spores) was assessed through the cell-free DTT assay. Results show that bioaerosols induce Reactive Oxygen Species (ROS) production, varying along the microorganism type, species, and concentration. Fungal spores show up to 10 times more ROS generation than bacterial cells. At the highest concentrations, fungal spores present as much oxidative reactivity as the most redox-active airborne chemicals (Copper, Naphtoquinone). Moreover, bioaerosols substantially influence OP of ambient PM and that of its chemical constituents: in presence of A. fumigatus spores, the OP of Cu/NQ is increased by a factor of 2 to 5, whereas, 10(4) and 10(5) S. epidermidis bacterial cells. mL(-1) halves the OP of Cu/NQ. Finally, viable and gamma-rays-killed model bioaerosols present similar oxidative reactivity, suggesting a metabolism-independent cellular mechanism. These results reveal the importance of bioaerosols for PM reactivity. PM toxicity can be modified due to bioaerosols contribution or by their ability to modulate the OP of toxic chemicals present in PM.
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Sammonds, P., Montagnat, M., Bons, P., & Schneebeli, M. (2017). Ice microstructures and microdynamics. Philosophical Transactions Of The Royal Society A-Mathematical Physical And Engineering Sciences, 375(2086). |
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Sammonds, P., Montagnat, M., Bons, P., & Schneebeli, M. (2017). Microdynamics of ice. Philosophical Transactions Of The Royal Society A-Mathematical Physical And Engineering Sciences, 375(2086). |
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Schwander, M., Bronnimann, S., Delaygue, G., Rohrer, M., Auchmann, R., & Brugnara, Y. (2017). Reconstruction of Central European daily weather types back to 1763. International Journal Of Climatology, 37, 30–44.
Abstract: Weather type classifications (WTCs) are a simple tool to analyse variations in weather patterns. Long series of WTCs could be used to address decadal changes in weather as a basis for studying changes in variability or extremes or for addressing contributions of sea-surface temperature or external forcings using climate models. However, there is no long series of daily objective weather types (WTs). A new method (Shortest Mahalanobis Distance, SMD) using daily European weather data is developed to reconstruct WTCs back in time. Here the SMD method is applied on the Cluster Analysis of Principal Components (CAP9) classification used by MeteoSwiss. The CAP9 daily WT time series (computed with ERA-40) is used as reference over the 1958-1998 period. Daily data (temperature, mean sea level pressure and pressure tendency) from 13 European stations covering the period 1763-2009 are used for the reconstruction. The reference CAP9 is reduced from nine to seven types so the new daily WTC is called CAP7. As an assessment, CAP7 is compared to the original classification CAP9 and to the same WTs computed with the Twentieth Century Reanalysis (20CR and 20CRv2c). Over the reference period up to 90% of all the daily WTs can be correctly reproduced in the new WTC compared to the original series, with higher reliability in winter than in summer. In addition, the reliability of the classification is increasing from 1763 onward. The annual occurrence of each type reveals some trends, mostly a decrease in the number of cyclonic days and an increase of cyclonic days.
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Schwier, A. N., Sellegri, K., Mas, S., Charriere, B., Pey, J., Rose, C., et al. (2017). Primary marine aerosol physical flux and chemical composition during a nutrient enrichment experiment in mesocosms in the Mediterranean Sea. Atmospheric Chemistry And Physics, 17(23), 14645–14660.
Abstract: While primary marine aerosol (PMA) is an important part of global aerosol total emissions, its chemical composition and physical flux as a function of the biogeo-chemical properties of the seawater still remain highly un-characterized due to the multiplicity of physical, chemical and biological parameters that are involved in the emission process. Here, two nutrient-enriched mesocosms and one control mesocosm, both filled with Mediterranean seawater, were studied over a 3-week period. PMA generated from the mesocosm waters were characterized in term of chemical composition, size distribution and size-segregated cloud condensation nuclei (CCN), as a function of the seawater chlorophyll a (Chl a) concentration, pigment composition, virus and bacteria abundances. The aerosol number size distribution flux was primarily affected by the seawater temperature and did not vary significantly from one mesocosm to the other. The aerosol number size distribution flux was primarily affected by the seawater temperature and did not vary significantly from one mesocosm to the other. Particle number and CCN aerosol fluxes increase by a factor of 2 when the temperature increases from 22 to 32 degrees C, for all particle submicron sizes. This effect, rarely observed in previous studies, could be specific to oligotrophic waters and/or to this temperature range. In all mesocosms (enriched and control mesocosms), we detected an enrichment of calcium (+ 500 %) and a deficit in chloride (36 %) in the submicron PMA mass compared to the literature inorganic composition of the seawater. There are indications that the chloride deficit and calcium enrichment are linked to biological processes, as they are found to be stronger in the enriched mesocosms. This implies a non-linear transfer function between the seawater composition and PMA composition, with complex processes taking place at the interface during the bubble bursting. We found that the artificial phytoplankton bloom did not affect the CCN activation diameter (D-p,D- 50,D- (average) = 59.85 +/- 3.52 nm and D-p,D-50,D- average = 93.42 +/- 5.14 nm for supersaturations of 0.30 and 0.15% respectively) or the organic fraction of the submicron PMA (average organic to total mass = 0.31 +/- 0.07) compared to the control mesocosm. Contrary to previous observations in natural bloom mesocosm experiments, the correlation between the particle organic fraction and the seawater Chl a was poor, indicating that Chl a is likely not a straightforward proxy for predicting, on a daily scale, PMA organi
c fraction in models for all types of sea and ocean waters. Instead, the organic fraction of the Aitken mode particles were more significantly linked to heterotrophic flagellates, viruses and dissolved organic carbon (DOC). We stress that different conclusions may be obtained in natural (non-enriched) or non-oligotrophic systems. |
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Serazin, G., Jaymond, A., Leroux, S., Penduff, T., Bessieres, L., Llovel, W., et al. (2017). A global probabilistic study of the ocean heat content low-frequency variability: Atmospheric forcing versus oceanic chaos. Geophysical Research Letters, 44(11), 5580–5589.
Abstract: A global 1/4 degrees ocean/sea ice 50-member ensemble simulation is used to disentangle the low-frequency imprints of the atmospherically forced oceanic variability and of the chaotic intrinsic oceanic variability (IOV) on the large-scale (10 degrees x10 degrees) ocean heat content (OHC) between 1980 and 2010. The IOV explains most of the interannual-to-decadal large-scale OHC variance over substantial fractions of the global ocean area that increase with depth: 9%, 22%, and 31% in the 0-700m, 700-2000m and 2000m bottom layers, respectively. Such areas concern principally eddy-active regions, mostly found in the Southern Ocean and in western boundary current extensions, and also concern the subtropical gyres at intermediate and deep levels. The oceanic chaos may also induce random multidecadal fluctuations so that large-scale regional OHC trends computed on the 1980-2010 period cannot be unambiguously attributed to the atmospheric forcing in several oceanic basins at various depths. These results are likely to raise detection and attribution issues from real observations.
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Shabou, S., Ruin, I., Lutoff, C., Debionne, S., Anquetin, S., Creutin, J. D., et al. (2017). MobRISK: a model for assessing the exposure of road users to flash flood events. Natural Hazards And Earth System Sciences, 17(9), 1631–1651.
Abstract: Recent flash flood impact studies highlight that road networks are often disrupted due to adverse weather and flash flood events. Road users are thus particularly exposed to road flooding during their daily mobility. Previous exposure studies, however, do not take into consideration population mobility. Recent advances in transportation research provide an appropriate framework for simulating individual travelactivity patterns using an activity-based approach. These activity-based mobility models enable the prediction of the sequence of activities performed by individuals and locating them with a high spatial-temporal resolution. This paper describes the development of the MobRISK microsimulation system: a model for assessing the exposure of road users to extreme hydrometeorological events. MobRISK aims at providing an accurate spatiotemporal exposure assessment by integrating travel-activity behaviors and mobility adaptation with respect to weather disruptions. The model is applied in a flash-flood-prone area in southern France to assess motorists' exposure to the September 2002 flash flood event. The results show that risk of flooding mainly occurs in principal road links with considerable traffic load. However, a lag time between the timing of the road submersion and persons crossing these roads contributes to reducing the potential vehicle-related fatal accidents. It is also found that sociodemographic variables have a significant effect on individual exposure. Thus, the proposed model demonstrates the benefits of considering spatiotemporal dynamics of population exposure to flash floods and presents an important improvement in exposure assessment methods. Such improved characterization of road user exposures can present valuable information for flood risk management services.
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Sherpa, S. F., Wagnon, P., Brun, F., Berthier, E., Vincent, C., Lejeune, Y., et al. (2017). Contrasted surface mass balances of debris-free glaciers observed between the southern and the inner parts of the Everest region (2007-15). Journal Of Glaciology, 63(240), 637–651.
Abstract: Three debris-free glaciers with strongly differing annual glaciological glacier-wide mass balances (MBs) are monitored in the Everest region (central Himalaya, Nepal). The mass budget of Mera Glacier (5.1 km(2) in 2012), located in the southern part of this region, was balanced during 2007-15, whereas Pokalde (0.1 km(2) in 2011) and West Changri Nup glaciers (0.9 km(2) in 2013), similar to 30 km further north, have been losing mass rapidly with annual glacier-wide MBs of -0.69 +/- 0.28 m w.e. a(-1) (2009-15) and -1.24 +/- 0.27 m w.e. a(-1) (2010-15), respectively. An analysis of high-elevation meteorological variables reveals that these glaciers are sensitive to precipitation, and to occasional severe cyclonic storms originating from the Bay of Bengal. We observe a negative horizontal gradient of annual precipitation in south-to-north direction across the range (<= -21 mm km(-1), i.e. -2% km(-1)). This contrasted mass-balance pattern over rather short distances is related (i) to the low maximum elevation of Pokalde and West Changri Nup glaciers, resulting in years where their accumulation area ratio is reduced to zero and (ii) to a steeper vertical gradient of MB for glaciers located in the inner arid part of the range.
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Simonin, M., Martins, J. M. F., Le Roux, X., Uzu, G., Calas, A., & Richaume, A. (2017). Toxicity of TiO2 nanoparticles on soil nitrification at environmentally relevant concentrations: Lack of classical dose-response relationships. Nanotoxicology, 11(2), 247–255.
Abstract: Titanium-dioxide nanoparticles (TiO2-NPs) are increasingly released in agricultural soils through, e.g. biosolids, irrigation or nanoagrochemicals. Soils are submitted to a wide range of concentrations of TiO2-NPs depending on the type of exposure. However, most studies have assessed the effects of unrealistically high concentrations, and the dose-response relationships are not well characterized for soil microbial communities. Here, using soil microcosms, we assessed the impact of TiO2-NPs at concentrations ranging from 0.05 to 500mg kg(-1)dry-soil, on the activity and abundance of ammonia-oxidizing archaea (AOA) and bacteria (AOB), and nitrite-oxidizing bacteria (Nitrobacter and Nitrospira). In addition, aggregation and oxidative potential of TiO2-NPs were measured in the spiking suspensions, as they can be important drivers of TiO2-NPs toxicity. After 90days of exposure, non-classical dose-response relationships were observed for nitrifier abundance or activity, making threshold concentrations impossible to compute. Indeed, AOA abundance was reduced by 40% by TiO2-NPs whatever the concentration, while Nitrospira was never affected. Moreover, AOB and Nitrobacter abundances were decreased mainly at intermediate concentrations nitrification was reduced by 25% at the lowest (0.05mgkg(-1)) and the highest (100 and 500mgkg(-1)) TiO2-NPs concentrations. Path analyses indicated that TiO2-NPs affected nitrification through an effect on the specific activity of nitrifiers, in addition to indirect effects on nitrifier abundances. Altogether these results point out the need to include very low concentrations of NPs in soil toxicological studies, and the lack of relevance of classical dose-response tests and ecotoxicological dose metrics (EC50, IC50...) for TiO2-NPs impact on soil microorganisms.
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Slimani, R., Oxarango, L., Sbartai, B., Tinet, A. J., Olivier, F., & Dias, D. (2017). Leachate flow around a well in MSW landfill: Analysis of field tests using Richards model. Waste Management, 63, 122–130.
Abstract: During the lifespan of a Municipal Solid Waste landfill, its leachate drainage system may get clogged. Then, as a consequence of rainfall, leachate generation and possibly leachate injection, the moisture content in the landfill increases to the point that a leachate mound could be created. Therefore, pumping the leachate becomes a necessary solution. This paper presents an original analysis of leachate pumping and injection in an instrumented well. The water table level around the well is monitored by nine piezometers which allow the leachate flow behaviour to be captured. A numerical model based on Richards equation and an exponential relationship between saturated hydraulic conductivity and depth is used to analyze the landfill response to pumping and injection. Decreasing permeability with depth appears to have a major influence on the behaviour of the leachate flow. It could have a drastic negative impact on the pumping efficiency with a maximum quasi-stationary pumping rate limited to approximately 1 m(3)/11 for the tested well and the radius of influence is less than 20 m. The numerical model provides a reasonable description of both pumping and injection tests. However, an anomalous behaviour observed at the transition between pumping and recovery phases is observed. This could be due to a limitation of the Richards model in that it neglects the gas phase behaviour and other double porosity heterogeneous effects. (C) 2016 Elsevier Ltd. All rights reserved.
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Soro, D. D., Koita, M., Biaou, C. A., Outoumbe, E., Vouillamoz, J. M., Yacouba, H., et al. (2017). Geophysical demonstration of the absence of correlation between lineaments and hydrogeologically usefull fractures: Case study of the Sanon hard rock aquifer (central northern Burkina Faso). Journal Of African Earth Sciences, 129, 842–852.
Abstract: The conceptualization of hard rock aquifers in terms of their geometry and structure has undergone considerable progress over the last two decades. Despite these advances, hydrogeologists are still divided by the models used to describe two central concepts: (i) the influence of weathering processes on hydraulic conductivity; (ii) the influence of tectonics on the hydraulic conductivity of hard rock aquifers. In order to provide further insight into this debate, the present study proposes a conceptual model for hard rock aquifers, based on an integrated hydrogeological and geophysical approach, using information acquired at different scales. The data and observations used for this case study were derived from the Sanon experimental site, located in Burkina Faso, which is presently exposed to a Sudano-Sahelian climate. The methodological approach consisted firstly in developing a description of the site's weathering profile at the scale of a borehole, based on lithologs and electrical resistivity logs. In a second step, the site's ridge to ridge (longitudinal) weathering profile was established from several 2D resistivity sections crossing a maximum number of lineament structures, which in some prior studies were considered to be the superficial manifestation of tectonic fractures. The results show that at that scale the weathering profile is comprised of three main layers, which from top to bottom are referred to as: the saprolite, the fissured layer and the fresh rock. This weathering profile model is consistent with other models proposed in recent years, suggesting that the hydraulic conductivity of hard rock aquifers is a consequence of weathering processes, rather than tectonic fracturing. Tectonic fractures are not visible on the 2D sections of the ridge to ridge profiles, and the lineaments originally thought to be overground representations of tectonic fractures are likely to have different origins. The lack of a substantial correlation between tectonic lineaments and fractures appears to account for the high incidence of negative boreholes in hard rock aquifers, where the siting of drillings has systematically been based on lineament studies and on geophysical studies looking for vertical fractures such as profiling and vertical electrical sounding. There is thus a need to revise current hydrogeological concepts and methodologies to site wells based on tectonic fractures represented by lineaments. (C) 2017 Elsevier Ltd. All rights reserved.
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Sprovieri, F., Pirrone, N., Bencardino, M., D'Amore, F., Angot, H., Barbante, C., et al. (2017). Five-year records of mercury wet deposition flux at GMOS sites in the Northern and Southern hemispheres. Atmospheric Chemistry And Physics, 17(4), 2689–2708.
Abstract: The atmospheric deposition of mercury (Hg) occurs via several mechanisms, including dry and wet scavenging by precipitation events. In an effort to understand the atmospheric cycling and seasonal depositional characteristics of Hg, wet deposition samples were collected for approximately 5 years at 17 selected GMOS monitoring sites located in the Northern and Southern hemispheres in the framework of the Global Mercury Observation System (GMOS) project. Total mercury (THg) exhibited annual and seasonal patterns in Hg wet deposition samples. Interannual differences in total wet deposition are mostly linked with precipitation volume, with the greatest deposition flux occurring in the wettest years. This data set provides a new insight into baseline concentrations of THg concentrations in precipitation worldwide, particularly in regions such as the Southern Hemisphere and tropical areas where wet deposition as well as atmospheric Hg species were not investigated before, opening the way for future and additional simultaneous measurements across the GMOS network as well as new findings in future modeling studies.
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Stenni, B., Curran, M. A. J., Abram, N. J., Orsi, A., Goursaud, S., Masson-Delmotte, V., et al. (2017). Antarctic climate variability on regional and continental scales over the last 2000 years. Climate Of The Past, 13(11), 1609–1634.
Abstract: Climate trends in the Antarctic region remain poorly characterized, owing to the brevity and scarcity of direct climate observations and the large magnitude of interannual to decadal-scale climate variability. Here, within the framework of the PAGES Antarctica2k working group, we build an enlarged database of ice core water stable isotope records from Antarctica, consisting of 112 records. We produce both unweighted and weighted isotopic (delta O-18) composites and temperature reconstructions since 0 CE, binned at 5- and 10-year resolution, for seven climatically distinct regions covering the Antarctic continent. Following earlier work of the Antarctica2k working group, we also produce composites and reconstructions for the broader regions of East Antarctica, West Antarctica and the whole continent. We use three methods for our temperature reconstructions: (i) a temperature scaling based on the delta O-18-temperature relationship output from an ECHAM5-wiso model simulation nudged to ERA-Interim atmospheric reanalyses from 1979 to 2013, and adjusted for the West Antarctic Ice Sheet region to borehole temperature data, (ii) a temperature scaling of the isotopic normalized anomalies to the variance of the regional reanalysis temperature and (iii) a composite-plus-scaling approach used in a previous continent-scale reconstruction of Antarctic temperature since 1 CE but applied to the new Antarctic ice core database. Our new reconstructions confirm a significant cooling trend from 0 to 1900 CE across all Antarctic regions where records extend back into the 1st millennium, with the exception of the Wilkes Land coast and Weddell Sea coast regions. Within this long-term cooling trend from 0 to 1900 CE, we find that the warmest period occurs between 300 and 1000 CE, and the coldest interval occurs from 1200 to 1900 CE. Since 1900 CE, significant warming trends are identified for the West Antarctic Ice Sheet, the Dronning Maud Land coast and the Antarctic Peninsula regions, and these trends are robust across the distribution of records that contribute to the unweighted isotopic composites and also significant in the weighted temperature reconstructions. Only for the Antarctic Peninsula is this most recent century-scale trend unusual in the context of natural variability over the last 2000 years. However, projected warming of the Antarctic continent during the 21st century may soon see significant and unusual warming develop across other parts of the Antarctic continent. The extended Antarctica2k ice core isotope database developed by this working group opens up many avenues for developing a deeper understanding of the response of Antarctic climate to natural and anthropogenic climate forcings. The first long-term quantification of regional climate in Antarctica presented herein is a basis for data-model comparison and assessments of past, present and future driving factors of Antarctic climate.
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Storto, A., Masina, S., Balmaseda, M., Guinehut, S., Xue, Y., Szekely, T., et al. (2017). Steric sea level variability (1993-2010) in an ensemble of ocean reanalyses and objective analyses. Climate Dynamics, 49(3), 709–729.
Abstract: Quantifying the effect of the seawater density changes on sea level variability is of crucial importance for climate change studies, as the sea level cumulative rise can be regarded as both an important climate change indicator and a possible danger for human activities in coastal areas. In this work, as part of the Ocean Reanalysis Intercomparison Project, the global and regional steric sea level changes are estimated and compared from an ensemble of 16 ocean reanalyses and 4 objective analyses. These estimates are initially compared with a satellite-derived (altimetry minus gravimetry) dataset for a short period (2003-2010). The ensemble mean exhibits a significant high correlation at both global and regional scale, and the ensemble of ocean reanalyses outperforms that of objective analyses, in particular in the Southern Ocean. The reanalysis ensemble mean thus represents a valuable tool for further analyses, although large uncertainties remain for the inter-annual trends. Within the extended intercomparison period that spans the altimetry era (1993-2010), we find that the ensemble of reanalyses and objective analyses are in good agreement, and both detect a trend of the global steric sea level of 1.0 and 1.1 ± 0.05 mm/year, respectively. However, the spread among the products of the halosteric component trend exceeds the mean trend itself, questioning the reliability of its estimate. This is related to the scarcity of salinity observations before the Argo era. Furthermore, the impact of deep ocean layers is non-negligible on the steric sea level variability (22 and 12 % for the layers below 700 and 1500 m of depth, respectively), although the small deep ocean trends are not significant with respect to the products spread.
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Strady, E., Dang, V. B. H., Nemery, J., Guedron, S., Dinh, Q. T., Denis, H., et al. (2017). Baseline seasonal investigation of nutrients and trace metals in surface waters and sediments along the Saigon River basin impacted by the megacity of Ho Chi Minh (Vietnam). Environmental Science And Pollution Research, 24(4), 3226–3243.
Abstract: The Saigon River, Southern Vietnam, crosses one of the most dynamic developing Megacity in Southeast Asia: Ho Chi Minh City (HCMC). The increased economic, industrial, and domestic developments may affect the environmental quality of water and halicutic resources. In this study, we evaluated the seasonal (dry and wet seasons) biogeochemical state of the Saigon River during two snapshot campaigns conducted along the river basin upstream from HCMC; the Saigon River was characterized by slightly acidic (pH 5.7-7.7) and oxygen-depleted water (dissolved oxygen (DO), 0.36-5.18 mg l(-1)). Nutrients (N-NH4 = 0.01-2.41, N-NO3 = 0.14-2.72, and P-PO4 = similar to 0-0.42 mg l(-1)), DOC (2.2-8.0 mg l(-1)), POC, and trace metal(oid) (As, Cd, Cr, Cu, Zn, and Hg) concentrations were low showing a good quality of the upstream river. In the urban center area, DO dropped to 0.03 mg l(-1) accompanied with a rise of nutrient concentrations (e.g., N-NH4, up to 17.7 mg l(-1)) likely originating from wastewater discharges. Trace metal concentrations also rose sharply (e.g., Cr and Hg rose up to 10-fold higher) in both water and sediments but remained under the World Health Organization (WHO) and Vietnamese concentration guidelines. In the downstream estuarine area, the intrusion of marine waters diluted water flowing from HCMC, leading water quality to return close to the state observed upstream from HCMC. In general, levels of nutrient and metal contaminations along the Saigon River during both seasons appear moderate regarding to Vietnamese and WHO guidelines although the urban area is highlighted as the major contributor for metal(oid) emissions. Finally, we showed that apart from wastewater and industrial discharges that affect the river quality, metal(oid) partitioning between solid and solution is controlled by the change in water geochemistry along the continuum during both seasons, such as DO (e.g., for As and Cr) and pH (e.g., for Pb) which drives their sorption/dissolution dynamics.
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Strady, E., Dinh, Q. T., Nemery, J., Nguyen, T. N., Guedron, S., Nguyen, N. S., et al. (2017). Spatial variation and risk assessment of trace metals in water and sediment of the Mekong Delta. Chemosphere, 179, 367–378.
Abstract: The Mekong Delta, is home to 17 million inhabitants and faces numerous challenges relating to climate change, environmental degradation and water issues. In this study, we assess trace metals concentrations (Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Mo, Cd, Hg, Pb) in the water, suspended particulate matter and surface sediments of the Tien River, the Northern branch of the Mekong Delta, during both dry and rainy seasons. Metal concentrations in the dissolved and suspended particle phases remain in the low concentration range of the main Asian Tropical River. During transportation in the riverine part, we evidenced that V, Cr, Co, As and Pb are dominant in the particulate phase while Mo, Ni and Cu dominate in the dissolved fraction. In the salinity gradient, dissolved U, V, Mo exhibit conservative behaviour while Ni, Cu, As, Co and Cd showed additive behaviour suggesting desorption processes. In the surface sediment, metal concentrations are controlled by the particle-size, POC contents and Fe, Al and Mn – oxy(hydr)oxides. Calculated Enrichment Factor and Geoaccumulation Index evidenced As enrichment while the calculated mean effect range median quotients evidenced a low to medium ecotoxicological potential effects range in the surface sediments. (C) 2017 Elsevier Ltd. All rights reserved.
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Sylvestre, A., Mizzi, A., Mathiot, S., Masson, F., Jaffrezo, J. L., Dron, J., et al. (2017). Comprehensive chemical characterization of industrial PM2.5 from steel industry activities. Atmospheric Environment, 152, 180–190.
Abstract: Industrial sources are among the least documented PM (Particulate Matter) source in terms of chemical composition, which limits our understanding of their effective impact on ambient PM concentrations. We report 4 chemical emission profiles of PM2.5 for multiple activities located in a vast metallurgical complex. Emissions profiles were calculated as the difference of species concentrations between an upwind and a downwind site normalized by the absolute PM2.5 enrichment between both sites. We characterized the PM2.5 emissions profiles of the industrial activities related to the cast iron (complex 1) and the iron ore conversion processes (complex 2), as well as 2 storage areas: a blast furnace slag area (complex 3) and an ore terminal (complex 4). PM2.5 major fractions (Organic Carbon (OC) and Elemental Carbon (EC), major ions), organic markers as well as metals/trace elements are reported for the 4 industrial complexes. Among the trace elements, iron is the most emitted for the complex 1 (146.0 mg g(-1) of PM2.5), the complex 2 (70.07 mg g(-1)) and the complex 3 (124.4 mg g(-1)) followed by Al, Mn and Zn. A strong emission of Polycyclic Aromatic Hydrocarbons (PAH), representing 1.3% of the Organic Matter (OM), is observed for the iron ore transformation complex (complex 2) which merges the activities of coke and iron sinter production and the blast furnace processes. In addition to unsubstituted PAHs, sulfur containing PAHs (SPAHs) are also significantly emitted (between 0.011 and 0.068 mg g(-1)) by the complex 2 and could become very useful organic markers of steel industry activities. For the complexes 1 and 2 (cast iron and iron ore converters), a strong fraction of sulfate ranging from 0.284 to 0336 g g(-1)) and only partially neutralized by ammonium, is observed indicating that sulfates, if not directly emitted by the industrial activity, are formed very quickly in the plume. Emission from complex 4 (Ore terminal) are characterized by high contribution of Al (125.7 mg g(-1) of PM2.5) but also, in a lesser extent, of Fe, Mn, Ti and Zn. We also highlighted high contribution of calcium ranging from 0.123 to 0.558 g g(-1) for all of the industrial complexes under study. Since calcium is also widely used as a proxy of the dust contributions in source apportionment studies, our results suggest that this assumption should be reexamined in environments impacted by industrial emissions. (C) 2016 Elsevier Ltd. All rights reserved.
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Tano, B. F. G., Dias, D., Stoltz, G., Touze-Foltz, N., & Olivier, F. (2017). Numerical modelling to identify key factors controlling interface behaviour of geosynthetic lining systems. Geosynthetics International, 24(2), 167–183.
Abstract: Geosynthetics have been extensively used in landfills as a lining system to prevent leachate infiltration into groundwater. In piggy-back landfill expansion (PBLE), consisting of building a new landfill over an existing one, a lining system is implemented between the old and new waste. In this context, interface failure (stability) and deformation (integrity) of the lining system should be considered for the design. Such stability and integrity mainly depend on the PBLE geometry and the mechanical properties of the geosynthetics. Comprehensive numerical modelling simulations were performed to show how these factors influence the shear stresses, shear displacements, translational stability and the axial strains/ forces within the various geosynthetics. The numerical modelling was conducted using the finite difference code FLAC 2D, focusing on a typical PBLE and considering geosynthetic interface strain softening, the nonlinear stiffness of geosynthetics, and the differentiation between the compressive and tensile behaviours of geosynthetics. The simulations showed that the lateral length of the PBLE, the type of geomembrane (textured or smooth) and the level of the leachate table in new waste are the factors that most influenced the mechanical behaviour of the lining system and its stability. Finally, a parameter called the stability ratio was proposed as a complement to the traditional factor of safety, to analyse the progressive slippage along the geosynthetic interfaces. The numerical results indicated that interface failure concurrently begins at the rightmost part of the lower flat area of the PBLE and near the corner of the inner slope before spreading out to the left as backfilling progresses.
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Tano, B. F. G., Stoltz, G., Touze-Foltz, N., Dias, D., & Olivier, F. (2017). A numerical modelling technique for geosynthetics validated on a cavity model test. Geotextiles And Geomembranes, 45(4), 339–349.
Abstract: Numerical modelling approaches can aid in designing geotechnical constructions involving geosynthetics. However, the reliability of numerical results depends on how the model is developed, the constitutive model, and the set of parameters used. By comparing the numerical results with experiment, the present work verifies a numerical modelling technique developed to model multilayered geosynthetic lining systems for landfills. The numerical modelling technique involves strain softening at interfaces and allows the axial stiffness of the geosynthetics to evolve as a function of strain. This work focuses on a two-dimensional finite-difference model, which is used to simulate three types of experimental tests: conventional uniaxial tensile tests, direct shear tests, and a large-scale test that was used to assess the overall mechanical behaviour of a reinforced geosynthetic system that spanned over a cavity. This reinforced geosynthetic system consisted of a 50 kNim polyvinyl alcohol geogrid reinforcement embedded in a layer of sand, a geosynthetic clay liner, a high-density polyethylene geomembrane, and a non-woven needle-punched geotextile. The uniaxial tensile tests, direct shear tests, and the large-scale test were numerically modelled and the numerical results were compared with experimental results. The results of the numerical modelling technique presented very closely match the results of the three experimental tests, which indicates that the numerical model correctly predicted the measured data. (C) 2017 Elsevier Ltd. All rights reserved.
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Taylor, C. M., Belusic, D., Guichard, F., Arker, D. J. P., Vischel, T., Bock, O., et al. (2017). Frequency of extreme Sahelian storms tripled since 1982 in satellite observations. Nature, 544(7651), 475–+.
Abstract: The hydrological cycle is expected to intensify under global warming(1), with studies reporting more frequent extreme rain events in many regions of the world(2-4), and predicting increases in future flood frequency(5). Such early, predominantly mid-latitude observations are essential because of shortcomings within climate models in their depiction of convective rainfall(6,7). A globally important group of intense storms-mesoscale convective systems (MCSs) 8-poses a particular challenge, because they organize dynamically on spatial scales that cannot be resolved by conventional climate models. Here, we use 35 years of satellite observations from the West African Sahel to reveal a persistent increase in the frequency of the most intense MCSs. Sahelian storms are some of the most powerful on the planet(9), and rain gauges in this region have recorded a rise in 'extreme'(17) daily rainfall totals. We find that intense MCS frequency is only weakly related to the multidecadal recovery of Sahel annual rainfall, but is highly correlated with global land temperatures. Analysis of trends across Africa reveals that MCS intensification is limited to a narrow band south of the Sahara desert. During this period, wet-season Sahelian temperatures have not risen, ruling out the possibility that rainfall has intensified in response to locally warmer conditions. On the other hand, the meridional temperature gradient spanning the Sahel has increased in recent decades, consistent with anthropogenic forcing driving enhanced Saharan warming(10). We argue that Saharan warming intensifies convection within Sahelian MCSs through increased wind shear and changes to the Saharan air layer. The meridional gradient is projected to strengthen throughout the twenty-first century, suggesting that the Sahel will experience particularly marked increases in extreme rain. The remarkably rapid intensification of Sahelian MCSs since the 1980s sheds new light on the response of organized tropical convection to global warming, and challenges conventional projections made by general circulation models.
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Terti, G., Ruin, I., Anquetin, S., & Gourley, J. J. (2017). A Situation-Based Analysis Of Flash Flood Fatalities In The United States. Bulletin Of The American Meteorological Society, 98(2), 333–345. |
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Tomaz, S., Jaffrezo, J. L., Favez, O., Perraudin, E., Villenave, E., & Albinet, A. (2017). Sources and atmospheric chemistry of oxy- and nitro-PAHs in the ambient air of Grenoble (France). Atmospheric Environment, 161, 144–154.
Abstract: Total individual concentrations (in both gaseous and particulate phases) of 80 polycyclic aromatic compounds (PACs) including 32 nitro-PAHs, 27 oxy-PAHs (polycyclic aromatic hydrocarbons) and 21 parent PAHs have been investigated over a year in the ambient air of Grenoble (France) together with an extended aerosol chemical characterization. The results indicated that their concentrations were strongly affected by primary emissions in cold period, especially from residential heating (i.e. biomass burning). Besides, secondary processes occurred in summer but also in cold period under specific conditions such as during long thermal inversion layer periods and severe PM pollution events. Different secondary processes were involved during both PM pollution events observed in March April and in December 2013. During the first one, long range transport of air masses, nitrate chemistry and secondary nitro-PAH formation seemed linked. During the second one, the accumulation of primary pollutants over several consecutive days enhanced secondary chemical processes notably highlighted by the dramatic increase of oxy-PAH concentrations. The study of the time trends of ratios of individual nitro- or oxy-PAHs to parent PAHs, in combination with key primary or secondary aerosol species and literature data, allowed the identification of potential molecular markers of PAH oxidation. Finally, 6H-dibenzo[b,d]pyran-6-one, biphenyl-2,2'-dicarboxaldehyde and 3-nitrophenanthrene have been selected to be the best candidates as markers of PAH oxidation processes in ambient air. (C) 2017 Elsevier Ltd. All rights reserved.
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Traversi, R., Becagli, S., Brogioni, M., Caiazzo, L., Ciardini, V., Giardi, F., et al. (2017). Multi-year record of atmospheric and snow surface nitrate in the central Antarctic plateau. Chemosphere, 172, 341–354.
Abstract: Continuous all year-round samplings of atmospheric aerosol and surface snow at high (daily to 4-day) resolution were carried out at Dome C since 2004-05 to 2013 and nitrate records are here presented. Basing on a larger statistical data set than previous studies, results confirm that nitrate seasonal pattern is characterized by maxima during austral summer for both aerosol and surface snow, occurring in-phase with solar UV irradiance. This temporal pattern is likely due to a combination of nitrate sources and post-depositional processes whose intensity usually enhances during the summer. Moreover, it should be noted that a case study of the synoptic conditions, which took place during a major nitrate event, showed the occurrence of a stratosphere-troposphere exchange. The sampling of both matrices at the same time with high resolution allowed the detection of a an about one-month long recurring lag of summer maxima in snow with respect to aerosol. This result can be explained by deposition and post-deposition processes occurring at the atmosphere-snow interface, such as a net uptake of gaseous nitric acid and a replenishment of the uppermost surface layers driven by a larger temperature gradient in summer. This hypothesis was preliminarily tested by a comparison with surface layers temperature data in the 2012-13 period. The analysis of the relationship between the nitrate concentration in the gas phase and total nitrate obtained at Dome C (2012-13) showed the major role of gaseous HNO3 to the total nitrate budget suggesting the need to further investigate the gas-to-particle conversion processes. (C) 2017 Elsevier Ltd. All rights reserved.
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Travnikov, O., Angot, H., Artaxo, P., Bencardino, M., Bieser, J., D'Amore, F., et al. (2017). Multi-model study of mercury dispersion in the atmosphere: atmospheric processes and model evaluation. Atmospheric Chemistry And Physics, 17(8), 5271–5295.
Abstract: Current understanding of mercury (Hg) behavior in the atmosphere contains significant gaps. Some key characteristics of Hg processes, including anthropogenic and geogenic emissions, atmospheric chemistry, and air-surface exchange, are still poorly known. This study provides a complex analysis of processes governing Hg fate in the atmosphere involving both measured data from ground-based sites and simulation results from chemical transport models. A variety of long-term measurements of gaseous elemental Hg (GEM) and reactive Hg (RM) concentration as well as Hg wet deposition flux have been compiled from different global and regional monitoring networks. Four contemporary global-scale transport models for Hg were used, both in their state-of-the-art configurations and for a number of numerical experiments to evaluate particular processes. Results of the model simulations were evaluated against measurements. As follows from the analysis, the interhemispheric GEM gradient is largely formed by the prevailing spatial distribution of anthropogenic emissions in the Northern Hemisphere. The contributions of natural and secondary emissions enhance the south-to-north gradient, but their effect is less significant. Atmospheric chemistry has a limited effect on the spatial distribution and temporal variation of GEM concentration in surface air. In contrast, RM air concentration and wet deposition are largely defined by oxidation chemistry. The Br oxidation mechanism can reproduce successfully the observed seasonal variation of the RM/GEM ratio in the near-surface layer, but it predicts a wet deposition maximum in spring instead of in summer as observed at monitoring sites in North America and Europe. Model runs with OH chemistry correctly simulate both the periods of maximum and minimum values and the amplitude of observed seasonal variation but shift the maximum RM/GEM ratios from spring to summer. O-3 chemistry does not predict significant seasonal variation of Hg oxidation. Hence, the performance of the Hg oxidation mechanisms under study differs in the extent to which they can reproduce the various observed parameters. This variation implies possibility of more complex chemistry and multiple Hg oxidation pathways occurring concurrently in various parts of the atmosphere.
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Treguier, A. M., Lique, C., Deshayes, J., & Molines, J. M. (2017). The North Atlantic Eddy Heat Transport and Its Relation with the Vertical Tilting of the Gulf Stream Axis. Journal Of Physical Oceanography, 47(6), 1281–1289.
Abstract: Correlations between temperature and velocity fluctuations are a significant contribution to the North Atlantic meridional heat transport, especially at the northern boundary of the subtropical gyre. In satellite observations and in a numerical model at 1/12 degrees resolution, a localized pattern of positive eddy heat flux is found northwest of the Gulf Stream, downstream of its separation at Cape Hatteras. It is confined to the upper 500 m. A simple kinematic model of a meandering jet can explain the surface eddy flux, taking into account a spatial shift between the maximum velocity of the jet and the maximum cross-jet temperature gradient. In the Gulf Stream such a spatial shift results from the nonlinear temperature profile and the vertical tilting of the velocity profile with depth. The numerical model suggests that the meandering of the Gulf Stream could account, at least in part, for the large eddy heat transport (of order 0.3 PW) near 36 degrees N in the North Atlantic and for its compensation by the mean flow.
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Tuzet, F., Dumont, M., Lafaysse, M., Picard, G., Arnaud, L., Voisin, D., et al. (2017). A multilayer physically based snowpack model simulating direct and indirect radiative impacts of light-absorbing impurities in snow. Cryosphere, 11(6), 2633–2653.
Abstract: Light-absorbing impurities (LAIs) decrease snow albedo, increasing the amount of solar energy absorbed by the snowpack. Its most intuitive and direct impact is to accelerate snowmelt. Enhanced energy absorption in snow also modifies snow metamorphism, which can indirectly drive further variations of snow albedo in the near-infrared part of the solar spectrum because of the evolution of the near-surface snow microstructure. New capabilities have been implemented in the detailed snowpack model SURFEX/ISBACrocus (referred to as Crocus) to account for impurities' deposition and evolution within the snowpack and their direct and indirect impacts. Once deposited, the model computes impurities' mass evolution until snow melts out, accounting for scavenging by meltwater. Taking advantage of the recent inclusion of the spectral radiative transfer model TARTES (Two-stream Analytical Radiative TransfEr in Snow model) in Crocus, the model explicitly represents the radiative impacts of light-absorbing impurities in snow. The model was evaluated at the Col de Porte experimental site (French Alps) during the 2013-2014 snow season against in situ standard snow measurements and spectral albedo measurements. In situ meteorological measurements were used to drive the snowpack model, except for aerosol deposition fluxes. Black carbon (BC) and dust deposition fluxes used to drive the model were extracted from simulations of the atmospheric model ALADIN-Climate. The model simulates snowpack evolution reasonably, providing similar performances to our reference Crocus version in terms of snow depth, snow water equivalent (SWE), near-surface specific surface area (SSA) and shortwave albedo. Since the reference empirical albedo scheme was calibrated at the Col de Porte, improvements were not expected to be significant in this study. We show that the deposition fluxes from the ALADIN-Climate model provide a reasonable estimate of the amount of light-absorbing impurities deposited on the snowpack except for extreme deposition events which are greatly underestimated. For this particular season, the simulated melt-out date advances by 6 to 9 days due to the presence of light-absorbing impurities. The model makes it possible to apportion the relative importance of direct and indirect impacts of light-absorbing impurities on energy absorption in snow. For the snow season considered, the direct impact in the visible part of the solar spectrum accounts for 85% of the total impact, while the indirect impact related to accelerated snow metamorphi
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Valdivieso, M., Haines, K., Balmaseda, M., Chang, Y. S., Drevillon, M., Ferry, N., et al. (2017). An assessment of air-sea heat fluxes from ocean and coupled reanalyses. Climate Dynamics, 49(3), 983–1008.
Abstract: Sixteen monthly air-sea heat flux products from global ocean/coupled reanalyses are compared over 19932009 as part of the Ocean Reanalysis Intercomparison Project (ORA-IP). Objectives include assessing the global heat closure, the consistency of temporal variability, comparison with other flux products, and documenting errors against in situ flux measurements at a number of OceanSITES moorings. The ensemble of 16 ORA-IP flux estimates has a global positive bias over 1993-2009 of 4.2 +/- 1.1 W m(-2). Residual heat gain (i. e., surface flux + assimilation increments) is reduced to a small positive imbalance (typically, + 1-2 W m(-2)). This compensation between surface fluxes and assimilation increments is concentrated in the upper 100 m. Implied steady meridional heat transports also improve by including assimilation sources, except near the equator. The ensemble spread in surface heat fluxes is dominated by turbulent fluxes (>40 W m(-2) over the western boundary currents). The mean seasonal cycle is highly consistent, with variability between products mostly <10 W m-2. The interannual variability has consistent signal-to-noise ratio (similar to 2) throughout the equatorial Pacific, reflecting ENSO variability. Comparisons at tropical buoy sites (10 degrees S-15 degrees N) over 2007-2009 showed too little ocean heat gain (i. e., flux into the ocean) in ORA-IP (up to 1/3 smaller than buoy measurements) primarily due to latent heat flux errors in ORA-IP. Comparisons with the Stratus buoy (20 degrees S, 85 degrees W) over a longer period, 2001-2009, also show the ORA-IP ensemble has 16 W m(-2) smaller net heat gain, nearly all of which is due to too much latent cooling caused by differences in surface winds imposed in ORA-IP.
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Valois, R., Vouillamoz, J. M., Lun, S., & Arnout, L. (2017). Assessment of water resources to support the development of irrigation in northwest Cambodia: a water budget approach. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques, 62(11), 1840–1855.
Abstract: Water availability is the primary constraint on the improvement of food security in rural areas in northwestern Cambodia. A 4-year study was carried out in the upper Stung Sreng watershed to assess water resources. Four sub-watersheds with different land cover types, ranging in size from 1.5 to 185km(2), were monitored using dedicated weather stations and rain- and streamgauges. Geophysics and observation boreholes were used to characterize aquifers. Rainwater is mostly split into evapotranspiration (annual mean of 54% rainfall) and streamflow components (49%), because groundwater recharge is low (1%). Thus, rainwater and streamflow are the main sources for irrigation development. Groundwater can be used only in specific locations for low water-demand crops. A total of 186 household ponds and three village-scale dams were built and 31 wells were installed. The household pond was determined to be the best solution for irrigation development because of its simple management.
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van de Wiel, B. J. H., Vignon, E., Baas, P., van Hooijdonk, I. G. S., van der Linden, S. J. A., van Hooft, J. A., et al. (2017). Regime Transitions in Near-Surface Temperature Inversions: A Conceptual Model. Journal Of The Atmospheric Sciences, 74(4), 1057–1073.
Abstract: A conceptual model is used in combination with observational analysis to understand regime transitions of near-surface temperature inversions at night as well as in Arctic conditions. The model combines a surface energy budget with a bulk parameterization for turbulent heat transport. Energy fluxes or feedbacks due to soil and radiative heat transfer are accounted for by a "lumped parameter closure,'' which represents the ``coupling strength'' of the system. Observations from Cabauw, Netherlands, and Dome C, Antarctica, are analyzed. As expected, inversions are weak for strong winds, whereas large inversions are found under weak-wind conditions. However, a sharp transition is found between those regimes, as it occurs within a narrow wind range. This results in a typical S-shaped dependency. The conceptual model explains why this characteristic must be a robust feature. Differences between the Cabauw and Dome C cases are explained from differences in coupling strength (being weaker in the Antarctic). For comparison, a realistic column model is run. As findings are similar to the simple model and the observational analysis, it suggests generality of the results. Theoretical analysis reveals that, in the transition zone near the critical wind speed, the response time of the system to perturbations becomes large. As resilience to perturbations becomes weaker, it may explain why, within this wind regime, an increase of scatter is found. Finally, the so-called heat flux duality paradox is analyzed. It is explained why numerical simulations with prescribed surface fluxes show a dynamical response different from more realistic surface-coupled systems.
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van Hooijdonk, I. G. S., Clercx, H. J. H., Abraham, C., Holdsworth, A. M., Monahan, A. H., Vignon, E., et al. (2017). Near-Surface Temperature Inversion Growth Rate during the Onset of the Stable Boundary Layer. Journal Of The Atmospheric Sciences, 74(10), 3433–3449.
Abstract: This study aims to find the typical growth rate of the temperature inversion during the onset of the stable boundary layer around sunset. The sunset transition is a very challenging period for numerical weather prediction, since neither accepted theories for the convective boundary layer nor those for the stable boundary layer appear to be applicable. To gain more insight in this period, a systematic investigation of the temperature inversion growth rate is conducted. A statistical procedure is used to analyze almost 16 years of observations from the Cabauw observational tower, supported by observations from two additional sites (Dome C and Karlsruhe). The results show that, on average, the growth rate of the temperature inversion (normalized by the maximum inversion during the night) weakly declines with increasing wind speed. The observed growth rate is quantitatively consistent among the sites, and it appears insensitive to various other parameters. The results were also insensitive to the afternoon decay rate of the net radiation except when this decay rate was very weak. These observations are compared to numerical solutions of three models with increasing complexity: a bulk model, an idealized single-column model (SCM), and an operational-level SCM. It appears only the latter could reproduce qualitative features of the observations using a first-order closure. Moreover, replacing this closure with a prognostic TKE scheme substantially improved the quantitative performance. This suggests that idealized models assuming instantaneous equilibrium flux-profile relations may not aid in understanding this period, since history effects may qualitatively affect the dynamics.
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Ventrillard, I., Xueref-Remy, I., Schmidt, M., Kwok, C. Y., Fain, X., & Romanini, D. (2017). Comparison of optical-feedback cavity-enhanced absorption spectroscopy and gas chromatography for ground-based and airborne measurements of atmospheric CO concentration. Atmospheric Measurement Techniques, 10(5), 1803–1812.
Abstract: We present the first comparison of carbon monoxide (CO) measurements performed with a portable laser spectrometer that exploits the optical-feedback cavity-enhanced absorption spectroscopy (OF-CEAS) technique, against a high-performance automated gas chromatograph (GC) with a mercuric oxide reduction gas detector (RGD). First, measurements of atmospheric CO mole fraction were continuously collected in a Paris (France) suburb over 1 week. Both instruments showed an excellent agreement within typically 2 ppb (part per billion in volume), fulfilling the World Meteorological Organization (WMO) recommendation for CO inter-laboratory comparison. The compact size and robustness of the OF-CEAS instrument allowed its operation aboard a small aircraft employed for routine tropospheric air analysis over the French Orleans forest area. Direct OF-CEAS real-time CO measurements in tropospheric air were then compared with later analysis of flask samples by the gas chromatograph. Again, a very good agreement was observed. This work establishes that the OF-CEAS laser spectrometer can run unattended at a very high level of sensitivity (< 1 ppb) and stability without any periodic calibration.
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Vignon, E., Genthon, C., Barral, H., Amory, C., Picard, G., Gallee, H., et al. (2017). Momentum- and Heat-Flux Parametrization at Dome C, Antarctica: A Sensitivity Study. Boundary-Layer Meteorology, 162(2), 341–367.
Abstract: An extensive meteorological observational dataset at Dome C, East Antarctic Plateau, enabled estimation of the sensitivity of surface momentum and sensible heat fluxes to aerodynamic roughness length and atmospheric stability in this region. Our study reveals that (1) because of the preferential orientation of snow micro-reliefs (sastrugi), the aerodynamic roughness length varies by more than two orders of magnitude depending on the wind direction; consequently, estimating the turbulent fluxes with a realistic but constant of 1 mm leads to a mean friction velocity bias of in near-neutral conditions; (2) the dependence of the ratio of the roughness length for heat to on the roughness Reynolds number is shown to be in reasonable agreement with previous models; (3) the wide range of atmospheric stability at Dome C makes the flux very sensitive to the choice of the stability functions; stability function models presumed to be suitable for stable conditions were evaluated and shown to generally underestimate the dimensionless vertical temperature gradient; as these models differ increasingly with increases in the stability parameter z / L, heat flux and friction velocity relative differences reached when ; (4) the shallowness of the stable boundary layer is responsible for significant sensitivity to the height of the observed temperature and wind data used to estimate the fluxes. Consistent flux results were obtained with atmospheric measurements at heights up to 2 m. Our sensitivity study revealed the need to include a dynamical parametrization of roughness length over Antarctica in climate models and to develop new parametrizations of the surface fluxes in very stable conditions, accounting, for instance, for the divergence in both radiative and turbulent fluxes in the first few metres of the boundary layer.
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Vignon, E., Hourdin, F., Genthon, C., Gallee, H., Bazile, E., Lefebvre, M. P., et al. (2017). Antarctic boundary layer parametrization in a general circulation model: 1-D simulations facing summer observations at Dome C. Journal Of Geophysical Research-Atmospheres, 122(13), 6818–6843.
Abstract: The parametrization of the atmospheric boundary layer (ABL) is critical over the Antarctic Plateau for climate modelling since it affects the climatological temperature inversion and the negatively buoyant near-surface flow over the ice-sheet. This study challenges state-of-the-art parametrizations used in general circulation models to represent the clear-sky summertime diurnal cycle of the ABL at Dome C, Antarctic Plateau. The Laboratoire de Meteorologie Dynamique-Zoom model is run in a 1-D configuration on the fourth Global Energy and Water Cycle Exchanges Project Atmospheric Boundary Layers Study case. Simulations are analyzed and compared to observations, giving insights into the sensitivity of one model that participates to the intercomparison exercise. Snow albedo and thermal inertia are calibrated leading to better surface temperatures. Using the so-called “thermal plume model” improves the momentum mixing in the diurnal ABL. In stable conditions, four turbulence schemes are tested. Best simulations are those in which the turbulence cuts off above 35 m in the middle of the night, highlighting the contribution of the longwave radiation in the ABL heat budget. However, the nocturnal surface layer is not stable enough to distinguish between surface fluxes computed with different stability functions. The absence of subsidence in the forcings and an underestimation of downward longwave radiation are identified to be likely responsible for a cold bias in the nocturnal ABL. Apart from model-specific improvements, the paper clarifies on which are the critical aspects to improve in general circulation models to correctly represent the summertime ABL over the Antarctic Plateau.
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Vignon, E., van de Wiel, B. J. H., van Hooijdonk, I. G. S., Genthon, C., van der Linden, S. J. A., van Hooft, J. A., et al. (2017). Stable boundary-layer regimes at Dome C, Antarctica: observation and analysis. Quarterly Journal Of The Royal Meteorological Society, 143(704), 1241–1253.
Abstract: Investigation of meteorological measurements along a 45m tower at Dome C on the high East Antarctic Plateau revealed two distinct stable boundary layer (SBL) regimes at this location. The first regime is characterized by strong winds and continuous turbulence. It results in full vertical coupling of temperature, wind magnitude and wind direction in the SBL. The second regime is characterized by weak winds, associated with weak turbulent activity and very strong temperature inversions reaching up to 25 K in the lowest 10 m. Vertical temperature profiles are generally exponentially shaped (convex) in the first regime and 'convex-concave-convex' in the second. The transition between the two regimes is particularly abrupt when looking at the near-surface temperature inversion and it can be identified by a 10 m wind-speed threshold. With winds under this threshold, the turbulent heat supply toward the surface becomes significantly lower than the net surface radiative cooling. The threshold value (including its range of uncertainty) appears to agree with recent theoretical predictions from the so-called 'minimum wind speed for sustainable turbulence' (MWST) theory. For the quasi-steady, clear-sky winter cases, the relation between the near-surface inversion amplitude and the wind speed takes a characteristic 'S' shape. Closer analysis suggests that this relation corresponds to a 'critical transition' between a steady turbulent and a steady 'radiative' regime, with a dynamically unstable branch in the transition zone. These fascinating characteristics of the Antarctic boundary layer challenge present and future numerical models to represent this region in a physically correct manner.
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Vincent, C., Fischer, A., Mayer, C., Bauder, A., Galos, S. P., Funk, M., et al. (2017). Common climatic signal from glaciers in the European Alps over the last 50 years. Geophysical Research Letters, 44(3), 1376–1383. |
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Wegmann, M., Orsolini, Y., Dutra, E., Bulygina, O., Sterin, A., & Bronnimann, S. (2017). Eurasian snow depth in long-term climate reanalyses. Cryosphere, 11(2), 923–935.
Abstract: Snow cover variability has significant effects on local and global climate evolution. By changing surface energy fluxes and hydrological conditions, changes in snow cover can alter atmospheric circulation and lead to remote climate effects. To document such multi-scale climate effects, atmospheric reanalysis and derived products offer the opportunity to analyze snow variability in great detail far back to the early 20th century. So far only little is know about their quality. Comparing snow depth in four long-term reanalysis datasets with Russian in situ snow depth data, we find a moderately high daily correlation (around 0.6-0.7), which is comparable to correlations for the recent era (1981-2010), and a good representation of sub-decadal variability. However, the representation of pre-1950 inter-decadal snow variability is questionable, since reanalysis products divert towards different base states. Limited availability of independent long-term snow data makes it difficult to assess the exact cause for this bifurcation in snow states, but initial investigations point towards representation of the atmosphere rather than differences in assimilated data or snow schemes. This study demonstrates the ability of long-term reanalysis to reproduce snow variability accordingly.
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Weiss, J., & Dansereau, V. (2017). Linking scales in sea ice mechanics. Philosophical Transactions Of The Royal Society A-Mathematical Physical And Engineering Sciences, 375(2086).
Abstract: Mechanics plays a key role in the evolution of the sea ice cover through its control on drift, on momentum and thermal energy exchanges between the polar oceans and the atmosphere along cracks and faults, and on ice thickness distribution through opening and ridging processes. At the local scale, a significant variability of the mechanical strength is associated with the microstructural heterogeneity of saline ice, however characterized by a small correlation length, below the ice thickness scale. Conversely, the sea ice mechanical fields (velocity, strain and stress) are characterized by long-ranged (more than 1000 km) and long-lasting (approx. few months) correlations. The associated space and time scaling laws are the signature of the brittle character of sea ice mechanics, with deformation resulting from a multi-scale accumulation of episodic fracturing and faulting events. To translate the short-range-correlated disorder on strength into long-range-correlated mechanical fields, several key ingredients are identified: long-ranged elastic interactions, slow driving conditions, a slow viscous-like relaxation of elastic stresses and a restoring/healing mechanism. These ingredients constrained the development of a new continuum mechanics modelling framework for the sea ice cover, called Maxwell-elasto-brittle. Idealized simulations without advection demonstrate that this rheological framework reproduces the main characteristics of sea ice mechanics, including anisotropy, spatial localization and intermittency, as well as the associated scaling laws. This article is part of the themed issue 'Microdynamics of ice'.
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Wilhelm, B., Vogel, H., & Anselmetti, F. S. (2017). A multi-centennial record of past floods and earthquakes in Valle d'Aosta, Mediterranean Italian Alps. Natural Hazards And Earth System Sciences, 17(5), 613–625.
Abstract: Mediterranean Alpine populations are particularly exposed to natural hazards like floods and earthquakes because of both the close Mediterranean humidity source and the seismically active Alpine region. Knowledge of longterm variability in flood and earthquake occurrences is of high value since it can be useful to improve risk assessment and mitigation. In this context, we explore the potential of a lake-sediment sequence from Lago Inferiore de Laures in Valle d'Aosta (Northern Italy) as a long-term record of past floods and earthquakes. The high-resolution sedimentological study revealed 76 event layers over the last ca. 270 years; 8 are interpreted as most probably induced by earthquakes and 68 by flood events. Comparison to historical seismic data suggests that the recorded earthquakes are strong (epicentral Medvedev-Sponheuer-Karnik (MSK) intensity of VI-IX) and/or close to the lake (distance of 25-120 km). Compared to other lake-sediment sequences, Lago Inferiore de Laures sediments appear to be regionally the most sensitive to earthquake shaking, offering a great potential to reconstruct the past regional seismicity further back in time. Comparison to historical and palaeoflood records suggests that the flood signal reconstructed from Lago Inferiore de Laures sediments represents the regional and (multi-) decadal variability of summer-autumn floods well, in connection to Mediterranean mesoscale precipitation events. Overall, our results reveal the high potential of Lago Inferiore de Laures sediments to extend the regional earthquake and flood catalogues far back in time.
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Wubda, M., Descloitres, M., Yalo, N., Ribolzi, O., Vouillamoz, J. M., Boukari, M., et al. (2017). Time-lapse electrical surveys to locate infiltration zones in weathered hard rock tropical areas. Journal Of Applied Geophysics, 142, 23–37.
Abstract: In West Africa, infiltration and groundwater recharge processes in hard rock areas are depending on climatic, surface and subsurface conditions, and are poorly documented. Part of the reason is that identification, location and monitoring of these processes is still a challenge. Here, we explore the potential for time-lapse electrical surveys to bring additional information on these processes for two different climate situations: a semi-arid Sahelian site (north of Burkina and a humid Sudanian site (north of Benin), respectively focusing on indirect (localized) and direct (diffuse) recharge processes. The methodology is based on surveys in dry season and rainy season on typical pond or gully using Electrical Resistivity Tomography (ERT) and frequency electromagnetic (FEM) apparent conductivity mapping. The results show that in the Sahelian zone an indirect recharge occurs as expected, but infiltration doesn't takes place at the center of the pond to the aquifer, but occurs laterally in the banks. In Sudanian zone, the ERT survey shows a direct recharge process as expected, but also a complicated behavior of groundwater dilution, as well as the role of hardpans for fast infiltration. These processes are ascertained by groundwater monitoring in adjacent observing wells. At last, FEM time lapse mapping is found to be difficult to quantitatively interpreted due to the non-uniqueness of the model, clearly evidenced comparing FEM result to auger holes monitoring. Finally, we found that time-lapse ERT can be an efficient way to track infiltration processes across ponds and gullies in both climatic conditions, the Sahelian setting providing results easier to interpret, due to significant resistivity contrasts between dry and rain seasons. Both methods can be used for efficient implementation of punctual sensors for complementary studies. However, FEM time-lapse mapping remains difficult to practice without external information that renders this method less attractive for quantitative interpretation purposes. (C) 2017 Elsevier B.V. All rights reserved.
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Xavier, P., Rauly, D., Chamberod, E., & Martins, J. M. F. (2017). Theoretical Evidence of Maximum Intracellular Currents Versus Frequency in an Escherichia coli Cell Submitted to AC Voltage. Bioelectromagnetics, 38(3), 213–219.
Abstract: In this work, the problem of intracellular currents in longilinear bacteria, such as Escherichia coli, suspended in a physiological medium and submitted to a harmonic voltage (AC), is analyzed using the Finite-Element-based software COMSOL Multiphysics. Bacterium was modeled as a cylindrical capsule, ended by semi-spheres and surrounded by a dielectric cell wall. An equivalent single-layer cell wall was defined, starting from the well-recognized three-shell modeling approach. The bacterium was considered immersed in a physiological medium, which was also taken into account in the modeling. A new complex transconductance was thus introduced, relating the complex ratio between current inside the bacterium and voltage applied between two parallel equipotential planes, separated by a realistic distance. When voltage was applied longitudinally relative to the bacterium main axis, numerical results in terms of frequency response in the 1-20 MHz range for E. coli cells revealed that transconductance magnitude exhibited a maximum at a frequency depending on the cell wall capacitance. This occurred in spite of the purely passive character of the model and could be explained by an equivalent electrical network giving very similar results and showing special conditions for lateral paths of the currents through the cell wall. It is shown that the main contribution to this behavior is due to the conductive part of the current. (C) 2016 Wiley Periodicals, Inc.
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Xia, J. Y., McGuire, A. D., Lawrence, D., Burke, E., Chen, G. S., Chen, X. D., et al. (2017). Terrestrial ecosystem model performance in simulating productivity and its vulnerability to climate change in the northern permafrost region. Journal Of Geophysical Research-Biogeosciences, 122(2), 430–446.
Abstract: Realistic projection of future climate-carbon (C) cycle feedbacks requires better understanding and an improved representation of the C cycle in permafrost regions in the current generation of Earth system models. Here we evaluated 10 terrestrial ecosystem models for their estimates of net primary productivity (NPP) and responses to historical climate change in permafrost regions in the Northern Hemisphere. In comparison with the satellite estimate from the Moderate Resolution Imaging Spectroradiometer (MODIS; 246 +/- 6gCm(-2) yr (-1)), most models produced higher NPP (309 +/- 12 g Cm-2 yr(-1)) over the permafrost region during 2000-2009. By comparing the simulated gross primary productivity (GPP) with a flux tower-based database, we found that although mean GPP among the models was only overestimated by 10% over 1982-2009, there was a twofold discrepancy among models (380 to 800 g Cm-2 yr(-1)), which mainly resulted from differences in simulated maximum monthly GPP (GPP(max)). Most models overestimated C use efficiency (CUE) as compared to observations at both regional and site levels. Further analysis shows that model variability of GPP and CUE are nonlinearly correlated to variability in specific leaf area and the maximum rate of carboxylation by the enzyme Rubisco at 25 degrees C (V-cmax_(25)), respectively. Themodels also varied in their sensitivities of NPP, GPP, and CUE to historical changes in climate and atmospheric CO2 concentration. These results indicate that model predictive ability of the C cycle in permafrost regions can be improved by better representation of the processes controlling CUE and GPP(max) as well as their sensitivity to climate change.
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Yan, Y., Barth, A., Beckers, J. M., Brankart, J. M., Brasseur, P., & Candille, G. (2017). Comparison of different incremental analysis update schemes in a realistic assimilation system with Ensemble Kalman Filter. Ocean Modelling, 115, 27–41.
Abstract: In this paper, three incremental analysis update schemes (IAU 0, IAU 50 and IAU 100) are compared in the same assimilation experiments with a realistic eddy permitting primitive equation model of the North Atlantic Ocean using the Ensemble Kalman Filter. The difference between the three IAU schemes lies on the position of the increment update window. The relevance of each IAU scheme is evaluated through analyses on both thermohaline and dynamical variables. The validation of the assimilation results is performed according to both deterministic and probabilistic metrics against different sources of observations. For deterministic validation, the ensemble mean and the ensemble spread are compared to the observations. For probabilistic validation, the continuous ranked probability score (CRPS) is used to evaluate the ensemble forecast system according to reliability and resolution. The reliability is further decomposed into bias and dispersion by the reduced centred random variable (RCRV) score. The obtained results show that 1) the IAU 50 scheme has the same performance as the IAU 100 scheme 2) the IAU 50/100 schemes outperform the IAU 0 scheme in error covariance propagation for thermohaline variables in relatively stable region, while the IAU 0 scheme outperforms the IAU 50/100 schemes in dynamical variables estimation in dynamically active region 3) in case with sufficient number of observations and good error specification, the impact of IAU schemes is negligible. The differences between the IAU 0 scheme and the IAU 50/100 schemes are mainly due to different model integration time and different instability (density inversion, large vertical velocity, etc.) induced by the increment update. The longer model integration time with the IAU 50/100 schemes, especially the free model integration, on one hand, allows for better re-establishment of the equilibrium model state, on the other hand, smooths the strong gradients in dynamically active region. (C) 2017 Elsevier Ltd. All rights reserved.
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Young, D. A., Roberts, J. L., Ritz, C., Frezzotti, M., Quartini, E., Cavitte, M. G. P., et al. (2017). High-resolution boundary conditions of an old ice target near Dome C, Antarctica. Cryosphere, 11(4), 1897–1911.
Abstract: A high-resolution (1 km line spacing) aerogeophysical survey was conducted over a region near the East Antarctic Ice Sheet's Dome C that may hold a 1.5 Myr climate record. We combined new ice thickness data derived from an airborne coherent radar sounder with unpublished data that was in part unavailable for earlier compilations, and we were able to remove older data with high positional uncertainties. We generated a revised high-resolution digital elevation model (DEM) to investigate the potential for an old ice record in this region, and used laser altimetry to confirm a Cryosat-2 derived DEM for inferring the glaciological state of the candidate area. By measuring the specularity content of the bed, we were able to find an additional 50 subglacial lakes near the candidate site, and by Doppler focusing the radar data, we were able to map out the roughness of the bed at length scales of hundreds of meters. We find that the primary candidate region contains elevated rough topography interspersed with scattered subglacial lakes and some regions of smoother bed. Free subglacial water appears to be restricted from bed overlain by ice thicknesses of less than 3000 m. A site near the ice divide was selected for further investigation. The high resolution of this ice thickness data set also allows us to explore the nature of ice thickness uncertainties in the context of radar geometry and processing
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Zhang, T., Price, S., Ju, L. L., Leng, W., Brondex, J., Durand, G., et al. (2017). A comparison of two Stokes ice sheet models applied to the Marine Ice Sheet Model Intercomparison Project for plan view models (MISMIP3d). Cryosphere, 11(1), 179–190.
Abstract: We present a comparison of the numerics and simulation results for two “full” Stokes ice sheet models, FELIX-S (Leng et al., 2012) and Elmer/Ice (Gagliardini et al., 2013). The models are applied to the Marine Ice Sheet Model Intercomparison Project for plan view models (MIS-MIP3d). For the diagnostic experiment (P75D) the two models give similar results (<2% difference with respect to along-flow velocities) when using identical geometries and computational meshes, which we interpret as an indication of inherent consistencies and similarities between the two models. For the standard (Stnd), P75S, and P75R prognostic experiments, we find that FELIX-S (Elmer/Ice) grounding lines are relatively more retreated (advanced), results that are consistent with minor differences observed in the diagnostic experiment results and that we show to be due to different choices in the implementation of basal boundary conditions in the two models. While we are not able to argue for the relative favorability of either implementation, we do show that these differences decrease with increasing horizontal (i.e., both along-and across-flow) grid resolution and that grounding-line positions for FELIX-S and Elmer/Ice converge to within the estimated truncation error for Elmer/Ice. Stokes model solutions are often treated as an accuracy metric in model intercomparison experiments, but computational cost may not always allow for the use of model resolution within the regime of asymptotic convergence. In this case, we propose that an alternative estimate for the uncertainty in the grounding-line position is the span of grounding-line positions predicted by multiple Stokes models.
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Zolina, O., Dufour, A., Gulev, S. K., & Stenchikov, G. (2017). Regional Hydrological Cycle over the Red Sea in ERA-Interim. Journal Of Hydrometeorology, 18(1), 65–83.
Abstract: The major sources of atmospheric moisture over the Red Sea are analyzed using ERA-Interim for the 1979-2013 period. The vertical structure of moisture transports across the coastlines has been computed separately for the western and eastern coasts of the Red Sea. The vertical structure of the moisture transport from the Red Sea to the continents is dominated by a breeze-like circulation in the near-surface layer and the Arabian high above 850 hPa. The lower-layer, breeze-like circulation is acting to export the moisture to the northwest of Africa and to the Arabian Peninsula and contributes about 80% of the moisture exports from the Red Sea, dominating over the transport in the upper layer, where the moisture is advected to the Arabian Peninsula in the northern part of the sea and to the African continent in the southern part. Integrated moisture divergence over the Red Sea decreased from the early 1980s to 1997 and then increased until the 2010s. Associated changes in the moisture export were provided primarily by the increasing intensity of the breeze-associated transports. The transports above the boundary layer, while being strong across the western and the eastern coasts, have a smaller effect on the net moisture export. The interannual variability of the moisture export in the near-surface layer was found to be closely correlated with the variability in sea surface temperature, especially in summer. Implications of the observed changes in the moisture advection for the hydrological cycle of the Middle East are discussed.
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Ago, E. E., Agbossou, E. K., Cohard, J. M., Galle, S., & Aubinet, M. (2016). Response of CO2 fluxes and productivity to water availability in two contrasting ecosystems in northern Benin (West Africa). Annals Of Forest Science, 73(2), 483–500.
Abstract: CO (2) fluxes were measured during 18 months in a forest and a savannah in northern Benin. Higher values of carbon fluxes were found during the wet season at each site. A strong dependency of carbon fluxes on water relations was found in two contrasting sites. The forest sequestered 640 +/- 50 and the savannah 190 +/- 40 g C m (-2) year (-1) . In West Africa, the main mechanisms or factors governing the dynamics of ecosystems, especially the dynamics of the carbon fluxes and productivity, still remain less known. This study reports the carbon fluxes over two contrasting ecosystems, notably a protected forest (lat 9.79A degrees N, long 1.72A degrees E, alt 414 m) and a cultivated savannah (lat 9.74A degrees N, long 1.60A degrees E, alt 449 m) in northern Benin. The two sites were among those equipped by the AMMA-CATCH observatory and Ou,m, 2025 project. Flux data were analyzed at the daily and seasonal scales in order to understand their controlling variables. We discussed the patterns of CO2 fluxes and the characteristics of the two ecosystems. The study also focused on the different water usage strategies developed by the two ecosystems since the alternation between dry and wet seasons highly influenced the seasonal dynamics. Finally, the annual carbon sequestration was estimated together with its uncertainty. The carbon fluxes were measured during 18 months (July 2008-December 2009) by an eddy-covariance system over two contrasting sites in northern Benin. Fluxes data were computed following the standard procedure. The responses of CO2 fluxes to the principal climatic and edaphic factors, and the canopy conductance were studied. A clear CO2 fluxes response to main environmental factors was observed, however with difference according to the seasons and vegetation types. The ecosystem respiration showed the highest values during the wet season and a progressive decrease from wet to dry periods. Also, the carbon uptake values were high during the wet period, but low during the dry period. However, the CO2 fluxes for the protected forest were always higher than that for the cultivated savannah within each defined period. This was due to the seasonal changes not only in phenology and physiology but also to the acclimation to environmental conditions, especially to the soil water availability. The water use efficiency was influenced by VPD during the day conditions for two ecosystems. However, the VPD response curve of water usage was relatively constant for the protected forest during the transitional and wet seasons. In contrary, for the cultivated savannah the VPD response decreased about 46 % from transitional dry-wet to wet seasons and remained relatively constant until transitional wet-dry season. The close relationships between the net CO2 assimilation and the canopy conductance were found for the two ecosystems. This suggests a regulation of the stomata by a partial stomatal closure besides the radiation control. Finally, the forest sequestered at the annual scale 640 +/- 50 and the savannah 190 +/- 40 g C m(-2) year(-1). We conclude with a strong relation between the carbon fluxes and water in the two investigated ecosystems. Apart from the radiation control, the stomata also play an important role in the regulation of the CO2 assimilation in the two ecosystems.
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Akuetevi, C. Q. C., Barnier, B., Verron, J., Molines, J. M., & Lecointre, A. (2016). Interactions between the Somali Current eddies during the summer monsoon: insights from a numerical study. Ocean Science, 12(1), 185–205.
Abstract: Three hindcast simulations of the global ocean circulation differing by resolution (1/4 or 1/12 degrees) or parametrization or atmospheric forcing are used to describe the interactions between the large anticyclonic eddies generated by the Somali Current system during the Southwest Monsoon. The present investigation of the Somalian coherent eddy structures allows us to identify the origin and the subsequent development of the cyclones flanked upon the Great Whirl (GW) previously identified by Beal and Donohue (2013) in satellite observations and to establish that similar cyclones are also flanked upon the Southern Gyre (SG). These cyclones are identified as potential actors in mixing water masses within the large eddies and offshore the coast of Somalia. All three simulations bring to light that during the period when the Southwest Monsoon is well established, the SG moves northward along the Somali coast and encounters the GW. The interaction between the SG and the GW is a collision without merging, in a way that has not been described in observations up to now. During the collision the GW is pushed to the east of Socotra Island, sheds several smaller patches of anticyclonic vorticity, and often reforms into the Socotra Eddy, thus proposing a formation mechanism for that eddy. During this process the GW gives up its place to the SG. This process is robust throughout the three simulations.
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Alastuey, A., Querol, X., Aas, W., Lucarelli, F., Pérez, N., Moreno, T., et al. (2016). Geochemistry of PM10 over Europe during the EMEP intensive measurement periods in summer 2012 and winter 2013. Acp, 16(10), 6107–6129. |
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Alcoba, M., Gosset, M., Kacou, M., Cazenave, F., & Fontaine, E. (2016). Characterization of Hydrometeors in Sahelian Convective Systems with an X-Band Radar and Comparison with In Situ Measurements. Part II: A Simple Brightband Method to Infer the Density of Icy Hydrometeors. Journal of Applied Meteorology and Climatology, 55(2), 251–263.
Abstract: AbstractA simple scheme that is based on the shape and intensity of the radar bright band is used to infer the density of hydrometeors just above the freezing level in Sahelian mesoscale convective systems (MCS). Four MCS jointly observed by a ground-based X-band radar and by an instrumented aircraft as part of the Megha-Tropiques algorithm-validation campaign during August 2010 in Niamey, Niger, are analyzed. The instrumented aircraft (with a 94-GHz radar and various optical probes on board) provided mass?diameter laws for the particles sampled during the flights. The mass?diameter laws derived from the ground-radar vertical profile of reflectivity (VPR) for each flight are compared with those derived from the airborne measurements. The density laws derived by both methods are consistent and encourage further use of the simple VPR scheme to quantify hydrometeor density laws and their variability for various analyses (microphysical processes and icy-hydrometeor scattering and radiative properties).
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Amory, C., Naaim-Bouvet, F., Gallee, H., & Vignon, E. (2016). Brief communication: Two well-marked cases of aerodynamic adjustment of sastrugi. Cryosphere, 10(2), 743–750.
Abstract: In polar regions, sastrugi are a direct manifestation of drifting snow and form the main surface roughness elements. In turn, sastrugi alter the generation of atmospheric turbulence and thus modify the wind field and the aeolian snow mass fluxes. Little attention has been paid to these feedback processes, mainly because of experimental difficulties. As a result, most polar atmospheric models currently ignore sastrugi over snow-covered regions. This paper aims at quantifying the potential influence of sastrugi on the local wind field and on snow erosion over a sastrugi-covered snowfield in coastal Ad,lie Land, East Antarctica. We focus on two erosion events during which sastrugi responses to shifts in wind direction have been interpreted from temporal variations in drag and aeolian snow mass flux measurements during austral winter 2013. Using this data set, it is shown that (i) neutral stability, 10aEuro-m drag coefficient (C-DN10) values are in the range of 1.3-1.5 x 10(-3) when the wind is well aligned with the sastrugi, (ii) as the wind shifts by only 20-30A degrees away from the streamlined direction, C-DN10 increases (by 30-120aEuro-%) and the aeolian snow mass flux decreases (by 30-80aEuro-%), thereby reflecting the growing contribution of the sastrugi form drag to the total surface drag and its inhibiting effect on snow erosion, (iii) the timescale of sastrugi aerodynamic adjustment can be as short as 3aEuro-h for friction velocities greater than 1aEuro-maEuro-s(-1) and during strong drifting snow conditions and (iv) knowing C-DN10 is not sufficient to estimate the snow erosion flux that results from drag partitioning at the surface because C-DN10 includes the contribution of the sastrugi form drag.
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Angot, H., Dastoor, A., De Simone, F., Gardfeldt, K., Gencarelli, C. N., Hedgecock, I. M., et al. (2016). Chemical cycling and deposition of atmospheric mercury in polar regions: review of recent measurements and comparison with models. Atmospheric Chemistry And Physics, 16(16), 10735–10763.
Abstract: Mercury (Hg) is a worldwide contaminant that can cause adverse health effects to wildlife and humans. While atmospheric modeling traces the link from emissions to deposition of Hg onto environmental surfaces, large uncertainties arise from our incomplete understanding of atmospheric processes (oxidation pathways, deposition, and re-emission). Atmospheric Hg reactivity is exacerbated in high latitudes and there is still much to be learned from polar regions in terms of atmospheric processes. This paper provides a synthesis of the atmospheric Hg monitoring data available in recent years (2011-2015) in the Arctic and in Antarctica along with a comparison of these observations with numerical simulations using four cutting-edge global models. The cycle of atmospheric Hg in the Arctic and in Antarctica presents both similarities and differences. Coastal sites in the two regions are both influenced by springtime atmospheric Hg depletion events and by summertime snowpack re-emission and oceanic evasion of Hg. The cycle of atmospheric Hg differs between the two regions primarily because of their different geography. While Arctic sites are significantly influenced by northern hemispheric Hg emissions especially in winter, coastal Antarctic sites are significantly influenced by the reactivity observed on the East Antarctic ice sheet due to katabatic winds. Based on the comparison of multi-model simulations with observations, this paper discusses whether the processes that affect atmospheric Hg seasonality and inter-annual variability are appropriately represented in the models and identifies research gaps in our understanding of the atmospheric Hg cycling in high latitudes.
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Angot, H., Dion, I., Vogel, N., Legrand, M., Magand, O., & Dommergue, A. (2016). Multi-year record of atmospheric mercury at Dumont d'Urville, East Antarctic coast: continental outflow and oceanic influences. Atmospheric Chemistry And Physics, 16(13), 8265–8279.
Abstract: Under the framework of the Global Mercury Observation System (GMOS) project, a 3.5-year record of atmospheric gaseous elemental mercury (Hg(0)) has been gathered at Dumont d'Urville (DDU, 66 degrees 40'S, 140 degrees 01'E, 43m above sea level) on the East Antarctic coast. Additionally, surface snow samples were collected in February 2009 during a traverse between Concordia Station located on the East Antarctic plateau and DDU. The record of atmospheric Hg(0) at DDU reveals particularities that are not seen at other coastal sites: a gradual decrease of concentrations over the course of winter, and a daily maximum concentration around midday in summer. Additionally, total mercury concentrations in surface snow samples were particularly elevated near DDU (up to 194.4 ng L-1) as compared to measurements at other coastal Antarctic sites. These differences can be explained by the more frequent arrival of inland air masses at DDU than at other coastal sites. This confirms the influence of processes observed on the Antarctic plateau on the cycle of atmospheric mercury at a continental scale, especially in areas subject to recurrent katabatic winds. DDU is also influenced by oceanic air masses and our data suggest that the ocean plays a dual role on Hg(0) concentrations. The open ocean may represent a source of atmospheric Hg(0) in summer whereas the sea-ice surface may provide reactive halogens in spring that can oxidize Hg(0). This paper also discusses implications for coastal Antarctic ecosystems and for the cycle of atmospheric mercury in high southern latitudes.
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Angot, H., Magand, O., Helmig, D., Ricaud, P., Quennehen, B., Gallee, H., et al. (2016). New insights into the atmospheric mercury cycling in central Antarctica and implications on a continental scale. Atmospheric Chemistry And Physics, 16(13), 8249–8264.
Abstract: Under the framework of the GMOS project (Global Mercury Observation System) atmospheric mercury monitoring has been implemented at Concordia Station on the high-altitude Antarctic plateau (75 degrees 06'S, 123 degrees 20'E, 3220m above sea level). We report here the first year-round measurements of gaseous elemental mercury (Hg(0)) in the atmosphere and in snowpack interstitial air on the East Antarctic ice sheet. This unique data set shows evidence of an intense oxidation of atmospheric Hg(0) in summer (24-hour daylight) due to the high oxidative capacity of the Antarctic plateau atmosphere in this period of the year. Summertime Hg(0) concentrations exhibited a pronounced daily cycle in ambient air with maximal concentrations around midday. Photochemical reactions and chemical exchange at the air-snow interface were prominent, highlighting the role of the snowpack on the atmospheric mercury cycle. Our observations reveal a 20 to 30% decrease of atmospheric Hg(0) concentrations from May to mid-August (winter, 24 h darkness). This phenomenon has not been reported elsewhere and possibly results from the dry deposition of Hg(0) onto the snowpack. We also reveal the occurrence of multi-day to weeklong atmospheric Hg(0) depletion events in summer, not associated with depletions of ozone, and likely due to a stagnation of air masses above the plateau triggering an accumulation of oxidants within the shallow boundary layer. Our observations suggest that the inland atmospheric reservoir is depleted in Hg(0) in summer. Due to katabatic winds flowing out from the Antarctic plateau down the steep vertical drops along the coast and according to observations at coastal Antarctic stations, the striking reactivity observed on the plateau most likely influences the cycle of atmospheric mercury on a continental scale.
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Arnaud, F., Poulenard, J., Giguet-Covex, C., Wilhelm, B., Revillon, S., Jenny, J. P., et al. (2016). Erosion under climate and human pressures: An alpine lake sediment perspective. Quaternary Science Reviews, 152, 1–18.
Abstract: We review the scientific efforts over the last decades to reconstruct erosion from continuous alpine lake sediment records. We focused both on methodological issues, showing the growing importance of non-destructive high resolution approaches (XRF core-scanner) as well as progresses in the understanding of processes leading to the creation of an “erosion signal” in lakes. We distinguish “continuous records” from “event-records”. Both provide complementary information but need to be studied with different approaches. Continuous regionally-relevant records proved to be particularly pertinent to document regional erosion patterns throughout the Holocene, in particular applying the source to sink approach. Event-based approaches demonstrated and took advantage of the strong non-linearity of sediment transport in high altitude catchment areas. This led to flood frequency and intensity reconstructions, highlighting the influence of climate change upon flood dynamics in the mountain. The combination of different record types, both in terms of location (high vs. low elevation), sedimentology (high vs. low terrigenous contribution) and significance (local vs. regional) is one of the main outputs of this paper. It allows the establishment of comprehensive histories of NW French Alps erosion, but also and consequently, soil dynamics and hydrological patterns throughout the Holocene. We also discuss the influence of glacier dynamics, one of the major agents of erosion in the Alps. A major feature is the growing human influence upon erosion at a local scale since at least the middle of the Bronze Age (3500 cal. BP). However and according to the regional record from Lake Bourget, only few periods of rising erosion at local scales generated a regional record that can be discriminated from wetter climatic periods. Among them, the period between 200 BCE and 400 AD appeared to be marked by a generalised rise in human-triggered erosion at local scales in the northern French Alps. This review highlights the importance of modern high-resolution and interdisciplinary studies of lake sediments, in order to better understand the complex relationships between humans, climate and the Earth system in general. We strongly argue that regional integration of data is now required to move a step further. Such an integration is easier with cost- and time-effective methods as well as after a better definition of approaches and their limits. This should lead to a stronger collaboration between paleo-data producers and modellers in the near future. (C) 2016 Elsevier Ltd. All rights reserved.
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Arshad, M., Merlina, G., Uzu, G., Sobanska, S., Sarret, G., Dumat, C., et al. (2016). Phytoavailability of lead altered by two Pelargonium cultivars grown on contrasting lead-spiked soils. Journal Of Soils And Sediments, 16(2), 581–591.
Abstract: This study assesses the potential of two contrasted fragrant Pelargonium cultivars to induce pH and dissolved organic carbon (DOC) changes in the soil solution, Pb speciation, and their subsequent effects on rhizosphere phytoavailable Pb. Rooted plantlets were grown in special devices, floating on aerated nutrient solution in PVC tanks. This setup allows roots to be physically separated, through a mesh, from a 3-mm soil matrix layer that can be considered as rhizosphere soil. Two contrasted soils, each spiked with Pb-rich particles, emitted from a battery recycling industry, were used at total burdens of 500 and 1500 mg Pb kg(-1) in addition to a control unspiked soil. Soil solution pH, phytoavailable Pb, DOC, Pb adsorption, precipitation on roots, and Pb phases in soil and plant were investigated. Attar of Roses (Attar) cultivar acidified its rhizosphere by 0.4 pH units in both spiked soils. Concolor Lace (Concolor) was unable to change soil solution pH on soil-1 and increased it by 0.7 units on soil 2. Concentrations of Pb in soil solution from Attar plants were always higher than those of Concolor ones. DOC contents of both unspiked soil-1 and soil-2 without plants were not significantly different. In the case of spiked samples, DOC contents in the rhizosphere soil were increased by three and two times for Attar and Concolor, respectively, compared to the unspiked soil without plant. Both cultivars were able to increase DOC contents, independent of soil type and level of contamination. Accumulation of Pb in shoots and roots was higher in Attar as compared to Concolor due to enhanced available Pb as a result of pH and DOC modifications of the rhizosphere soil. Significant amounts of Pb were adsorbed on roots of both cultivars. X-ray elemental analysis of precipitates on roots revealed the association of Pb with P in cylinder-like structures. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that Pb was present, to a major extent in the inorganic form, mainly as PbSO4 in the soil, whereas it was complexed with organic species within plant tissues. The conversion of Pb into organic species could decrease toxicity, may enhance plant tolerance, and could increase translocation. Plant-induced changes were responsible for the modification of lead phases within the soil. Immobile forms present in the source leaded particles as well as in the soils were converted into soluble species, ultimately improving the phytoavailable or soil solubilized Pb.
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Asay-Davis, X. S., Cornford, S. L., Durand, G., Galton-Fenzi, B. K., Gladstone, R. M., Gudmundsson, G. H., et al. (2016). Experimental design for three interrelated marine ice sheet and ocean model intercomparison projects: MISMIP v. 3 (MISMIP+), ISOMIP v. 2 (ISOMIP+) and MISOMIP v. 1 (MISOMIP1). Geoscientific Model Development, 9(7), 2471–2497.
Abstract: Coupled ice sheet-ocean models capable of simulating moving grounding lines are just becoming available. Such models have a broad range of potential applications in studying the dynamics of marine ice sheets and tidewater glaciers, from process studies to future projections of ice mass loss and sea level rise. The Marine Ice Sheet-Ocean Model Intercomparison Project ( MISOMIP) is a community effort aimed at designing and coordinating a series of model intercomparison projects ( MIPs) for model evaluation in idealized setups, model verification based on observations, and future projections for key regions of the West Antarctic Ice Sheet ( WAIS). Here we describe computational experiments constituting three interrelated MIPs for marine ice sheet models and regional ocean circulation models incorporating ice shelf cavities. These consist of ice sheet experiments under the Marine Ice Sheet MIP third phase ( MISMIP+), ocean experiments under the Ice Shelf-Ocean MIP second phase ( ISOMIP+) and coupled ice sheet-ocean experiments under the MISOMIP first phase ( MISOMIP1). All three MIPs use a shared domain with idealized bedrock topography and forcing, allowing the coupled simulations ( MISOMIP1) to be compared directly to the individual component simulations ( MISMIP+ and ISOMIP+). The experiments, which have qualitative similarities to Pine Island Glacier Ice Shelf and the adjacent region of the Amundsen Sea, are designed to explore the effects of changes in ocean conditions, specifically the temperature at depth, on basal melting and ice dynamics. In future work, differences between model results will form the basis for the evaluation of the participating models.
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Attard, G., Rossier, Y., & Eisenlohr, L. (2016). Urban groundwater age modeling under unconfined condition – Impact of underground structures on groundwater age: Evidence of a piston effect. Journal Of Hydrology, 535, 652–661.
Abstract: In this paper, underground structures are shown to have a major influence on the groundwater mean age distribution described as a dispersive piston effect. Urban underground development does not occur without impacts on subsoil resources. In particular, groundwater resources can be vulnerable and generate disturbances when this space is exploited. Groundwater age spatial distribution data are fundamental for resource management as it can provide operational sustainability indicators. However, the application of groundwater age modeling is neglected regarding the potential effect of underground structures in urban areas. A three dimensional modeling approach was conducted to quantify the impact of two underground structures: (1) an impervious structure and (2) a draining structure. Both structures are shown to cause significant mixing processes occurring between shallow and deeper aquifers. The design technique used for draining structures is shown to have the greatest impact, generating a decrease in mean age of more than 80% under the structure. Groundwater age modeling is shown to be relevant for highlighting the role played by underground structures in advective-dispersive flows in urban areas. (C) 2016 Elsevier B.V. All rights reserved.
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Attard, G., Rossier, Y., Winiarski, T., Cuvillier, L., & Eisenlohr, L. (2016). Deterministic modelling of the cumulative impacts of underground structures on urban groundwater flow and the definition of a potential state of urban groundwater flow: example of Lyon, France. Hydrogeology Journal, 24(5), 1213–1229.
Abstract: Underground structures have been shown to have a great influence on subsoil resources in urban aquifers. A methodology to assess the actual and the potential state of the groundwater flow in an urban area is proposed. The study develops a three-dimensional modeling approach to understand the cumulative impacts of underground infrastructures on urban groundwater flow, using a case in the city of Lyon (France). All known underground structures were integrated in the numerical model. Several simulations were run: the actual state of groundwater flow, the potential state of groundwater flow (without underground structures), an intermediate state (without impervious structures), and a transient simulation of the actual state of groundwater flow. The results show that underground structures fragment groundwater flow systems leading to a modification of the aquifer regime. For the case studied, the flow systems are shown to be stable over time with a transient simulation. Structures with drainage systems are shown to have a major impact on flow systems. The barrier effect of impervious structures was negligible because of the small hydraulic gradient of the area. The study demonstrates that the definition of a potential urban groundwater flow and the depiction of urban flow systems, which involves understanding the impact of underground structures, are important issues with respect to urban underground planning.
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Attard, G., Rossier, Y., Winiarski, T., & Eisenlohr, L. (2016). Deterministic modeling of the impact of underground structures on urban groundwater temperature. Science Of The Total Environment, 572, 986–994.
Abstract: Underground structures have a major influence on groundwater temperature and have a major contribution on the anthropogenic heat fluxes into urban aquifers. Groundwater temperature is crucial for resource management as it can provide operational sustainability indicators for groundwater quality and geothermal energy. Here, a three dimensional heat transport modeling approach was conducted to quantify the thermally affected zone (TAZ, i.e. increase in temperature of more than +0.5 degrees C) caused by two common underground structures: (1) an impervious structure and (2) a draining structure. These design techniques consist in (1) ballasting the underground structure in order to resist hydrostatic pressure, or (2) draining the groundwater under the structure in order to remove the hydrostatic pressure. The volume of the TAZ caused by these underground structures was shown to range from 14 to 20 times the volume of the underground structure. Additionally, the cumulative impact of underground structures was assessed under average thermal conditions at the scale of the greater Lyon area (France). The heat island effect caused by underground structures was highlighted in the business center of the city. Increase in temperature of more than +4.5 degrees C were locally put in evidence. The annual heat flow from underground structures to the urban aquifer was computed deterministically and represents 4.5 GW.h. Considering these impacts, the TAZ of deep underground structures should be taken into account in the geothermal potential mapping. Finally, the amount of heat energy provided should be used as an indicator of heating potential in these areas. (C) 2016 Elsevier B.V. All rights reserved.
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Attard, G., Winiarski, T., Rossier, Y., & Eisenlohr, L. (2016). Review: Impact of underground structures on the flow of urban groundwater. Hydrogeology Journal, 24(1), 5–19.
Abstract: Property economics favours the vertical development of cities but flow of groundwater can be affected by the use of underground space in them. This review article presents the state of the art regarding the impact of disturbances caused by underground structures (tunnels, basements of buildings, deep foundations, etc.) on the groundwater flow in urban aquifers. The structures built in the underground levels of urban areas are presented and organised in terms of their impact on flow: obstacle to the flow or disturbance of the groundwater budget of the flow system. These two types of disturbance are described in relation to the structure area and the urban area. The work reviewed shows, on one hand, the individual impacts of different urban underground structures, and on the other, their cumulative impacts on flow, using real case studies. Lastly, the works are placed in perspective regarding the integration of underground structures with the aim of operational management of an urban aquifer. The literature presents deterministic numerical modelling as a tool capable of contributing to this aim, in that it helps to quantify the effect of an underground infrastructure project on groundwater flow, which is crucial for decision-making processes. It can also be an operational decision-aid tool for choosing construction techniques or for formulating strategies to manage the water resource.
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Audebert, M., Oxarango, L., Duquennoi, C., Touze-Foltz, N., Forquet, N., & Clement, R. (2016). Understanding leachate flow in municipal solid waste landfills by combining time-lapse ERT and subsurface flow modelling – Part II: Constraint methodology of hydrodynamic models. Waste Management, 55, 176–190.
Abstract: Leachate recirculation is a key process in the operation of municipal solid waste landfills as bioreactors. To ensure optimal water content distribution, bioreactor operators need tools to design leachate injection systems. Prediction of leachate flow by subsurface flow modelling could provide useful information for the design of such systems. However, hydrodynamic models require additional data to constrain them and to assess hydrodynamic parameters. Electrical resistivity tomography (ERT) is a suitable method to study leachate infiltration at the landfill scale. It can provide spatially distributed information which is useful for constraining hydrodynamic models. However, this geophysical method does not allow ERT users to directly measure water content in waste. The MICS (multiple inversions and clustering strategy) methodology was proposed to delineate the infiltration area precisely during time-lapse ERT survey in order to avoid the use of empirical petrophysical relationships, which are not adapted to a heterogeneous medium such as waste. The infiltration shapes and hydrodynamic information extracted with MICS were used to constrain hydrodynamic models in assessing parameters. The constraint methodology developed in this paper was tested on two hydrodynamic models: an equilibrium model where, flow within the waste medium is estimated using a single continuum approach and a non-equilibrium model where flow is estimated using a dual continuum approach. The latter represents leachate flows into fractures. Finally, this methodology provides insight to identify the advantages and limitations of hydrodynamic models. Furthermore, we suggest an explanation for the large volume detected by MICS when a small volume of leachate is injected. (C) 2016 Elsevier Ltd. All rights reserved.
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Avanzi, F., De Michele, C., Morin, S., Carmagnola, C. M., Ghezzi, A., & Lejeune, Y. (2016). Model complexity and data requirements in snow hydrology: seeking a balance in practical applications. Hydrological Processes, 30(13), 2106–2118.
Abstract: We investigate the problem of balancing model complexity and input data requirements in snow hydrology. For this purpose, we analyze the performance of two models of different complexity in estimating variables of interest in snow hydrology applications. These are snow depth, bulk snow density, snow water equivalent and snowmelt run-off. We quantify the differences between data and model prediction using 18years of measurements from an experimental site in the French Alps (Col de Porte, 1325m AMSL). The models involved in this comparison are a one-layer temperature-index model (HyS) and a multilayer model (Crocus). Results show that the expected loss in performance in the one-layer temperature-index model with respect to the multilayer model is low when considering snow depth, snow water equivalent and bulk snow density. As for run-off, the comparison returns less clear indications for identification of a balance. In particular, differences between the models' prediction and data with an hourly resolution are higher when considering the Crocus model than the HyS model. However, Crocus is better at reproducing sub-daily cycles in this variable. In terms of daily run-off, the multilayer physically based model seems to be a better choice, while results in terms of cumulative run-off are comparable. The better reproduction of daily and sub-daily variability of run-off suggests that use of the multilayer model may be preferable for this purpose. Variation in performance is discussed as a function of both the calibration solution chosen and the time of year. Copyright (c) 2016 John Wiley & Sons, Ltd.
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Azam, M. F., Ramanathan, A., Wagnon, P., Vincent, C., Linda, A., Berthier, E., et al. (2016). Meteorological conditions, seasonal and annual mass balances of Chhota Shigri Glacier, western Himalaya, India. Annals Of Glaciology, 57(71), 328–338.
Abstract: We present the updated glaciological mass balance (MB) of Chhota Shigri Glacier, the longest continuous annual MB record in the Hindu-Kush Karakoram Himalaya (HKH) region. Additionally, 4 years of seasonal MBs are presented and analyzed using the data acquired at an automatic weather station (AWS-M) installed in 2009 on a lateral moraine (4863ma.s.l.). The glaciological MB series since 2002 is first recalculated using an updated glacier hypsometry and then validated against geodetic MB derived from satellite stereo-imagery between 2005 (SPOT5) and 2014 (Pleiades). Chhota Shigri Glacier lost mass between 2002 and 2014 with a cumulative glaciological MB of -6.72 m w. e. corresponding to a mean annual glacier-wide MB (B-a) of -0.56m w. e. a(-1). Equilibriumline altitude (ELA(0)) for the steady-state condition is calculated as similar to 4950 m a.s.l., corresponding to an accumulation-area ratio (AAR(0)) of similar to 61%. Analysis of seasonal MBs between 2009 and 2013 with air temperature from AWS-M and precipitation from the nearest meteorological station at Bhuntar (1050 m a.s.l.) suggests that the summer monsoon is the key season driving the interannual variability of Ba for this glacier. The intensity of summer snowfall events controls the B-a evolution via controlling summer glacier-wide MB (B-s).
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Banerjee, A., & Azam, M. F. (2016). Temperature reconstruction from glacier length fluctuations in the Himalaya. Annals Of Glaciology, 57(71), 189–198.
Abstract: A temperature reconstruction in the glacierized Himalaya over the past centuries using glacial length fluctuation records is challenging due to the abundance of debris-covered glaciers and a scarcity of glacial length fluctuation data. Using idealized flowline model simulations, we show that supraglacial debris cover significantly alters the length fluctuations only when the debris cover is very thick. An expanded database of length fluctuation records for 43 glaciers in the Himalaya and Karakoram is compiled and a standard linear inversion procedure is applied to a subset of 34 glaciers in this database. The reconstructed temperature anomaly during 1860-2010 indicates a continued warming of the region with a total temperature change of similar to 1.6 K. A close resemblance of the regional temperature anomaly to global trends is seen.
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Bannour, H., Touze-Foltz, N., & Pierson, P. (2016). Transient hydraulic behavior of two GMBs-GCLs composite liners. Geotextiles And Geomembranes, 44(1), 51–58.
Abstract: In general, interface transmissivity is obtained based on measurements of the steady-state flow in composite geomembranes (GMBs) – geosynthetic clay liners (GCLs). However, transient flow rate has not been studied prior to this study. It has been already noticed that its consideration can lead to an important increase in predicted effluent volumes penetrating the liner, the subgrade soil and the ground water. The goal of the study presented herein is to study the transient state by quantifing the reduction in flow rate with time for two different composite liners. In these composite liners, a needle-punched GCL was combined to a high-density polyethylene GMB. The first GCL contained granular sodium bentonite and the second GCL contained powdered sodium bentonite. While the GCLs hydrated their water content and deformation were quantified. To this aim, various tests were conducted over various periods of time, corresponding to different flow rate values through the composite liner. Relationships between the flow rate and water uptake of the GCL on one side and swell on the other side are given for both GCLs. The results show that the mass per unit area in addition to the granulometric distribution of bentonite in GCL specimens may affect the hydration process and the flow rate evolution through composite liners. In addition, For both GCLs, the deformation follows the same trend of evolution with water content. (C) 2015 Elsevier Ltd. All rights reserved.
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Basantes-Serrano, R., Rabatel, A., Francou, B., Vincent, C., Maisincho, L., Caceres, B., et al. (2016). Slight mass loss revealed by reanalyzing glacier mass-balance observations on Glaciar Antisana 15 alpha (inner tropics) during the 1995-2012 period. Journal Of Glaciology, 62(231), 124–136.
Abstract: In this paper, we reanalyze the glacier mass balance on Glaciar Antisana 15 alpha over the 1995-2012 period. Annual glacier mass balances were quantified on the basis of monthly glaciological measurements using an adaptation of Lliboutry's statistical approach. The geodetic mass balance was computed between 1997 and 2009 giving a cumulative balance of -1.39 +/- 1.97 m w.e. and a slightly negative adjusted annual glaciological mass balance (-0.12 +/- 0.16 m w.e. a(-1)). Despite a careful analysis of uncertainties, we found a large discrepancy between the cumulative glaciological and the geodetic mass balances over the common period, of 4.66 m w.e. This discrepancy can mainly be explained by underestimated net accumulation in the glacier upper reaches, which could be due to the peculiar climate conditions of the equatorial zone with year round accumulation, thereby preventing clear identification of annual layers. An increase of similar to 70% in measured rates of net accumulation would be needed to balance the glaciological and geodetic mass balances; a hypothesis confirmed by estimated ice flux in the vicinity of the ELA. Consequently, the vertical gradient of precipitation may be higher than previously estimated and the accumulation processes (including the role of frost deposition) need to be carefully analyzed.
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Bazin, L., Landais, A., Capron, E., Masson-Delmotte, V., Ritz, C., Picard, G., et al. (2016). Phase relationships between orbital forcing and the composition of air trapped in Antarctic ice cores. Climate Of The Past, 12(3), 729–748.
Abstract: Orbital tuning is central for ice core chronologies beyond annual layer counting, available back to 60 ka (i.e. thousands of years before 1950) for Greenland ice cores. While several complementary orbital tuning tools have recently been developed using delta O-18(atm), delta O-2/N-2 and air content with different orbital targets, quantifying their uncertainties remains a challenge. Indeed, the exact processes linking variations of these parameters, measured in the air trapped in ice, to their orbital targets are not yet fully understood. Here, we provide new series of delta O-2/N-2 and delta O-18(atm) data encompassing Marine Isotopic Stage (MIS) 5 (between 100 and 160 ka) and the oldest part (340-800 ka) of the East Antarctic EPICA Dome C (EDC) ice core. For the first time, the measurements over MIS 5 allow an inter-comparison of delta O-2/N-2 and delta O-18(atm) records from three East Antarctic ice core sites (EDC, Vostok and Dome F). This comparison highlights some site-specific delta O-2/N-2 variations. Such an observation, the evidence of a 100 ka periodicity in the delta O-2/N-2 signal and the difficulty to identify extrema and mid-slopes in delta O-2/N-2 increase the uncertainty associated with the use of delta O-2/N-2 as an orbital tuning tool, now calculated to be 3-4 ka. When combining records of delta O-18(atm) and delta O-2/N-2 from Vostok and EDC, we find a loss of orbital signature for these two parameters during periods of minimum eccentricity (similar to 400 ka, 720-800 ka). Our data set reveals a time-varying offset between delta O-2/N-2 and delta O-18(atm) records over the last 800 ka that we interpret as variations in the lagged response of delta O-18(atm) to precession. The largest offsets are identified during Terminations II, MIS 8 and MIS 16, corresponding to periods of destabilization of the Northern polar ice sheets. We therefore suggest that the occurrence of Heinrich like events influences the response of delta O-18(atm) to precession.
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Ben Daoud, A., Sauquet, E., Bontron, G., Obled, C., & Lang, M. (2016). Daily quantitative precipitation forecasts based on the analogue method: Improvements and application to a French large river basin. Atmospheric Research, 169, 147–159.
Abstract: This paper presents some improvements of a probabilistic quantitative precipitation forecasting method based on analogues, formerly developed on small basins located in South-Eastern France. The scope is extended to large scale basins mainly influenced by frontal systems, considering a case study area related to the Saone river, a large basin in eastern France. For a given target situation, this method consists in searching for the most similar situations observed in a historical meteorological archive. Precipitation amounts observed during analogous situations are then collected to derive an empirical predictive distribution function, i.e. the probabilistic estimation of the precipitation amount expected for the target day. The former version of this forecasting method (Bontron, 2004) has been improved by introducing two innovative variables: temperature, that allows taking seasonal effects into account and vertical velocity, which enables a better characterization of the vertical atmospheric motion. The new algorithm is first applied in a perfect prognosis context (target situations come from a meteorological reanalysis) and then in an operational forecasting context (target situations come from weather forecasts) for a three years period. Results show that this approach yields useful forecasts, with a lower false alarm rate and improved performances from the present day D to day D + 2. (C) 2015 Elsevier B.V. All rights reserved.
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Berger, A., Crucifix, M., Hodell, D. A., Mangili, C., McManus, J. F., Otto-Bliesner, B., et al. (2016). Interglacials of the last 800,000years. Reviews Of Geophysics, 54(1), 162–219.
Abstract: Interglacials, including the present (Holocene) period, are warm, low land ice extent (high sea level), end-members of glacial cycles. Based on a sea level definition, we identify eleven interglacials in the last 800,000years, a result that is robust to alternative definitions. Data compilations suggest that despite spatial heterogeneity, Marine Isotope Stages (MIS) 5e (last interglacial) and 11c (similar to 400ka ago) were globally strong (warm), while MIS 13a (similar to 500ka ago) was cool at many locations. A step change in strength of interglacials at 450ka is apparent only in atmospheric CO2 and in Antarctic and deep ocean temperature. The onset of an interglacial (glacial termination) seems to require a reducing precession parameter (increasing Northern Hemisphere summer insolation), but this condition alone is insufficient. Terminations involve rapid, nonlinear, reactions of ice volume, CO2, and temperature to external astronomical forcing. The precise timing of events may be modulated by millennial-scale climate change that can lead to a contrasting timing of maximum interglacial intensity in each hemisphere. A variety of temporal trends is observed, such that maxima in the main records are observed either early or late in different interglacials. The end of an interglacial (glacial inception) is a slower process involving a global sequence of changes. Interglacials have been typically 10-30ka long. The combination of minimal reduction in northern summer insolation over the next few orbital cycles, owing to low eccentricity, and high atmospheric greenhouse gas concentrations implies that the next glacial inception is many tens of millennia in the future.
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Berthier, E., Cabot, V., Vincent, C., & Six, D. (2016). Decadal region-wide and glacier-wide mass balances derived from multi-temporal ASTER satellite digital elevation models. Validation over the Mont-Blanc area. Frontiers in Earth Science, 4, 63. |
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Berthou, S., Mailler, S., Drobinski, P., Arsouze, T., Bastin, S., Beranger, K., et al. (2016). Influence of submonthly air-sea coupling on heavy precipitation events in the Western Mediterranean basin. Quarterly Journal Of The Royal Meteorological Society, 142, 453–471.
Abstract: Heavy precipitation events (HPEs) can be a major cause of damage and casualties in the Mediterranean basin. With the use of atmosphere-ocean coupled regional climate models (AORCMs) and the advantage of 24 years of simulation (1989-2012), we identified regions with potential impact of submonthly air-sea coupling on HPEs from among the regions hit by heavy rain during the Hydrological Cycle in the Mediterranean Experiment (HyMeX) first special observation period (SOP1): Valencia in Spain, the Cevennes in Southern France, Liguria in Northwestern Italy, Calabria in Southern Italy and Northeastern Italy. A first evaluation of the two AORCMs (MORCE and CNRM-RCSM4) against gridded precipitation datasets showed that 70-90% of the 30 most intense HPEs simulated were observed HPEs for most regions. The Cevennes, Valencia and Calabria were the only three regions to show a statistical relationship between rain differences and sea-surface temperature (SST) differences where the low-level jets that feed the events most frequently blow. This sensitivity of precipitation to SST changes is due to low-level wind changes related to changes in surface heat fluxes. Based on the calculation of submonthly variations in these regions during HyMeX, HPEs on 28 September 2012 and 12 October 2012 in Valencia and on 25-26 October 2012 in the Cevennes have most probably been affected by submonthly air-sea coupling.
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Bichet, A., Kushner, P. J., & Mudryk, L. (2016). Estimating the Continental Response to Global Warming Using Pattern-Scaled Sea Surface Temperatures and Sea Ice. Journal Of Climate, 29(24), 9125–9139.
Abstract: Better constraining the continental climate response to anthropogenic forcing is essential to improve climate projections. In this study, pattern scaling is used to extract, from observations, the patterned response of sea surface temperature (SST) and sea ice concentration (SICE) to anthropogenically dominated long-term global warming. The SST response pattern includes a warming of the tropical Indian Ocean, the high northern latitudes, and the western boundary currents. The SICE pattern shows seasonal variations of the main locations of sea ice loss. These SST-SICE response patterns are used to drive an ensemble of an atmospheric general circulation model, the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5 (CAM5), over the period 1980-2010 along with a standard AMIP ensemble using observed SST-SICE. The simulations enable attribution of a variety of observed trends of continental climate to global warming. On the one hand, the warming trends observed in all seasons across the entire Northern Hemisphere extratropics result from global warming, as does the snow loss observed over the northern midlatitudes and northwestern Eurasia. On the other hand, 1980-2010 precipitation trends observed in winter over North America and in summer over Africa result from the recent decreasing phase of the Pacific decadal oscillation and the recent increasing phase of the Atlantic multidecadal oscillation, respectively, which are not part of the global warming signal. The method holds promise for near-term decadal climate prediction but as currently framed cannot distinguish regional signals associated with oceanic internal variability from aerosol forcing and other sources of short-term forcing.
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Blanchet, J., Ceresetti, D., Molinie, G., & Creutin, J. D. (2016). A regional GEV scale-invariant framework for Intensity-Duration-Frequency analysis. Journal Of Hydrology, 540, 82–95.
Abstract: We propose in this paper a regional formulation of Intensity-Duration-Frequency curves of point-rainfall maxima in a scale-invariant Generalized Extreme Value (GEV) framework. The two assumptions we make is that extreme daily rainfall is GEV-distributed – which is justified by Extreme Value Theory (EVT) – and that extremes of aggregated daily rainfall follow simple-scaling relationships. Following these assumptions, we develop in a unified way a GEV simple-scaling model for extremes of aggregated daily rainfall over the range of durations where scaling applies. Then we propose a way of correcting this model for measurement frequency, giving a new GEV-scaling model for extremes of aggregated hourly rainfall. This model deviates from the simple-scaling assumption. This framework is applied to the Mediterranean region of Cevennes-Vivarais, France. A network of about 300 daily raingage stations covering the last 50 years and accumulated to span the range 1 day-1 week is used to fit the scale invariant GEV-model locally. By means of spatial interpolation of the model parameters, and correction for measurement frequency, we are able to build a regional model with good performances down to 1 h duration, even though only one hourly station is used to build the model. Finally we produce mean and return level maps within the region in the range 1 h-1 week and comment on the potential rain storms leading to these maps. (C) 2016 Elsevier B.V. All rights reserved.
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Bock, J., Savarino, J., & Picard, G. (2016). Air-snow exchange of nitrate: a modelling approach to investigate physicochemical processes in surface snow at Dome C, Antarctica. Atmospheric Chemistry And Physics, 16(19), 12531–12550.
Abstract: Snowpack is a multiphase (photo) chemical reactor that strongly influences the air composition in polar and snow-covered regions. Snowpack plays a special role in the nitrogen cycle, as it has been shown that nitrate undergoes numerous recycling stages (including photolysis) in the snow before being permanently buried in the ice. However, the current understanding of these physicochemical processes remains very poor. Several modelling studies have attempted to reproduce (photo) chemical reactions inside snow grains, but these have relied on strong assumptions to characterise snow reactive properties, which are not well defined. Air-snow exchange processes such as adsorption, solid-state diffusion, or co-condensation also affect snow chemical composition. Here, we present a physically based model of these processes for nitrate. Using as input a 1-year-long time series of atmospheric nitrate concentration measured at Dome C, Antarctica, our model reproduces with good agreement the nitrate measurements in the surface snow. By investigating the relative importance of the main exchange processes, this study shows that, on the one hand, the combination of bulk diffusion and co-condensation allows a good reproduction of the measurements (correlation coefficient r = 0.95), with a correct amplitude and timing of summer peak concentration of nitrate in snow. During winter, nitrate concentration in surface snow is mainly driven by thermodynamic equilibrium, whilst the peak observed in summer is explained by the kinetic process of co-condensation. On the other hand, the adsorption of nitric acid on the surface of the snow grains, constrained by an already existing parameterisation for the isotherm, fails to fit the observed variations. During winter and spring, the modelled concentration of adsorbed nitrate is respectively 2.5 and 8.3-fold higher than the measured one. A strong diurnal variation driven by the temperature cycle and a peak occurring in early spring are two other major features that do not match the measurements. This study clearly demonstrates that co-condensation is the most important process to explain nitrate incorporation in snow undergoing temperature gradient metamorphism. The parameterisation developed for this process can now be used as a foundation piece in snowpack models to predict the inter-relationship between snow physical evolution and snow nitrate chemistry.
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Boike, J., Grau, T., Heim, B., Gunther, F., Langer, M., Muster, S., et al. (2016). Satellite-derived changes in the permafrost landscape of central Yakutia, 2000-2011: Wetting, drying, and fires. Global And Planetary Change, 139, 116–127.
Abstract: The focus of this research has been on detecting changes in lake areas, vegetation, land surface temperatures, and the area covered by snow, using data from remote sensing. The study area covers the main (central) part of the Lena River catchment in the Yakutia region of Siberia (Russia), extending from east of Yakutsk to the central Siberian Plateau, and from the southern Lena River to north of the Vilyui River. Approximately 90% of the area is underlain by continuous permafrost. Remote sensing products were used to analyze changes in water bodies, land surface temperature (1ST), and leaf area index (LAI), as well as the occurrence and extent of forest fires, and the area and duration of snow cover. The remote sensing analyses (for 1ST, snow cover, LAI, and fire) were based on MODIS-derived NASA products (250-1000 m) for 2000 to 2011. Changes in water bodies were calculated from two mosaics of (USGS) Landsat (30 m) satellite images from 2002 and 2009. Within the study area's 315,000 km(2) the total area covered by lakes increased by 17.9% between 2002 and 2009, but this increase varied in different parts of the study area, ranging between 11% and 42%. The land surface temperatures showed a consistent warming trend, with an average increase of about 0.12 degrees C/year. The average rate of warming during the April-May transition period was 0.17 degrees C/year and 0.19 degrees C/year in the September-October period, but ranged up to 0.49 degrees C/year during September-October. Regional differences in the rates of land surface temperature change, and possible reasons for the temperature changes, are discussed with respect to changes in the land cover. Our analysis of a broad spectrum of variables over the study area suggests that the spring warming trend is very likely to be due to changes in the area covered by snow. The warming trend observed in fall does not, however, appear to be directly related to any changes in the area of snow cover, or to the atmospheric conditions, or to the proportion of the land surface that is covered by water (i.e., to wetting and drying). Supplementary data (original data, digitized version of the maps, metadata) are archived under PANGAEA (http://dx.doi.org/10.1594/PANGAEA.855124). (C) 2016 Elsevier B.V. All rights reserved.
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Bonvalot, L., Tuna, T., Fagault, Y., Jaffrezo, J. L., Jacob, V., Chevrier, F., et al. (2016). Estimating contributions from biomass burning, fossil fuel combustion, and biogenic carbon to carbonaceous aerosols in the Valley of Chamonix: a dual approach based on radiocarbon and levoglucosan. Atmospheric Chemistry And Physics, 16(21), 13753–13772.
Abstract: Atmospheric particulate matter (PM) affects the climate in various ways and has a negative impact on human health. In populated mountain valleys in Alpine regions, emissions from road traffic contribute to carbonaceous aerosols, but residential wood burning can be another source of PM during winter. We determine the contribution of fossil and non-fossil carbon sources by measuring radiocarbon in aerosols using the recently installed AixMICADAS facility. The accelerator mass spectrometer is coupled to an elemental analyzer (EA) by means of a gas interface system directly connected to the gas ion source. This system provides rapid and accurate radiocarbon measurements for small samples (10-100 μgC) with minimal preparation from the aerosol filters. We show how the contamination induced by the EA protocol can be quantified and corrected for. Several standards and synthetic samples are then used to demonstrate the precision and accuracy of aerosol measurements over the full range of expected C-14/C-12 ratios, ranging from modern carbon to fossil carbon depleted in C-14. Aerosols sampled in Chamonix and Passy (Arve River valley, French Alps) from November 2013 to August 2014 are analyzed for both radiocarbon (124 analyses in total) and levoglucosan, which is commonly used as a specific tracer for biomass burning. NOx concentration, which is expected to be associated with traffic emissions, is also monitored. Based on C-14 measurements, we can show that the relative fraction of non-fossil carbon is significantly higher in winter than in summer. In winter, non-fossil carbon represents about 85% of total carbon, while in summer this proportion is still 75% considering all samples. The largest total carbon and levoglucosan concentrations are observed for winter aerosols with values up to 50 and 8 μg m(-3), respectively. These levels are higher than those observed in many European cities, but are close to those for other polluted Alpine valleys. The non-fossil carbon concentrations are strongly correlated with the levoglucosan concentrations in winter samples, suggesting that almost all of the non-fossil carbon originates from wood combustion used for heating during winter. For summer samples, the joint use of C-14 and levoglucosan measurements leads to a new model to separately quantify the contributions of biomass burning and biogenic emissions in the non-fossil fraction. The comparison of the biogenic fraction with polyols (a proxy for primary soil biogenic emissions) and with the temperature suggests a major influence of the secondary biogenic aerosols. Significant correlations are found between the NOx concentration and the fossil carbon concentration for all seasons and sites, confirming the relation between road traffic emissions and fossil carbon. Overall, this dual approach combining radiocarbon and levoglucosan analyses strengthens the conclusion concerning the impact of biomass burning. Combining these geochemical data serves both to detect and quantify additional carbon sources. The Arve River valley provides the first illustration of aerosols of this model.
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Boudevillain, B., Delrieu, G., Wijbrans, A., & Confoland, A. (2016). A high-resolution rainfall re-analysis based on radar-raingauge merging in the Cevennes-Vivarais region, France. Journal Of Hydrology, 541, 14–23.
Abstract: This work aims at providing quantitative precipitation estimates (QPEs) for the Cevennes-Vivarais region, France, over temporal (1-6 h) and spatial (1-300 km(2)) scales relevant for flash-flood prediction in that region. A systematic implementation of three estimation methods (radar QPE, hourly raingauge Ordinary Kriging – OK – and merging of radar and raingauge data through ICriging with External Drift – KED) proves the ICED method to systematically outperform the concurrent approaches for the 131 main rain events selected during the period 2007-2014. Error models, assuming the standard deviation of the QPE error to be a bi-linear function of the rain rate and the kriging normalized estimation standard deviation, are parameterized for the KED and OK QPEs for the considered temporal and spatial scales. The error models are shown to depend on the type of rain event (Cevennes rain events, localized convection, widespread rainfall) and physical parameters such as the 0 degrees C isotherm altitude and the rain intermittency. The added-value of the radar network in terms of QPE with respect to the hourly raingauge network is larger for localized convection rain events as well as for the smallest space-time scales which are those of interest for flash-flood prediction in the region. (C) 2016 Elsevier B.V. All rights reserved.
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Boudhar, A., Boulet, G., Hanich, L., Sicart, J. E., & Chehbouni, A. (2016). Energy fluxes and melt rate of a seasonal snow cover in the Moroccan High Atlas. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques, 61(5), 931–943.
Abstract: In this study, we used an energy balance model and two simple methods based on readily available data to identify the processes driving the point-scale energy and mass balance of the snowpack. Data were provided from an experimental site located at 3200m. All models were evaluated by comparing observed and modelled snow water equivalents. Performances are variable from one season to the next and the energy balance model gives better results (mean of root mean square error, RMSE=25mm and r(2)=0.90) than the two simplified approaches (mean of RMSE=54mm and r(2)=0.70). There are significant amounts of snow sublimation but they are highly variable from season to season, depending on wind conditions (between 7 and 20% of the total). While the main source of energy for melting is net radiation, the amount of heat brought by sensible heat flux is significant for two of the most windy snow seasons.
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Brocca, L., Pellarin, T., Crow, W. T., Ciabatta, L., Massari, C., Ryu, D., et al. (2016). Rainfall estimation by inverting SMOS soil moisture estimates: A comparison of different methods over Australia. Journal Of Geophysical Research-Atmospheres, 121(20), 12062–12079.
Abstract: Remote sensing of soil moisture has reached a level of maturity and accuracy for which the retrieved products can be used to improve hydrological and meteorological applications. In this study, the soil moisture product from the Soil Moisture and Ocean Salinity (SMOS) satellite is used for improving satellite rainfall estimates obtained from the Tropical Rainfall Measuring Mission multisatellite precipitation analysis product (TMPA) using three different bottom up techniques: SM2RAIN, Soil Moisture Analysis Rainfall Tool, and Antecedent Precipitation Index Modification. The implementation of these techniques aims at improving the well-known top down rainfall estimate derived from TMPA products (version 7) available in near real time. Ground observations provided by the Australian Water Availability Project are considered as a separate validation data set. The three algorithms are calibrated against the gauge-corrected TMPA reanalysis product, 3B42, and used for adjusting the TMPA real-time product, 3B42RT, using SMOS soil moisture data. The study area covers the entire Australian continent, and the analysis period ranges from January 2010 to November 2013. Results show that all the SMOS-based rainfall products improve the performance of 3B42RT, even at daily time scale (differently from previous investigations). The major improvements are obtained in terms of estimation of accumulated rainfall with a reduction of the root-mean-square error of more than 25%. Also, in terms of temporal dynamic (correlation) and rainfall detection (categorical scores) the SMOS-based products provide slightly better results with respect to 3B42RT, even though the relative performance between the methods is not always the same. The strengths and weaknesses of each algorithm and the spatial variability of their performances are identified in order to indicate the ways forward for this promising research activity. Results show that the integration of bottom up and top down approaches has the potential to improve the quality of near-real-time rainfall estimates from remote sensing in the near future.
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Brovkin, V., Bruecher, T., Kleinen, T., Zaehle, S., Joos, F., Roth, R., et al. (2016). Comparative carbon cycle dynamics of the present and last interglacial. Quaternary Science Reviews, 137, 15–32.
Abstract: Changes in temperature and carbon dioxide during glacial cycles recorded in Antarctic ice cores are tightly coupled. However, this relationship does not hold for interglacials. While climate cooled towards the end of both the last (Eemian) and present (Holocene) interglacials, CO2 remained stable during the Eemian while rising in the Holocene. We identify and review twelve biogeochemical mechanisms of terrestrial (vegetation dynamics and CO2 fertilization, land use, wildfire, accumulation of peat, changes in permafrost carbon, subaerial volcanic outgassing) and marine origin (changes in sea surface temperature, carbonate compensation to deglaciation and terrestrial biosphere regrowth, shallow-water carbonate sedimentation, changes in the soft tissue pump, and methane hydrates), which potentially may have contributed to the CO2 dynamics during interglacials but which remain not well quantified. We use three Earth System Models (ESMs) of intermediate complexity to compare effects of selected mechanisms on the interglacial CO2 and delta(CO2)-C-13 changes, focusing on those with substantial potential impacts: namely carbonate sedimentation in shallow waters, peat growth, and (in the case of the Holocene) human land use. A set of specified carbon cycle forcings could qualitatively explain atmospheric CO2 dynamics from 8 ka BP to the pre-industrial. However, when applied to Eemian boundary conditions from 126 to 115 ka BP, the same set of forcings led to disagreement with the observed direction of CO2 changes after 122 ka BP. This failure to simulate late-Eemian CO2 dynamics could be a result of the imposed forcings such as prescribed CaCO3 accumulation and/or an incorrect response of simulated terrestrial carbon to the surface cooling at the end of the interglacial. These experiments also reveal that key natural processes of interglacial CO2 dynamics – shallow water CaCO3 accumulation, peat and permafrost carbon dynamics are not well represented in the current ESMs. Global-scale modeling of these long-term carbon cycle components started only in the last decade, and uncertainty in parameterization of these mechanisms is a main limitation in the successful modeling of interglacial CO2 dynamics. (C) 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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Brun, F., Buri, P., Miles, E. S., Wagnon, P., Steiner, J., Berthier, E., et al. (2016). Quantifying volume loss from ice cliffs on debris-covered glaciers using high-resolution terrestrial and aerial photogrammetry. Journal Of Glaciology, 62(234), 684–695.
Abstract: Mass losses originating from supraglacial ice cliffs at the lower tongues of debris-covered glaciers are a potentially large component of the mass balance, but have rarely been quantified. In this study, we develop a method to estimate ice cliff volume losses based on high-resolution topographic data derived from terrestrial and aerial photogrammetry. We apply our method to six cliffs monitored in May and October 2013 and 2014 using four different topographic datasets collected over the debris-covered Lirung Glacier of the Nepalese Himalayas. During the monsoon, the cliff mean backwasting rate was relatively consistent in 2013 (3.8 +/- 0.3 cm w.e. d(-1)) and more heterogeneous among cliffs in 2014 (3.1 +/- 0.7 cm w.e. d(-1)), and the geometric variations between cliffs are larger. Their mean backwasting rate is significantly lower in winter (October 2013-May 2014), at 1.0 +/- 0.3 cm w.e. d(-1). These results are consistent with estimates of cliff ablation from an energy-balance model developed in a previous study. The ice cliffs lose mass at rates six times higher than estimates of glacier-wide melt under debris, which seems to confirm that ice cliffs provide a large contribution to total glacier melt.
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Brunner, M. I., Seibert, J., & Favre, A. - C. (2016). Bivariate return periods and their importance for flood peak and volume estimation. Wiley Interdisciplinary Reviews: Water, 3(6), 819–833.
Abstract: Estimates of flood event magnitudes with a certain return period are required for the design of hydraulic structures. While the return period is clearly defined in a univariate context, its definition is more challenging when the problem at hand requires considering the dependence between two or more variables in a multivariate framework. Several ways of defining a multivariate return period have been proposed in the literature, which all rely on different probability concepts. Definitions use the conditional probability, the joint probability, or can be based on the Kendall's distribution or survival function. In this study, we give a comprehensive overview on the tools that are available to define a return period in a multivariate context. We especially address engineers, practitioners, and people who are new to the topic and provide them with an accessible introduction to the topic. We outline the theoretical background that is needed when one is in a multivariate setting and present the reader with different definitions for a bivariate return period. Here, we focus on flood events and the different probability concepts are explained with a pedagogical, illustrative example of a flood event characterized by the two variables peak discharge and flood volume. The choice of the return period has an important effect on the magnitude of the design variable quantiles, which is illustrated with a case study in Switzerland. However, this choice is not arbitrary and depends on the problem at hand.
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Buri, P., Miles, E. S., Steiner, J. F., Immerzeel, W. W., Wagnon, P., & Pellicciotti, F. (2016). A physically based 3-D model of ice cliff evolution over debris-covered glaciers. Journal Of Geophysical Research-Earth Surface, 121(12), 2471–2493.
Abstract: We use high-resolution digital elevation models (DEMs) from unmanned aerial vehicle (UAV) surveys to document the evolution of four ice cliffs on the debris-covered tongue of Lirung Glacier, Nepal, over one ablation season. Observations show that out of four cliffs, three different patterns of evolution emerge: (i) reclining cliffs that flatten during the ablation season; (ii) stable cliffs that maintain a self-similar geometry; and (iii) growing cliffs, expanding laterally. We use the insights from this unique data set to develop a 3-D model of cliff backwasting and evolution that is validated against observations and an independent data set of volume losses. The model includes ablation at the cliff surface driven by energy exchange with the atmosphere, reburial of cliff cells by surrounding debris, and the effect of adjacent ponds. The cliff geometry is updated monthly to account for the modifications induced by each of those processes. Model results indicate that a major factor affecting the survival of steep cliffs is the coupling with ponded water at its base, which prevents progressive flattening and possible disappearance of a cliff. The radial growth observed at one cliff is explained by higher receipts of longwave and shortwave radiation, calculated taking into account atmospheric fluxes, shading, and the emission of longwave radiation from debris surfaces. The model is a clear step forward compared to existing static approaches that calculate atmospheric melt over an invariant cliff geometry and can be used for long-term simulations of cliff evolution and to test existing hypotheses about cliffs' survival.
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Calle, L., Canadell, J. G., Patra, P., Ciais, P., Ichii, K., Tian, H. Q., et al. (2016). Regional carbon fluxes from land use and land cover change in Asia, 1980-2009. Environmental Research Letters, 11(7).
Abstract: We present a synthesis of the land-atmosphere carbon flux from land use and land cover change (LULCC) in Asia using multiple data sources and paying particular attention to deforestation and forest regrowth fluxes. The data sources are quasi-independent and include the U.N. Food andAgriculture Organization-Forest Resource Assessment (FAO-FRA2015; country-level inventory estimates), the Emission Database for Global Atmospheric Research (EDGARv4.3), the 'Houghton' bookkeeping model that incorporates FAO-FRA data, an ensemble of 8 state-of-the-art Dynamic Global Vegetation Models (DGVM), and 2 recently published independent studies using primarily remote sensing techniques. The estimates are aggregated spatially to Southeast, East, and South Asia and temporally for three decades, 1980-1989, 1990-1999 and 2000-2009. Since 1980, net carbon emissions from LULCC in Asia were responsible for 20%-40% of global LULCC emissions, with emissions from Southeast Asia alone accounting for 15%-25% of global LULCC emissions during the same period. In the 2000s and for all Asia, three estimates (FAO-FRA, DGVM, Houghton) were in agreement of a net source of carbon to the atmosphere, with mean estimates ranging between 0.24 to 0.41 Pg Cyr(-1), whereas EDGARv4.3 suggested a net carbon sink of -0.17 Pg Cyr(-1). Three of 4 estimates suggest that LULCC carbon emissions declined by at least 34% in the preceding decade (1990-2000). Spread in the estimates is due to the inclusion of different flux components and their treatments, showing the importance to include emissions from carbon rich peatlands and land management, such as shifting cultivation and wood harvesting, which appear to be consistently underreported.
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Callens, D., Drews, R., Witrant, E., Philippe, M., & Pattyn, F. (2016). Temporally stable surface mass balance asymmetry across an ice rise derived from radar internal reflection horizons through inverse modeling. Journal Of Glaciology, 62(233), 525–534.
Abstract: Ice rises are locally grounded parts of Antarctic ice shelves that play an important role in regulating ice flow from the continent towards the ocean. Because they protrude out of the otherwise horizontal ice shelves, ice rises induce an orographic uplift of the atmospheric flow, resulting in an asymmetric distribution of the surface mass balance (SMB). Here, we combine younger and older internal reflection horizons (IRHs) from radar to quantify this distribution in time and space across Derwael Ice Rise (DIR), Dronning Maud Land, Antarctica. We employ two methods depending on the age of the IRHs, i.e. the shallow layer approximation for the younger IRHs near the surface and an optimization technique based on an ice flow model for the older IRHs. We identify an SMB ratio of 2.5 between the flanks and the ice divide with the SMB ranging between 300 and 750 kg m(-2) a(-1). The SMB maximum is located on the upwind side, similar to 4 km offset to today's topographic divide. The large-scale asymmetry is consistently observed in time until 1966. The SMB from older IRHs is less-well constrained, but the asymmetry has likely persisted for >ka, indicating that DIR has been a stable features over long time spans.
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Carton, X., Ciani, D., Verron, J., Reinaud, J., & Sokolovskiy, M. (2016). Vortex merger in surface quasi-geostrophy. Geophysical And Astrophysical Fluid Dynamics, 110(1), 1–22.
Abstract: The merger of two identical surface temperature vortices is studied in the surface quasi-geostrophic model. The motivation for this study is the observation of the merger of submesoscale vortices in the ocean. Firstly, the interaction between two point vortices, in the absence or in the presence of an external deformation field, is investigated. The rotation rate of the vortices, their stationary positions and the stability of these positions are determined. Then, a numerical model provides the steady states of two finite-area, constant-temperature, vortices. Such states are less deformed than their counterparts in two-dimensional incompressible flows. Finally, numerical simulations of the nonlinear surface quasi-geostrophic equations are used to investigate the finite-time evolution of initially identical and symmetric, constant temperature vortices. The critical merger distance is obtained and the deformation of the vortices before or after merger is determined. The addition of external deformation is shown to favor or to oppose merger depending on the orientation of the vortex pair with respect to the strain axes. An explanation for this observation is proposed. Conclusions are drawn towards an application of this study to oceanic vortices.
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Casado, M., Landais, A., Masson-Delmotte, V., Genthon, C., Kerstel, E., Kassi, S., et al. (2016). Continuous measurements of isotopic composition of water vapour on the East Antarctic Plateau. Atmospheric Chemistry And Physics, 16(13), 8521–8538.
Abstract: Water stable isotopes in central Antarctic ice cores are critical to quantify past temperature changes. Accurate temperature reconstructions require one to understand the processes controlling surface snow isotopic composition. Isotopic fractionation processes occurring in the atmosphere and controlling snowfall isotopic composition are well understood theoretically and implemented in atmospheric models. However, post-deposition processes are poorly documented and understood. To quantitatively interpret the isotopic composition of water archived in ice cores, it is thus essential to study the continuum between surface water vapour, precipitation, surface snow and buried snow. Here, we target the isotopic composition of water vapour at Concordia Station, where the oldest EPICA Dome C ice cores have been retrieved. While snowfall and surface snow sampling is routinely performed, accurate measurements of surface water vapour are challenging in such cold and dry conditions. New developments in infrared spectroscopy enable now the measurement of isotopic composition in water vapour traces. Two infrared spectrometers have been deployed at Concordia, allowing continuous, in situ measurements for 1 month in December 2014-January 2015. Comparison of the results from infrared spectroscopy with laboratory measurements of discrete samples trapped using cryo-genic sampling validates the relevance of the method to measure isotopic composition in dry conditions. We observe very large diurnal cycles in isotopic composition well correlated with temperature diurnal cycles. Identification of different behaviours of isotopic composition in the water vapour associated with turbulent or stratified regime indicates a strong impact of meteorological processes in local vapour/snow interaction. Even if the vapour isotopic composition seems to be, at least part of the time, at equilibrium with the local snow, the slope of delta D against delta O-18 prevents us from identifying a unique origin leading to this isotopic composition.
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Casse, C., Gosset, M., Vischel, T., Quantin, G., & Tanimoun, B. A. (2016). Model-based study of the role of rainfall and land use-land cover in the changes in the occurrence and intensity of Niger red floods in Niamey between 1953 and 2012. Hydrology And Earth System Sciences, 20(7), 2841–2859.
Abstract: Since 1950, the Niger River basin has gone through three main climatic periods: a wet period (1950-1960), an extended drought (1970-1980) and since 1990 a recent partial recovery of annual rainfall. Hydrological changes co-occur with these rainfall fluctuations. In most of the basin, the rainfall deficit caused an enhanced discharge deficit, but in the Sahelian region the runoff increased despite the rainfall deficit. Since 2000 the Sahelian part of the Niger has been hit by an increase of flood hazards during the so-called red flood period. In Niamey city, the highest river levels and the longest flooded period ever recorded occurred in 2003, 2010, 2012 and 2013, with heavy casualties and property damage. The reasons for these changes, and the relative role of climate versus land use-land cover (LULC) changes are still debated and are investigated in this paper. The evolution of the Niger red flood in Niamey from 1950 to 2012 is analysed based on long-term records of rainfall (three data sets based on in situ and/or satellite data) and discharge, and a hydrological model. The model is first run with the present LULC conditions in order to analyse solely the effect of rainfall variability. The impact of LULC and drainage area modification is investigated in a second step. The simulations based on the current surface conditions are able to reproduce the observed trend in the red flood occurrence and intensity since the 1980s. This has been verified with three independent rainfall data sets and implies that rainfall variability is the main driver for the red flood intensification observed over the last 30 years. The simulation results since 1953 have revealed that LULC and drainage area changes need to be invoked to explain the changes over a 60-year period.
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Causse, B., Spadini, L., Sarret, G., Faure, A., Travelet, C., Madern, D., et al. (2016). Xanthan Exopolysaccharide: Cu2+ Complexes Affected from the pH-Dependent Conformational State; Implications for Environmentally Relevant Biopolymers. Environmental Science & Technology, 50(7), 3477–3485.
Abstract: The conformational impact of environmental biopolymers on metal sorption was studied through Cu sorption on xanthan. The apparent Cu2+ complexation constant (logK; Cu2+ + L- <-> CuL+) decreased from 2.9 +/- 0.1 at pH 3.5 to 2.5 +/- 0.1 at pH 5.5 (ionic strength I = 0.1). This behavior is in apparent contradiction with basic thermodynamics, as usually the higher the pH the more cations bind. Our combined titration, circular dichroism and dynamic light scattering study indicated that the change observed in Cu bond strength relates to a conformational change of the structure of xanthan, which generates more chelating sites at pH 3.5 than at pH 5.5. This hypothesis was validated by the fact that the Cu sorption constants on xanthan were always higher than those measured on a mixture of pyruvic and glucuronic acids (logK = 2.2), which are the two constitutive ligands present in the xanthan monomer. This study shows the role of the structural conformation of natural biopolymers in metal bond strength. This finding may help to better predict the fate of Cu and other metals in acidic environmental settings such as aquatic media affected by acid mine drainage, as well as peats and acidic soils, and to better define optimal conditions for bioremediation processes.
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Cavalli, F., Alastuey, A., Areskoug, H., Ceburnis, D., Cech, J., Genberg, J., et al. (2016). A European aerosol phenomenology-4: Harmonized concentrations of carbonaceous aerosol at 10 regional background sites across Europe. Atmospheric Environment, 144, 133–145.
Abstract: Although particulate organic and elemental carbon (OC and EC) are important constituents of the suspended atmospheric particulate matter (PM), measurements of OC and EC are much less common and More uncertain than measurements of e.g. the ionic components of PM. In the framework of atmospheric research infrastructures supported by the European Union, actions have been undertaken to determine and mitigate sampling artefacts, and assess the comparability of OC and EC data obtained in a network of 10 atmospheric observatories across Europe. Positive sampling artefacts (from 0:4 to 2.8 μg C/m(3)) and analytical discrepancies (between -50% and +40% for the EC/TC ratio) have been taken into account to generate a robust data set, from which we established the phenomenology of carbonaceous aerosols at regional background sites in Europe. Across the network, TC and EC annual average concentrations range from 0.4 to 9 μg C/m(3), and from 0.1 to 2 μg C/m(3), respectively. TC/PM10 annual mean ratios range from 0.11 at a Mediterranean site to 0.34 at the most polluted continental site, and TC/PM2.5 ratios are slightly greater at all sites (0.15-0.42). EC/TC annual mean ratios range from 0.10 to 0.22, and do not depend much on PM concentration levels, especially in winter. Seasonal variations in PM and TC concentrations, and in TC/PM and EC/TC ratios, differ across the network, which can be explained by seasonal changes in PM source contributions at some sites. (C) 2016 The Authors. Published by Elsevier Ltd.
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Cavitte, M. G. P., Blankenship, D. D., Young, D. A., Schroeder, D. M., Parrenin, F., Lemeur, E., et al. (2016). Deep radiostratigraphy of the East Antarctic plateau: connecting the Dome C and Vostok ice core sites. Journal Of Glaciology, 62(232), 323–334.
Abstract: Several airborne radar-sounding surveys are used to trace internal reflections around the European Project for Ice Coring in Antarctica Dome C and Vostok ice core sites. Thirteen reflections, spanning the last two glacial cycles, are traced within 200 km of Dome C, a promising region for million-year-old ice, using the University of Texas Institute for Geophysics High-Capacity Radar Sounder. This provides a dated stratigraphy to 2318 m depth at Dome C. Reflection age uncertainties are calculated from the radar range precision and signal-to-noise ratio of the internal reflections. The radar stratigraphy matches well with the Multichannel Coherent Radar Depth Sounder (MCoRDS) radar stratigraphy obtained independently. We show that radar sounding enables the extension of ice core ages through the ice sheet with an additional radar-related age uncertainty of similar to 1/3-1/2 that of the ice cores. Reflections are extended along the Byrd-Totten Glacier divide, using University of Texas/Technical University of Denmark and MCoRDS surveys. However, core-to-core connection is impeded by pervasive aeolian terranes, and Lake Vostok's influence on reflection geometry. Poor radar connection of the two ice cores is attributed to these effects and suboptimal survey design in affected areas. We demonstrate that, while ice sheet internal radar reflections are generally isochronal and can be mapped over large distances, careful survey planning is necessary to extend ice core chronologies to distant regions of the East Antarctic ice sheet.
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Cazenave, F., Gosset, M., Kacou, M., Alcoba, M., Fontaine, E., Duroure, C., et al. (2016). Characterization of Hydrometeors in Sahelian Convective Systems with an X-Band Radar and Comparison with In Situ Measurements. Part I: Sensitivity of Polarimetric Radar Particle Identification Retrieval and Case Study Evaluation. Journal Of Applied Meteorology And Climatology, 55(2), 231–249.
Abstract: The particle identification scheme developed by Dolan and Rutledge for X-band polarimetric radar is tested for the first time in Africa and compared with in situ measurements. The data were acquired during the Megha-Tropiques mission algorithm-validation campaign that occurred in Niger in 2010. The radar classification is compared with the in situ observations gathered by an instrumented aircraft for the 13 August 2010 squall-line case. An original approach has been developed for the radar-in situ comparison: it consists of simulating synthetic radar variables from the microphysical-probe information and comparing the two datasets in a common “radar space.” The consistency between the two types of observation is good considering the differences in sampling illustrated in the paper. The time evolution of the hydrometeor types and their relative proportion in the convective and stratiform regions are analyzed. The farther away from the convection one looks, the more aggregation dominates, riming diminishes, and hydrometeors are less dense. Particle identification based on the polarimetric radar will be applied to a 5-yr African dataset in the future.
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Cea, L., Legout, C., Grangeon, T., & Nord, G. (2016). Impact of model simplifications on soil erosion predictions: application of the GLUE methodology to a distributed event-based model at the hillslope scale. Hydrological Processes, 30(7), 1096–1113.
Abstract: In this paper, we analyse how the performance and calibration of a distributed event-based soil erosion model at the hillslope scale is affected by different simplifications on the parameterizations used to compute the production of suspended sediment by rainfall and runoff. Six modelling scenarios of different complexity are used to evaluate the temporal variability of the sedimentograph at the outlet of a 60m long cultivated hillslope. The six scenarios are calibrated within the generalized likelihood uncertainty estimation framework in order to account for parameter uncertainty, and their performance is evaluated against experimental data registered during five storm events. The Nash-Sutcliffe efficiency, percent bias and coverage performance ratios show that the sedimentary response of the hillslope in terms of mass flux of eroded soil can be efficiently captured by a model structure including only two soil erodibility parameters, which control the rainfall and runoff production of suspended sediment. Increasing the number of parameters makes the calibration process more complex without increasing in a noticeable manner the predictive capability of the model. Copyright (c) 2015 John Wiley & Sons, Ltd.
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Chang, J. F., Ciais, P., Herrero, M., Havlik, P., Campioli, M., Zhang, X. Z., et al. (2016). Combining livestock production information in a process-based vegetation model to reconstruct the history of grassland management. Biogeosciences, 13(12), 3757–3776.
Abstract: Grassland management type (grazed or mown) and intensity (intensive or extensive) play a crucial role in the greenhouse gas balance and surface energy budget of this biome, both at field scale and at large spatial scale. However, global gridded historical information on grassland management intensity is not available. Combining modelled grass-biomass productivity with statistics of the grass-biomass demand by livestock, we reconstruct gridded maps of grassland management intensity from 1901 to 2012. These maps include the minimum area of managed vs. maximum area of unmanaged grasslands and the fraction of mown vs. grazed area at a resolution of 0.5A degrees by 0.5A degrees. The grass-biomass demand is derived from a livestock dataset for 2000, extended to cover the period 1901-2012. The grass-biomass supply (i.e. forage grass from mown grassland and biomass grazed) is simulated by the process-based model ORCHIDEE-GM driven by historical climate change, risingaEuro-CO2 concentration, and changes in nitrogen fertilization. The global area of managed grassland obtained in this study increases from 6.1aEuro-aEuro parts per thousand x aEuro-10(6)aEuro-km(2) in 1901 to 12.3aEuro-aEuro parts per thousand x aEuro-10(6)aEuro-km(2) in 2000, although the expansion pathway varies between different regions. ORCHIDEE-GM also simulated augmentation in global mean productivity and herbage-use efficiency over managed grassland during the 20th century, indicating a general intensification of grassland management at global scale but with regional differences. The gridded grassland management intensity maps are model dependent because they depend on modelled productivity. Thus specific attention was given to the evaluation of modelled productivity against a series of observations from site-level net primary productivity (NPP) measurements to two global satellite products of gross primary productivity (GPP) (MODIS-GPP and SIF data). Generally, ORCHIDEE-GM captures the spatial pattern, seasonal cycle, and interannual variability of grassland productivity at global scale well and thus is appropriate for global applications presented here.
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Chardon, J., Favre, A. C., & Hingray, B. (2016). Effects of Spatial Aggregation on the Accuracy of Statistically Downscaled Precipitation Predictions. Journal Of Hydrometeorology, 17(5), 1561–1578.
Abstract: The effects of spatial aggregation on the skill of downscaled precipitation predictions obtained over an 8 x 8 km(2) grid from circulation analogs for metropolitan France are explored. The Safran precipitation reanalysis and an analog approach are used to downscale the precipitation where the predictors are taken from the 40-yr ECMWF Re-Analysis (ERA-40). Prediction skillcharacterized by the continuous ranked probability score (CRPS), its skill score, and its decompositionis generally found to continuously increase with spatial aggregation. The increase is also greater when the spatial correlation of precipitation is lower. This effect is shown from an empirical experiment carried out with a fully uncorrelated dataset, generated from a space-shake experiment, where the precipitation time series of each grid cell is randomly assigned to another grid cell. The underlying mechanisms of this effect are further highlighted with synthetic predictions simulated using a stochastic spatiotemporal generator. It is shown 1) that the skill increase with spatial aggregation jointly results from the higher and lower values obtained for the resolution and uncertainty terms of the CRPS decomposition, respectively, and 2) that the lower spatial correlation of precipitation is beneficial for both terms. Results obtained for France suggest that the prediction skill indefinitely increases with aggregation. A last experiment is finally proposed to show that this is not expected to be always the case. A prediction skill optimum is, for instance, obtained when the mean areal precipitation is estimated over a region where local precipitations of different grid cells originate from different underlying meteorological processes.
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Charrois, L., Cosme, E., Dumont, M., Lafaysse, M., Morin, S., Libois, Q., et al. (2016). On the assimilation of optical reflectances and snow depth observations into a detailed snowpack model. Cryosphere, 10(3), 1021–1038.
Abstract: This paper examines the ability of optical reflectance data assimilation to improve snow depth and snow water equivalent simulations from a chain of models with the SAFRAN meteorological model driving the detailed multilayer snowpack model Crocus now including a two-stream radiative transfer model for snow, TARTES. The direct use of reflectance data, allowed by TARTES, instead of higher level snow products, mitigates uncertainties due to commonly used retrieval algorithms.Data assimilation is performed with an ensemble-based method, the Sequential Importance Resampling Particle filter, to represent simulation uncertainties. In snowpack modeling, uncertainties of simulations are primarily assigned to meteorological forcings. Here, a method of stochastic perturbation based on an autoregressive model is implemented to explicitly simulate the consequences of these uncertainties on the snowpack estimates.Through twin experiments, the assimilation of synthetic spectral reflectances matching the MODerate resolution Imaging Spectroradiometer (MODIS) spectral bands is examined over five seasons at the Col du Lautaret, located in the French Alps. Overall, the assimilation of MODIS-like data reduces by 45aEuro-% the root mean square errors (RMSE) on snow depth and snow water equivalent. At this study site, the lack of MODIS data on cloudy days does not affect the assimilation performance significantly. The combined assimilation of MODIS-like reflectances and a few snow depth measurements throughout the 2010/2011 season further reduces RMSEs by roughly 70aEuro-%. This work suggests that the assimilation of optical reflectances has the potential to become an essential component of spatialized snowpack simulation and forecast systems. The assimilation of real MODIS data will be investigated in future works.
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Chikhi, N., Clavier, R., Laurent, J. P., Fichot, F., & Quintard, M. (2016). Pressure drop and average void fraction measurements for two-phase flow through highly permeable porous media. Annals Of Nuclear Energy, 94, 422–432.
Abstract: The modeling of pressure drop for two-phase flows through porous media is a key point to assess the coolability of debris beds resulting from nuclear severe accidents. Models involve several parameters which are non-linear functions of the void fraction, e.g. relative permeabilities. Their identification requires that experimental data include the measurement of void fraction. This paper presents a new technique developed to reach this objective. The method is based on the use of a capacitance probe and has been validated by comparison with a weighing method. The validation has shown that the accuracy is better than 10%. The measurement device has been implemented in the CALIDE facility, at IRSN, which has been designed to-perform air-water flow through debris bed. Tests have been carried out with beds made of single size 4 mm and 8 mm beads. Measurements of pressure drop and average void fraction are reported in the paper, for air and water flow rates representative of flows that would result of either the reflooding of the damaged core or the cooling of corium debris in a stagnant pool of water. Finally, the pressure drop models used in severe accident simulation codes, based on generalizations of the single-phase Ergun law, have been assessed against the new data. It has been observed that generalized Ergun laws including an interfacial drag term accurately predict the pressure drop and the void fraction for flows with a zero net water velocity. (C) 2016 Elsevier Ltd. All rights reserved.
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Chiron, S., & Duwig, C. (2016). Biotic nitrosation of diclofenac in a soil aquifer system (Katari watershed, Bolivia). Science Of The Total Environment, 565, 473–480.
Abstract: Up till now, the diclofenac (DCF) transformation into its nitrogen-derivatives, N-nitroso-DCF (NO-DCF) and 5-nitro-DCF (NO2-DCF), has been mainly investigated in waste water treatment plant under nitrification or denitrification processes. This work reports, for the first time, an additional DCF microbial mediated nitrosation pathway of DCF in soil under strictly anoxic conditions probably involving codenitrification processes and fungal activities. This transformation pathway was investigated by using field observations data at a soil aquifer system (Katari watershed, Bolivia) and by carrying out soil slurry batch experiments. It was also observed for diphenylamine (DPA). Field measurements revealed the occurrence of NO-DCF, NO2-DCF and NO-DPA in groundwater samples at concentration levels in the 6-68 s/L range. These concentration levels are more significant than those previously reported in waste water treatment plant effluents taking into account dilution processes in soil. Interestingly, the p-benzoquinone imine of 5-OH-DCF was also found to be rather stable in surface water. In laboratory batch experiments under strictly anoxic conditions, the transformation of DCF and DPA into their corresponding N-nitroso derivatives was well correlated to denitrification processes. It was also observed that NO-DCF evolved into NO2-DCF while NO-DPA was stable. In vitro experiments showed that the Fisher-Hepp rearrangement could not account for NO2-DCF formation. One possible mechanism might be that NO-DCF underwent spontaneous NO loss to give the resulting intermediates diphenylaminyl radical or nitrenium cation which might evolve into NO2-DCF in presence of NO2 radical or nitrite ion, respectively. (C) 2016 Elsevier B.V. All rights reserved.
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Chuvieco, E., Yue, C., Heil, A., Mouillot, F., Alonso-Canas, I., Padilla, M., et al. (2016). A new global burned area product for climate assessment of fire impacts. Global Ecology And Biogeography, 25(5), 619–629.
Abstract: Aim This paper presents a new global burned area (BA) product developed within the framework of the European Space Agency's Climate Change Initiative (CCI) programme, along with a first assessment of its potentials for atmospheric and carbon cycle modelling. Innovation Methods are presented for generating a new global BA product, along with a comparison with existing BA products, in terms of BA extension, fire size and shapes and emissions derived from biomass burnings. Main conclusions Three years of the global BA product were produced, accounting for a total BA of between 360 and 380 Mha year(-1). General omission and commission errors for BA were 0.76 and 0.64, but they decreased to 0.51 and 0.52, respectively, for sites with more than 10% BA. Intercomparison with other existing BA datasets found similar spatial and temporal trends, mainly with the BA included in the Global Fire Emissions Database (GFED4), although regional differences were found (particularly in the 2006 fires of eastern Europe). The simulated carbon emissions from biomass burning averaged 2.1 Pg C year(-1).
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Ciani, D., Carton, X., & Verron, J. (2016). On the merger of subsurface isolated vortices. Geophysical And Astrophysical Fluid Dynamics, 110(1), 23–49.
Abstract: Vortex merger is a phenomenon characterizing the whole class of geophysical vortices, from atmospheric storms and large oceanic eddies up to small scale turbulence. Here we focus on the merger of subsurface oceanic anticyclones in an idealized primitive equations model. This study has been motivated by past and recent observations of colliding lens-like anticyclones off of Gibraltar Strait. The critical conditions for merger (critical merger distance and time needed for merger) are determined. We will show that the predictions of classical two-dimensional merger are not verified for subsurface isolated vortices. For instance, critical merger distances will be reduced because of the vortex potential vorticity (PV) structure. The post-merger characteristics of the vortex (radius, extension and PV), are also determined. Merger-related effects, like production of peripheral filaments and small-scale eddies are also investigated and suggest the contribution of merger in both direct and inverse energy cascades.
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Crichton, K. A., Bouttes, N., Roche, D. M., Chappellaz, J., & Krinner, G. (2016). Permafrost carbon as a missing link to explain CO2 changes during the last deglaciation. Nature Geoscience, 9(9), 683–+.
Abstract: The atmospheric concentration of CO2 increased from 190 to 280 ppm between the last glacial maximum 21,000 years ago and the pre-industrial era(1,2). This CO2 rise and its timing have been linked to changes in the Earth's orbit, ice sheet configuration and volume, and ocean carbon storage(2,3). The ice-core record of delta(CO2)-C-13 (refs 2,4) in the atmosphere can help to constrain the source of carbon, but previous modelling studies have failed to capture the evolution of delta(CO2)-C-13 over this period(5). Here we show that simulations of the last deglaciation that include a permafrost carbon component can reproduce the ice core records between 21,000 and 10,000 years ago. We suggest that thawing permafrost, due to increasing summer insolation in the northern hemisphere, is the main source of CO2 rise between 17,500 and 15,000 years ago, a period sometimes referred to as the Mystery Interval(6). Together with a fresh water release into the North Atlantic, much of the CO2 variability associated with the Bolling-Allerod/Younger Dryas period similar to 15,000 to similar to 12,000 years ago can also be explained. In simulations of future warming we find that the permafrost carbon feedback increases global mean temperature by 10-40% relative to simulations without this feedback, with the magnitude of the increase dependent on the evolution of anthropogenic carbon emissions.
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Dansereau, V., Weiss, J., Saramito, P., & Lattes, P. (2016). A Maxwell elasto-brittle rheology for sea ice modelling. Cryosphere, 10(3), 1339–1359.
Abstract: A new rheological model is developed that builds on an elasto-brittle (EB) framework used for sea ice and rock mechanics, with the intent of representing both the small elastic deformations associated with fracturing processes and the larger deformations occurring along the faults/leads once the material is highly damaged and fragmented. A viscous-like relaxation term is added to the linear-elastic constitutive law together with an effective viscosity that evolves according to the local level of damage of the material, like its elastic modulus. The coupling between the level of damage and both mechanical parameters is such that within an undamaged ice cover the viscosity is infinitely large and deformations are strictly elastic, while along highly damaged zones the elastic modulus vanishes and most of the stress is dissipated through permanent deformations. A healing mechanism is also introduced, counterbalancing the effects of damaging over large timescales. In this new model, named Maxwell-EB after the Maxwell rheology, the irreversible and reversible deformations are solved for simultaneously; hence drift velocities are defined naturally. First idealized simulations without advection show that the model reproduces the main characteristics of sea ice mechanics and deformation: strain localization, anisotropy, intermittency and associated scaling laws.
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de Lavergne, C., Madec, G., Le Sommer, J., Nurser, A. J. G., & Garabato, A. C. N. (2016). On the Consumption of Antarctic Bottom Water in the Abyssal Ocean. Journal Of Physical Oceanography, 46(2), 635–661.
Abstract: The abyssal ocean is primarily filled by cold, dense waters formed around Antarctica and collectively referred to as Antarctic Bottom Water (AABW). At steady state, AABW must be consumed in the ocean interior at the same rate it is produced, but how and where this consumption is achieved remains poorly understood. Here, estimates of abyssal water mass transformation by geothermal heating and parameterized internal wave-driven mixing are presented. This study uses maps of the energy input to internal waves by tidal and geostrophic motions interacting with topography combined with assumptions about the distribution of energy dissipation to evaluate dianeutral transports induced by breaking internal tides and lee waves. Geothermal transformation is assessed based on a map of geothermal heat fluxes. Under the hypotheses underlying the constructed climatologies of buoyancy fluxes, the authors calculate that locally dissipating internal tides and geothermal heating contribute, respectively, about 8 and 5 Sverdrups (Sv; 1 Sv equivalent to 10(6) m(3) s(-1)) of AABW consumption (upwelling), mostly north of 30 degrees S. In contrast, parameterized lee wave-driven mixing causes significant transformation only in the Southern Ocean, where it forms about 3 Sv of AABW, decreasing the mean density but enhancing the northward flow of abyssal waters. The possible role of remotely dissipating internal tides in complementing AABW consumption is explored based on idealized distributions of mixing energy. Depending mostly on the chosen vertical structure, such mixing could drive 1 to 28 Sv of additional AABW upwelling, highlighting the need to better constrain the spatial distribution of remote dissipation. Though they carry large uncertainties, these climatological transformation estimates shed light on the qualitative functioning and key unknowns of the diabatic overturning.
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de Lavergne, C., Madec, G., Le Sommer, J., Nurser, A. J. G., & Garabato, A. C. N. (2016). The Impact of a Variable Mixing Efficiency on the Abyssal Overturning. Journal Of Physical Oceanography, 46(2), 663–681.
Abstract: In studies of ocean mixing, it is generally assumed that small-scale turbulent overturns lose 15%-20% of their energy in eroding the background stratification. Accumulating evidence that this energy fraction, or mixing efficiency R-f, significantly varies depending on flow properties challenges this assumption, however. Here, the authors examine the implications of a varying mixing efficiency for ocean energetics and deep-water mass transformation. Combining current parameterizations of internal wave-driven mixing with a recent model expressing R-f as a function of a turbulence intensity parameter Re-b = epsilon(nu)/nu N-2, the ratio of dissipation epsilon(nu) to stratification N-2 and molecular viscosity nu, it is shown that accounting for reduced mixing efficiencies in regions of weak stratification or energetic turbulence (high Re-b) strongly limits the ability of breaking internal waves to supply oceanic potential energy and drive abyssal upwelling. Moving from a fixed R-f = 1/6 to a variable efficiency R-f(Re-b) causes Antarctic Bottom Water upwelling induced by locally dissipating internal tides and lee waves to fall from 9 to 4 Sverdrups (Sv; 1 Sv equivalent to 10(6) m(3) s(-1)) and the corresponding potential energy source to plunge from 97 to 44 GW. When adding the contribution of remotely dissipating internal tides under idealized distributions of energy dissipation, the total rate of Antarctic Bottom Water upwelling is reduced by about a factor of 2, reaching 5-15 Sv, compared to 10-33 Sv for a fixed efficiency. The results suggest that distributed mixing, overflow-related boundary processes, and geothermal heating are more effective in consuming abyssal waters than topographically enhanced mixing by breaking internal waves. These calculations also point to the importance of accurately constraining R-f (Re-b) and including the effect in ocean models.
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Debionne, S., Ruin, I., Shabou, S., Lutoff, C., & Creutin, J. D. (2016). Assessment of commuters' daily exposure to flash flooding over the roads of the Gard region, France. Journal Of Hydrology, 541, 636–648.
Abstract: Flash floods are responsible for a majority of natural disaster fatalities in the USA and Europe and most of them are vehicle-related. If human exposure to flood is generally assessed through the number of inhabitants per buildings located in flood prone zone, it is clear that this number varies dramatically throughout the day as people move from place to place to follow their daily program of activities. Knowing the number of motorists exposed on flood prone road sections or the factors determining their exposure would allow providing a more realistic evaluation of the degree of exposure. In order to bridge this gap and provide emergency managers with methods to assess the risk level for motorists, this paper describes two methods, a simple rough-and-ready estimate and a traffic attribution method, and applies both of them on datasets of the Gard departement, an administrative region of Southern France with about 700000 inhabitants over 5875 km(2). The first method to obtain an overall estimation of motorists flood exposure is to combine (i) the regional density of roads and rivers to derive a count of potential road cuts and (ii) the average daily kilometers driven by commuters of the study area to derive the number of people passing these potential cuts. If useful as a first approximation, this method fails to capture the spatial heterogeneities introduced by the geometry of river and road networks and the distribution of commuters' itineraries. To address this point, this paper (i) uses a pre-established detailed identification of road cuts (Naulin et al., 2013) and (ii) applies a well-known traffic attribution method to existing and freely available census datasets. Both methods indicate that commuters' exposure is much larger than the number of commuters itself, illustrating the risk amplification effect of mobility. Comparing the results from both methods shows that (i) the road network geometry plays a significant role in reducing the risk of river-road dangerous intersections and (ii) not all commuters are equally exposed. Evidently commuters who have longer routes are more exposed, but residents of rural municipalities as well as professionals with highly qualified jobs are also more exposed. Finally, these exposure assessment methods applied to the Gard area allows locating road sections where commuters' exposure to flood is high. It also sets the first step toward the implementation of a modeling platform able to combine the estimation of daily travel patterns exposure and behavioral response of motorists to road flooding, a critical input for emergency services and services in charge of the management of road networks in flash flood prone areas. (C) 2016 Elsevier B.V. All rights reserved.
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Dedieu, J. P., Carlson, B. Z., Bigot, S., Sirguey, P., Vionnet, V., & Choler, P. (2016). On the Importance of High-Resolution Time Series of Optical Imagery for Quantifying the Effects of Snow Cover Duration on Alpine Plant Habitat. Remote Sensing, 8(6).
Abstract: We investigated snow cover dynamics using time series of moderate (MODIS) to high (SPOT-4/5, Landsat-8) spatial resolution satellite imagery in a 3700 km(2) region of the southwestern French Alps. Our study was carried out in the context of the SPOT (Take 5) Experiment initiated by the Centre National d'Etudes Spatiales (CNES), with the aim of exploring the utility of high spatial and temporal resolution multispectral satellite imagery for snow cover mapping and applications in alpine ecology. Our three objectives were: (i) to validate remote sensing observations of first snow free day derived from the Normalized Difference Snow Index (NDSI) relative to ground-based measurements; (ii) to generate regional-scale maps of first snow free day and peak standing biomass derived from the Normalized Difference Vegetation Index (NDVI); and (iii) to examine the usefulness of these maps for habitat mapping of herbaceous vegetation communities above the tree line. Imagery showed strong agreement with ground-based measurements of snow melt-out date, although R-2 was higher for SPOT and Landsat time series (0.92) than for MODIS (0.79). Uncertainty surrounding estimates of first snow free day was lower in the case of MODIS, however (+/- 3 days as compared to +/- 9 days for SPOT and Landsat), emphasizing the importance of high temporal as well as high spatial resolution for capturing local differences in snow cover duration. The main floristic differences between plant communities were clearly visible in a two-dimensional habitat template defined by the first snow free day and NDVI at peak standing biomass, and these differences were accentuated when axes were derived from high spatial resolution imagery. Our work demonstrates the enhanced potential of high spatial and temporal resolution multispectral imagery for quantifying snow cover duration and plant phenology in temperate mountain regions, and opens new avenues to examine to what extent plant community diversity and functioning are controlled by snow cover duration.
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Derin, Y., Anagnostou, E., Berne, A., Borga, M., Boudevillain, B., Buytaert, W., et al. (2016). Multiregional Satellite Precipitation Products Evaluation over Complex Terrain. Journal Of Hydrometeorology, 17(6), 1817–1836.
Abstract: An extensive evaluation of nine global-scale high-resolution satellite-based rainfall (SBR) products is performed using aminimumof 6 years (within the period of 2000-13) of reference rainfall data derived from rain gauge networks in nine mountainous regions across the globe. The SBR products are compared to a recently released global reanalysis dataset from the European Centre for Medium-Range Weather Forecasts (ECMWF). The study areas include the eastern Italian Alps, the Swiss Alps, the western Black Sea of Turkey, the French Cevennes, the Peruvian Andes, the Colombian Andes, the Himalayas over Nepal, the Blue Nile in East Africa, Taiwan, and the U.S. Rocky Mountains. Evaluation is performed at annual, monthly, and daily time scales and 0.258 spatial resolution. The SBR datasets are based on the following retrieval algorithms: Tropical Rainfall Measuring Mission Multisatellite Precipitation Analysis (TMPA), the NOAA/Climate Prediction Center morphing technique (CMORPH), Precipitation Estimation from Remotely Sensed Information Using Artificial Neural Networks (PERSIANN), and Global Satellite Mapping of Precipitation (GSMaP). SBRproducts are categorized into those that include gauge adjustment versus unadjusted. Results show that performance of SBR is highly dependent on the rainfall variability. Many SBR products usually underestimate wet season and overestimate dry season precipitation. The performance of gauge adjustment to the SBR products varies by region and depends greatly on the representativeness of the rain gauge network.
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Despax, A., Perret, C., Garcon, R., Hauet, A., Belleville, A., Le Coz, J., et al. (2016). Considering sampling strategy and cross-section complexity for estimating the uncertainty of discharge measurements using the velocity-area method. Journal Of Hydrology, 533, 128–140.
Abstract: Streamflow time series provide baseline data for many hydrological investigations. Errors in the data mainly occur through uncertainty in gauging (measurement uncertainty) and uncertainty in the determination of the stage-discharge relationship based on gaugings (rating curve uncertainty). As the velocity area method is the measurement technique typically used for gaugings, it is fundamental to estimate its level of uncertainty. Different methods are available in the literature (ISO 748, Q+, IVE), all with their own limitations and drawbacks. Among the terms forming the combined relative uncertainty in measured discharge, the uncertainty component relating to the limited number of verticals often includes a large part of the relative uncertainty. It should therefore be estimated carefully. In ISO 748 standard, proposed values of this uncertainty component only depend on the number of verticals without considering their distribution with respect to the depth and velocity cross-sectional profiles. The Q+ method is sensitive to a user-defined parameter while it is questionable whether the IVE method is applicable to stream-gaugings performed with a limited number of verticals. To address the limitations of existing methods, this paper presents a new methodology, called FLow Analog UnceRtainty Estimation (Fume), to estimate the uncertainty component relating to the limited number of verticals. High-resolution reference gaugings (with 31 and more verticals) are used to assess the uncertainty component through a statistical analysis. Instead of subsampling purely randomly the verticals of these reference stream-gaugings, a subsampling method is developed in a way that mimicks the behavior of a hydrometric technician. A sampling quality index (SQI) is suggested and appears to be a more explanatory variable than the number of verticals. This index takes into account the spacing between verticals and the variation of unit flow between two verticals. To compute the uncertainty component for any routine gauging, the four most similar gaugings among the reference stream-gaugings dataset are selected using an analog approach, where analogy includes both riverbed shape and flow distribution complexity. This new method was applied to 3185 stream-gaugings with various flow conditions and compared with the other methods (ISO 748, IVE, with a simple automated parametrization). Results show that FLAME is overall consistent with the Q+ method but not with ISO 748 and NE methods, which produce clearly overestimated uncertainties fo
r discharge measurements with less than 15 verticals. The FLAURE approach therefore appears to be a consistent method. An advantage is the explicit link made between the estimation of cross-sectional interpolation errors and the study of high-resolution reference gaugings. (C) 2015 Elsevier B.V. All rights reserved. |
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Dieppois, B., Lawler, D. M., Slonosky, V., Massei, N., Bigot, S., Fournier, M., et al. (2016). Multidecadal climate variability over northern France during the past 500 years and its relation to large-scale atmospheric circulation. International Journal Of Climatology, 36(15), 4679–4696.
Abstract: We examine secular changes and multidecadal climate variability on a seasonal scale in northern France over the last 500 years and examine the extent to which they are driven by large-scale atmospheric variability. Multiscale trend analysis and segmentation procedures show statistically significant increases of winter and spring precipitation amounts in Paris since the end of the 19th century. This changes the seasonal precipitation distribution from one with a pronounced summer peak at the end of the Little Ice Age to an almost uniform distribution in the 20th century. This switch is linked to an early warming trend in winter temperature. Changes in spring precipitation are also correlated with winter precipitation for time scales greater than 50 years, which suggests a seasonal persistence. Hydrological modelling results show similar rising trends in river flow for the Seine at Paris. However, such secular trends in the seasonal climatic conditions over northern France are substantially modulated by irregular multidecadal (50-80 years) fluctuations. Furthermore, since the end of the 19th century, we find an increasing variance in multidecadal hydroclimatic winter and spring, and this coincides with an increase in the multidecadal North Atlantic Oscillation (NAO) variability, suggesting a significant influence of large-scale atmospheric circulation patterns. However, multidecadal NAO variability has decreased in summer. Using Empirical Orthogonal Function analysis, we detect multidecadal North Atlantic sea-level pressure anomalies, which are significantly linked to the NAO during the Modern period. In particular, a south-eastward (south-westward) shift of the Icelandic Low (Azores High) drives substantial multidecadal changes in spring. Wetter springs are likely to be driven by potential changes in moisture advection from the Atlantic, in response to northward shifts of North Atlantic storm tracks over European regions, linked to periods of positive NAO. Similar, but smaller, changes in rainfall are observed in winter.
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Domine, F., Barrere, M., & Morin, S. (2016). The growth of shrubs on high Arctic tundra at Bylot Island: impact on snow physical properties and permafrost thermal regime. Biogeosciences, 13(23), 6471–6486.
Abstract: With climate warming, shrubs have been observed to grow on Arctic tundra. Their presence is known to increase snow height and is expected to increase the thermal insulating effect of the snowpack. An important consequence would be the warming of the ground, which will accelerate permafrost thaw, providing an important positive feedback to warming. At Bylot Island (73 degrees N, 80 degrees W) in the Canadian high Arctic where bushes of willows (Salix richardsonii Hook) are growing, we have observed the snow stratigraphy and measured the vertical profiles of snow density, thermal conductivity and specific surface area (SSA) in over 20 sites of high Arctic tundra and in willow bushes 20 to 40 cm high. We find that shrubs increase snow height, but only up to their own height. In shrubs, snow density, thermal conductivity and SSA are all significantly lower than on herb tundra. In shrubs, depth hoar which has a low thermal conductivity was observed to grow up to shrub height, while on herb tundra, depth hoar only developed to 5 to 10 cm high. The thermal resistance of the snowpack was in general higher in shrubs than on herb tundra. More signs of melting were observed in shrubs, presumably because stems absorb radiation and provide hotspots that initiate melting. When melting was extensive, thermal conductivity was increased and thermal resistance was reduced, counteracting the observed effect of shrubs in the absence of melting. Simulations of the effect of shrubs on snow properties and on the ground thermal regime were made with the Crocus snow physics model and the ISBA (Interactions between Soil-Biosphere-Atmosphere) land surface scheme, driven by in situ and reanalysis meteorological data. These simulations did not take into account the summer impact of shrubs. They predict that the ground at 5 cm depth at Bylot Island during the 2014-2015 winter would be up to 13 degrees C warmer in the presence of shrubs. Such warming may however be mitigated by summer effects.
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Domine, F., Barrere, M., & Sarrazin, D. (2016). Seasonal evolution of the effective thermal conductivity of the snow and the soil in high Arctic herb tundra at Bylot Island, Canada. Cryosphere, 10(6), 2573–2588.
Abstract: The values of the snow and soil thermal conductivity, k(snow) and k(soil), strongly impact the thermal regime of the ground in the Arctic, but very few data are available to test model predictions for these variables. We have monitored k(snow) and k(soil) using heated needle probes at Bylot Island in the Canadian High Arctic (73 degrees N, 80 degrees W) between July 2013 and July 2015. Few k(snow) data were obtained during the 2013-2014 winter, because little snow was present. During the 2014-2015 winter k(snow) monitoring at 2, 12 and 22 cm heights and field observations show that a depth hoar layer with k(snow) around 0.02 Wm(-1) K-1 rapidly formed. At 12 and 22 cm, wind slabs with k(snow) around 0.2 to 0.3 Wm(-1) K-1 formed. The monitoring of ksoil at 10 cm depth shows that in thawed soil k(soil) was around 0.7 Wm(-1) K-1, while in frozen soil it was around 1.9 Wm(-1) (K-)1. The transition between both values took place within a few days, with faster thawing than freezing and a hysteresis effect evidenced in the thermal conductivity-liquid water content relationship. The fast transitions suggest that the use of a bimodal distribution of k(soil) for modelling may be an interesting option that deserves further testing. Simulations of k(snow) using the snow physics model Crocus were performed. Contrary to observations, Crocus predicts high k(snow) values at the base of the snowpack (0.12-0.27 Wm(-1) K-1) and low ones in its upper parts (0.02-0.12 Wm(-1) K-1). We diagnose that this is because Crocus does not describe the large upward water vapour fluxes caused by the temperature gradient in the snow and soil. These fluxes produce mass transfer between the soil and lower snow layers to the upper snow layers and the atmosphere. Finally, we discuss the importance of the structure and properties of the Arctic snowpack on subnivean life, as species such as lemmings live under the snow most of the year and must travel in the lower snow layer in search of food.
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Dommergue, A., Martinerie, P., Courteaud, J., Witrant, E., & Etheridge, D. M. (2016). A new reconstruction of atmospheric gaseous elemental mercury trend over the last 60 years from Greenland firn records. Atmospheric Environment, 136, 156–164.
Abstract: This study presents measurements of gaseous elemental mercury (GEM) concentrations in the 80 m of firn air at the international drilling site of NEEM in Greenland (2452 m, 77 degrees 25.8 N, 51 degrees 06.4 W). Using inverse modeling, we were able to reconstruct the atmospheric GEM trend at this Arctic site over the last 60 years. We show discrepancies between this record and the previous firn record of Summit. This could be attributed to experimental biases and/or differences in air mass transport. A multisite inverse model was used to derive an atmospheric scenario reconciling the two firn records. We show that GEM seasonal variations are very limited at these high altitude sites and thus probably unaffected by spring/summer photochemistry. The firn reconstructions suggest an increase of GEM concentrations since the 1950s peaking in the late 1960s and early 1970s. A decrease is then observed with minimum GEM concentrations around 1995-2000. The reconstruction compares well with historical mercury (Hg) releases and recent simulations of atmospheric Hg. Our optimal GEM scenario does not allow to categorically conclude on recent trends for GEM concentrations over the 2000-2010 decade. (C) 2016 Elsevier Ltd. All rights reserved.
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Dufour, A., Zolina, O., & Gulev, S. K. (2016). Atmospheric Moisture Transport to the Arctic: Assessment of Reanalyses and Analysis of Transport Components. Journal Of Climate, 29(14), 5061–5081.
Abstract: The atmospheric water cycle of the Arctic is evaluated via seven global reanalyses and in radiosonde observations covering the 1979-2013 period. In the regional moisture budget, evaporation and precipitation are the least consistent terms among different datasets. Despite the assimilation of radiosoundings, the reanalyses present a tendency to overestimate the moisture transport. Aside from this overestimation, the reanalyses exhibit a remarkable agreement with the radiosondes in terms of spatial and temporal patterns. The northern North Atlantic, subpolar North Pacific, and Labrador Sea stand out as the main gateways for moisture to the Arctic in all reanalyses. Because these regions correspond to the end of the storm tracks, the link between moisture transports and extratropical cyclones is further investigated by decomposing the moisture fluxes in the mean flow and transient eddy parts. In all reanalyses, the former term tends to cancel out when averaged over a latitude circle, leaving the latter to provide the bulk of the midlatitude moisture imports ( 89%-94% at 70 degrees N). Although the Arctic warms faster than the rest of the world, the impact of these changes on its water cycle remains ambiguous. In most datasets, evaporation, precipitation, and precipitable water increase in line with what is expected from a warming signal. At the same time, the moisture transports have decreased in all the reanalyses but not in the radiosonde observations, though none of these trends is statistically significant. The fluxes do not scale with the Clausius-Clapeyron relation because the increasing humidity is not correlated with the meridional wind, particularly near the surface.
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Dupire, S., Bourrier, F., Monnet, J. - M., Bigot, S., Borgniet, L., Berger, F., et al. (2016). Novel quantitative indicators to characterize the protective effect of mountain forests against rockfall. Ecological Indicators, 67, 98–107.
Abstract: Natural hazards are frequent in mountain areas where they regularly cause casualties and damages to human infrastructures. Mountain forests contribute in mitigating these hazards, in particular rockfalls. Assessing the protective effect of a forest against rockfall is a difficult task for both forest managers and rockfall experts. Accurate and simple tools are therefore required to efficiently evaluate the level of protection that results from the presence of forest. This study defines three novel indicators to quantify the protective effect of forests against rockfalls, regarding (1) the reduction of the frequency of rockfalls, (2) the reduction of their maximum intensity, and (3) the combination of the reduction of the frequency and the energy of the rocks. The first two indicators are relevant for rockfall experts whereas the third is mostly interesting for foresters as it summarizes the protective effect of forest. The Rockyfor3D model was adapted and used to simulate rockfalls propagation on 3886 different forest stands located in all the French Alps. The results of the simulations were used to calculate the three indicators for each forest stand. Finally, the relations between the forest structures and compositions and the indicators values were investigated. Our principal result shows that only three forest characteristics are required to accurately predict the indicators and evaluate the protective level of a forest against rockfall. The two first variables correspond to the basal area and the mean diameter at breast height (DBH) of the forest stand which are two parameters commonly used by forest managers. The third characteristic is the length of forest in the maximum slope direction which can be computed with a geographic information system (GIS). The method proposed in this study is easily reproducible and is suitable to evaluate the protective effect of European mountain forests at different scales. At local scale, the proposed indicators can enrich rockfall studies in which forests are usually set aside to simplify the evaluation. Moreover, the indicators may find direct applications with foresters by allowing them to identify the protective level of their forest and consequently to adapt their management. Finally, the indicators are convenient to perform spatial analysis and produce maps of the protective effect of mountain forests that could find many applications in land settlement or evaluation of ecosystem services. (C) 2016 Elsevier Ltd. All rights reserved.
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Dupire, S., Bourrier, F., Monnet, J. - M., Bigot, S., Borgniet, L., Berger, F., et al. (2016). The protective effect of forests against rockfalls across the French Alps: Influence of forest diversity. Forest Ecology and Management, 382, 269–279.
Abstract: The role of forests in the mitigation of natural hazards has been repeatedly demonstrated. The protective effect of mountain forests against rockfalls has especially been pointed out because it can constitute a natural and cost-effective protection measure in many situations. However, this particular ecosystem service may substantially differ according to the structure and the composition of the forest. Until now, the rockfall protection capability has always been studied at a local scale with only few forest types. Moreover, the comparison of the protective effect of the different forest types studied remains difficult because different methods and indicators were used. For the same reasons, it is not possible to draw conclusions about the influence of biological and structural diversities on the protection capabilities of forests from former works. The aims of this study were (1) to quantitatively assess the protective effect of forests at the French Alps scale and build a classification based on the protection capability, (2) to compare the protective effect of the different forest types present in the French Alps and (3) to analyze the relations between the protective effect and the forest diversity in terms of stand structure and tree composition. For this purpose, the model Rockyfor3D was used to simulate the propagation of rocks on 3886 different forest plots spread over the whole French Alps. Quantitative indicators characterizing the protective effect of each forest plot were then calculated from the simulation results and used to perform the different analyses. Our results emphasized the importance of taking into account the length of forest in the maximum slope direction for an accurate assessment of the protective effect. Thus, the minimum length of forest to get a reduction of 99% of the rockfall hazard was chosen as indicator to compare protective effect between forests. Using this indicator, half of the French Alpine forests presented a high level of protection after a short forested slope (190m). A decreasing gradient in the protection capabilities was observed from forest types dominated by broadleaved species to those dominated by conifer species. Moreover, considering an equivalent proportion of conifers, stands dominated by shade-tolerant tree species showed better ability to reduce rockfall hazard. Finally, our study highlighted that a high biodiversity and a structural heterogeneity within the forest have a positive effect on the reduction of rockfalls hazard.
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Erktan, A., Cecillon, L., Graf, F., Roumet, C., Legout, C., & Rey, F. (2016). Increase in soil aggregate stability along a Mediterranean successional gradient in severely eroded gully bed ecosystems: combined effects of soil, root traits and plant community characteristics. Plant And Soil, 398(1-2), 121–137.
Abstract: Our objectives were to evaluate changes in soil aggregate stability along a successional gradient, located in severely eroded Mediterranean gully bed ecosystems and to identify predictors of soil aggregate stability variations among several soil, root traits and plant community characteristics. We selected 75 plots in gully beds, representing five successional stages that differ in plant community composition, dominated by herbs, shrubs or trees according to successional stage. In each plot, we measured soil aggregate stability, basic soil characteristics, root traits and plant diversity indices. Soil aggregate stability increased along the successional gradient, being thrice higher in tree-dominated communities as compared to grass-dominated communities. This increase was mainly driven by soil organic carbon (SOC) accumulation. In early successional stages showing low SOC (below 24 g.kg(-1) or 12 g.kg(-1) in some cases), fine sand content and the percentage of fine roots acted as co-drivers enhancing soil aggregate stability while silt content decreased it. Plant succession in severely eroded Mediterranean gully bed ecosystems is accompanied by a strong stabilization of soil aggregates, mainly driven by SOC accumulation and for early successional stages, by soil granulometry and root traits as co-drivers. Stimulating succession thus appears as a promising restoration strategy for severely eroded ecosystems.
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Erktan, A., Legout, C., De Danieli, S., Daumergue, N., & Cecillon, L. (2016). Comparison of infrared spectroscopy and laser granulometry as alternative methods to estimate soil aggregate stability in Mediterranean badlands. Geoderma, 271, 225–233.
Abstract: Soil aggregate stability is a key indicator of soil resistance to erosion, but its measurement remains fastidious for large scale uses. Alternative time and cost-effective methods are thus needed. Our objective was to assess and compare the efficiency of laser granulometry (LG) and soil mid- and near-infrared spectroscopy (MIR/NIR) as alternative methods to assess soil aggregate stability in Mediterranean badland soils. A collection of 75 badland soil samples was used, showing wide variations in soil aggregate stability. Three different categories of measurements were performed: (i) the aggregate breakdown kinetics of the [<1 mm] size fraction under stirring and sonication, tracked by repeated particle size distribution measurements, using LG, (ii) mid-(diffuse-MIR-DR and attenuate transmitted reflectance MIR-ATR) and near-(NIR-DR) infrared spectra of the fine soil fraction [<2 mm] and (iii) the soil aggregate [3-5 mm] stability, using the standardized method (ISO/FDIS 10930, 2012). Partial least squares regression models were used to predict soil aggregate stability using LG data and infrared spectra. Results showed that NIR-DR and MIR-ATR data provided the best prediction model for soil aggregate stability values (RPD = 2.61 & 2.74; R-2 = 0.85 & 810.87), followed by MIR-DR data (RPD = 2.24; R-2 = 0.89) and finally LG data (RPD = 2.12; R-2 = 0.80). For a quantitative use of the models to assign soil samples to standardized soil aggregate stability classes (ISO/FDIS 10930, 2012), infrared spectra also provided the best accuracy, with a misclassification rate below 30% for NIR-DR and MIR-ATR models, while it reached 43% with the LG-based model. The combination of IR and LG data did not yield a better prediction model for soil aggregate stability values and classes, Infrared-based method also provided best results in terms of time-saving strategy, reducing the measurement time to 8 min only. To conclude, infrared spectra (NIR-DR and MIR-ATR) outperformed LG-data to predict soil aggregate stability. Further development of this technique would require calibrating a set of soil-type specific prediction models for a wide range of soil types. (C) 2016 Elsevier B.V. All rights reserved.
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Escudier, R., Renault, L., Pascual, A., Brasseur, P., Chelton, D., & Beuvier, J. (2016). Eddy properties in the Western Mediterranean Sea from satellite altimetry and a numerical simulation. Journal Of Geophysical Research-Oceans, 121(6), 3990–4006.
Abstract: Three different eddy detection and tracking methods are applied to the outputs of a high-resolution simulation in the Western Mediterranean Sea in order to extract mesoscale eddy characteristics. The results are compared with the same eddy statistics derived from satellite altimetry maps over the same period. Eddy radii are around 30 km in altimetry maps whereas, in the model, they are around 20 km. This is probably due to the inability of altimetry maps to resolve the smaller mesoscale in the region. About 30 eddies are detected per day in the basin with a very heterogeneous spatial distribution and relatively short lifespans (median life around 13 days). Unlike other areas of the open ocean, they do not have a preferred direction of propagation but appear to be advected by mean currents. The number of detected eddies seems to present an annual cycle when separated according to their lifespan. With the numerical simulation, we show that anticyclones extend deeper in the water column and have a more conic shape than cyclones.
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Evin, G., Blanchet, J., Paquet, E., Garavaglia, F., & Penot, D. (2016). A regional model for extreme rainfall based on weather patterns subsampling. Journal Of Hydrology, 541, 1185–1198.
Abstract: Many rainfall generators rely on the assumption that statistical properties of rainfall observations can be related to physical processes via weather patterns. The MEWP (Multi-Exponential Weather Pattern) model belongs to this class. In this daily rainfall model, extremes above a threshold are distributed exponentially, for each season and atmospheric circulation pattern. A wide range of applications of this rainfall compound distribution has demonstrated its robustness and reliability. However, recent investigations showed that MEWP tends to underestimate the most extreme rainfall events in specific regions (e.g. the South-East of France). In this paper, we apply different versions of a generalized MEWP model: the MDWP (Multi-Distribution Weather Pattern) model. In the MDWP model, the exponential distribution is replaced by distributions with a heavier tail, such as the Generalized Pareto Distribution (GPD). Unfortunately, local applications of the MDWP model reveal a lack of robustness and overfitting issues. To solve this issue, a regional version of the MDWP model is proposed. Different options of a regionalization approach for excesses are scrutinized (e.g. choice of the scale factor, testing of homogeneous regions based on neighborhoods around each site, choice of the distribution modelling extreme rainfall). We compare the performances of local and regional models on long daily rainfall series covering the southern half of France. These applications show that the local models with heavy-tailed distributions exhibit a lack of robustness. In comparison, an impressive improvement of model robustness is obtained with the regional version, without a loss of reliability. (C) 2016 Elsevier B.V. All rights reserved.
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Favier, V., Verfaillie, D., Berthier, E., Menegoz, M., Jomelli, V., Kay, J. E., et al. (2016). Atmospheric drying as the main driver of dramatic glacier wastage in the southern Indian Ocean. Scientific Reports, 6.
Abstract: The ongoing retreat of glaciers at southern sub-polar latitudes is particularly rapid and widespread. Akin to northern sub-polar latitudes, this retreat is generally assumed to be linked to warming. However, no long-term and well-constrained glacier modeling has ever been performed to confirm this hypothesis. Here, we model the Cook Ice Cap mass balance on the Kerguelen Islands (Southern Indian Ocean, 49 degrees S) since the 1850s. We show that glacier wastage during the 2000s in the Kerguelen was among the most dramatic on Earth. We attribute 77% of the increasingly negative mass balance since the 1960s to atmospheric drying associated with a poleward shift of the mid-latitude storm track. Because precipitation modeling is very challenging for the current generation of climate models over the study area, models incorrectly simulate the climate drivers behind the recent glacier wastage in the Kerguelen. This suggests that future glacier wastage projections should be considered cautiously where changes in atmospheric circulation are expected.
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Francois, B. (2016). Influence of winter North-Atlantic Oscillation on Climate-Related Energy penetration in Europe. Renewable Energy, 99, 602–613.
Abstract: When considering 100% renewable scenarios, backup generation is needed for stabilizing the network when Climate Related Energy (CRE) such as wind, solar or run-of-the river hydropower are not sufficient for supplying the load. Several studies show that, over relatively short time period (less than 10 years), backup generation needs are reduced by dissipating power densities either in space through grids or time through storage. This study looks at the impact of low time frequency variations of CRE with a specific focus on the time variability induced by the North Atlantic Oscillation (NAO) teleconnection pattern during winter season. A set of eleven regions in Europe and Tunisia is used for highlighting space variability of the winter NAO's impact. For each of these regions, we combine data from the Weather Research and Forecasting Model and the European Climate Assessment & Dataset for estimating solar power, wind-power, run-of-the-river hydro-power and the energy load over the 1980-2012 time period. Results show that NAO's impact on winter penetration rate depends on both the considered energy source and the location. They also highlight a non-linear relation between the NAO's impact on CRE penetration rates and the level of equipment used for harvesting the CRE sources. (C) 2016 Elsevier Ltd. All rights reserved.
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Francois, B., Borga, M., Creutin, J. D., Hingray, B., Raynaud, D., & Sauterleute, J. F. (2016). Complementarity between solar and hydro power: Sensitivity study to climate characteristics in Northern-Italy. Renewable Energy, 86, 543–553.
Abstract: Climate related energy sources such as wind, solar and runoff sources are variable in time and space, following their driving weather variables. High penetration of such energy sources might be facilitated by using their complementarity in order to increase the balance between energy load and generation. This study presents the analysis of the effect of a 100% renewable energy mix composed by solar and run-of-the-river energy in Northern Italy where these two energy sources are the main alternative energy sources. Along a climate gradient from the Alpine crest (snow melt dominated area) to the Veneto plain (rainfall dominated area), solar power is generated in the flat plain, and run-of-the-river hydropower at two mountainous locations. Covering all possible mixes of these two sources, we analyze their complementarity across different temporal scales using two indicators: the standard deviation of the energy balance and the theoretical storage required for balancing generation and load. Results show that at small temporal scale (hourly), a high share of run-of-the-river power allows minimizing the energy balance variability. The opposite is obtained at larger temporal scales (daily and monthly) essentially because of lower variability of solar power generation, which also implies a lower storage requirement. (C) 2015 Elsevier Ltd. All rights reserved.
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Francois, B., Hingray, B., Raynaud, D., Borga, M., & Creutin, J. D. (2016). Increasing climate-related-energy penetration by integrating run-of-the river hydropower to wind/solar mix. Renewable Energy, 87, 686–696.
Abstract: The penetration rate of Climate Related Energy sources like solar-power, wind-power and hydro-power source is potentially low as a result of the large space and time variability of their driving climatic variables. Increased penetration rates can be achieved with mixes of sources. Optimal mixes, i.e. obtained with the optimal share for each source, are being identified for a number of regions worldwide. However, they often consider wind and solar power only. Based on 33 years of daily data (1980-2012) for a set of 12 European regions, we re-estimate the optimal mix when wild run-of-the-river energy is included in the solar/wind mix. It is found to be highly region dependent but the highest shares are often obtained for run-of-the-river, ranging from 35% to 65% in Belarus and England. High solar shares (>40%) are found in southern countries but solar shares drop to less than 15% in northern countries. Wind shares range from 10 to 35% with the exception of Norway where it reaches 50%. These results put in perspective the optimal 60%-40% wind/solar mix currently used for Europe. For all regions, including run-of-the-river in the mix allows increasing the penetration rate of CREs (from 1 to 8% points). (C) 2015 Elsevier Ltd. All rights reserved.
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Furst, J. J., Durand, G., Gillet-Chaulet, F., Tavard, L., Rankl, M., Braun, M., et al. (2016). The safety band of Antarctic ice shelves. Nature Climate Change, 6(5), 479–482.
Abstract: The floating ice shelves along the seaboard of the Antarctic ice sheet restrain the outflow of upstream grounded ice(1,2). Removal of these ice shelves, as shown by past ice-shelf recession and break-up, accelerates the outflow(3,4), which adds to sea-level rise. A key question in predicting future outflow is to quantify the extent of calving that might precondition other dynamic consequences and lead to loss of ice-shelf restraint. Here we delineate frontal areas that we label as 'passive shelf ice' and that can be removed without major dynamic implications, with contrasting results across the continent. The ice shelves in the Amundsen and Bellingshausen seas have limited or almost no 'passive' portion, which implies that further retreat of current ice-shelf fronts will yield important dynamic consequences. This region is particularly vulnerable as ice shelves have been thinning at high rates for two decades' and as upstream grounded ice rests on a backward sloping bed, a precondition to marine ice-sheet instability(6,7). In contrast to these ice shelves, Larsen C Ice Shelf, in the Weddell Sea, exhibits a large 'passive' frontal area, suggesting that the imminent calving of a vast tabular iceberg(8) will be unlikely to instantly produce much dynamic change.
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Gabrielli, P., Barbante, C., Bertagna, G., Berto, M., Binder, D., Carton, A., et al. (2016). Age of the Mt. Ortles ice cores, the Tyrolean Iceman and glaciation of the highest summit of South Tyrol since the Northern Hemisphere Climatic Optimum. Cryosphere, 10(6), 2779–2797.
Abstract: In 2011 four ice cores were extracted from the summit of Alto dell'Ortles (3859 m), the highest glacier of South Tyrol in the Italian Alps. This drilling site is located only 37 km southwest from where the Tyrolean Iceman, similar to 5.3 kyrs old, was discovered emerging from the ablating ice field of Tisenjoch (3210 m, near the Italian-Austrian border) in 1991. The excellent preservation of this mummy suggested that the Tyrolean Iceman was continuously embedded in prehistoric ice and that additional ancient ice was likely preserved elsewhere in South Tyrol. Dating of the ice cores from Alto dell'Ortles based on Pb-210, tritium, beta activity and C-14 determinations, combined with an empirical model (COPRA), provides evidence for a chronologically ordered ice stratigraphy from the modern glacier surface down to the bottom ice layers with an age of similar to 7 kyrs, which confirms the hypothesis. Our results indicate that the drilling site has continuously been glaciated on frozen bedrock since similar to 7 kyrs BP. Absence of older ice on the highest glacier of South Tyrol is consistent with the removal of basal ice from bedrock during the Northern Hemisphere Climatic Optimum (6-9 kyrs BP), the warmest interval in the European Alps during the Holocene. Borehole inclinometric measurements of the current glacier flow combined with surface ground penetration radar (GPR) measurements indicate that, due to the sustained atmospheric warming since the 1980s, an acceleration of the glacier Alto dell'Ortles flow has just recently begun. Given the stratigraphic-chronological continuity of the Mt. Ortles cores over millennia, it can be argued that this behaviour has been unprecedented at this location since the Northern Hemisphere Climatic Optimum.
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Gagliardini, O., Brondex, J., Gillet-Chaulet, F., Tavard, L., Peyaud, V., & Durand, G. (2016). Brief communication: Impact of mesh resolution for MISMIP and MISMIP3d experiments using Elmer/Ice. Cryosphere, 10(1), 307–312.
Abstract: The dynamical contribution of marine ice sheets to sea level rise is largely controlled by grounding line (GL) dynamics. Two marine ice sheet model intercomparison exercises, namely MISMIP and MISMIP3d, have been proposed to the community to test and compare the ability of models to capture the GL dynamics. Both exercises are known to present a discontinuity of the friction at the GL, which is believed to increase the model sensitivity to mesh resolution. Here, using Elmer/Ice, the only Stokes model which completed both intercomparisons, the sensitivity to the mesh resolution is studied from an extended MISMIP experiment in which the friction continuously decreases over a transition distance and equals zero at the GL. Using this MISMIP-like setup, it is shown that the sensitivity to the mesh resolution is not improved for a vanishing friction at the GL. For the original MISMIP experiment, i.e. for a discontinuous friction at the GL, we further show that the results are moreover very sensitive to the way the friction is interpolated in the close vicinity of the GL. In the light of these new insights, and thanks to increased computing resources, new results for the MISMIP3d experiments obtained for higher resolutions than previously published are made available for future comparisons as the Supplement.
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Garambois, S., Legchenko, A., Vincent, C., & Thibert, E. (2016). Ground-penetrating radar and surface nuclear magnetic resonance monitoring of an englacial water-filled cavity in the polythermal glacier of Tete Rousse. Geophysics, 81(1), WA131–WA146.
Abstract: In polythermal glaciers, specific climatic, topographic, and exposure conditions may lead to the formation of englacial lakes that can produce catastrophic effects downstream in the event of abrupt natural drainage. We have determined how a combination of ground-penetrating radar (GPR) and surface nuclear magnetic resonance (SNMR) surveys helped to locate and visualize the evolution of a water-filled cavity within the Tete Rousse glacier (French Alps). We have used GPR results to delineate the roof of the cavity and monitor the cavity deformation caused by artificial drainage. Because the glacier bed and cavity have complex 3D geometries, we needed dense acquisition lines and 3D GPR views to qualitatively identify out-of-plane reflections. This 3D approach made it possible to establish a precise map of the glacier bed topography, the accuracy of which was verified against borehole observations. Then, repetitive GPR measurements were used to obtain a quantitative estimate of the vertical deflection of the cavity's roof and changes in crevasse geometry observed in response to the decrease in the water pressure when 47; 800 m(3) of water was drained by pumping. We have used 3D SNMR imaging to locate water accumulation zones within the glacier and to estimate the volume of accumulated water. The SNMR monitoring revealed that in two years, the cavity lost approximately 73% of its initial volume, with 65% lost after the first drainage. Knowledge of the water contained in the ice provided a better understanding of GPR images and thus a more reliable interpretation of GPR data. However, SNMR imaging had a much lower resolution in comparison with GPR, and consequently GPR allowed a more accurate study of the evolution of cavity geometry caused by consecutive drainage and refilling. This study demonstrated the value of combining GPR data with SNMR data for the study of polythermal glaciers.
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Garnier, F., Brankart, J. M., Brasseur, P., & Cosme, E. (2016). Stochastic parameterizations of biogeochemical uncertainties in a 1/4 degrees NEMO/PISCES model for probabilistic comparisons with ocean color data. Journal Of Marine Systems, 155, 59–72.
Abstract: In spite of recent advances, biogeochemical models are still unable to represent the full complexity of natural ecosystems. Their formulations are mainly based on empirical laws involving many parameters. Improving biogeochemical models therefore requires to properly characterize model uncertainties and their consequences. Subsequently, this paper investigates the potential of using random processes to simulate some uncertainties of the 1/4 degrees coupled Physical-Biogeochemical NEMO/PISCES model of the North Atlantic ocean. Starting from a deterministic simulation performed with the original PISCES formulation, we propose a generic method based on AR(1) random processes to generate perturbations with temporal and spatial correlations. These perturbations are introduced into the model formulations to simulate 2 classes of uncertainties: the uncertainties on biogeochemical parameters and the uncertainties induced by unresolved scales in the presence of non-linear processes. Using these stochastic parameterizations, a probabilistic version of PISCES is designed and a 60-member ensemble simulation is performed. With respect to the simulation of chlorophyll, the relevance of the probabilistic configuration and the impacts of these stochastic parameterizations are assessed. In particular, it is shown that the ensemble simulation is in good agreement with the SeaWIFS ocean color data. Using these observations, the statistical consistency (reliability) of the ensemble is evaluated with rank histograms. Finally, the benefits expected from the probabilistic description of uncertainties (model error) are discussed in the context of future ocean color data assimilation. (C) 2015 Elsevier B.V. All rights reserved.
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Gautier, E., Savarino, J., Erbland, J., Lanciki, A., & Possenti, P. (2016). Variability of sulfate signal in ice core records based on five replicate cores. Climate Of The Past, 12(1), 103–113.
Abstract: Current volcanic reconstructions based on ice core analysis have significantly improved over the past few decades by incorporating multiple-core analyses with a high temporal resolution from different parts of the polar regions into a composite common volcanic eruption record. Regional patterns of volcanic deposition are based on composite records, built from cores taken at both poles. However, in many cases only a single record at a given site is used for these reconstructions. This assumes that transport and regional meteorological patterns are the only source of the dispersion of the volcanic products. Here we evaluate the local-scale variability of a sulfate profile in a low-accumulation site (Dome C, Antarctica), in order to assess the representativeness of one core for such a reconstruction. We evaluate the variability with depth, statistical occurrence, and sulfate flux deposition variability of volcanic eruptions detected in five ice cores, drilled 1m apart from each other. Local-scale variability, essentially attributed to snow drift and surface roughness at Dome C, can lead to a non-exhaustive record of volcanic events when a single core is used as the site reference, with a bulk probability of 30% of missing volcanic events and close to 65% uncertainty on one volcanic flux measurement (based on the standard deviation obtained from a five-core comparison). Averaging n records reduces the uncertainty of the deposited flux mean significantly (by a factor 1/root n); in the case of five cores, the uncertainty of the mean flux can therefore be reduced to 29 %.
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Genthon, C., Six, D., Scarchilli, C., Ciardini, V., & Frezzotti, M. (2016). Meteorological and snow accumulation gradients across Dome C, East Antarctic plateau. International Journal Of Climatology, 36(1), 455–466.
Abstract: In situ observations show that snow accumulation is similar to 10% larger 25 km north than south of the summit of Dome C on the east antarctic plateau. The mean wind direction is southerly. Although a slight slope-related diverging katabatic flow component is detectable, the area is an essentially flat (similar to 10m elevation change or less) homogeneous snow surface. The European Center for Medium-range Weather Forecasts meteorological analyses data reproduce a significant accumulation gradient and suggest that 90% of the the mean accumulation results from the 25% largest precipitation events. During these events, air masses originate from coastal areas in the north rather than from inland in the south. Radiative cooling condensation occurs on the way across the dome and as the moisture reservoir is depleted less snow is dumped 25 km south than north, with little direct impact from the local (50-km scale) topography. Air masses are warmer on average, and warmer north than south, when originating from the coast. This marginally affects the mean temperature gradients. The moisture gradients are more affected because moisture is nonlinearly related to temperature: the mean atmospheric moisture is larger north than south. Significant meteorological and hydrological gradients over such relatively small distances (50 km) over locally flat region may be an issue when interpreting ice cores: although cores are drilled at the top of domes and ridges where the slopes and elevation gradients are minimal, they sample small surfaces in areas affected by significant meteorological and hydrological spatial gradients.
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Gillet-Chaulet, F., Durand, G., Gagliardini, O., Mosbeux, C., Mouginot, J., Rémy, F., et al. (2016). Assimilation of surface velocities acquired between 1996 and 2010 to constrain the form of the basal friction law under Pine Island Glacier. Geophys. Res. Lett., 43(19), 10,311–10,321.
Abstract: Abstract In ice-sheet models, slip conditions at the base between the ice and the bed are parameterized by a friction law. The most common relation has two poorly constrained parameters, C and m. The basal slipperiness coefficient, C, depends on local unobserved quantities and is routinely inferred using inverse methods. While model results have shown that transient responses to external forcing are highly sensitive to the stress exponent m, no consensus value has emerged, with values commonly used ranging from 1 to ∞ depending on the slip processes. By assimilation of Pine Island Glacier surface velocities from 1996 to 2010, we show that observed accelerations are best reproduced with m>=5. We conclude that basal motion, in much of the fast flowing region, is governed by plastic deformation of the underlying sediments. This implies that the glacier bed in this area cannot deliver resistive stresses higher than today, making the drainage basin potentially more sensitive to dynamical perturbations than predicted with models using standard values m = 1 or 3.
Keywords: ice flow modeling; subglacial conditions; inverse modeling
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Goix, S., Uzu, G., Oliva, P., Barraza, F., Calas, A., Castet, S., et al. (2016). Metal concentration and bioaccessibility in different particle sizes of dust and aerosols to refine metal exposure assessment. Journal Of Hazardous Materials, 317, 552–562.
Abstract: Refined exposure assessments were realized for children, 7-9 yrs, in the mining/smelting city of Oruro, Bolivia. Aerosols (PM>2.5, PM1-2.5, PM0.4-1 and PM0.5) and dust (separated in different particle size fractions: 2000-200 μm, 200-50 μm, 50-20 μm, 20-2 μm and <2 μm) were sampled on football fields highly frequented by children in both the mining and smelting areas. Trace element concentrations (Ag, As, Cd, Cu, Pb, Sb, Sn and Zn) in each size fraction of dust and aerosols, lung bioaccessibility of metals in aerosols, and gastric bioaccessibility of metals in dust were measured. Exposure was assessed considering actual external exposure (i.e. exposure pathways: metals inhaled and ingested) and simulated internal exposure (i.e., complex estimation using gastric and lung bioaccessibility, deposition and clearance of particles in lungs). Significant differences between external and simulated internal exposure were attributed to dissemblances in gastric and lung bioaccessibilities, as well as metal distribution within particle size range, revealing the importance of both parameters in exposure assessment. (C) 2016 Elsevier B.V. All rights reserved.
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Gond, V., Dubiez, E., Boulogne, M., Gigaud, M., Peroches, A., Pennec, A., et al. (2016). Forest cover and carbon stock change dynamics in the Democratic Republic of Congo: Case of the wood-fuel supply basin of Kinshasa. Bois Et Forets Des Tropiques, (327), 19–28.
Abstract: To contribute to the development of methods for the sustainable management of forest ecosystems in Central Africa, the following research question was addressed: can analyses of land cover change help to understand and document the spatial organization and mechanisms of forest degradation? To answer this question in the Democratic Republic of Congo, the Makala project mapped the tree and forest resources of Kinshasa's wood-fuel supply basin and attempted to predict how they would evolve in the future. Maps were made for four periods (1984, 2001, 2006 and 2012) using a mosaic of four LANDSAT images. The above-ground biomass was estimated in 2012 using forest inventories in 317 plots distributed across the four types of plant cover found on the Bateke plateau (4,337 trees and 44 species were recorded). This inventory data combined with the satellite data allowed us to make the first comprehensive assessment of above-ground biomass in the study area. Between 2000 and 2012, the average volume of wood-fuel fell by more than 50%. Over the entire period studied (1984 to 2012), carbon stocks fell by 75%. In the wood-fuel supply area for Kinshasa, the drastic loss of forest cover, shortened fallow periods, savannah encroachment and the decline of biomass and carbon stocks are clear signals of degradation. However, these initial estimations were derived from a small sample that was extrapolated to the entire supply area. It would be very useful to increase sampling in order to obtain more accurate and realistic figures. The experience of the Makala project clearly shows that the analysis of land cover change helps to understand and document the spatial organization and mechanisms of forest degradation. However, only with a sound wood-fuel resource policy and sustainable community land management, combined with very dynamic tree reintroduction on agricultural land, will it be possible to initiate a sustainable process of restoration.
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Gosset, M., Kunstmann, H., Zougmore, F., Cazenave, F., Leijnse, H., Uijlenhoet, R., et al. (2016). Improving Rainfall Measurement in Gauge Poor Regions Thanks to Mobile Telecommunication Networks. Bulletin of the American Meteorological Society, 97(3), Es49–Es51. |
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Gratiot, N., & Anthony, E. J. (2016). Role of flocculation and settling processes in development of the mangrove-colonized, Amazon-influenced mud-bank coast of South America. Marine Geology, 373, 1–10.
Abstract: The Guianas coast downdrift (northwestward) of the Amazon River mouth is characterized by the periodic migration of mud banks that originate from the Amazon. The characteristic sizes of these banks as well as their rates of alongshore migration have been estimated from remote sensing. However, the physical mechanisms leading to their displacement are not yet fully understood. The present work is aimed at investigating a number of micro-scale processes involved in the coastal and estuarine dynamics of fine sediments, and expected to occur during the migration of mud banks. The relative magnitudes of flocculation, hindered settling and consolidation have been determined. Sampling of coastal mud was carried out during a field survey in 2001 that focussed on the fluid mud layer in the leading edge of a mud bank in French Guiana. Settling column experiments were conducted under quiescent conditions for various mean sediment concentrations in the range of 2.5-110 g.l(-1), which is typical of mud bank concentrations. The time dependent vertical profiles of suspended sediment concentration were monitored using an optical settling tank equipped with 32 pre-calibrated optical sensors. The corresponding settling velocities were deduced from the equation of the conservation of mass. The results show that the timescales of hindered settling and consolidation processes are much larger than the timescales of mixing mechanisms such as tides or propagating waves. The individual floc settling velocities are too small to counterbalance the turbulent mixing induced by breaking waves. Hindered settling, favoured by wave action and by the high background suspended sediment concentrations, is thus an overarching process in wave-driven mud bank migration. This pervasive hindered settling regime is characteristic of the wave-exposed outer and leading edges of mud banks where active mobilization of mud assures mud bank migration. As a result, consolidation is theoretically precluded. The experiments pointed out, however, that additional flocculation and differential settling should enhance sedimentation during slack water and under low wave conditions. We deduce from this that enhanced settling in the inner,subtidal-intertidal, parts of leading edges of banks during such conditions is important in the temporary sedimentation that generates gel-like fluid mud patches and mud bars. These low-energy inner, leading parts of banks form an accreted substrate colonized by mangroves, which further contribute to enhanced settling and rapid sedimentation. (C) 2015 Elsevier B.V. All rights reserved.
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Hamon, M., Beuvier, J., Somot, S., Lellouche, J. M., Greiner, E., Jorda, G., et al. (2016). Design and validation of MEDRYS, a Mediterranean Sea reanalysis over the period 1992-2013. Ocean Science, 12(2), 577–599.
Abstract: The French research community in the Mediterranean Sea modeling and the French operational ocean forecasting center Mercator Ocean have gathered their skill and expertise in physical oceanography, ocean modeling, atmospheric forcings and data assimilation to carry out a MEDiterranean sea ReanalYsiS (MEDRYS) at high resolution for the period 1992-2013. The ocean model used is NEMOMED12, a Mediterranean configuration of NEMO with a 1/12 degrees (similar to 7 km) horizontal resolution and 75 vertical z levels with partial steps. At the surface, it is forced by a new atmospheric-forcing data set (ALDERA), coming from a dynamical downscaling of the ERA-Interim atmospheric reanalysis by the regional climate model ALADIN-Climate with a 12 km horizontal and 3 h temporal resolutions. This configuration is used to carry a 34-year hindcast simulation over the period 1979-2013 (NM12-FREE), which is the initial state of the reanalysis in October 1992. MEDRYS uses the existing Mercator Ocean data assimilation system SAM2 that is based on a reduced-order Kalman filter with a threedimensional (3-D) multivariate modal decomposition of the forecast error. Altimeter data, satellite sea surface temperature (SST) and temperature and salinity vertical profiles are jointly assimilated. This paper describes the configuration we used to perform MEDRYS. We then validate the skills of the data assimilation system. It is shown that the data as-similation restores a good average temperature and salinity at intermediate layers compared to the hindcast. No particular biases are identified in the bottom layers. However, the reanalysis shows slight positive biases of 0.02 psu and 0.15 degrees C above 150 m depth. In the validation stage, it is also shown that the assimilation allows one to better reproduce water, heat and salt transports through the Strait of Gibraltar. Finally, the ability of the reanalysis to represent the sea surface high-frequency variability is shown.
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Hassane, B., Durand, A., Garba, Z., Dieppois, B., Sebag, D., Rajot, J. L., et al. (2016). Can daily meteorological measurement of near-surface wind detect climate changes in the Sahel (SE Niger, 1950-1992)? Journal Of Arid Environments, 124, 91–101.
Abstract: Using three daily measurements of wind speed and direction from synoptic weather station data in SE Niger, we examined the diurnal, seasonal and interannual time-scale of Sahel climate variability between 1950 and 1992. The seasonal wind patterns are closely related to the temperatures and West African monsoon dynamics. The transitions between the two seasons are marked by an important increase in calms (wind speed < 0.5 m s(-1)). Such variations are related to meridional shifts of the Inter Tropical Discontinuity (ITD) and Inter Tropical Convergence Zone (ITCZ). Interannual fluctuations of annual wind speeds are consistent with Sahel rainfall variability. Dry years, such as in 1969-1973 and 1983-1986 periods, are associated with negative anomalies in wind speeds mainly due to an increase in calms and dry conditions. Nevertheless, we note several differences: the first period is associated with a yearly increase in the annual mean speed, while the second is associated with a decrease. Differences could be related to changes in atmospheric circulation, especially regarding the strength and latitudinal position of Tropical and African Easterly jets. (C) 2015 Elsevier Ltd. All rights reserved.
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Hattori, S., Savarino, J., Kamezaki, K., Ishino, S., Dyckmans, J., Fujinawa, T., et al. (2016). Automated system measuring triple oxygen and nitrogen isotope ratios in nitrate using the bacterialmethod and N2O decomposition by microwave discharge. Rapid Communications In Mass Spectrometry, 30(24), 2635–2644.
Abstract: RATIONALE: Triple oxygen and nitrogen isotope ratios in nitrate are powerful tools for assessing atmospheric nitrate formation pathways and their contribution to ecosystems. N2O decomposition using microwave-induced plasma (MIP) has been used only for measurements of oxygen isotopes to date, but it is also possible to measure nitrogen isotopes during the same analytical run. METHODS: The main improvements to a previous system are (i) an automated distribution system of nitrate to the bacterial medium, (ii) N2O separation by gas chromatography before N2O decomposition using the MIP, (iii) use of a corundumtube for microwave discharge, and (iv) development of an automated system for isotopic measurements. Three nitrate standards with sample sizes of 60, 80, 100, and 120 nmol were measured to investigate the sample size dependence of the isotope measurements. RESULTS: The delta O-17, delta O-18, and Delta O-17 values increased with increasing sample size, although the delta N-15 value showed no significant size dependency. Different calibration slopes and intercepts were obtained with different sample amounts. The slopes and intercepts for the regression lines in different sample amounts were dependent on sample size, indicating that the extent of oxygen exchange is also dependent on sample size. The sample-size-dependent slopes and intercepts were fitted using natural log (ln) regression curves, and the slopes and intercepts can be estimated to apply to any sample size corrections. When using 100 nmol samples, the standard deviations of residuals from the regression lines for this system were 0.5 parts per thousand, 0.3 parts per thousand, and 0.1 parts per thousand, respectively, for the delta O-18, Delta O-17, and delta N-15 values, results that are not inferior to those from other systems using gold tube or gold wire. CONCLUSIONS: An automated system was developed to measure triple oxygen and nitrogen isotopes in nitrate using N2O decomposition by MIP. This system enables us to measure both triple oxygen and nitrogen isotopes in nitrate with comparable precision and sample throughput (23 min per sample on average), and minimal manual treatment. Copyright (C) 2016 John Wiley & Sons, Ltd.
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Hector, B., & Hinderer, J. (2016). pyGrav, a Python-based program for handling and processing relative gravity data. Computers & Geosciences, 91, 90–97.
Abstract: pyGrav is a Python-based open-source software dedicated to the complete processing of relative-gravity data. It is particularly suited for time-lapse gravity surveys where high precision is sought. Its purpose is to bind together single-task processing codes in a user-friendly interface for handy and fast treatment of raw gravity data from many stations of a network. The intuitive object-based implementation allows to easily integrate additional functions (reading/writing routines, processing schemes, data plots) related to the appropriate object (a station, a loop, or a survey). This makes pyGrav an evolving tool. Raw data can be corrected for tides and air pressure effects. The data selection step features a double table-plot graphical window with either manual or automatic selection according to specific thresholds on data channels (tilts, gravity values, gravity standard deviation, duration of measurements, etc.). Instrumental drifts and gravity residuals are obtained by least square analysis of the dataset. This first step leads to the gravity simple differences between a reference point and any point of the network. When different repetitions of the network are done, the software computes then the gravity double differences and associated errors. The program has been tested on two specific case studies: a large dataset acquired for the study of water storage changes on a small catchment in West Africa, and a dataset operated and processed by several different users for geothermal studies in northern Alsace, France. In both cases, pyGrav proved to be an efficient and easy-to-use solution for the effective processing of relative-gravity data. (C) 2016 Elsevier Ltd. All rights reserved.
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Ivankovic, T., du Roscoat, S. R., Geindreau, C., Sechet, P., Huang, Z. J., & Martins, J. M. F. (2016). Development and evaluation of an experimental protocol for 3-D visualization and characterization of the structure of bacterial biofilms in porous media using laboratory X-ray tomography. Biofouling, 32(10), 1235–1244.
Abstract: The development of a reliable model allowing accurate predictions of biofilm growth in porous media relies on a good knowledge of the temporal evolution of biofilm structure within the porous network. Since little is known about the real 3-D structure of biofilms in porous media, this work was aimed at developing a new experimental protocol to visualize the 3-D microstructure of the inside of a porous medium using laboratory X-ray microtomography. A reliable and reproducible methodology is proposed for (1) growing a biofilm inside a porous medium, and (2) X-ray tomography-based characterization of the temporal development of the biofilm at the inlet of the biofilter. The statistical analysis proposed here also validates the results presented in the literature based on a biofilm structure single measurement.
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Jomelli, V., Lane, T., Favier, V., Masson-Delmotte, V., Swingedouw, D., Rinterknecht, V., et al. (2016). Paradoxical cold conditions during the medieval climate anomaly in the Western Arctic. Scientific Reports, 6.
Abstract: In the Northern Hemisphere, most mountain glaciers experienced their largest extent in the last millennium during the Little Ice Age (1450 to 1850 CE, LIA), a period marked by colder hemispheric temperatures than the Medieval Climate Anomaly (950 to 1250 CE, MCA), a period which coincided with glacier retreat. Here, we present a new moraine chronology based on 36Cl surface exposure dating from Lyngmarksbraeen glacier, West Greenland. Consistent with other glaciers in the western Arctic, Lyngmarksbraeen glacier experienced several advances during the last millennium, the first one at the end of the MCA, in similar to 1200 CE, was of similar amplitude to two other advances during the LIA. In the absence of any significant changes in accumulation records from South Greenland ice cores, we attribute this expansion to multi-decadal summer cooling likely driven by volcanic and/or solar forcing, and associated regional sea-ice feedbacks. Such regional multi-decadal cold conditions at the end of the MCA are neither resolved in temperature reconstructions from other parts of the Northern Hemisphere, nor captured in last millennium climate simulations.
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Jordan, T. M., Bamber, J. L., Williams, C. N., Paden, J. D., Siegert, M. J., Huybrechts, P., et al. (2016). An ice-sheet-wide framework for englacial attenuation from ice-penetrating radar data. Cryosphere, 10(4), 1547–1570.
Abstract: Radar inference of the bulk properties of glacier beds, most notably identifying basal melting, is, in general, derived from the basal reflection coefficient. On the scale of an ice sheet, unambiguous determination of basal reflection is primarily limited by uncertainty in the englacial attenuation of the radio wave, which is an Arrhenius function of temperature. Existing bed-returned power algorithms for deriving attenuation assume that the attenuation rate is regionally constant, which is not feasible at an ice-sheet-wide scale. Here we introduce a new semi-empirical framework for deriving englacial attenuation, and, to demonstrate its efficacy, we apply it to the Greenland Ice Sheet. A central feature is the use of a prior Arrhenius temperature model to estimate the spatial variation in englacial attenuation as a first guess input for the radar algorithm. We demonstrate regions of solution convergence for two input temperature fields and for independently analysed field campaigns. The coverage achieved is a trade-off with uncertainty and we propose that the algorithm can be “tuned” for discrimination of basal melt (attenuation loss uncertainty similar to 5 dB). This is supported by our physically realistic (similar to 20 dB) range for the basal reflection coefficient. Finally, we show that the attenuation solution can be used to predict the temperature bias of thermomechanical ice sheet models and is in agreement with known model temperature biases at the Dye 3 ice core.
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Jouanno, J., Ochoa, J., Pallas-Sanz, E., Sheinbaum, J., Andrade-Canto, F., Candela, J., et al. (2016). Loop Current Frontal Eddies: Formation along the Campeche Bank and Impact of Coastally Trapped Waves. Journal Of Physical Oceanography, 46(11), 3339–3363.
Abstract: Velocity data from a mooring array deployed northeast of the Campeche Bank (CB) show the presence of subinertial, high-frequency (below 15 days) velocity fluctuations within the core of the northward flowing Loop Current. These fluctuations are associated with the presence of surface-intensified Loop Current frontal eddies (LCFEs), with cyclonic vorticity and diameter, 100 km. These eddies are well reproduced by a high-resolution numerical simulation of the Gulf of Mexico, and the model analysis suggests that they originate along and north of the CB, their main energy source being the mixed baroclinic-barotropic instability of the northward flow along the shelf break. There is no indication that these high-frequency LCFEs contribute to the LC eddy detachment in contrast to the low-frequency LCFEs (periods. 30 days) that have been linked to Caribbean eddies and the LC separation process. Model results show that wind variability associated with winter cold surges are responsible for the emergence of high-frequency LCFEs in a narrow band of periods (6-10 day) in the region of the CB. The dynamical link between the formation of these LCFEs and the wind variability is not direct: (i) the large-scale wind perturbations generate sea level anomalies on the CB as well as first baroclinic mode, coastally trapped waves in the western Gulf of Mexico; (ii) these waves propagate cyclonically along the coast; and (iii) the interaction of these anomalies with the Loop Current triggers cyclonic vorticity perturbations that grow in intensity as they propagate downstream and develop into cyclonic eddies when they flow north of the Yucatan shelf.
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Jouini, M., Beranger, K., Arsouze, T., Beuvier, J., Thiria, S., Crepon, M., et al. (2016). The Sicily Channel surface circulation revisited using a neural clustering analysis of a high-resolution simulation. Journal Of Geophysical Research-Oceans, 121(7), 4545–4567.
Abstract: The Sicily Channel surface circulation is investigated by analyzing the outputs of a high-resolution ocean model MED12 forced during 46 years by the ARPERA atmospheric fields. Applying a neural network classifier, we show that the surface circulation in the Sicily Channel can be decomposed into 8 modes characterizing the major patterns of that circulation, particularly the Algerian Current separation at the entrance to the Sicily Channel, the features of the Atlantic Tunisian Current and of the Atlantic Ionian Stream. These modes reflect the variability of the circulation in space and time at seasonal and inter-annual scales. Some modes preferably occur in winter whilst others are characteristic of summer. The mode sequence presents an inter-annual variability in good agreement with observations. The topography of the Sicily Channel sill plays a major role in steering the circulation. In particular the summer upwelling along the southern coast of Sicily, which is present in several modes, could be explained by a large-scale density forcing. A combination of barotropic/baroclinic double Kelvin waves generated on both sides of the sill provides a mechanism for explaining the complexity of the surface circulation advecting the surface waters from the Western Mediterranean toward the Eastern Mediterranean, the most salient features of which are the Atlantic Tunisian Current, the Atlantic Ionian Stream and the Tyrrhenian Sicilian Current which is a new feature highlighted by the present study.
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Jourdain, N. C., Lengaigne, M., Vialard, J., Izumo, T., & Sen Gupta, A. (2016). Further Insights on the Influence of the Indian Ocean Dipole on the Following Year's ENSO from Observations and CMIP5 Models. Journal Of Climate, 29(2), 637–658.
Abstract: Recent observational studies have suggested that negative and positive Indian Ocean dipole (IOD) events (nIOD and pIOD, respectively) favor a transition toward, respectively, El Nino and La Nina events one year later. These statistical inferences are however limited by the length and uncertainties in the observational records. This paper compares observational datasets with twenty-one 155-yr historical simulations from phase 5 of CMIP (CMIP5) to assess IOD and El Nino-Southern Oscillation (ENSO) properties along with their synchronous and delayed relationships. In the observations and most CMIP5 models, it is shown that El Ninos tend to be followed by La Ninas but not the opposite, that pIODs co-occur more frequently with El Ninos than nIODs with La Ninas, that nIODs tend to be followed by El Ninos one year later less frequently than pIODs by La Ninas, and that including an IOD index in a linear prediction based on the Pacific warm water volume improves ENSO peak hindcasts at 14 months lead. The IOD-ENSO delayed relationship partly results from a combination of ENSO intrinsic properties (e.g., the tendency for El Ninos to be followed by La Ninas) and from the synchronous IOD-ENSO relationship. The results, however, reveal that this is not sufficient to explain the high prevalence of pIOD-Nina transitions in the observations and 75% of the CMIP5 models, and of nIOD-Nino transitions in 60% of CMIP5 models. This suggests that the tendency of IOD to lead ENSO by one year should be explained by a physical mechanism that, however, remains elusive in the CMIP5 models. The ability of many CMIP5 models to reproduce the delayed influence of the IOD on ENSO is nonetheless a strong incentive to explore extended-range dynamical forecasts of ENSO.
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Junquas, C., Li, L., Vera, C. S., Le Treut, H., & Takahashi, K. (2016). Influence of South America orography on summertime precipitation in Southeastern South America. Climate Dynamics, 46(11-12), 3941–3963.
Abstract: Impacts of the main South American orographic structures (the Andes, the Brazilian Plateau and the Guiana shield) on the regional climate and associated global teleconnection are investigated through numerical experiments in which some of these features are suppressed. Simulations are performed with a “two-way nesting” system coupling interactively the regional and global versions of the LMDZ4 atmospheric general circulation model. At regional scale, the simulations confirm previous studies, showing that both the Andes and the Brazilian Plateau exert a control on the position and strength of the South Atlantic convergence zone (SACZ), mainly through their impact on the low-level jet and the coastal branch of the subtropical anticyclones. The northern topography of South America appears to be crucial to determine the leading mode of rainfall variability in eastern South America, which manifests itself as a dipole-like pattern between Southeastern South America and the SACZ region. The suppression of South America orography also shows global-scale effects, corresponding to an adjustment of the global circulation system. Changes in atmospheric circulation and precipitation are found in remote areas on the globe, being the consequences of various teleconnection mechanisms. When the Brazilian Plateau and the Andes are suppressed, there is a decrease of precipitation in the SACZ region, associated with a weakening of the large-scale ascendance. Changes are described in terms of anomalies in the Walker circulation, meridional displacements of the mid-latitude jet stream, Southern annular mode anomalies and modifications of Rossby wave train teleconnection processes.
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Junquas, C., Li, L., Vera, C. S., Le Treut, H., & Takahashi, K. (2016). Influence of South America orography on summertime precipitation in Southeastern South America (vol 46, pg 3941, 2016). Climate Dynamics, 47(9-10), 3389–3390. |
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Kerr, Y. H., Al-Yaari, A., Rodriguez-Fernandez, N., Parrens, M., Molero, B., Leroux, D., et al. (2016). Overview of SMOS performance in terms of global soil moisture monitoring after six years in operation. Remote Sensing Of Environment, 180, 40–63.
Abstract: The Soil Moisture and Ocean Salinity satellite (SMOS) was launched in November 2009 and started delivering data in January 2010. The commissioning phase ended in May 2010. Subsequently, the satellite has been in operation for over six years while the retrieval algorithms from Level 1 (L1) to Level 2 (L2) underwent significant evolutions as knowledge improved. Moreover, other approaches for retrieval at L2 over land were investigated while Level 3 (L3) and Level 4 (L4) were initiated. In this paper, these improvements were assessed by inter comparisons of the current L2 (V620) against the previous version (V551) and new products (using neural networks referred to as SMOS-NN) and 1.3 (referred to as SMOS-L3). In addition, a global evaluation of different SMOS soil moisture (SM) products (SMOS-L2, SMOS-L3, and SMOS-NN) was performed comparing products with those of model simulations and other satellites. Finally, all products were evaluated against in situ measurements of soil moisture (SM). To achieve such a goal a set of metrics to evaluate different satellite products are suggested. The study demonstrated that the V620 shows a significant improvement (including those at L1 improving L2) with respect to the earlier version V551. Results also show that neural network based approaches can often yield excellent results over areas where other products are poor. Finally, global compa'rison indicates that SMOS behaves very well when compared to other sensors/approaches and gives consistent results over all surfaces from very dry (African Sahel, Arizona), to wet (tropical rain forests). RFI (Radio Frequency Interference) is still an issue even though detection has been greatly improved through the significant reduction of RFI sources in several areas of the world. When compared to other satellite products, the analysis shows that SMOS achieves its expected goals and is globally consistent over different eco climate regions from low to high latitudes and throughout the seasons. (C) 2016 Elsevier Inc All rights reserved.
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Kim, Y., Sartelet, K., Seigneur, C., Charron, A., Besombes, J. L., Jaffrezo, J. L., et al. (2016). Effect of measurement protocol on organic aerosol measurements of exhaust emissions from gasoline and diesel vehicles. Atmospheric Environment, 140, 176–187.
Abstract: Exhaust emissions of semi-volatile organic compounds (SVOC) from passenger vehicles are usually estimated only for the particle phase via the total particulate matter measurements. However, they also need to be estimated for the gas phase, as they are semi-volatile. To better estimate SVOC emission factors of passenger vehicles, a measurement campaign using a chassis dynamometer was conducted with different instruments: (1) a constant volume sampling (CVS) system in which emissions were diluted with filtered air and sampling was performed on filters and polyurethane foams (PUF) and (2) a Dekati Fine Particle Sampler (FPS) in which emissions were diluted with purified air and sampled with on-line instruments (PTR-ToF-MS, HR-ToF-AMS, MAAP, CPC). Significant differences in the concentrations of organic carbon (OC) measured by the instruments are observed. The differences can be explained by sampling artefacts, differences between (1) the time elapsed during sampling (in the case of filter and PUF sampling) and (2) the time elapsed from emission to measurement (in the case of on-line instruments), which vary from a few seconds to 15 min, and by the different dilution factors. To relate elapsed times and measured concentrations of OC, the condensation of SVOC between the gas and particle phases is simulated with a dynamic aerosol model. The simulation results allow us to understand the relation between elapsed times and concentrations in the gas and particle phases. They indicate that the characteristic times to reach thermodynamic equilibrium between gas and particle phases may be as long as 8 min. Therefore, if the elapsed time is less than this characteristic time to reach equilibrium, gas phase SVOC are not at equilibrium with the particle phase and a larger fraction of emitted SVOC will be in the gas phase than estimated by equilibrium theory, leading to an underestimation of emitted OC if only the particle phase is considered or if the gas-phase SVOC are estimated by equilibrium theory. Current European emission inventories for passenger cars do not yet estimate gas-phase SVOC emissions, although they may represent 60% of total emitted SVOC (gas + particle phases). (C) 2016 The Authors. Published by Elsevier Ltd.
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Klutse, N. A. B., Sylla, M. B., Diallo, I., Sarr, A., Dosio, A., Diedhiou, A., et al. (2016). Daily characteristics of West African summer monsoon precipitation in CORDEX simulations. Theoretical And Applied Climatology, 123(1-2), 369–386.
Abstract: We analyze and intercompare the performance of a set of ten regional climate models (RCMs) along with the ensemble mean of their statistics in simulating daily precipitation characteristics during the West African monsoon (WAM) period (June-July-August-September). The experiments are conducted within the framework of the COordinated Regional Downscaling Experiments for the African domain. We find that the RCMs exhibit substantial differences that are associated with a wide range of estimates of higher-order statistics, such as intensity, frequency, and daily extremes mostly driven by the convective scheme employed. For instance, a number of the RCMs simulate a similar number of wet days compared to observations but greater rainfall intensity, especially in oceanic regions adjacent to the Guinea Highlands because of a larger number of heavy precipitation events. Other models exhibit a higher wet-day frequency but much lower rainfall intensity over West Africa due to the occurrence of less frequent heavy rainfall events. This indicates the existence of large uncertainties related to the simulation of daily rainfall characteristics by the RCMs. The ensemble mean of the indices substantially improves the RCMs' simulated frequency and intensity of precipitation events, moderately outperforms that of the 95th percentile, and provides mixed benefits for the dry and wet spells. Although the ensemble mean improved results cannot be generalized, such an approach produces encouraging results and can help, to some extent, to improve the robustness of the response of the WAM daily precipitation to the anthropogenic greenhouse gas warming.
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Kupiszewski, P., Zanatta, M., Mertes, S., Vochezer, P., Lloyd, G., Schneider, J., et al. (2016). Ice residual properties in mixed-phase clouds at the high-alpine Jungfraujoch site. Journal Of Geophysical Research-Atmospheres, 121(20), 12343–12362.
Abstract: Ice residual (IR) and total aerosol properties were measured in mixed-phase clouds (MPCs) at the high-alpine Jungfraujoch research station. Black carbon (BC) content and coating thickness of BC-containing particles were determined using single-particle soot photometers. The ice activated fraction (IAF), derived from a comparison of IR and total aerosol particle size distributions, showed an enrichment of large particles in the IR, with an increase in the IAF from values on the order of 10(-4) to 10(-3) for 100nm (diameter) particles to 0.2 to 0.3 for 1m (diameter) particles. Nonetheless, due to the high number fraction of submicrometer particles with respect to total particle number, IR size distributions were still dominated by the submicrometer aerosol. A comparison of simultaneously measured number size distributions of BC-free and BC-containing IR and total aerosol particles showed depletion of BC by number in the IR, suggesting that BC does not play a significant role in ice nucleation in MPCs at the Jungfraujoch. The potential anthropogenic climate impact of BC via the glaciation effect in MPCs is therefore likely to be negligible at this site and in environments with similar meteorological conditions and a similar aerosol population. The IAF of the BC-containing particles also increased with total particle size, in a similar manner as for the BC-free particles, but on a level 1 order of magnitude lower. Furthermore, BC-containing IR were found to have a thicker coating than the BC-containing total aerosol, suggesting the importance of atmospheric aging for ice nucleation.
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Kutuzov, S. S., Mikhalenko, V. N., Grachev, A. M., Ginot, P., Lavrentiev, I. I., Kozachek, A. V., et al. (2016). First geophysical and shallow ice core investigation of the Kazbek plateau glacier, Caucasus Mountains. Environmental Earth Sciences, 75(23).
Abstract: First-ever ice core drilling at Mt. Kazbek (Caucasus Mountains) took place in the summer of 2014. A shallow ice core (18 m) was extracted from a plateau at similar to 4500 m a.s.l. in the vicinity of the Mt. Kazbek summit (5033 m a.s.l.). A detailed radar survey showed that the maximum ice thickness at this location is similar to 250 m. Borehole temperature of -7 degrees C was measured at 10 m depth. The ice core was analyzed for oxygen and deuterium isotopes and dust concentration. From the observed seasonal cycle, it was determined that the ice core covers the time interval of 2009-2014, with a mean annual snow accumulation rate of 1800 mm w.eq. Multiple melt layers have been detected. delta O-18 values vary from -25 to -5%. The dust content was determined using a particle sizing and counting analyzer. The dust layers were investigated using scanning electron microscopy and X-ray diffraction analysis. Dust can be separated into two categories by its origin: local and distant. Samples reflecting predominantly local origin consisted mainly of magmatic rocks, while clay minerals were a characteristic of dust carried over large distances, from the deserts of the Middle East and Sahara. The calculated average dust flux over three years at Kazbek was of 1.3 mg/cm(2) a(-1). Neither delta O-18 nor dust records appear to have been affected by summer melting. Overall, the conditions on Kazbek plateau and the available data suggest that the area offers good prospects of future deep drilling in order to obtain a unique environmental record.
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Langer, M., Westermann, S., Boike, J., Kirillin, G., Grosse, G., Peng, S., et al. (2016). Rapid degradation of permafrost underneath waterbodies in tundra landscapes-Toward a representation of thermokarst in land surface models. Journal Of Geophysical Research-Earth Surface, 121(12), 2446–2470.
Abstract: Waterbodies such as lakes and ponds are abundant in vast Arctic landscapes and strongly affect the thermal state of the surrounding permafrost. In order to gain a better understanding of the impact of small-and medium-sized waterbodies on permafrost and the formation of thermokarst, a land surface model was developed that can represent the vertical and lateral thermal interactions between waterbodies and permafrost. The model was validated using temperature measurements from two typical waterbodies located within the Lena River delta in northern Siberia. Impact simulations were performed under current climate conditions as well as under a moderate and a strong climate-warming scenario. The performed simulations demonstrate that small waterbodies can rise the sediment surface temperature by more than 10 degrees C and accelerate permafrost thaw by a factor of between 4 and 5. Up to 70% of this additional heat flux into the ground was found to be dissipated into the surrounding permafrost by lateral ground heat flux in the case of small, shallow, and isolated waterbodies. Under moderate climate warming, the lateral heat flux was found to reduce permafrost degradation underneath waterbodies by a factor of 2. Under stronger climatic warming, however, the lateral heat flux was too small to prevent rapid permafrost degradation. The lateral heat flux was also found to strongly impede the formation of thermokarst. Despite this stabilizing effect, our simulations have demonstrated that underneath shallow waterbodies (<1 m), thermokarst initiation happens 30 to 40 years earlier than in simulations without preexisting waterbody.
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Laube, J. C., Hanif, N. M., Martinerie, P., Gallacher, E., Fraser, P. J., Langenfelds, R., et al. (2016). Tropospheric observations of CFC-114 and CFC-114a with a focus on long-term trends and emissions. Atmospheric Chemistry And Physics, 16(23), 15347–15358.
Abstract: Chlorofluorocarbons (CFCs) are ozone-depleting substances as well as strong greenhouse gases, and the control of their production and use under the Montreal Protocol has had demonstrable benefits to both mitigation of increasing surface UV radiation and climate forcing. A global ban on consumption came into force in 2010, but there is evidence of continuing emissions of certain CFCs from a range of sources. One compound has received little attention in the literature, namely CFC-114 (C2Cl2F4). Of particular interest here is the differentiation between CFC-114 (CClF2CClF2) and its asymmetric isomeric form CFC-114a (CF3CCl2F) as atmospheric long-term measurements in the peer-reviewed literature to date have been assumed to represent the sum of both isomers with a time-invariant isomeric speciation. Here we report the first long-term measurements of the two isomeric forms separately, and find that they have different origins and trends in the atmosphere. Air samples collected at Cape Grim (41 degrees S), Australia, during atmospheric background conditions since 1978, combined with samples collected from deep polar snow (firn) enable us to obtain a near-complete record of both gases since their initial production and release in the 1940s. Both isomers were present in the unpolluted atmosphere in comparably small amounts before 1960. The mixing ratio of CFC-114 doubled from 7.9 to 14.8 parts per trillion (ppt) between the start of the Cape Grim record in 1978 and the end of our record in 2014, while over the same time CFC-114a trebled from 0.35 to 1.03 ppt. Mixing ratios of both isomers are slowly decreasing by the end of this period. This is consistent with measurements of recent aircraft-based samples showing no significant interhemispheric mixing ratio gradient. We also find that the fraction of CFC-114a mixing ratio relative to that of CFC-114 increased from 4.2 to 6.9% over the 37-year period. This contradicts the current tacit assumption used in international climate change and ozone depletion assessments that both isomers have been largely co-emitted and that their atmospheric concentration ratio has remained approximately constant in time. Complementary observations of air collected in Taiwan indicate a persisting source of CFC-114a in South East Asia which may have been contributing to the changing balance between the two isomers. In addition we present top-down global annual emission estimates of CFC-114 and CFC-114a derived from these measurements using a two-dimensional atmospheric chemistry-transport model. In general, the emissions for both compounds grew steadily during the 1980s, followed by a substantial reduction from the late 1980s onwards, which is consistent with the reduction of emission in response to the Montreal Protocol, and broadly consistent with bottom-up estimates derived by industry. However, we find that small but significant emissions of both isomers remain in 2014. Moreover the inferred changes to the ratio of emissions of the two isomers since the 1990s also indicate that the sources of the two gases are, in part, independent.
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Le Coz, J., Blanquart, B., Pobanz, K., Dramais, G., Pierrefeu, G., Hauet, A., et al. (2016). Estimating the Uncertainty of Streamgauging Techniques Using In Situ Collaborative Interlaboratory Experiments. Journal Of Hydraulic Engineering, 142(7).
Abstract: While the application of uncertainty propagation methods to hydrometry is still challenging, in situ collaborative interlaboratory experiments are a valuable tool for empirically estimating the uncertainty of stream gauging techniques in given measurement conditions. The authors propose a simple procedure for organizing such experiments and processing the results according to the authoritative ISO standards related to interlaboratory experiments, which are of common practice in many metrological fields. Beyond the computation and interpretation of the results, some issues are discussed regarding the estimation of the stream gauging technique bias in the absence of accurate enough discharge references in rivers; the uncertainty of the uncertainty estimates, according to the number of participants and repeated measurements; the criteria related to error sources that are possibly meaningful for categorizing measurement conditions. The interest and limitations of the in situ collaborative interlaboratory experiments are exemplified by an application to the hydro-acoustic profiler (ADCP) stream gauging technique conducted in 2010 at two different sites downstream of Genissiat hydropower plant in the Rhone river, France. Typically, the expanded uncertainty (with a probability level of 95%) of the average discharge over six successive transects varied from +/- 5% at one site with favorable conditions to +/- 9% at the other site due to unstable flow conditions.
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Legchenko, A., Vouillamoz, J. M., Lawson, F. M. A., Alle, C., Descloitres, M., & Boucher, M. (2016). Interpretation of magnetic resonance measurements in the varying earth's magnetic field. Geophysics, 81(4), WB23–WB31.
Abstract: At the scale of a magnetic resonance sounding (MRS) field setup, the earth's magnetic field in the subsurface may vary laterally with depth and over time. These variations can be caused by different natural factors and generally cannot be compensated for by accurate tuning of the measuring device. The varying geomagnetic field (GMF) causes nonresonance conditions of excitation that affect the amplitude and phase of the MRS signal. Usually, variations of the GMF do not exceed a few hertz and their effect on the amplitude is relatively small, permitting us to assume near-resonance conditions for inversion. However, in some cases, the results may be erroneous if a varying GMF is not taken into account. Motivated by possible improvements in MRS inversion, we have developed a procedure for measuring and interpreting MRS data that considers a varying GMF. Our results showed that it is relatively easy to take a time-varying GMF into account. As a demonstration, we have developed the inversion of MRS data measured in Benin (West Africa). A depth-varying GMF is a more complex problem, and to consider this, we have developed an algorithm of nonlinear inversion. We have tested this approach on synthetic data, which resulted in an improved inversion. Field validation of this procedure awaits the discovery of a suitable test site with known variations of the earth's magnetic field in the subsurface.
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Legrand, M., McConnell, J., Fischer, H., Wolff, E. W., Preunkert, S., Arienzo, M., et al. (2016). Boreal fire records in Northern Hemisphere ice cores: a review. Climate Of The Past, 12(10).
Abstract: Here, we review different attempts made since the early 1990s to reconstruct past forest fire activity using chemical signals recorded in ice cores extracted from the Greenland ice sheet and a few mid-northern latitude, high-elevation glaciers. We first examined the quality of various inorganic (ammonium, nitrate, potassium) and organic (black carbon, various organic carbon compounds including levoglucosan and numerous carboxylic acids) species proposed as fire proxies in ice, particularly in Greenland. We discuss limitations in their use during recent vs. pre-industrial times, atmospheric lifetimes, and the relative importance of other non-biomass-burning sources. Different high-resolution records from several Greenland drill sites and covering various timescales, including the last century and Holocene, are discussed. We explore the extent to which atmospheric transport can modulate the record of boreal fires from Canada as recorded in Greenland ice. Ammonium, organic fractions (black and organic carbon), and specific organic compounds such as formate and vanillic acid are found to be good proxies for tracing past boreal fires in Greenland ice. We show that use of other species – potassium, nitrate, and carboxylates (except formate) – is complicated by either post-depositional effects or existence of large non-biomass-burning sources. The quality of levoglucosan with respect to other proxies is not addressed here because of a lack of high-resolution profiles for this species, preventing a fair comparison. Several Greenland ice records of ammonium consistently indicate changing fire activity in Canada in response to past climatic conditions that occurred during the last millennium and since the last large climatic transition. Based on this review, we make recommendations for further study to increase reliability of the reconstructed history of forest fires occurring in a given region.
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Legrand, M., Preunkert, S., Savarino, J., Frey, M. M., Kukui, A., Helmig, D., et al. (2016). Inter-annual variability of surface ozone at coastal (Dumont d'Urville, 2004-2014) and inland (Concordia, 2007-2014) sites in East Antarctica. Atmospheric Chemistry And Physics, 16(12), 8053–8069.
Abstract: Surface ozone has been measured since 2004 at the coastal East Antarctic site of Dumont d'Urville (DDU), and since 2007 at the Concordia station located on the high East Antarctic plateau. This paper discusses long-term changes, seasonal and diurnal cycles, as well as inter-annual summer variability observed at these two East Antarctic sites. At Concordia, near-surface ozone data were complemented by balloon soundings and compared to similar measurements done at the South Pole. The DDU record is compared to those obtained at the coastal site of Syowa, also located in East Antarctica, as well as the coastal sites of Neumayer and Halley, both located on the coast of the Weddell Sea in West Antarctica. Surface ozone mixing ratios exhibit very similar seasonal cycles at Concordia and the South Pole. However, in summer the diurnal cycle of ozone is different at the two sites with a drop of ozone in the afternoon at Concordia but not at the South Pole. The vertical distribution of ozone above the snow surface also differs. When present, the ozone-rich layer located near the ground is better mixed and deeper at Concordia (up to 400aEuro-m) than at the South Pole during sunlight hours. These differences are related to different solar radiation and wind regimes encountered at these two inland sites. DDU appears to be the coastal site where the impact of the late winter/spring bromine chemistry is the weakest, but where the impact of elevated ozone levels caused by NOx snow emissions from the high Antarctic plateau is the highest. The highest impact of the bromine chemistry is seen at Halley and Neumayer, and to a lesser extent at Syowa. These three sites are only weakly impacted by the NOx chemistry and the net ozone production occurring on the high Antarctic plateau. The differences in late winter/spring are attributed to the abundance of sea ice offshore from the sites, whereas those in summer are related to the topography of East Antarctica that promotes the katabatic flow bringing oxidant-rich inland air masses to the site. There appears to be a decreasing change in summer surface ozone at the two East Antarctic sites of Concordia and DDU over the most recent period (2004-2014 and 2007-2014). Further research, including continued monitoring, is needed at these two sites to better separate the effect of synoptic transport from possible change of NOx snow emissions in response to recovery of the stratospheric ozone layer leading to penetration of more UV radiation to the surface.
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Legrand, M., Yang, X., Preunkert, S., & Theys, N. (2016). Year-round records of sea salt, gaseous, and particulate inorganic bromine in the atmospheric boundary layer at coastal (Dumont d'Urville) and central (Concordia) East Antarctic sites. Journal Of Geophysical Research-Atmospheres, 121(2), 997–1023.
Abstract: Multiple year-round records of bulk and size-segregated compositions of aerosol were obtained at the coastal Dumont d'Urville (DDU) and inland Concordia sites located in East Antarctica. They document the sea-salt aerosol load and composition including, for the first time in Antarctica, the bromide depletion of sea-salt aerosol relative to sodium with respect to seawater. In parallel, measurements of bromide trapped in mist chambers and denuder tubes were done to investigate the concentrations of gaseous inorganic bromine species. These data are compared to simulations done with an off-line chemistry transport model, coupled with a full tropospheric bromine chemistry scheme and a process-based sea-salt production module that includes both sea-ice-sourced and open-ocean-sourced aerosol emissions. Observed and simulated sea-salt concentrations sometime differ by up to a factor of 2 to 3, particularly at DDU possibly due to local wind pattern. In spite of these discrepancies, both at coastal and inland Antarctica, the dominance of sea-ice-related processes with respect to open ocean emissions for the sea-salt aerosol load in winter is confirmed. For summer, observations and simulations point out sea salt as the main source of gaseous inorganic bromine species. Investigations of bromide in snow pit samples do not support the importance of snowpack bromine emissions over the Antarctic Plateau. To evaluate the overall importance of the bromine chemistry over East Antarctica, BrO simulations were also discussed with respect data derived from GOME-2 satellite observations over Antarctica.
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Leroux, D. J., Pellarin, T., Vischel, T., Cohard, J. M., Gascon, T., Gibon, F., et al. (2016). Assimilation of SMOS soil moisture into a distributed hydrological model and impacts on the water cycle variables over the Ouémé catchment in Benin. Hydrol. Earth Syst. Sci., 20(7), 2827–2840. |
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Leroux, S., Bellon, G., Roehrig, R., Caian, M., Klingaman, N. P., Lafore, J. P., et al. (2016). Inter-model comparison of subseasonal tropical variability in aquaplanet experiments: Effect of a warm pool. Journal of Advances in Modeling Earth Systems, 8(4), 1526–1551.
Abstract: This study compares the simulation of subseasonal tropical variability by a set of six state-of-the-art AGCMs in two experiments in aquaplanet configuration: a zonally symmetric experiment, and an experiment with a warm pool centered on the equator. In all six models, the presence of the warm pool generates zonal asymmetries in the simulated mean states in the form of a "Gill-type'' response, made more complex by feedbacks between moisture, convective heating and circulation. Noticeable differences appear from one model to another. Only half the models simulate mean low-level equatorial westerlies over the warm pool area. The presence of the warm pool can also favor the development of large-scale variability consistent with observed Madden-Julian Oscillation (MJO) characteristics, but this happens only in half the models. Our results do not support the idea that the presence of the warm pool and/or of mean low-level equatorial westerlies are sufficient conditions for MJO-like variability to arise in the models. Comparing spectral characteristics of the simulated Convectively Coupled Equatorial Waves (CCEWs) in the aquaplanet experiments and the corresponding coupled atmosphere-ocean (i. e., CMIP) and atmosphere-only (i. e., AMIP) simulations, we also show that there is more consistency for a given model across its configurations, than for a given configuration across the six models. Overall, our results confirm that the simulation of subseasonal variability by given model is significantly influenced by the parameterization of subgrid physical processes (most-likely cloud processes), both directly and through modulation of the mean state.
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Linsbauer, A., Frey, H., Haeberli, W., Machguth, H., Azam, M. F., & Allen, S. (2016). Modelling glacier-bed overdeepenings and possible future lakes for the glaciers in the Himalaya-Karakoram region. Annals Of Glaciology, 57(71), 119–130.
Abstract: Surface digital elevation models (DEMs) and slope-related estimates of glacier thickness enable modelling of glacier-bed topographies over large ice-covered areas. Due to the erosive power of glaciers, such bed topographies can contain numerous overdeepenings, which when exposed following glacier retreat may fill with water and form new lakes. In this study, the bed overdeepenings for similar to 28 000 glaciers (40 775 km(2)) of the Himalaya-Karakoram region are modelled using GlabTop2 (Glacier Bed Topography model version 2), in which ice thickness is inferred from surface slope by parameterizing basal shear stress as a function of elevation range for each glacier. The modelled ice thicknesses are uncertain (+/- 30%), but spatial patterns of ice thickness and bed elevation primarily depend on surface slopes as derived from the DEM and, hence, are more robust. About 16 000 overdeepenings larger than 104 m(2) were detected in the modelled glacier beds, covering an area of similar to 2200 km(2) and having a volume of similar to 120 km(3) (3-4% of present-day glacier volume). About 5000 of these overdeepenings (1800 km(2)) have a volume larger than 10(6) m(3). The results presented here are useful for anticipating landscape evolution and potential future lake formation with associated opportunities (tourism, hydropower) and risks (lake outbursts).
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Lipenkov, V. Y., Ekaykin, A. A., Polyakova, E. V., & Raynaud, D. (2016). Characterization of subglacial Lake Vostok as seen from physical and isotope properties of accreted ice. Philosophical Transactions Of The Royal Society A-Mathematical Physical And Engineering Sciences, 374(2059).
Abstract: Deep drilling at the Vostok Station has reached the surface of subglacial Lake Vostok (LV) twice-in February 2012 and January 2015. As a result, three replicate cores from boreholes 5G-1, 5G-2 and 5G-3 became available for detailed and revalidation analyses of the 230 m thickness of the accreted ice, down to its contact with water at 3769 m below the surface. The study reveals that the concentration of gases in the lake water beneath Vostok is unexpectedly low. A clear signature of the melt water in the surface layer of the lake, which is subject to refreezing on the icy ceiling of LV, has been discerned in the three different properties of the accreted ice: the ice texture, the isotopic and the gas content of the ice. These sets of data indicate in concert that poor mixing of the melt (and hydrothermal) water with the resident lake water and pronounced spatial and/or temporal variability of local hydrological conditions are likely to be the characteristics of the southern end of the lake. The latter implies that the surface water may be not representative enough to study LV's behaviour, and that direct sampling of the lake at different depths is needed in order to move ahead with our understanding of the lake's hydrological regime.
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Lutoff, C., Creutin, J. D., Ruin, I., & Borga, M. (2016). Anticipating flash-floods: Multi-scale aspects of the social response. Journal Of Hydrology, 541, 626–635.
Abstract: This paper aims at exploring the anticipation phase before a flash flood, corresponding to the time between the first climatic signs and the peak-flow. We focus the analysis on people's behaviors observing how they use this period to organize themselves for facing the event. The analysis is made through the definition of three specific scales: the timeliness scale, an analytical scale of anticipatory actions and the scale of human response network. Using a cross-scale and cross level analysis enables to define different phases in the anticipation period where different kind of environmental precursors are mobilized by the actors in order to make sense of the situation and adapt. Three main points deserve attention at the end: firstly, the concepts of timeliness, anticipatory actions and crisis network scales enable to understand differently what happens both physically and socially during an extreme event; secondly, analyzing the precursors shows that each level of crisis network uses different kinds of signs for estimating the situation, organizing and reacting; thirdly, there is a potential for improvement in observation on both social and physical processes at different scales, for verifying the theory of the anticipatory phases. (C) 2016 Elsevier B.V. All rights reserved.
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Macelloni, G., Leduc-Leballeur, M., Brogioni, M., Ritz, C., & Picard, G. (2016). Analyzing and modeling the SMOS spatial variations in the East Antarctic Plateau. Remote Sensing Of Environment, 180, 193–204.
Abstract: The SMOS brightness temperature (T-B) collected on the East Antarctic Plateau revealed spatial signatures at L-band that have never before been observed when only higher-frequency passive microwave observations were available, and this has opened up a new field of research. Because of the much greater penetration depth, modeling the microwave ice sheet emission requires taldng into account not only snow conditions on the surface, but should also include glaciological information. Even if the penetration depth of the L-band is not well known due to the uncertainty on the imaginary part of the ice permittivity, it is likely to be of the order of several hundreds of meters, which means that the temperature of the ice over a depth of nearly 1000 m influences the emission. Over such a depth, the temperature is related to both the surface conditions and to the ice sheet thickness, which in turn depends on the bedrock topography and on other glaciological variables. The present paper aims to provide a thorough theoretical explanation of the observed T-B spatial variation close to the Brewster angle at vertical polarization, in order to limit the effect of surface and vertical density variability in the firn. In order to provide reliable inputs to the microwave emission models used for simulating T-B data, an in-depth analysis of the temperature profiles was performed by means of glaciological models. The comparison between simulated and observed data over three transects totalling 2000 km in East Antarctica pointed out that, whereas the emission models are capable of explaining the T-B spatial variations of several kelvins (0.7 and 2.9 K), they are unable to predict its absolute value correctly. This study also shows that the main limiting factor in simulating low-frequency microwave data is the uncertainty in the currently available imaginary part of the ice permittivity. (C) 2016 Elsevier Inc All rights reserved.
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Makris, A., Prieur, C., Vischel, T., Quantin, G., Lebel, T., & Roca, R. (2016). Stochastic tracking of mesoscale convective systems: evaluation in the West African Sahel. Stochastic Environmental Research And Risk Assessment, 30(2), 681–691.
Abstract: In this work we apply a recently proposed Bayesian multiple target tracking model to mesoscale convective systems tracking. This stochastic model follows the multiple hypothesis tracking paradigm and can handle a varying number of targets while detecting the target birth, death, split, and merge events. The model is tested experimentally with real MCS targets detected from meteosat IR data over the Sahelian region. The performance of the stochastic tracking is evaluated by comparing it qualitatively and quantitatively with well established deterministic methods.
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Malam-Abdou, M., Vandervaere, J. P., Bouzou-Moussa, I., Descroix, L., Mamadou, I., & Faran-Maiga, O. (2016). Flow production within two small crystalline basement catchments of Western Niger: multiscale standpoint for the hydrodynamical functionning. Geomorphologie-Relief Processus Environnement, 22(4), 363–375.
Abstract: The observed increase of the catchments flow values in the Sahel during the drought years has exacerbated the interest for the studies on the superficial hydrodynamic properties of soils and their impact onto runoff volumes. This work focuses on the processes responsible for the flow production within two experimental catchments situated in the granitic basement of Western Niger. It aims at characterizing the hydrodynamical functioning at three nested spatial scales. At point scale (50 cm(2)), soil surface infiltrability (0-10 cm) is determined for different surface features through hydraulic conductivity (Ks) measurements carried out in situ. At the plot scale, runoff is measured from 10 m(2) experimental plots while at catchment scale (5 ha), stream flow is controlled by stream gauges at the exutories. Measurements show low values of hydraulic conductivity ranging from 10 mm.h(-1) (minimal value measured on erosion crusts ERO) to 40 mm.h(-1) (maximal value measured on cultivated sites in seasonal average). Runoff coefficients are inversely ranked, from 0.60 on ERO to 0.25 on cultivated surface. Catchment scale runoff coefficients are 0.41 and 0.28 for the not cultivated and the cultivated basins, respectively. Results obtained at these three scales are consistent and reveal the strong runoff production ability of these granitic catchments resulting from both low point scale infiltrability and small re-infiltration of runoff water within the stream network. This last component represents only 4% of annual rainfall while it may reach more than 50% for sedimentary catchments of similar size. Within these catchments, thus, a 2-4 mm rainfall is sufficient to generate a flow at the outlet due to rather small (less than 20 cm) sand deposits covering the stream beds. Finally, both at the plot and at the catchment scales, hydrodynamical functioning was found quasi independent on the initial water content.
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Mamadou, O., Galle, S., Cohard, J. M., Peugeot, C., Kounouhewa, B., Biron, R., et al. (2016). Dynamics of water vapor and energy exchanges above two contrasting Sudanian climate ecosystems in Northern Benin (West Africa). Journal Of Geophysical Research-Atmospheres, 121(19), 11269–11286.
Abstract: Natural ecosystems in sub-Saharan Africa are experiencing intense changes that will probably modify land surface feedbacks and consequently the regional climate. In this study, we have analyzed water vapor (Q(LE)) and sensible heat (Q(H)) fluxes over a woodland (Bellefoungou, BE) and a cultivated area (Nalohou, NA) in the Sudanian climate of Northern Benin, using 2years (from July 2008 to June 2010) of eddy covariance measurements. The evaporative fraction (EF) response to environmental and surface variables was investigated at seasonal scale. Soil moisture was found to be the main environmental factor controlling energy partitioning. During the wet seasons, EF was rather stable with an average of 0.750.07 over the woodland and 0.700.025 over the cultivated area. This means that 70-75% of the available energy was changed into actual evapotranspiration during the investigated wet seasons depending on the vegetation type. The cumulative annual actual evapotranspiration (AET) varied between 73050mmyr(-1) at the NA site and 104070mmyr(-1) at the BE site. With similar weather conditions at the two sites, the BE site showed 30% higher AET values than the NA site. The sensible heat flux Q(H) at the cultivated site was always higher than that of the woodland site, but observed differences were much less than those of Q(LE). In a land surface conversion context, these differences are expected to impact both atmospheric dynamics and the hydrological cycle.
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Mariac, C., Ousseini, I. S., Alio, A. K., Jugde, H., Pham, J. L., Bezancon, G., et al. (2016). Spatial and Temporal Variation in Selection of Genes Associated with Pearl Millet Varietal Quantitative Traits In situ. Frontiers In Genetics, 7.
Abstract: Ongoing global climate changes imply new challenges for agriculture. Whether plants and crops can adapt to such rapid changes is still a widely debated question. We previously showed adaptation in the form of earlier flowering in pearl millet at the scale of a whole country over three decades. However, this analysis did not deal with variability of year to year selection. To understand and possibly manage plant and crop adaptation, we need more knowledge of how selection acts in situ. Is selection gradual, abrupt, and does it vary in space and over time? In the present study, we tracked the evolution of allele frequency in two genes associated with pearl millet phenotypic variation in situ. We sampled 17 populations of cultivated pearl millet over a period of 2 years. We tracked changes in allele frequencies in these populations by genotyping more than seven thousand individuals. We demonstrate that several allele frequencies changes are compatible with selection, by correcting allele frequency changes associated with genetic drift. We found marked variation in allele frequencies from year to year, suggesting a variable selection effect in space and over time. We estimated the strength of selection associated with variations in allele frequency. Our results suggest that the polymorphism maintained at the genes we studied is partially explained by the spatial and temporal variability of selection. In response to environmental changes, traditional pearl millet varieties could rapidly adapt thanks to this available functional variability.
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Marra, F., Nikolopoulos, E. I., Creutin, J. D., & Borga, M. (2016). Space-time organization of debris flows-triggering rainfall and its effect on the identification of the rainfall threshold relationship. Journal Of Hydrology, 541, 246–255.
Abstract: Debris flow occurrence is generally forecasted by means of empirical rainfall depth-duration thresholds based on raingauge observations. Rainfall estimation errors related to the sparse nature of raingauge data are enhanced in case of convective rainfall events characterized by limited spatial extent. Such errors have been shown to cause underestimation of the rainfall thresholds and, thus, less efficient forecasts of debris flows occurrence. This work examines the spatial organization of debris flows-triggering rainfall around the debris flow initiation points using high-resolution, carefully corrected radar data for a set of short duration (<30 h) storm events occurred in the eastern Italian Alps. On average, triggering rainfall presents a local peak corresponding to the debris flow initiation point, with rain depth at 5 km (10 km) distance being on average around 70% (40%) of rain depth observed at the debris flow initiation points. The peak is consistently enhanced for events characterized by short durations and causes a systematic underestimation of the rainfall depth-duration thresholds when rainfall is measured away from the debris flow initiation points. We develop an analytical framework that exploits the general characteristics of the spatial rainfall organization to predict the systematic underestimation of the depth-duration thresholds when rainfall is sampled away from the initiation points. Predictions obtained based on this analytical framework are assessed using a Monte Carlo sampling technique. (C) 2015 Elsevier B.V. All rights reserved.
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Maurel, A., Mercier, J. F., & Montagnat, M. (2016). Critical investigation of calculation methods for the elastic velocities in anisotropic ice polycrystals. Cryosphere, 10(6), 3063–3070.
Abstract: Crystallographic texture (or fabric) evolution with depth along ice cores can be evaluated using borehole sonic logging measurements. These measurements provide the velocities of elastic waves that depend on the ice polycrystal anisotropy, and they can further be related to the ice texture. To do so, elastic velocities need to be inverted from a modeling approach that relate elastic velocities to ice texture. So far, two different approaches can be found. A classical model is based on the effective medium theory; the velocities are derived from elastic wave propagation in a homogeneous medium characterized by an average elasticity tensor. Alternatively, a velocity averaging approach was used in the glaciology community that averages the velocities from a given population of single crystals with different orientations. In this paper, we show that the velocity averaging method is erroneous in the present context. This is demonstrated for the case of waves propagating along the clustering direction of a highly textured polycrystal, characterized by crystallographic c axes oriented along a single maximum (cluster). In this case, two different shear wave velocities are obtained while a unique velocity is theoretically expected. While making use of this velocity averaging method, reference work by Bennett (1968) does not end with such an unphysical result. We show that this is due to the use of erroneous expressions for the shear wave velocities in a single crystal, as the starting point of the averaging process. Because of the weak elastic anisotropy of ice single crystal, the inversion of the measured velocities requires accurate modeling approaches. We demonstrate here that the inversion method based on the effective medium theory provides physically based results and should therefore be favored.
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Mazzilli, N., Boucher, M., Chalikakis, K., Legchenko, A., Jourde, H., & Champollion, C. (2016). Contribution of magnetic resonance soundings for characterizing water storage in the unsaturated zone of karst aquifers. Geophysics, 81(4), WB49–WB61.
Abstract: Understanding the role of the unsaturated zone in aquifer recharge and contaminant attenuation processes is a major challenge for the protection and management of karstic water resources. We present the potential of the magnetic resonance soundings (MRS) geophysical method for characterizing the vadose zone of karst aquifers composed of epikarst and infiltration layers. To investigate the hydraulic functioning of the Durzon karst system located on the Larzac plateau (southern France), we used the MRS method at 16 sites. TheMRS results have been compared with available geologic information and to core water content measurements. The remarkable spatial variability of the MRS response observed in the study area makes it possible to determine ranges of water storage properties in relation to the lithology of the investigated carbonate formations (dolomite, marly, and siliceous limestone). All soundings found either constant or increasing MRS water content with depth, which demonstrates that the infiltration zone might be the major water storage entity for permanent water storage, with important consequences for recharge quality and quantity. These results show the feasibility and potential of the MRS method for the characterization of the karst unsaturated zone and for understanding the vertical distribution of water content, which impacts the overall functioning of karst systems.
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McGuire, A. D., Koven, C., Lawrence, D. M., Clein, J. S., Xia, J. Y., Beer, C., et al. (2016). Variability in the sensitivity among model simulations of permafrost and carbon dynamics in the permafrost region between 1960 and 2009. Global Biogeochemical Cycles, 30(7), 1015–1037.
Abstract: A significant portion of the large amount of carbon (C) currently stored in soils of the permafrost region in the Northern Hemisphere has the potential to be emitted as the greenhouse gases CO2 and CH4 under a warmer climate. In this study we evaluated the variability in the sensitivity of permafrost and C in recent decades among land surface model simulations over the permafrost region between 1960 and 2009. The 15 model simulations all predict a loss of near-surface permafrost (within 3m) area over the region, but there are large differences in the magnitude of the simulated rates of loss among the models (0.2 to 58.8x10(3)km(2)yr(-1)). Sensitivity simulations indicated that changes in air temperature largely explained changes in permafrost area, although interactions among changes in other environmental variables also played a role. All of the models indicate that both vegetation and soil C storage together have increased by 156 to 954TgCyr(-1) between 1960 and 2009 over the permafrost region even though model analyses indicate that warming alone would decrease soil C storage. Increases in gross primary production (GPP) largely explain the simulated increases in vegetation and soil C. The sensitivity of GPP to increases in atmospheric CO2 was the dominant cause of increases in GPP across the models, but comparison of simulated GPP trends across the 1982-2009 period with that of a global GPP data set indicates that all of the models overestimate the trend in GPP. Disturbance also appears to be an important factor affecting C storage, as models that consider disturbance had lower increases in C storage than models that did not consider disturbance. To improve the modeling of C in the permafrost region, there is the need for the modeling community to standardize structural representation of permafrost and carbon dynamics among models that are used to evaluate the permafrost C feedback and for the modeling and observational communities to jointly develop data sets and methodologies to more effectively benchmark models.
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Merino, N., Le Sommer, J., Durand, G., Jourdain, N. C., Madec, G., Mathiot, P., et al. (2016). Antarctic icebergs melt over the Southern Ocean: Climatology and impact on sea ice. Ocean Modelling, 104, 99–110.
Abstract: Recent increase in Antarctic freshwater release to the Southern Ocean is suggested to contribute to change in water masses and sea ice. However, climate models differ in their representation of the freshwater sources. Recent improvements in altimetry-based detection of small icebergs and in estimates of the mass loss of Antarctica may help better constrain the values of Antarctic freshwater releases. We propose a model-based seasonal climatology of iceberg melt over the Southern Ocean using state-of-the-art observed glaciological estimates of the Antarctic mass loss. An improved version of a Lagrangian iceberg model is coupled with a global, eddy-permitting ocean/sea ice model and compared to small icebergs observations. Iceberg melt increases sea ice cover, about 10% in annual mean sea ice volume, and decreases sea surface temperature over most of the Southern Ocean, but with distinctive regional patterns. Our results underline the importance of improving the representation of Antarctic freshwater sources. This can be achieved by forcing ocean/sea ice models with a climatological iceberg fresh-water flux. (C) 2016 Elsevier Ltd. All rights reserved.
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Meynadier, R., de Coetlogon, G., Leduc-Leballeur, M., Eymard, L., & Janicot, S. (2016). Seasonal influence of the sea surface temperature on the low atmospheric circulation and precipitation in the eastern equatorial Atlantic. Climate Dynamics, 47(3-4), 1127–1142.
Abstract: The air-sea interaction in the Gulf of Guinea and its role in setting precipitation at the Guinean coast is investigated in the present paper. This study is based on satellite observations and WRF simulations forced by different sea surface temperature (SST) patterns. It shows that the seasonal cold tongue setup in the Gulf of Guinea, along with its very active northern front, tends to strongly constrain the low level atmospheric dynamics between the equator and the Guinean coast. Underlying mechanisms including local SST effect on the marine boundary layer stability and hydrostatically-changed meridional pressure gradient through changes in SST gradient are quantified in WRF regarding observations and CFSR reanalyses. Theses mechanisms strongly impact moisture flux convergence near the coast, leading to the installation of the first rainy season of the West African Monsoon (WAM) system. The current study details the mechanisms by which the Atlantic Equatorial cold tongue plays a major role in the pre-onset of the boreal WAM.
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Moron, V., Oueslati, B., Pohl, B., Rome, S., & Janicot, S. (2016). Trends of mean temperatures and warm extremes in northern tropical Africa (1961-2014) from observed and PPCA-reconstructed time series. Journal Of Geophysical Research-Atmospheres, 121(10), 5298–5319.
Abstract: Trends in daily maximum (TX) and minimum (TN) temperatures and indices of warm extremes are studied in tropical North Africa, west of the eastern African highlands, from 1961 to 2014. The analysis is based on the concatenation and cross-checking of two observed databases. Due to the large number of missing entries (similar to 25%), a statistical infilling using probabilistic principal component analysis was applied. Averaged over 90 stations, the linear trends of annual mean TX and TN equal respectively +0.021 degrees C/yr and +0.028 degrees C/yr. The frequency of very hot days (TX > 35 degrees C) and tropical nights (TN> 20 degrees C), as well as the frequency of daily TX and TN above the 90th percentile (p90) (“warm days” and “warm nights”), roughly follows the variations of mean TX and TN, respectively. Heat spells of TX or TN> p90 are often short (usually <2-3 days), and the interannual variation of their mean duration is noisier than for the other indices. Nevertheless, heat spells tend to last longer, with almost constantly positive anomalies since the mid-1990s. The trends in March-June, the warmest season across the Sahelian and Sudanian belts, show similar variations as annual means. Overall, the local-scale warming in annual temperatures, and in March-June, may be viewed merely as a simple shift of the probability distribution function of daily TX and TN. The correlations between the thermal indices and the 2m temperatures suggest that the low-frequency (>8 years) variations may be viewed as a regional-scale fingerprint of the global warming, with largest correlations in the tropical Atlantic and Indian basins, while the high-frequency (<8 years) variations should be mostly viewed as a delayed remote impact of El Nino-Southern Oscillation (ENSO) events over the region, with warm (cold) anomalies tending to follow warm (cold) ENSO events.
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Morri, M., Soualmia, A., & Belleudy, P. (2016). Mean Velocity Predictions in Vegetated Flows. Journal Of Applied Fluid Mechanics, 9(3), 1273–1283.
Abstract: Vegetation plays an important role in influencing the hydrodynamic behavior, ecological equilibrium and environmental characteristics of water bodies. Several previous models have been developed, to predict hydraulic conditions in vegetated rivers, but only few are actually used in practice. In This paper six analytic model derived for submerged vegetation are compared and evaluate: Klopstra et al. (1997); Stone and Shen (2002); Van velzen (2003); Baptist et al. (2007); Huthoff et al. (2007) and Yang and Choi (2010). The evaluation of the flow formulas is based on the comparison with experimental data from literature using the criteria of deviation. Most descriptors show a good performance for predicting the mean velocity for rigid vegetation. However, the flow formulas proposed by Klopstra et al. (1997) and Huthoff et al. (2007) show the best fit to experimental data. Only for experiments with law density, these models indicate an underestimation. Velocity predicted for flexible vegetation by the six models is less accurate than the prediction in the case of rigid vegetation.
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Mosbeux, C., Gillet-Chaulet, F., & Gagliardini, O. (2016). Comparison of adjoint and nudging methods to initialise ice sheet model basal conditions. Geoscientific Model Development, 9(7), 2549–2562.
Abstract: Ice flow models are now routinely used to forecast the ice sheets' contribution to 21st century sea-level rise. For such short term simulations, the model response is greatly affected by the initial conditions. Data assimilation algorithms have been developed to invert for the friction of the ice on its bedrock using observed surface velocities. A drawback of these methods is that remaining uncertainties, especially in the bedrock elevation, lead to non-physical ice flux divergence anomalies resulting in undesirable transient effects. In this study, we compare two different assimilation algorithms based on adjoints and nudging to constrain both bedrock friction and elevation. Using synthetic twin experiments with realistic observation errors, we show that the two algorithms lead to similar performances in reconstructing both variables and allow the flux divergence anomalies to be significantly reduced.
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Mourre, L., Condom, T., Junquas, C., Lebel, T., Sicart, J. E., Figueroa, R., et al. (2016). Spatio-temporal assessment of WRF, TRMM and in situ precipitation data in a tropical mountain environment (Cordillera Blanca, Peru). Hydrology And Earth System Sciences, 20(1), 125–141.
Abstract: The estimation of precipitation over the broad range of scales of interest for climatologists, meteorologists and hydrologists is challenging at high altitudes of tropical regions, where the spatial variability of precipitation is important while in situ measurements remain scarce largely due to operational constraints. Three different types of rainfall products – ground based (kriging interpolation), satellite derived (TRMM3B42), and atmospheric model outputs (WRF – Weather Research and Forecasting) – are compared for 1 hydrological year in order to retrieve rainfall patterns at timescales ranging from sub-daily to annual over a watershed of approximately 10 000 km(2) in Peru. An ensemble of three different spatial resolutions is considered for the comparison (27, 9 and 3 km), as long as well as a range of timescales (annual totals, daily rainfall patterns, diurnal cycle). WRF simulations largely overestimate the annual totals, especially at low spatial resolution, while reproducing correctly the diurnal cycle and locating the spots of heavy rainfall more realistically than either the ground-based KED or the Tropical Rainfall Measuring Mission (TRMM) products. The main weakness of kriged products is the production of annual rainfall maxima over the summit rather than on the slopes, mainly due to a lack of in situ data above 3800 ma.s.l. This study also confirms that one limitation of TRMM is its poor performance over ice-covered areas because ice on the ground behaves in a similar way as rain or ice drops in the atmosphere in terms of scattering the microwave energy. While all three products are able to correctly represent the spatial rainfall patterns at the annual scale, it not surprisingly turns out that none of them meets the challenge of representing both accumulated quantities of precipitation and frequency of occurrence at the short timescales (sub-daily and daily) required for glacio-hydrological studies in this region. It is concluded that new methods should be used to merge various rainfall products so as to make the most of their respective strengths.
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Nakano, S., Suzuki, K., Kawamura, K., Parrenin, F., & Higuchi, T. (2016). A sequential Bayesian approach for the estimation of the age-depth relationship of the Dome Fuji ice core. Nonlinear Processes In Geophysics, 23(1), 31–44.
Abstract: A technique for estimating the age-depth relationship in an ice core and evaluating its uncertainty is presented. The age-depth relationship is determined by the accumulation of snow at the site of the ice core and the thinning process as a result of the deformation of ice layers. However, since neither the accumulation rate nor the thinning process is fully known, it is essential to incorporate observational information into a model that describes the accumulation and thinning processes. In the proposed technique, the age as a function of depth is estimated by making use of age markers and delta O-18 data. The age markers provide reliable age information at several depths. The data of delta O-18 are used as a proxy of the temperature for estimating the accumulation rate. The estimation is achieved using the particle Markov chain Monte Carlo (PMCMC) method, which is a combination of the sequential Monte Carlo (SMC) method and the Markov chain Monte Carlo method. In this hybrid method, the posterior distributions for the parameters in the models for the accumulation and thinning process are computed using the Metropolis method, in which the likelihood is obtained with the SMC method, and the posterior distribution for the age as a function of depth is obtained by collecting the samples generated by the SMC method with Metropolis iterations. The use of this PMCMC method enables us to estimate the age-depth relationship without assuming either linearity or Gaussianity. The performance of the proposed technique is demonstrated by applying it to ice core data from Dome Fuji in Antarctica.
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Narcisi, B., Petit, J. R., Langone, A., & Stenni, B. (2016). A new Eemian record of Antarctic tephra layers retrieved from the Talos Dome ice core (Northern Victoria Land). Global And Planetary Change, 137, 69–78.
Abstract: Polar ice sheets are remarkable repositories of tephra layers. The Talos Dome ice core (72 degrees 49'5, 159 degrees 11'E), drilled at the edge of the East Antarctic Plateau, close to Late Quaternary volcanoes, offers considerable potential to extend the current tephra time-stratigraphic framework. A tephrochronological study was undertaken of the ice core sections related to the Last Interglacial and the transition to the subsequent glacial period. Thirteen macroscopically visible layers, interpreted to be related to primary deposition of fallout tephra, have been analysed for quantitative grain size and glass shard geochemistry. The layers, precisely framed within the climate (6180) record for the core, span in age from 111.6 +/- 1.9 to 123.3 +/- 22 ka. Coarse particle size suggests origin from regional sources. Indeed, the vast majority of the samples display an alkaline affinity and trachytic composition that are both typical geochemical features of rifting Antarctic volcanism. Using subtle differences in the geochemical signatures and the comparison with data from previous studies, a few layers are attributed to known coeval Mt. Melbourne eruptions. Another sample subset is consistent with derivation from The Pleiades and Mt. Rittmann volcanoes. One peculiar trachytic glass population appears to be related to activity of the more distant Marie Byrd land volcanoes. The newly detected tephras provide stratigraphic markers that could facilitate future synchronisation and dating of palaeoclimatic records. The Talos Dome tephra inventory also contributes significantly to the reconstruction of the Northern Victoria Land explosive volcanism, for which chronostratigraphic data for the last Interglacial temporal segment are poor. (C) 2015 Elsevier B.V. All rights reserved.
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Nemery, J., & Garnier, J. (2016). BIOGEOCHEMISTRY The fate of phosphorus. Nature Geoscience, 9(5), 343–344. |
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Nemery, J., Gratiot, N., Doan, P. T. K., Duvert, C., Alvarado-Villanueva, R., & Duwig, C. (2016). Carbon, nitrogen, phosphorus, and sediment sources and retention in a small eutrophic tropical reservoir. Aquatic Sciences, 78(1), 171–189.
Abstract: Rapid urbanization and the absence of efficient water management policies are increasingly degrading the water quality of tropical reservoirs in developing countries. The small tropical reservoir of Cointzio, located in the Trans-Mexican Volcanic Belt, is a warm monomictic water body (surface area = 6 km(2) with short water residence time < 1 year) that is strategic to the drinking water supply of the city of Morelia and to downstream irrigation during the dry season (6 months of the year). The reservoir faces two threats: (a) reduced water storage capacity due to sediment accumulation and (b) eutrophication caused by excess nutrients that likely come from untreated wastewaters in the upstream watershed. Intensive field measurements of water and sediment were conducted in 2009 to characterize the trophic status of the reservoir and to estimate nitrogen (N) and phosphorus (P) sources, total suspended sediment (TSS) (N), (P), and carbon (C) loads, and their accumulation or removal in the reservoir. We found that point sources represent the majority of N and P inputs to the reservoir. The trophic status is clearly eutrophic given the high chlorophyll a peaks (up to 70 A μg L-1) and a long period of anoxia (from May to October). Most of the TSS, C, N, and P were conveyed to the reservoir between June and October during the wet season. The TSS yield from the watershed was estimated at 35 +/- A 19 t km(-2) year(-1), of which more than 90 % was trapped in the reservoir (sediment accumulation rate = 7800 +/- A 2100 g m(-2) of reservoir year(-1)). The export load of C, N, and P downstream at the reservoir outlet was reduced by 31, 46 and 30 % respectively in comparison to the load at the reservoir inlet. This study reveals the effect of climatic seasonality on inputs to tropical reservoirs and accumulation; it also highlights the need to both reduce nutrient input to combat eutrophication and mitigate erosion to maintain the water storage capacity of the reservoir over the long term.
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Nerentorp Mastromonaco, M. G., Gardfeldt, K., Langer, S., & Dommergue, A. (2016). Seasonal Study of Mercury Species in the Antarctic Sea Ice Environment. Environmental Science & Technology, 50(23), 12705–12712.
Abstract: Limited studies have been conducted on mercury concentrations in the polar cryosphere and the factors affecting the distribution of mercury within sea ice and snow are poorly understood. Here we present the first comprehensive seasonal study of elemental and total mercury concentrations in the Antarctic sea ice environment covering data from measurements in air, sea ice, seawater, snow, frost flowers, and brine. The average concentration of total mercury in sea ice decreased from winter (9.7 ng L-1) to spring (4.7 ng L-1) while the average elemental mercury concentration increased from winter (0.07 ng L-1) to summer (0.105 ng L-1). The opposite trends suggest potential photo or dark oxidation/reduction processes within the ice and an eventual loss of mercury via brine drainage or gas evasion of elemental mercury. Our results indicate a seasonal variation of mercury species in the polar sea ice environment probably due to varying factors such as solar radiation, temperature, brine volume, and atmospheric deposition. This study shows that the sea ice environment is a significant interphase between the polar ocean and the atmosphere and should be accounted for when studying how climate change may affect the mercury cycle in polar regions.
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Nerentorp Mastromonaco, M. N., Gardfeldt, K., Jourdain, B., Abrahamsson, K., Granfors, A., Ahnoff, M., et al. (2016). Antarctic winter mercury and ozone depletion events over sea ice. Atmospheric Environment, 129, 125–132.
Abstract: During atmospheric mercury and ozone depletion events in the springtime in polar regions gaseous elemental mercury and ozone undergo rapid declines. Mercury is quicldy transformed into oxidation products, which are subsequently removed by deposition. Here we show that such events also occur during Antarctic winter over sea ice areas, leading to additional deposition of mercury. Over four months in the Weddell Sea we measured gaseous elemental, oxidized, and particulate-bound mercury, as well as ozone in the troposphere and total and elemental mercury concentrations in snow, demonstrating a series of depletion and deposition events between July and September. The winter depletions in July were characterized by stronger correlations between mercury and ozone and larger formation of particulate-bound mercury in air compared to later spring events. It appears that light at large solar zenith angles is sufficient to initiate the photolytic formation of halogen radicals. We also propose a dark mechanism that could explain observed events in air masses coming from dark regions. Br-2 that could be the main actor in dark conditions was possibly formed in high concentrations in the marine boundary layer in the dark. These high concentrations may also have caused the formation of high concentrations of CHBr3 and CH2I2 in the top layers of the Antarctic sea ice observed during winter. These new findings show that the extent of depletion events is larger than previously believed and that winter depletions result in additional deposition of mercury that could be transferred to marine and terrestrial ecosystems. (C) 2016 Elsevier Ltd. All rights reserved.
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Nicolet, G., Eckert, N., Morin, S., & Blanchet, J. (2016). Decreasing spatial dependence in extreme snowfall in the French Alps since 1958 under climate change. Journal Of Geophysical Research-Atmospheres, 121(14), 8297–8310.
Abstract: Whereas changes in magnitude of geophysical extremes under climate change have received significant attention, potential concomitant changes in spatial dependence structures have remained unexplored so far. Here we provide first evidence of such an effect, highlighting a significant trend in the spatial dependence structure of snowfall extremes in the French Alps at decadal time scale. Specifically, we process a comprehensive data set of winter maximum snowfall from all over the French Alps collected in 90 stations from 1958 to 2012. We estimate extremal dependence over 20year moving estimation windows taking into account possible anisotropy potentially related to orographic effects and/or patterns in atmospheric flows. For each window, we derive a range representing the distance above which extremes are almost independent. We show that snowfall extremes tended to become less spatially dependent over time, with the dependence range reduced roughly by half during the study period. We demonstrate the connection between this trend and local and synoptic climatic variables associated with the current climate change context. In details, the decreasing pattern in extremal dependence is concomitant with a trend toward less harsh winter conditions. It is attributable at first to the increase in temperature and its major control on the snow/rain partitioning. Yet a magnitude effect, with less dependent extremes due to a decrease in intensity of precipitation, also exists. Finally, we show that our results are largely insensitive to the minimal modeling assumptions necessary to our data-based approach. This robustness makes it potentially suitable for various other studies in the field of geophysical extremes.
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Noack, L., Honing, D., Rivoldini, A., Heistracher, C., Zimov, N., Journaux, B., et al. (2016). Water-rich planets: How habitable is a water layer deeper than on Earth? Icarus, 277, 215–236.
Abstract: Water is necessary for the origin and survival of life as we know it. In the search for life-friendly worlds, water-rich planets therefore are obvious candidates and have attracted increasing attention in recent years. The surface H2O layer on such planets (containing a liquid water ocean and possibly high-pressure ice below a specific depth) could potentially be hundreds of kilometres deep depending on the water content and the evolution of the proto-atmosphere. We study possible constraints for the habitability of deep water layers and introduce a new habitability classification relevant for water-rich planets (from Mars-size to super-Earth-size planets). A new ocean model has been developed that is coupled to a thermal evolution model of the mantle and core. Our interior structure model takes into account depth-dependent thermodynamic properties and the possible formation of high-pressure ice. We find that heat flowing out of the silicate mantle can melt an ice layer from below (in some cases episodically), depending mainly on the thickness of the ocean-ice shell, the mass of the planet, the surface temperature and the interior parameters (e.g. radioactive mantle heat sources). The high pressure at the bottom of deep water-ice layers could also impede volcanism at the water-mantle boundary for both stagnant lid and plate tectonics silicate shells. We conclude that water-rich planets with a deep ocean, a large planet mass, a high average density or a low surface temperature are likely less habitable than planets with an Earth-like ocean. (C) 2016 Elsevier Inc. All rights reserved.
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Paci, A., Staquet, C., Allard, J., Barral, H., Canut, G., Cohard, J. M., et al. (2016). La campagne Passy-2015 : dynamique atmosphérique et qualité de l'air dans la vallée de l'Arve. Pollution atmosphérique, 231-232(8), 271–289.
Abstract: Wintertime anticyclonic conditions, associated with clear sky and cold nights, trigger the formation of persistent layers of stable air over the ground. In an urban area, these persistent layers lead to poor air quality, especially when the terrain is mountainous. This is particularly the case in the Arve River Valley near the city of Passy, located 20 km downstream of Chamonix-Mont-Blanc, where air quality stands among the poorest ones in France.
Beyond the monitoring of air quality, as performed by the Auvergne-Rhône-Alpes air quality agency or within the scientific project DECOMBIO led by the Institute for Geosciences and the Environment (IGE), knowledge of the atmospheric dynamics at the valley scale should be gained to understand how pollutants are dispersed. This is the motivation of the Passy project, which started in 2014. It relies on the Passy-2015 field experiment, whereof presentation, along with the discussion of a few results, is the purpose of the present paper. The objective of this field experiment is to document the atmospheric dynamics in the Arve River Valley during wintertime pollution episodes. The work conducted during the Passy project and the analysis of the Passy-2015 field experiment will benefit from a several-year long collaboration among the different partners. The knowledge thus gained will contribute to refine weather forecast and air quality prediction in the Arve River Valley and, more generally, in mountain urban areas under stable conditions. From an operational perspective, our goal is to improve our ability to forecast critical events such as low temperatures, ice and fog formation, pollution events or locations subject to high pollutant concentration. |
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Parrenin, F., Fujita, S., Abe-Ouchi, A., Kawamura, K., Masson-Delmotte, V., Motoyama, H., et al. (2016). Climate dependent contrast in surface mass balance in East Antarctica over the past 216 ka. Journal Of Glaciology, 62(236), 1037–1048.
Abstract: Documenting past changes in the East Antarctic surface mass balance is important to improve ice core chronologies and to constrain the ice-sheet contribution to global mean sea-level change. Here we reconstruct past changes in the ratio of surface mass balance (SMB ratio) between the EPICA Dome C (EDC) and Dome Fuji (DF) East Antarctica ice core sites, based on a precise volcanic synchronization of the two ice cores and on corrections for the vertical thinning of layers. During the past 216 000 a, this SMB ratio, denoted SMBEDC/SMBDF, varied between 0.7 and 1.1, being small during cold periods and large during warm periods. Our results therefore reveal larger amplitudes of changes in SMB at EDC compared with DF, consistent with previous results showing larger amplitudes of changes in water stable isotopes and estimated surface temperature at EDC compared with DF. Within the last glacial inception (Marine Isotope Stages, MIS-5c and MIS-5d), the SMB ratio deviates by up to 0.2 from what is expected based on differences in water stable isotope records. Moreover, the SMB ratio is constant throughout the late parts of the current and last interglacial periods, despite contrasting isotopic trends.
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Passalacqua, O., Gagliardini, O., Parrenin, F., Todd, J., Gillet-Chaulet, F., & Ritz, C. (2016). Performance and applicability of a 2.5-D ice-flow model in the vicinity of a dome. Geoscientific Model Development, 9(7), 2301–2313.
Abstract: Three-dimensional ice flow modelling requires a large number of computing resources and observation data, such that 2-D simulations are often preferable. However, when there is significant lateral divergence, this must be accounted for (2.5-D models), and a flow tube is considered (volume between two horizontal flowlines). In the absence of velocity observations, this flow tube can be derived assuming that the flowlines follow the steepest slope of the surface, under a few flow assumptions. This method typically consists of scanning a digital elevation model (DEM) with a moving window and computing the curvature at the centre of this window. The ability of the 2.5-D models to account properly for a 3-D state of strain and stress has not clearly been established, nor their sensitivity to the size of the scanning window and to the geometry of the ice surface, for example in the cases of sharp ridges. Here, we study the applicability of a 2.5-D ice flow model around a dome, typical of the East Antarctic plateau conditions. A twin experiment is carried out, comparing 3-D and 2.5-D computed velocities, on three dome geometries, for several scanning windows and thermal conditions. The chosen scanning window used to evaluate the ice surface curvature should be comparable to the typical radius of this curvature. For isothermal ice, the error made by the 2.5-D model is in the range 0-10aEuro-% for weakly diverging flows, but is 2 or 3 times higher for highly diverging flows and could lead to a non-physical ice surface at the dome. For non-isothermal ice, assuming a linear temperature profile, the presence of a sharp ridge makes the 2.5-D velocity field unrealistic. In such cases, the basal ice is warmer and more easily laterally strained than the upper one, the walls of the flow tube are not vertical, and the assumptions of the 2.5-D model are no longer valid.
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Pearce, D. A., Alekhina, I. A., Terauds, A., Wilmotte, A., Quesada, A., Edwards, A., et al. (2016). Aerobiology Over Antarctica – A New Initiative for Atmospheric Ecology. Frontiers in Microbiology, 7, 16.
Abstract: The role of aerial dispersal in shaping patterns of biodiversity remains poorly understood, mainly due to a lack of coordinated efforts in gathering data at appropriate temporal and spatial scales. It has been long known that the rate of dispersal to an ecosystem can significantly influence ecosystem dynamics, and that aerial transport has been identified as an important source of biological input to remote locations. With the considerable effort devoted in recent decades to understanding atmospheric circulation in the south polar region, a unique opportunity has emerged to investigate the atmospheric ecology of Antarctica, from local to continental scales. This concept note identifies key questions in Antarctic microbial biogeography and the need for standardized sampling and analysis protocols to address such questions. A consortium of polar aerobiologists is established to bring together researchers with a common interest in the airborne dispersion of microbes and other propagules in the Antarctic, with opportunities for comparative studies in the Arctic.
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Pellarin, T., Mialon, A., Biron, R., Coulaud, C., Gibon, F., Kerr, Y., et al. (2016). Three years of L-band brightness temperature measurements in a mountainous area: Topography, vegetation and snowmelt issues. Remote Sensing Of Environment, 180, 85–98.
Abstract: L-band passive measurements (1.4 GHz) over continental areas are known to be related to surface soil moisture. Two satellite missions were recently launched to measure land surface emissions at this frequency band (SMOS-Soil Moisture and Ocean Salinity in 2009 and SMAP-Soil Moisture Active/Passive in 2015). In order to improve soil moisture retrievals from satellite data, ground-based radiometer systems operating at the same frequency were deployed over specified areas to investigate the L-band emission of various land covers under various climatological conditions. In this study, three years of L-band passive measurements from a radiometer installed on top of a steep mountain in the French Alps were analyzed and compared to L-band passive simulations. The innovative radiometer location led to large footprints due to the distance between the radiometer and the area under study. This experiment also produced microwave measurements affected by various potential difficulties typically encountered in SMOS/SMAP satellite missions: topography, heterogeneous footprints, dry/wet snow events, dew and vegetation litter. Based on in situ and modeling data, this paper investigates the potential of a radiative transfer model (L-band Microwave Emission of the Biosphere, L-MEB) to simulate L-band measurements and analyzes the differences with ELBARA observations. First, it was found that the topography generated a mixing of the horizontal and vertical polarizations. In addition, a large positive bias was found on ELBARA measurements (31 K and 12 K in horizontal and vertical polarizations respectively). Investigations showed that the sky reflection measured by the radiometer was partially substituted by land reflection coming from the surrounding topography. Second, the low-vegetation emission was investigated and highlighted the inability of the MODIS NDVI product to correctly represent the vegetation dynamics. Finally, dry snow conditions were found to have non-negligible impact at L-band and a particular signature was found during snow melting periods, with potential applications at the SMOS/SMAP spatial scales (similar to 40 km). (C) 2016 Elsevier Inc. All rights reserved.
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Peng, S., Ciais, P., Krinner, G., Wang, T., Gouttevin, I., McGuire, A. D., et al. (2016). Simulated high-latitude soil thermal dynamics during the past 4 decades. Cryosphere, 10(1), 179–192.
Abstract: Soil temperature (T-s) change is a key indicator of the dynamics of permafrost. On seasonal and interannual timescales, the variability of T-s determines the active-layer depth, which regulates hydrological soil properties and biogeochemical processes. On the multi-decadal scale, increasing T-s not only drives permafrost thaw/retreat but can also trigger and accelerate the decomposition of soil organic carbon. The magnitude of permafrost carbon feedbacks is thus closely linked to the rate of change of soil thermal regimes. In this study, we used nine process-based ecosystem models with permafrost processes, all forced by different observation-based climate forcing during the period 1960-2000, to characterize the warming rate of T-s in permafrost regions. There is a large spread of T-s trends at 20 cm depth across the models, with trend values ranging from 0.010 +/- 0.003 to 0.031 +/- 0.005 degrees C yr(-1). Most models show smaller increase in T-s with increasing depth. Air temperature (T-a) and longwave downward radiation (LWDR) are the main drivers of T-s trends, but their relative contributions differ amongst the models. Different trends of LWDR used in the forcing of models can explain 61% of their differences in T-s trends, while trends of T a only explain 5% of the differences in T-s trends. Uncertain climate forcing contributes a larger uncertainty in T-s trends (0.021 +/- 0.008 degrees C yr(-1), mean +/- standard deviation) than the uncertainty of model structure (0.012 +/- 0.001 degrees C yr(-1)), diagnosed from the range of response between different models, normalized to the same forcing. In addition, the loss rate of near-surface permafrost area, defined as total area where the maximum seasonal active-layer thickness (ALT) is less than 3m loss rate, is found to be significantly correlated with the magnitude of the trends of T-s at 1m depth across the models (R = -0.85, P = 0.003), but not with the initial total nearsurface permafrost area (R = -0.30, P = -0.438). The sensitivity of the total boreal near-surface permafrost area to T-s at 1m is estimated to be of -2.80 +/- 0.67 million km(2) degrees C-1. Finally, by using two long-term LWDR data sets and relationships between trends of LWDR and T-s across models, we infer an observation-constrained total boreal near-surface permafrost area decrease comprising between 39 +/- 14 x 10(3) and 75 +/- 14 x 10(3) km(2) yr(-1) from 1960 to 2000. This corresponds to 9-18% degradation of the current permafrost area.
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Philippon, N., de Lapparent, B., Gond, V., Seze, G., Martiny, N., Camberlin, P., et al. (2016). Analysis of the diurnal cycles for a better understanding of the mean annual cycle of forests greenness in Central Africa. Agricultural And Forest Meteorology, 223, 81–94.
Abstract: Central Africa hosts the world's second largest tropical forest after the Amazonian basin. However, as compared to its Amazonian counterpart, the Central Africa forests receive much less rain (similar to 1500 mm/year in mean spread over two rainy seasons in March-May and September-November, and two dry seasons). They also experience a slower deforestation rate, so that the main threat for the next decades might come from climate variations. Nonetheless, their response to the annual cycle of solar radiation and rainfall/clouds is still poorly known. Analysing high resolution remote sensing data of Enhanced Vegetation Index, rainfall, cloudiness, and solar radiation for a target region located between 0 and 5 degrees N and 12-19 degrees E, we explore the climatic drivers of the forests greenness mean annual cycle. Three main points emerge; first, the diurnal cycle is a key-scale for understanding the mean annual cycles of rainfall and incoming solar radiation at surface, then how climate shapes the greenness mean annual evolution; second, neither the two dry seasons nor the two rainy seasons resemble each other in terms of cloud cover, solar radiation and rainfall, and their links with greenness levels; third, whereas the first rainy season (March-May) appears optimal for greenness especially because of favorable light conditions, water availability is the main controlling factor during the main dry season and at the start of the first vegetative season (February). Regarding the little dry season (mid-June-mid-August) and the second rainy season (September-October), light availability might be the main limiting factor. These findings pave the way for further studies of the climate interannual variability and change impacts on the Central Africa forests, taking into account time-scale interactions. (C) 2016 Elsevier B.V. All rights reserved.
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Picard, G., Arnaud, L., Panel, J. M., & Morin, S. (2016). Design of a scanning laser meter for monitoring the spatio-temporal evolution of snow depth and its application in the Alps and in Antarctica. Cryosphere, 10(4), 1495–1511.
Abstract: Although both the temporal and spatial variations of the snow depth are usually of interest for numerous applications, available measurement techniques are either space-oriented (e.g. terrestrial laser scans) or time-oriented (e.g. ultrasonic ranging probe). Because of snow heterogeneity, measuring depth in a single point is insufficient to provide accurate and representative estimates. We present a cost-effective automatic instrument to acquire spatio-temporal variations of snow depth. The device comprises a laser meter mounted on a 2-axis stage and can scan approximate to 200 000 points over an area of 100-200 m(2) in 4 h. Two instruments, installed in Antarctica (Dome C) and the French Alps (Col de Porte), have been operating continuously and unattended over 2015 with a success rate of 65 and 90% respectively. The precision of single point measurements and long-term stability were evaluated to be about 1 cm and the accuracy to be 5 cm or better. The spatial variability in the scanned area reached 7-10 cm (root mean square) at both sites, which means that the number of measurements is sufficient to average out the spatial variability and yield precise mean snow depth. With such high precision, it was possible for the first time at Dome C to (1) observe a 3-month period of regular and slow increase of snow depth without apparent link to snowfalls and (2) highlight that most of the annual accumulation stems from a single event although several snowfall and strong wind events were predicted by the ERA-Interim reanalysis. Finally the paper discusses the benefit of laser scanning compared to multiplying single-point sensors in the context of monitoring snow depth.
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Picard, G., Libois, Q., & Arnaud, L. (2016). Refinement of the ice absorption spectrum in the visible using radiance profile measurements in Antarctic snow. Cryosphere, 10(6), 2655–2672.
Abstract: Ice is a highly transparent material in the visible. According to the most widely used database (IA2008; Warren and Brandt, 2008), the ice absorption coefficient reaches values lower than 10(-3) m(-1) around 400 nm. These values were obtained from a vertical profile of spectral radiance measured in a single snow layer at Dome C in Antarctica. We reproduced this experiment using an optical fiber inserted in the snow to record 56 profiles from which 70 homogeneous layers were identified. Applying the same estimation method on every layer yields 70 ice absorption spectra. They present a significant variability but absorption coefficients are overall larger than IA2008 by 1 order of magnitude at 400-450 nm. We devised another estimation method based on Bayesian inference that treats all the profiles simultaneously. It reduces the statistical variability and confirms the higher absorption, around 2 x 10(-2) m(-1) near the minimum at 440 nm. We explore potential instrumental artifacts by developing a 3-D radiative transfer model able to explicitly account for the presence of the fiber in the snow. The simulation shows that the radiance profile is indeed perturbed by the fiber intrusion, but the error on the ice absorption estimate is not larger than a factor of 2. This is insufficient to explain the difference between our new estimate and IA2008. The same conclusion applies regarding the plausible contamination by black carbon or dust, concentrations reported in the literature are insufficient. Considering the large number of profiles acquired for this study and other estimates from the Antarctic Muon and Neutrino Detector Array (AMANDA), we nevertheless estimate that ice absorption values around 10(-2) m(-1) at the minimum are more likely than under 10(-3) m(-1). A new estimate in the range 400-600 nm is provided for future modeling of snow, cloud, and sea-ice optical properties. Most importantly, we recommend that modeling studies take into account the large uncertainty of the ice absorption coefficient in the visible and that future estimations of the ice absorption coefficient should also thoroughly account for the impact of the measurement method.
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Picard, G., Libois, Q., Arnaud, L., Verin, G., & Dumont, M. (2016). Development and calibration of an automatic spectral albedometer to estimate near-surface snow SSA time series. Cryosphere, 10(3), 1297–1316.
Abstract: Spectral albedo of the snow surface in the visible/near-infrared range has been measured for 3 years by an automatic spectral radiometer installed at Dome C (75A degrees aEuro-S, 123A degrees aEuro-E) in Antarctica in order to retrieve the specific surface area (SSA) of superficial snow. This study focuses on the uncertainties of the SSA retrieval due to instrumental and data processing limitations. We find that when the solar zenith angle is high, the main source of uncertainties is the imperfect angular response of the light collectors. This imperfection introduces a small spurious wavelength-dependent trend in the albedo spectra which greatly affects the SSA retrieval. By modeling this effect, we show that for typical snow and illumination conditions encountered at Dome C, retrieving SSA with an accuracy better than 15aEuro-% (our target) requires the difference of response between 400 and 1100aEuro-nm to not exceed 2. Such a small difference can be achieved only by (i) a careful design of the collectors, (ii) an ad hoc correction of the spectra using the actual measured angular response of the collectors, and (iii) for solar zenith angles less than 75A degrees. The 3-year time series of retrieved SSA features a 3-fold decrease every summer which is significantly larger than the estimated uncertainties. This highlights the high dynamics of near-surface SSA at Dome C.
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Prado, B., Strozzi, A. G., Huerta, E., Duwig, C., Zamora, O., Delmas, P., et al. (2016). 2,4-D mobility in clay soils: Impact of macrofauna abundance on soil porosity. Geoderma, 279, 87–96.
Abstract: Pesticides in agriculture are commonly used to meet the growing food demand; however they compromise the quality of water and soil. The pesticide 2,4-D is one of the most widely used herbicides in the world, it belongs to the group of synthetic herbicides that control broadleaf weeds. In this paper the risk of groundwater contamination by 24-D and its major metabolite, 2,4-DCP, is studied in a context of high density of soil worms. We compared the adsorption, desorption, degradation and displacement of 2,4-D in soils from Tabasco Region having different properties: clay, organic matter, iron and aluminum oxides contents. In addition to the classical soil physicochemical characterizations, a 3D analysis of the soil structure and porosity was performed by analyzing images acquired by Computed Tomography. The objective was to evaluate the effect of soil properties and macroporosity produced by the macrofauna activity on solute movement. All the four soils studied sorbed the herbicides, more importantly 2,4-DCP, the metabolite, than 2,4-D itself. The distribution coefficients for 2,4-D sorption were linear and varied between 1 and 4 while those for 2,4-DCP were above 10. The contents of iron and aluminum have an important role in the adsorption of these two compounds. In aerobic conditions, the herbicides half-life was about 2 days. Water movement occurred in physical equilibrium in three of the four soils; soil dispersivity ranged from 1.2 to 7 cm, clay content being the main factor. After 20 to 60 days depending on the soil, no 2,4-D leaching was observed through the soil columns, except for one soil were there was preferential flow. Earthworms burrows were exhibited and quantified in the soils samples through the analysis of Computer Tomography (CT) images, they appeared as small, snail-shaped, rounded volume of 3 to 7 mm radius with a higher density with respect to the surrounding soil. They were extracted from the original data using a combination of image, processing and mathematical morphology operators. Based on the results obtained, it can be concluded that preferential flow caused by both high clay content and the presence of macrofauna pores significantly reduces the buffering capacity of the soil, increasing the risk of contamination by herbicides of the underlying aquifer. (C) 2016 Elsevier B.V. All rights reserved.
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Quennehen, B., Raut, J. C., Law, K. S., Daskalakis, N., Ancellet, G., Clerbaux, C., et al. (2016). Multi-model evaluation of short-lived pollutant distributions over east Asia during summer 2008. Atmospheric Chemistry And Physics, 16(17), 10765–10792.
Abstract: The ability of seven state-of-the-art chemistry-aerosol models to reproduce distributions of tropospheric ozone and its precursors, as well as aerosols over eastern Asia in summer 2008, is evaluated. The study focuses on the performance of models used to assess impacts of pollutants on climate and air quality as part of the EU ECLIPSE project. Models, run using the same ECLIPSE emissions, are compared over different spatial scales to in situ surface, vertical profiles and satellite data. Several rather clear biases are found between model results and observations, including overestimation of ozone at rural locations downwind of the main emission regions in China, as well as downwind over the Pacific. Several models produce too much ozone over polluted regions, which is then transported downwind. Analysis points to different factors related to the ability of models to simulate VOC-limited regimes over polluted regions and NOx limited regimes downwind. This may also be linked to biases compared to satellite NO2, indicating overestimation of NO2 over and to the north of the northern China Plain emission region. On the other hand, model NO2 is too low to the south and west of this region and over South Korea/Japan. Overestimation of ozone is linked to systematic underestimation of CO particularly at rural sites and downwind of the main Chinese emission regions. This is likely to be due to enhanced destruction of CO by OH. Overestimation of Asian ozone and its transport downwind implies that radiative forcing from this source may be overestimated. Model-observation discrepancies over Beijing do not appear to be due to emission controls linked to the Olympic Games in summer 2008. With regard to aerosols, most models reproduce the satellite-derived AOD patterns over eastern China. Our study nevertheless reveals an overestimation of ECLIPSE model mean surface BC and sulphate aerosols in urban China in summer 2008. The effect of the short-term emission mitigation in Beijing is too weak to explain the differences between the models. Our results rather point to an overestimation of SO2 emissions, in particular, close to the surface in Chinese urban areas. However, we also identify a clear underestimation of aerosol concentrations over northern India, suggesting that the rapid recent growth of emissions in India, as well as their spatial extension, is underestimated in emission inventories. Model deficiencies in the representation of pollution accumulation due to the Indian monsoon may also be playing a role. Comparison with vertical aerosol lidar measurements highlights a general underestimation of scattering aerosols in the boundary layer associated with overestimation in the free troposphere pointing to modelled aerosol lifetimes that are too long. This is likely linked to too strong vertical transport and/or insufficient deposition efficiency during transport or export from the boundary layer, rather than chemical processing (in the case of sulphate aerosols). Underestimation of sulphate in the boundary layer implies potentially large errors in simulated aerosol-cloud interactions, via impacts on boundary-layer clouds. This evaluation has important implications for accurate assessment of air pollutants on regional air quality and global climate based on global model calculations. Ideally, models should be run at higher resolution over source regions to better simulate urban-rural pollutant gradients and/or chemical regimes, and also to better resolve pollutant processing and loss by wet deposition as well as vertical transport. Discrepancies in vertical distributions require further quantification and improvement since these are a key factor in the determination of radiative forcing from short-lived pollutants.
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Rabatel, A., Dedieu, J. P., & Vincent, C. (2016). Spatio-temporal changes in glacier-wide mass balance quantified, by optical remote sensing on 30 glaciers in the French Alps for the period 1983-2014. Journal Of Glaciology, 62(236), 1153–1166.
Abstract: Remote sensing is a powerful method to reconstruct annual mass-balance series over past decades by exploiting archives of available images, as well as to study glaciers in inaccessible regions. We present the application of a methodological framework based only on optical satellite images to retrieve glacier-wide annual mass balances for 30 glaciers in the French Alps. The glacier-wide annual mass balance for the period 1983-2014 was reconstructed by combining changes in glacier volumes computed from remote-sensing derived DEMs with annual measurements of the snow line altitude on satellite images. Data from direct observations on two of the glaciers confirmed the accuracy of the annual mass balances quantified by remote sensing with an average difference of similar to 0.3 m w.e., within the uncertainty range of the methods. Our results confirm the significant increase in mass loss since the early 2000s, with a difference >1 m w.e. a(-1) between the periods 1983-2002 and 2003-14. The region-wide mass balance for the French Alps over the period 1979-2011 was -0.66 +/- 0.27 m w.e. a(-1), close to that of the European Alps. We also show that changes in glacier surface area or length are not representative of changes in mass balance at the scale of a few decades.
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Revuelto, J., Lopez-Moreno, J. I., Azorin-Molina, C., Alonso-Gonzalez, E., & Sanmiguel-Vallelado, A. (2016). Small-Scale Effect of Pine Stand Pruning on Snowpack Distribution in the Pyrenees Observed with a Terrestrial Laser Scanner. Forests, 7(8).
Abstract: Forests in snow-dominated areas have substantial effects on the snowpack and its evolution over time. Such interactions have significant consequences for the hydrological response of mountain rivers. Thus, the impact of forest management actions on the snow distribution, and hence the storage of water in the form of snow during winter and spring, is a major concern. The results of this study provide the first detailed comparison of the small-scale effect of forest characteristics on the snowpack distribution, assessed prior to and following major modification of the structure of the canopy by pruning of the lower branches of the trees to 3 m above the ground. This is a common management practice aimed at reducing the spread of forest fires. The snowpack distribution was determined using terrestrial laser scanning (LiDAR technology) at a high spatial resolution (0.25 m) over a 1000 m(2) study area during 23 survey dates over three snow seasons in a small study area in the central Pyrenees. The pruning was conducted during summer following the snow season in the second year of the study (i.e., the study duration encompassed two seasons prior to canopy pruning and one following). Principal component analysis (PCA) was used to identify recurring spatial patterns of snow distribution. The results showed that pruning reduced the average radius of the canopy of trees by 1.2 m, and increased the clearance around the trunks, as all the branches that formerly contacted the ground were removed. However, the impact on the snowpack was moderate. The PCA revealed that the spatial configuration of the snowpack did not change significantly, as the principal components included survey days from different periods of the snow season, and did not discriminate days surveyed prior to and following pruning. Nevertheless, removal of the lower branches reduced the area beneath the canopy by 36%, and led to an average increase in total snow depth of approximately 14%.
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Rhodes, R. H., Fain, X., Brook, E. J., McConnell, J. R., Maselli, O. J., Sigl, M., et al. (2016). Local artifacts in ice core methane records caused by layered bubble trapping and in situ production: a multi-site investigation. Climate Of The Past, 12(4), 1061–1077.
Abstract: Advances in trace gas analysis allow localised, non-atmospheric features to be resolved in ice cores, superimposed on the coherent atmospheric signal. These high frequency signals could not have survived the low-pass filter effect that gas diffusion in the firn exerts on the atmospheric history and therefore do not result from changes in the atmospheric composition at the ice sheet surface. Using continuous methane (CH4) records obtained from five polar ice cores, we characterise these non-atmospheric signals and explore their origin. Isolated samples, enriched in CH4 in the Tunul3 (Greenland) record are linked to the presence of melt layers. Melting can enrich the methane concentration due to a solubility effect, but we find that an additional in situ process is required to generate the full magnitude of these anomalies. Furthermore, in all the ice cores studied there is evidence of reproducible, decimetre-scale CH4 variability. Through a series of tests, we demonstrate that this is an artifact of layered bubble trapping in a heterogeneous density firn column; we use the term “trapping signal” for this phenomenon. The peak -to -peak amplitude of the trapping signal is typically 5 ppb, but may exceed 40 ppb. Signal magnitude increases with atmospheric CH4 growth rate and seasonal density contrast, and decreases with accumulation rate. Significant annual periodicity is present in the CH4 variability of two Greenland ice cores, suggesting that layered gas trapping at these sites is controlled by regular, seasonal variations in the physical properties of the firn. Future analytical campaigns should anticipate high -frequency artifacts at high -melt ice core sites or during time periods with high atmospheric CH4 growth rate in order to avoid misinterpretation of such features as past changes in atmospheric composition.
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Ringard, J., Dieppois, B., Rome, S., Diedhiou, A., Pellarin, T., Konare, A., et al. (2016). The intensification of thermal extremes in west Africa. Global And Planetary Change, 139, 66–77.
Abstract: This study aims in filling the gap in understanding the relationship between trend and extreme in diurnal and nocturnal temperatures (Tx and Tn) over the Gulf of Guinea area and the Sahel. Time-evolution and trend of Tx and Tn anomalies, extreme temperatures and heat waves are examined using regional and station-based indices over the 1900-2012 and 1950-2012 periods respectively. In investigating extreme temperature anomalies and heat waves, a percentile method is used. At the regional and local scales, rising trends in Tx and Tn anomalies, which appear more pronounced over the past 60 years, are identified over the two regions. The trends are characterized by an intensification of: i) nocturnal/Tn warming over the second half of the 20th century; and ii) diurnal/Tx warming over the post-1980s. This is the same scheme with extreme warm days and warm nights. Finally annual number of diurnal and nocturnal heat waves has increase over the Gulf of Guinea coastal regions over the second half of the 20th century, and even more substantially over the post-1980s period. Although this trend in extreme warm days and nights is always overestimated in the simulations, from the Coupled Model Intercomparison Project Phase 5 (CMIP5), those models display rising trends whatever the scenario, which are likely to be more and more pronounced over the two regions in the next 50 years. (C) 2016 Elsevier B.V. All rights reserved.
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Riveiros, N. V., Govin, A., Waelbroeck, C., Mackensen, A., Michel, E., Moreira, S., et al. (2016). Mg/Ca thermometry in planktic foraminifera: Improving paleotemperature estimations for G. bulloides and N. pachyderma left. Geochemistry Geophysics Geosystems, 17(4), 1249–1264.
Abstract: Planktic foraminiferal Mg/Ca ratios have become a fundamental seawater temperature proxy in past climate reconstructions, due to the temperature dependence of Mg uptake into foraminiferal calcite. However, empirical calibrations for single species from methodologically consistent data are still lacking. Here we present species -specific calibrations of Mg/Ca versus calcification temperature for two commonly used species of planktic foraminifera: Globigerina bulloides and Neogloboquadrina pachyderma left, based on a series of Southern Ocean and North Atlantic core tops. Combining these new data with previously published data, we derive an integrated G. bulloides Mg/Ca-temperature calibration for mid and high latitudes of both hemispheres between 2 and 18 degrees C, where Mg/Ca = 1.006 +/- 0.032* e(0.065 +/- 0.003)*(Tiso) (R-2=0.82). G. bulloides is found to calcify deeper in the Southern Ocean ( 200 m) than in the North Atlantic (top 50 m). We also propose a Mg/Ca temperature calibration to describe the temperature response in N. pachyderma left that calcified away from the influence of sea ice in the Southern Ocean, valid between similar to -1 and 9 degrees C, of the form Mg/Ca = 0.580 +/- 0.016 * e(0.084) (+/-) (0.006)*(Tisa) (R-2 = 0.70). These calibrations account for uncertainties on Mg/Ca measurements and calcification temperature that were carefully estimated and propagated using Monte Carlo iterations. The la propagated error in Mg/Ca-derived temperatures is 1.1 degrees C for G. bulloides and 0.9 degrees C for N. pachyderma left for the presented data sets. Geographical extension of genotypes must be assessed when choosing to develop regional or global calibrations.
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Roy, A., Royer, A., St-Jean-Rondeau, O., Montpetit, B., Picard, G., Mavrovic, A., et al. (2016). Microwave snow emission modeling uncertainties in boreal and subarctic environments. Cryosphere, 10(2), 623–638.
Abstract: This study aims to better understand and quantify the uncertainties in microwave snow emission models using the Dense Media Radiative Theory Multi-Layer model (DMRT-ML) with in situ measurements of snow properties. We use surface-based radiometric measurements at 10.67, 19 and 37aEuro-GHz in boreal forest and subarctic environments and a new in situ data set of measurements of snow properties (profiles of density, snow grain size and temperature, soil characterization and ice lens detection) acquired in the James Bay and Umiujaq regions of Northern Qu,bec, Canada. A snow excavation experiment – where snow was removed from the ground to measure the microwave emission of bare frozen ground – shows that small-scale spatial variability (less than 1aEuro-km) in the emission of frozen soil is small. Hence, in our case of boreal organic soil, variability in the emission of frozen soil has a small effect on snow-covered brightness temperature (T-B). Grain size and density measurement errors can explain the errors at 37aEuro-GHz, while the sensitivity of T-B at 19aEuro-GHz to snow increases during the winter because of the snow grain growth that leads to scattering. Furthermore, the inclusion of observed ice lenses in DMRT-ML leads to significant improvements in the simulations at horizontal polarization (H-pol) for the three frequencies (up to 20aEuro-K of root mean square error). However, representation of the spatial variability of T-B remains poor at 10.67 and 19aEuro-GHz at H-pol given the spatial variability of ice lens characteristics and the difficulty in simulating snowpack stratigraphy related to the snow crust. The results also show that, in our study with the given forest characteristics, forest emission reflected by the snow-covered surface can increase the T-B up to 40aEuro-K. The forest contribution varies with vegetation characteristics and a relationship between the downwelling contribution of vegetation and the proportion of pixels occupied by vegetation (trees) in fisheye pictures was found. We perform a comprehensive analysis of the components that contribute to the snow-covered microwave signal, which will help to develop DMRT-ML and to improve the required field measurements. The analysis shows that a better consideration of ice lenses and snow crusts is essential to improve T-B simulations in boreal forest and subarctic environments.
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Ruggiero, G. A., Cosme, E., Brankart, J. M., Le Sommer, J., & Ubelmann, C. (2016). An Efficient Way to Account for Observation Error Correlations in the Assimilation of Data from the Future SWOT High-Resolution Altimeter Mission. Journal Of Atmospheric And Oceanic Technology, 33(12), 2755–2768.
Abstract: Most data assimilation algorithms require the inverse of the covariance matrix of the observation errors. In practical applications, the cost of computing this inverse matrix with spatially correlated observation errors is prohibitive. Common practices are therefore to subsample or combine the observations so that the errors of the assimilated observations can be considered uncorrelated. As a consequence, a large fraction of the available observational information is not used in practical applications. In this study, a method is developed to account for the correlations of the errors that will be present in the wide-swath sea surface height measurements, for example, the Surface Water and Ocean Topography (SWOT) mission. It basically consists of the transformation of the observation vector so that the inverse of the corresponding covariance matrix can be replaced by a diagonal matrix, thus allowing to genuinely take into account errors that are spatially correlated in physical space. Numerical experiments of ensemble Kalman filter analysis of SWOT-like observations are conducted with three different observation error covariance matrices. Results suggest that the proposed method provides an effective way to account for error correlations in the assimilation of the future SWOT data. The transformation of the observation vector proposed herein yields both a significant reduction of the root-mean-square errors and a good consistency between the filter analysis error statistics and the true error statistics.
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Rysman, J. F., Lahellec, A., Vignon, E., Genthon, C., & Verrier, S. (2016). Characterization of Atmospheric Ekman Spirals at Dome C, Antarctica. Boundary-Layer Meteorology, 160(2), 363–373.
Abstract: We use wind speed and temperature measurements taken along a 45-m meteorological tower located at Dome C, Antarctica (, ) to highlight and characterize the Ekman spiral. Firstly, temperature records reveal that the atmospheric boundary layer at Dome C is stable during winter and summer nights (i.e., 85 % of the time). The wind vector, in both speed and direction, also shows a strong dependence with elevation. An Ekman model was then fitted to the measurements. Results show that the wind vector follows the Ekman spiral structure for more than 20 % of the year (2009). Most Ekman spirals have been detected during summer nights, that is, when the boundary layer is slightly stratified. During these episodes, the boundary-layer height ranged from 25 to 100 m, the eddy viscosity from 0.004 to , and the Richardson number from zero to 1.6.
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Sanchez, S., Fournier, A., Aubert, J., Cosme, E., & Gallet, Y. (2016). Modelling the archaeomagnetic field under spatial constraints from dynamo simulations: a resolution analysis. Geophysical Journal International, 207(2), 983–1002.
Abstract: Archaeomagnetic observations are key to recovering the behaviour of the geomagnetic field over the past few millennia. The corresponding data set presents a highly heterogeneous distribution in both space and time. Furthermore, the data are affected by substantial age and experimental uncertainties. In order to mitigate these detrimental properties, time-dependent global archaeomagnetic field models are usually constructed under spatial and temporal regularization constraints, with the use of bootstrap techniques to account for data uncertainties. The models so obtained are the product of an adjustable trade-off between goodness-of-fit and model complexity. The spatial complexity is penalized by means of a norm reflecting the minimization of Ohmic dissipation within the core. We propose in this study to resort to alternative spatial constraints relying on the statistics of a numerical dynamo simulation with Earth-like features. To that end, we introduce a dynamo norm in an ensemble least-squares iterative framework, the goal of which is to produce single-epoch models of the archaeomagnetic field. We first validate this approach using synthetic data. We next construct a redistributed archaeomagnetic data set between 1200 BC and 2000 AD by binning the data in windows of 40-yr width. Since the dynamo norm is not adjustable, we can legitimately calculate a resolution matrix to quantify the resolving power of the available archaeomagnetic data set. Gauss coefficients are resolved up to spherical harmonic degree 3 for the first thousand years of the interval, to degree 4 for the next thousand years and to degree 5 during the last millennium. These conclusions are based on the distribution and uncertainties that characterize the data set, and do not take into account the possible presence of outliers. Comparison between our model, called AmR, and previously published archaeomagnetic field models confirms the archaeomagnetic resolution analysis: it highlights the dichotomy between data-driven coefficients for which model predictions coincide (within their respective uncertainties), and prior-driven coefficients. This study opens the way to physics-based models of the archaeomagnetic field; future work will be devoted to integrating the framework here introduced into a time-dependent ensemble assimilation scheme.
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Savarino, J., Vicars, W. C., Legrand, M., Preunkert, S., Jourdain, B., Frey, M. M., et al. (2016). Oxygen isotope mass balance of atmospheric nitrate at Dome C, East Antarctica, during the OPALE campaign. Atmospheric Chemistry And Physics, 16(4), 2659–2673.
Abstract: Variations in the stable oxygen isotope composition of atmospheric nitrate act as novel tools for studying oxidative processes taking place in the troposphere. They provide both qualitative and quantitative constraints on the pathways determining the fate of atmospheric nitrogen oxides (NO + NO2 = NOx). The unique and distinctive O-17 excess (Delta O-17 = delta O-17-0.52 x delta O-18) of ozone, which is transferred to NOx via oxidation, is a particularly useful isotopic fingerprint in studies of NOx transformations. Constraining the propagation of O-17 excess within the NOx cycle is critical in polar areas, where there exists the possibility of extending atmospheric investigations to the glacial-interglacial timescale using deep ice core records of nitrate. Here we present measurements of the comprehensive isotopic composition of atmospheric nitrate collected at Dome C (East Antarctic Plateau) during the austral summer of 2011/2012. Nitrate isotope analysis has been here combined for the first time with key precursors involved in nitrate production (NOx, O-3, OH, HO2, RO2, etc.) and direct observations of the transferrable Delta O-17 of surface ozone, which was measured at Dome C throughout 2012 using our recently developed analytical approach. Assuming that nitrate is mainly produced in Antarctica in summer through the OH + NO2 pathway and using concurrent measurements of OH and NO2, we calculated a Delta O-17 signature for nitrate on the order of (21-22 + 3) %. These values are lower than the measured values that ranged between 27 and 31 %. This discrepancy between expected and observed Delta O-17(NO3-)values suggests the existence of an unknown process that contributes significantly to the atmospheric nitrate budget over this East Antarctic region. However, systematic errors or false isotopic balance transfer functions are not totally excluded.
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Schreck, E., Sarret, G., Oliva, P., Calas, A., Sobanska, S., Guedron, S., et al. (2016). Is Tillandsia capillaris an efficient bioindicator of atmospheric metal and metalloid deposition? Insights from five months of monitoring in an urban mining area. Ecological Indicators, 67, 227–237.
Abstract: Atmospheric pollution in megacities has a major impact on human health and environmental quality. Air quality bioindicators may have some advantages over standard devices such as impactors or filters. In this study we evaluated the reliability of Tillandsia sp. versus passive filters for monitoring the atmospheric deposition of metal(loid)s in an area affected by anthropogenic activities. We aimed to gain insight into the composition and origin of atmospheric particles and their fate after deposition on the plant. Three zones with different contamination levels were monitored for five months in 2012. For the highly contaminated area, a linear increase in metalloid) accumulation was found in passive filters, whereas for transplanted Tillandsia capillaris the increase was almost linear for As, Cd, Hg, and Sn, but not for Ag, Pb, Sb, and Zn. For the moderately contaminated zone, the results showed that the exposure time was not sufficient for metal(loid) concentrations to increase in either the plants or filters. However, natural specimens provided some indications of the levels of metal contamination. Metal particles were observed on the plant surface and also in the central disc underneath tillandsia trichomes, suggesting that this is a possible pathway for metals to enter the plant. X-ray absorption spectroscopy demonstrated chemical transformation for Pb and As, both in filters and plants. For Pb, sorbed Pb and/or cell wall complexes were identified in the plants. No As-III-S species, indicative of As detoxification, were identified in the plant. Arsenic was oxidized from As-III to As-V in both plants and filters. Thus, in the present study, passive filters proved more reliable than T. capillaris transplants, although natural specimens provided some insights into local contamination. Particulate contaminants underwent chemical transformation after being trapped in the plant, but there was no clear evidence of internalization and detoxification. (C) 2016 Elsevier Ltd. All rights reserved.
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Segura, H., Espinoza, J. C., Junquas, C., & Takahashi, K. (2016). Evidencing decadal and interdecadal hydroclimatic variability over the Central Andes. Environmental Research Letters, 11(9).
Abstract: In this study we identified a significant low frequency variability (8 to 20 years) that characterizes the hydroclimatology over the Central Andes. Decadal-interdecadal variability is related to the central-western Pacific Ocean (R-2 = 0.50) and the zonal wind at 200 hPa above the Central Andes (R-2 = 0.66). These two oceanic-atmospheric variables have a dominant decadal-interdecadal variability, and there is a strong relationship between them at a low frequency time scale (R-2 = 0.66). During warming decades in the central-western Pacific Ocean, westerlies are intensified at 200 hPa above the Central Andes, which produce decadal periods of hydrological deficit over this region. In contrast, when the central- western Pacific Ocean is cooler than usual, easterly anomalies prevail over the Central Andes, which are associated with decades of positive hydrological anomalies over this region. Our results indicate that impacts of El Nino on hydrology over the Central Andes could be influenced by the low frequency variability documented in this study.
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Serazin, G., Meyssignac, B., Penduff, T., Terray, L., Barnier, B., & Molines, J. M. (2016). Quantifying uncertainties on regional sea level change induced by multidecadal intrinsic oceanic variability. Geophysical Research Letters, 43(15), 8151–8159.
Abstract: A global eddy-permitting (1/4 degrees resolution) ocean general circulation model is shown to spontaneously generate intrinsic oceanic variability (IOV) up to multidecadal timescales (T > 20years) under a repeated seasonal atmospheric forcing. In eddy-active regions, the signature of this multidecadal eddy-driven IOV on sea level is substantial, weakly autocorrelated, and is comparable to (and may clearly exceed) the corresponding signature of internal climate variability (ICV) produced by current coupled climate modelswhose laminar ocean components may strongly underestimate IOV. Deriving sea level trends from finite-length time series in eddy-active regions yields uncertainties induced by this multidecadal IOV, which are of the same order of magnitude as those due to ICV. A white noise model is proposed to approximate the low-frequency tail of the IOV spectra and could be used to update ICV estimates from current climate simulations and projections.
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Shapero, D. R., Joughin, I. R., Poinar, K., Morlighem, M., & Gillet-Chaulet, F. (2016). Basal resistance for three of the largest Greenland outlet glaciers. Journal Of Geophysical Research-Earth Surface, 121(1), 168–180.
Abstract: Resistance at the ice-bed interface provides a strong control on the response of ice streams and outlet glaciers to external forcing, yet it is not observable by remote sensing. We used inverse methods constrained by satellite observations to infer the basal resistance to flow underneath three of the Greenland Ice Sheet's largest outlet glaciers. In regions of fast ice flow and high (>250kPa) driving stresses, ice is often assumed to flow over a strong bed. We found, however, that the beds of these three glaciers provide almost no resistance under the fast-flowing trunk. Instead, resistance to flow is provided by the lateral margins and stronger beds underlying slower-moving ice upstream. Additionally, we found isolated patches of high basal resistivity within the predominantly weak beds. Because these small-scale (<1 ice thickness) features may be artifacts of overfitting our solution to measurement errors, we tested their robustness to different degrees of regularization.
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Sicart, J. E., Espinoza, J. C., Queno, L., & Medina, M. (2016). Radiative properties of clouds over a tropical Bolivian glacier: seasonal variations and relationship with regional atmospheric circulation. International Journal Of Climatology, 36(8), 3116–3128.
Abstract: At low latitudes, strong seasonal changes in cloud cover and precipitation largely control the mass balance of glaciers. Measurements of shortwave and longwave radiation fluxes reaching Zongo glacier, Bolivia (16 degrees S, 5060m asl), were analysed from 2005 to 2013 to investigate cloud radiative properties. Cloud shortwave attenuation and longwave emission were greater in the wet summer season (DJF) than in the dry winter season (JJA) probably because most DJF clouds were low warm cumulus associated with local convection, whereas JJA clouds were frequently altostratus associated with extra-tropical perturbations. Solar irradiance was high all year round and cloud radiative forcing on down-welling fluxes was strongly negative, with monthly averages ranging from -60 to -110Wm(-2) from the dry to the wet season, respectively. In the wet season, high extraterrestrial solar irradiance and low shortwave transmissivity caused very negative cloud forcing despite the high longwave emissivity of convective clouds. Reanalysis of wind and geopotential height anomalies and outgoing longwave radiation satellite data were used to characterize the regional atmospheric circulation causing thick cloud covers (10% thickest clouds) during the dry (JJA), transition (SON), and wet (DJF) seasons. Around 87% (80%) of cloud events in JJA (SON) occurred during the incursion of low-level southern wind from southern South America to the Bolivian Andes, which caused 2-3 days of cold surge episodes in the Cordillera Real. Around 13% of cloudy days in JJA were associated with high-level low-pressure conditions over the Chilean coast around 45 degrees S, including cut-off lows. In SON, 20% of cloudy days were associated with summer conditions, characterized by an active Bolivian High and moist air advection from the Amazon basin. In the wet season, only 46% of thick cloud events were associated with low-level southern wind incursions, the other events being associated with the South American Monsoon.
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Sigl, M., Fudge, T. J., Winstrup, M., Cole-Dai, J., Ferris, D., McConnell, J. R., et al. (2016). The WAIS Divide deep ice core WD2014 chronology – Part 2: Annual-layer counting (0-31 ka BP). Climate Of The Past, 12(3), 769–786.
Abstract: We present the WD2014 chronology for the upper part (0-2850 m; 31.2 ka BP) of the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core. The chronology is based on counting of annual layers observed in the chemical, dust and electrical conductivity records. These layers are caused by seasonal changes in the source, transport, and deposition of aerosols. The measurements were interpreted manually and with the aid of two automated methods. We validated the chronology by comparing to two high-accuracy, absolutely dated chronologies. For the Holocene, the cosmogenic isotope records of Be-10 from WAIS Divide and C-14 for IntCal13 demonstrated that WD2014 was consistently accurate to better than 0.5% of the age. For the glacial period, comparisons to the Hulu Cave chronology demonstrated that WD2014 had an accuracy of better than 1% of the age at three abrupt climate change events between 27 and 31 ka. WD2014 has consistently younger ages than Greenland ice core chronologies during most of the Holocene. For the Younger Dryas-Preboreal transition (11.595 ka; 24 years younger) and the Bolling-Allerod Warming (14.621 ka; 7 years younger), WD2014 ages are within the combined uncertainties of the timescales. Given its high accuracy, WD2014 can become a reference chronology for the Southern Hemisphere, with synchronization to other chronologies feasible using high-quality proxies of volcanism, solar activity, atmospheric mineral dust, and atmospheric methane concentrations.
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Simonin, M., Martins, J. M. F., Uzu, G., Vince, E., & Richaume, A. (2016). Combined Study of Titanium Dioxide Nanoparticle Transport and Toxicity on Microbial Nitrifying Communities under Single and Repeated Exposures in Soil Columns. Environmental Science & Technology, 50(19), 10693–10699.
Abstract: Soils are exposed to nanoparticles (NPs) as a result of their increasing use in many commercial products. Adverse effects of NPs on soil microorganisms have, been reported in several ecotoxicological studies using microcosms. Although repeated exposures are more likely to occur in soils, most of these previous studies were performed as a single exposure to NPs. Contrary to single, contamination, the study of multiple NP contaminations in soils requires the use of specialized setups. Using a soil column experiment, we compared the influence of single and repeated exposures (one, two, or three exposures that resulted in the same final concentration applied) on the transport of titanium dioxide (TiO2) NPs through soil and the effect of these different exposure scenarios on the abundance and activity of soil nitrifying microbial communities after a 2 month incubation. The transport of TiO2 NPs was very limited under both single and repeated exposures and was highest for the lowest concentration injected during the first application. Significant decreases in nitrification-activity and ammonia-oxidizing archaea and bacteria populations were observed only for the repeated exposure scenario (three TiO2 NP contaminations). These results suggest that, under repeated exposures, the transport of TiO2 NPs to deep soil layers and groundwater is limited and that a chronic contamination is more harmful for the soil microbiological functioning than a single exposure.
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Simonin, M., Richaume, A., Guyonnet, J. P., Dubost, A., Martins, J. M. F., & Pommier, T. (2016). Titanium dioxide nanoparticles strongly impact soil microbial function by affecting archaeal nitrifiers. Scientific Reports, 6.
Abstract: Soils are facing new environmental stressors, such as titanium dioxide nanoparticles (TiO2-NPs). While these emerging pollutants are increasingly released into most ecosystems, including agricultural fields, their potential impacts on soil and its function remain to be investigated. Here we report the response of the microbial community of an agricultural soil exposed over 90 days to TiO2-NPs (1 and 500 mg kg(-1) dry soil). To assess their impact on soil function, we focused on the nitrogen cycle and measured nitrification and denitrification enzymatic activities and by quantifying specific representative genes (amoA for ammonia-oxidizers, nirK and nirS for denitrifiers). Additionally, diversity shifts were examined in bacteria, archaea, and the ammonia-oxidizing clades of each domain. With strong negative impacts on nitrification enzyme activities and the abundances of ammonia-oxidizing microorganism, TiO2-NPs triggered cascading negative effects on denitrification enzyme activity and a deep modification of the bacterial community structure after just 90 days of exposure to even the lowest, realistic concentration of NPs. These results appeal further research to assess how these emerging pollutants modify the soil health and broader ecosystem function.
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Sirguey, P., Still, H., Cullen, N. J., Dumont, M., Arnaud, Y., & Conway, J. P. (2016). Reconstructing the mass balance of Brewster Glacier, New Zealand, using MODIS-derived glacier-wide albedo. Cryosphere, 10(5), 2465–2484.
Abstract: In New Zealand, direct measurements of mass balance are sparse due to the inaccessibility of glaciers in the Southern Alps and the logistical difficulties associated with maintaining a mass balance record. In order to explore the benefit of remotely sensed imaging to monitor mass balance in the Southern Alps, this research assesses the relationship between measurements of glacier surface albedo derived from Moderate Resolution Imaging Spectroradiometer (MODIS) and mass balance observations using the glaciological method on Brewster Glacier over the 2005-2013 period. We confirm that minimum glacier-wide albedo is a reliable predictor for annual mass balance in this maritime environment (R-2 = 0.93). Furthermore, we show that regular monitoring of glacier-wide albedo enables a new metric of winter accumulation to be derived, namely the cumulative winter albedo, which is found to correlate strongly with winter mass balance (R-2 = 0.88), thus enabling the reconstruction of separate winter and summer mass balance records. This allows the mass balance record for Brewster Glacier to be extended back to the start of MODIS observations in 2000 and to confirm that the annual balance of Brewster Glacier is largely controlled by summer balance (R-2 = 92 %). An application of the extended record is proposed whereby the relationship between mass balance and the photographic record of the end-of-summer snowline altitude is assessed. This allowed the annual balance record of Brewster Glacier to be reconstructed over the period 1977-2013, thus providing the longest record of mass balance for a glacier in New Zealand. Over the 37-year period, our results show that Brewster Glacier gained a significant mass of up to 14.5 +/- 2.7 m w.e. by 2007. This gain was offset by a marked shift toward negative balances after 2008, yielding a loss of 5.1 +/- 1.2 m w.e., or 35% of the gain accumulated over the previous 30 years. The good correspondence between mass balance of Brewster Glacier and the phase of the Pacific (Inter-)Decadal Oscillation (PDO/IPO), associated with the fast terminus retreat observed between 1978 and 1998, strongly suggests that the observed mass gain of Brewster Glacier since 1977 is only offsetting a longer sequence of dominantly negative balances.
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Somers, L. D., Gordon, R. P., McKenzie, J. M., Lautz, L. K., Wigmore, O., Glose, A. M., et al. (2016). Quantifying groundwater-surface water interactions in a proglacial valley, Cordillera Blanca, Peru. Hydrological Processes, 30(17), 2915–2929. |
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Sprovieri, F., Pirrone, N., Bencardino, M., D'Amore, F., Carbone, F., Cinnirella, S., et al. (2016). Atmospheric mercury concentrations observed at ground-based monitoring sites globally distributed in the framework of the GMOS network. Atmospheric Chemistry And Physics, 16(18), 11915–11935.
Abstract: Long-term monitoring of data of ambient mercury (Hg) on a global scale to assess its emission, transport, atmospheric chemistry, and deposition processes is vital to understanding the impact of Hg pollution on the environment. The Global Mercury Observation System (GMOS) project was funded by the European Commission (http://www.gmos.eu) and started in November 2010 with the overall goal to develop a coordinated global observing system to monitor Hg on a global scale, including a large network of ground-based monitoring stations, ad hoc periodic oceanographic cruises and measurement flights in the lower and upper troposphere as well as in the lower stratosphere. To date, more than 40 ground-based monitoring sites constitute the global network covering many regions where little to no observational data were available before GMOS. This work presents atmospheric Hg concentrations recorded worldwide in the framework of the GMOS project (2010-2015), analyzing Hg measurement results in terms of temporal trends, seasonality and comparability within the network. Major findings highlighted in this paper include a clear gradient of Hg concentrations between the Northern and Southern hemispheres, confirming that the gradient observed is mostly driven by local and regional sources, which can be anthropogenic, natural or a combination of both.
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Tano, B. F. G., Dias, D., Fowmes, G. J., Olivier, F., Stoltz, G., & Touze-Foltz, N. (2016). Numerical modeling of the nonlinear mechanical behavior of multilayer geosynthetic system for piggyback landfill expansions. Geotextiles And Geomembranes, 44(6), 782–798.
Abstract: This study examines the extent to which the results of numerical calculations can be influenced both by the differing compressive and tensile behavior of multiple geosynthetics GSYs and by the assumption of strain softening at interfaces between GSYs. Several numerical models are implemented using the finite difference code FLAC 2D on a typical piggyback landfill expansion (PBLE) that involves four GSYs and six interfaces. The present work applies comprehensive, state-of-the-art numerical modeling to study the interactions, between multiple layers of GSYs. It also investigates the nonlinear axial stiffness of GSYs through a series of uniaxial tensile tests. The numerical results,show that, if the GSY axial compressive and tensile characteristics are the same, then tensile force is minimized, which induces significant compressive force in the GSYs. The results also indicate that neglecting strain softening at the interface between GSYs affects interface shear stresses, displacements of GSYs at the interface, and the GSY force distribution, potentially rendering the model unrealistic. Including strain softening, however, allows the assessment (location) of unstable areas along the interface where large displacements occur. (C) 2016 Elsevier Ltd. All rights reserved.
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Tomaz, S., Shahpoury, P., Jaffrezo, J. L., Lammel, G., Perraudin, E., Villenave, E., et al. (2016). One-year study of polycyclic aromatic compounds at an urban site in Grenoble (France): Seasonal variations, gas/particle partitioning and cancer risk estimation. Science Of The Total Environment, 565, 1071–1083.
Abstract: 21 PAHs, 27 oxy-PAHs and 32 nitro-PAHs were measured every third day over a year in both gaseous (G) and particulate PM10 (P) phases in ambient air of Grenoble (France). Mean total concentrations (G + P) of PAHs and oxy-PAHs were in the same range and about 10 ng m(-3). Nitro-PAHs were 50 to 100 times less concentrated averaging 100 pg m(-3). Polycyclic aromatic compound (PAC) concentrations were 5 to 7 times higher in “cold” period (October to March) than in “warm” period (April to September). Seasonal variations may be explained by higher primary emissions from residential heating, especially biomass burning in “cold” season. Meteorological conditions and influence of the geomorphology around Grenoble, with the formation of thermal inversion layers leading to the stagnation of pollutants, were additional key parameters. Maximum individual PAC concentrations were observed during two PM10 pollution events in December and February-March. Chemical processes and secondary formation of oxy-and nitro-PAH were probably enhanced by the accumulation of the pollutants during these events. PAC gas/particle partitioning depended on compound molecular weight and vapour pressure. Gas/particle partitioning of oxy- and nitro-PAHs were evaluated using a multi-phase poly-parameter linear free energy relationship model. The PAC cancer risk was assessed using toxic equivalency factors available in the literature (19 PAHs, 10 nitro-PAHs and 1 oxy- PAH). Overall, particle-bound PACs contributed about 76% of the cancer risk. While PAHs accounted for most of the total PAC cancer risk, oxy- and nitro-PAHs could account for up to 24%. The risk quantification across substance classes is limited by toxicological data availability. (C) 2016 Elsevier B.V. All rights reserved.
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Touzeau, A., Landais, A., Stenni, B., Uemura, R., Fukui, K., Fujita, S., et al. (2016). Acquisition of isotopic composition for surface snow in East Antarctica and the links to climatic parameters. Cryosphere, 10(2), 837–852.
Abstract: The isotopic compositions of oxygen and hydrogen in ice cores are invaluable tools for the reconstruction of past climate variations. Used alone, they give insights into the variations of the local temperature, whereas taken together they can provide information on the climatic conditions at the point of origin of the moisture. However, recent analyses of snow from shallow pits indicate that the climatic signal can become erased in very low accumulation regions, due to local processes of snow reworking. The signal-to-noise ratio decreases and the climatic signal can then only be retrieved using stacks of several snow pits. Obviously, the signal is not completely lost at this stage, otherwise it would be impossible to extract valuable climate information from ice cores as has been done, for instance, for the last glaciation. To better understand how the climatic signal is passed from the precipitation to the snow, we present here results from varied snow samples from East Antarctica. First, we look at the relationship between isotopes and temperature from a geographical point of view, using results from three traverses across Antarctica, to see how the relationship is built up through the distillation process. We also take advantage of these measures to see how second-order parameters (d-excess and O-17-excess) are related to delta O-18 and how they are controlled. d-excess increases in the interior of the continent (i.e., when delta O-18 decreases), due to the distillation process, whereas O-17-excess decreases in remote areas, due to kinetic fractionation at low temperature. In both cases, these changes are associated with the loss of original information regarding the source. Then, we look at the same relationships in precipitation samples collected over 1 year at Dome C and Vostok, as well as in surface snow at Dome C. We note that the slope of the delta O-18 vs. temperature (T) relationship decreases in these samples compared to those from the traverses, and thus caution is advocated when using spatial slopes for past climate reconstruction. The second-order parameters behave in the same way in the precipitation as in the surface snow from traverses, indicating that similar processes are active and that their interpretation in terms of source climatic parameters is strongly complicated by local temperature effects in East Antarctica. Finally we check if the same relationships between delta O-18 and second-order parameters are also found in the snow from four snow pits. While the d-excess remains opposed to delta O-18 in most snow pits, the O-17-excess is no longer positively correlated to delta O-18 and even shows anti-correlation to delta O-18 at Vostok. This may be due to a stratospheric influence at this site and/or to post-deposition processes.
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Trudinger, C. M., Fraser, P. J., Etheridge, D. M., Sturges, W. T., Vollmer, M. K., Rigby, M., et al. (2016). Atmospheric abundance and global emissions of perfluorocarbons CF4, C2F6 and C3F8 since 1800 inferred from ice core, firn, air archive and in situ measurements. Atmospheric Chemistry And Physics, 16(18), 11733–11754.
Abstract: Perfluorocarbons (PFCs) are very potent and long-lived greenhouse gases in the atmosphere, released predominantly during aluminium production and semiconductor manufacture. They have been targeted for emission controls under the United Nations Framework Convention on Climate Change. Here we present the first continuous records of the atmospheric abundance of CF4 (PFC-14), C2F6 (PFC-116) and C3F8 (PFC-218) from 1800 to 2014. The records are derived from high-precision measurements of PFCs in air extracted from polar firn or ice at six sites (DE08, DE08-2, DSSW20K, EDML, NEEM and South Pole) and air archive tanks and atmospheric air sampled from both hemispheres. We take account of the age characteristics of the firn and ice core air samples and demonstrate excellent consistency between the ice core, firn and atmospheric measurements. We present an inversion for global emissions from 1900 to 2014. We also formulate the inversion to directly infer emission factors for PFC emissions due to aluminium production prior to the 1980s. We show that 19th century atmospheric levels, before significant anthropogenic influence, were stable at 34.1 +/- 0.3 ppt for CF4 and below detection limits of 0.002 and 0.01 ppt for C2F6 and C3F8, respectively. We find a significant peak in CF4 and C2F6 emissions around 1940, most likely due to the high demand for aluminium during World War II, for example for construction of aircraft, but these emissions were nevertheless much lower than in recent years. The PFC emission factors for aluminium production in the early 20th century were significantly higher than today but have decreased since then due to improvements and better control of the smelting process. Mitigation efforts have led to decreases in emissions from peaks in 1980 (CF4) or early-to-mid-2000s (C2F6 and C3F8) despite the continued increase in global aluminium production; however, these decreases in emissions appear to have recently halted. We see a temporary reduction of around 15% in CF4 emissions in 2009, presumably associated with the impact of the global financial crisis on aluminium and semiconductor production.
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Tschamber, V., Trouve, G., Leyssens, G., Le-Dreff-Lorimier, C., Jaffrezo, J. L., Genevray, P., et al. (2016). Domestic Wood Heating Appliances with Environmental High Performance: Chemical Composition of Emission and Correlations between Emission Factors and Operating Conditions. Energy & Fuels, 30(9), 7241–7255.
Abstract: If the use of biomass and wood in particular replaces the fossil fuels for the heat production, this has to be made in conditions controlled to minimize the environmental and health impacts. Two recent French domestic appliances presenting high technology of adjustment of different hot air entrances (secondary and postcombustion) were tested with regard to their particulate and gaseous pollutants (total suspended particles (TSP), particulate matter with diameter below 2.5 μm (PM2.5), carbon monoxide (CO), and total hydrocarbons compounds (THC)) for different heat output and combustion phases. Characterization of particulate composition consisted of determining the total carbon (TC) fraction, and its repartition between organic (OC) and elementary (EC) carbon, polycyclic aromatic hydrocarbons (PAH), and wood tracers. Analyses of PAH in the gas phase were also performed. Differences in the proportion of EC/OC in TSP were observed during a wood load: particles are mainly constituted of organic carbon during the inflammation phase. The carbon fraction of the particles at the end of the load decreases to about 20% with approximately half of organic carbon. Levoglucosan is the major biomass tracer present in the solid phase of TSP. Light PAH are predominant in the gas phase, with the naphthalene representing 75% of the total, whereas heavy PAH with cycle numbers from 5 to 7 are mainly present in the solid phase of TSP. However, considering the toxic equivalent factor, the human health impact of adsorbed and gaseous PAH is almost the same. In these conditions, emission factors of CO and TSP were below the minimal values imposed by the highest level of the environmental French label “Flamme Verte” and future European regulations that should come into force in 2022.
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Vakulenko, N. V., Kotlyakov, V. M., Parrenin, F., & Sonechkin, D. M. (2016). A study of different-scale relationship between changes of the surface air temperature and the CO2 concentration in the atmosphere. Led I Sneg-Ice And Snow, 56(4), 533–544.
Abstract: A concept of the anthropogenic origin of the current global climate warming assumes that growth of concentration of the atmospheric carbon dioxide and other greenhouse gases is of great concern in this process. However, all earlier performed analyses of the Antarctic ice cores, covering the time interval of several glacial cycles for about 1 000 000 years, have demonstrated that the carbon dioxide concentration changes had a certain lag relative to the air temperature changes by several hundred years during every beginning of the glacial terminations as well as at endings of interglacials. In contrast to these findings, a recently published careful analysis of Antarctic ice cores (Parrenin et al., 2013) had shown that both, the carbon dioxide concentration and global temperature, varied almost synchronously during the transition from the last glacial maximum to the Holocene. To resolve this dilemma, a special technique for analysis of the paleoclimatic time series, based on the wavelets, had been developed and applied to the same carbon dioxide concentration and temperature time series which were used in the above paper of Parrenin et al., 2013. Specifically, a stack of the Antarctic delta O-18 time series (designated as ATS) and the deuterium Dome C – EPICA ones (dD) were compared to one another in order to: firstly, to quantitatively estimate differences between time scales of these series; and, secondly, to clear up the lead-lag relationships between different scales variations within these time series. It was found that accuracy of the mutual ATS and dD time series dating lay within the range of 80-160 years. Perhaps, the mutual dating of the temperature and carbon dioxide concentration series was even worse due to the assumed displacement of air bubbles within the ice. It made us to limit our analysis by the time scales of approximately from 800 to 6000 years. But it should be taken into account that any air bubble movement changes the time scale of the carbon dioxide series as a whole. Therefore, if a difference between variations in any temperature and the carbon dioxide time series is found to be longer than 80-160 years, and if these variations are timescale-dependent, it means that the bubble displacements are not essential, and so these advancing and delays are characteristic of the time series being compared. Our wavelet-based comparative and different-scale analysis confirms that the relationships between the carbon dioxide concentration and temperature variations were essentially timescale-dependent during the transition from the last glacial maximum to the Holocene. The carbon dioxide concentration variations were ahead of the temperature ones during transition from the glacial maximum to the Boelling – Alleroud warming as well as from the Young Drias cooling to the Holocene optimum. However, the temperature variations were ahead during the transition from the Boelling – Alleroud warming to the Young Drias cooling and during the transition from the Holocene optimum to the present-day climate.
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van den Hurk, B., Kim, H. J., Krinner, G., Seneviratne, S. I., Derksen, C., Oki, T., et al. (2016). LS3MIP (v1.0) contribution to CMIP6: the Land Surface, Snow and Soil moisture Model Intercomparison Project – aims, setup and expected outcome. Geoscientific Model Development, 9(8), 2809–2832.
Abstract: The Land Surface, Snow and Soil Moisture Model Intercomparison Project (LS3MIP) is designed to provide a comprehensive assessment of land surface, snow and soil moisture feedbacks on climate variability and climate change, and to diagnose systematic biases in the land modules of current Earth system models (ESMs). The solid and liquid water stored at the land surface has a large influence on the regional climate, its variability and predictability, including effects on the energy, water and carbon cycles. Notably, snow and soil moisture affect surface radiation and flux partitioning properties, moisture storage and land surface memory. They both strongly affect atmospheric conditions, in particular surface air temperature and precipitation, but also large-scale circulation patterns. However, models show divergent responses and representations of these feedbacks as well as systematic biases in the underlying processes. LS3MIP will provide the means to quantify the associated uncertainties and better constrain climate change projections, which is of particular interest for highly vulnerable regions (densely populated areas, agricultural regions, the Arctic, semi-arid and other sensitive terrestrial ecosystems). The experiments are subdivided in two components, the first addressing systematic land biases in offline mode (“LMIP”, building upon the 3rd phase of Global Soil Wetness Project; GSWP3) and the second addressing land feedbacks attributed to soil moisture and snow in an integrated framework (“LFMIP”, building upon the GLACE-CMIP blueprint).
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Vannier, O., Anquetin, S., & Braud, I. (2016). Investigating the role of geology in the hydrological response of Mediterranean catchments prone to flash-floods: Regional modelling study and process understanding. Journal Of Hydrology, 541, 158–172.
Abstract: In this study, a regional distributed hydrological model is used to perform long-term and flash-flood event simulations, over the Cevennes-Vivarais region (south of France). The objective is to improve our understanding on the role played by geology on the hydrological processes of catchments during two past flash-flood events. This modelling work is based on Vannier et al. (“Regional estimation of catchment-scale soil properties by means of streamflow recession analysis for use in distributed hydrological models”, Hydrological Processes, 2014), where streamflow recessions are analysed to estimate the thickness and hydraulic conductivity of weathered rock layers, depending on the geological nature of catchments. Weathered rock layers are thus implemented into the hydrological model CVN-p, and the contribution of these layers is assessed during flash-flood events simulations as well as during inter event periods. The model is used without any calibration, to test hypotheses on the active hydrological processes. The results point out two different hydrological behaviours, depending on the geology: on crystalline rocks (granite and gneiss), the addition of a weathered rock layer considerably improves the simulated discharges, during flash-flood events as well as during recession periods, and makes the model able to remarkably reproduce the observed streamflow dynamics. For other geologies (schists especially), the benefits are real, but not sufficient to properly simulate the observed streamflow dynamics. These results probably underline the existence of poorly known processes (flow paths, non-linear spilling process) associated with the planar structure of schisty rocks. On a methodological point of view, this study proposes a simple way to account for the additional storage associated with each geological entity, through the addition of a weathered porous rock layer situated below the traditionally considered upper soil horizons, and shows its applicability and benefits for the simulation of flash flood events at the regional scale. (C) 2016 Elsevier B.V. All rights reserved.
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Veylon, G., Stoltz, G., Meriaux, P., Faure, Y. H., & Touze-Foltz, N. (2016). Performance of geotextile filters after 18 years' service in drainage trenches. Geotextiles And Geomembranes, 44(4), 515–533.
Abstract: This study evaluates the long-term performance of two types of geotextiles that were used for 18 years in drainage trenches to stabilize slopes in the French Alps. The flow rate analysis of each trench enabled estimating an average permittivity at trench scale of between 10(-5) and 10(-6) s(-1). After exhuming the geotextiles, their hydraulic and mechanical properties were assessed. The hydraulic tests performed on geotextiles alone gave permittivities greater than 10(-3) s(-1). Gradient ratio tests were performed on undisturbed soil/geotextile/drainage specimens and gave results in the order of 10(-8) s(-1). The aged specimens were examined using scanning electron microscopy. Quantifying the overall performance of the geotextile filter is complicated because of the brittleness of the calcite crust and the subsequent difficulty of characterizing undisturbed interfaces. Various possible explanatory mechanisms involved in the deterioration of trench performance were reviewed: filter cake blinding, internal clogging and downstream chemical clogging. Existing analytical models were used in order to predict the loss of hydraulic performance due to each of these clogging mechanisms. By comparing the measured permittivities to the calculated permittivities, we demonstrated that chemical clogging due to calcite precipitation on the downstream face of the geotextiles was probably the preponderant mechanism responsible for the poorer performance of geotextile filters at trench scale. (C) 2016 Elsevier Ltd. All rights reserved.
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Vidal, J. P., Hingray, B., Magand, C., Sauquet, E., & Ducharne, A. (2016). Hierarchy of climate and hydrological uncertainties in transient low-flow projections. Hydrology And Earth System Sciences, 20(9), 3651–3672.
Abstract: This paper proposes a methodology for estimating the transient probability distribution of yearly hydrological variables conditional to an ensemble of projections built from multiple general circulation models (GCMs), multiple statistical downscaling methods (SDMs), and multiple hydrological models (HMs). The methodology is based on the quasi-ergodic analysis of variance (QE-ANOVA) framework that allows quantifying the contributions of the different sources of total uncertainty, by critically taking account of large-scale internal variability stemming from the transient evolution of multiple GCM runs, and of small-scale internal variability derived from multiple realizations of stochastic SDMs. This framework thus allows deriving a hierarchy of climate and hydrological uncertainties, which depends on the time horizon considered. It was initially developed for long-term climate averages and is here extended jointly to (1) yearly anomalies and (2) low-flow variables. It is applied to better understand possible transient futures of both winter and summer low flows for two snow-influenced catchments in the southern French Alps. The analysis takes advantage of a very large data set of transient hydrological projections that combines in a comprehensive way 11 runs from four different GCMs, three SDMs with 10 stochastic realizations each, as well as six diverse HMs. The change signal is a decrease in yearly low flows of around -20 % in 2065, except for the more elevated catchment in winter where low flows barely decrease. This signal is largely masked by both large- and small-scale internal variability, even in 2065. The time of emergence of the change signal is however detected for low-flow averages over 30-year time slices starting as early as 2020. The most striking result is that a large part of the total uncertainty – and a higher one than that due to the GCMs – stems from the difference in HM responses. An analysis of the origin of this substantial divergence in HM responses for both catchments and in both seasons suggests that both evapotranspiration and snowpack components of HMs should be carefully checked for their robustness in a changed climate in order to provide reliable outputs for informing water resource adaptation strategies.
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Vincendon, B., Edouard, S., Dewaele, H., Ducrocq, V., Lespinas, F., Delrieu, G., et al. (2016). Modeling flash floods in southern France for road management purposes. Journal Of Hydrology, 541, 190–205.
Abstract: Flash-floods are among the most devastating hazards in the Mediterranean. A major subset of damage and casualties caused by flooding is related to road submersion. Distributed hydrological nowcasting can be used for road flooding monitoring. This requires rainfall-runoff simulations at a high space and time resolution. Distributed hydrological models, such as the ISBA-TOP coupled system used in this study, are designed to simulate discharges for any cross-section of a river but they are generally calibrated for certain outlets and give deteriorated results for the sub-catchment outlets. The paper first analyses ISBA-TOP discharge simulations in the French Mediterranean region for target points different from the outlets used for calibration. The sensitivity of the model to its governing factors is examined to highlight the validity of results obtained for ungauged river sections compared with those obtained for the main gauged outlets. The use of improved model inputs is found beneficial for sub-catchments simulation. The calibration procedure however provides the parameters' values for the main outlets only and these choices influence the simulations for ungauged catchments or sub-catchments. As a result, a new version of ISBA-TOP system without any parameter to calibrate is used to produce diagnostics relevant for quantifying the risk of road submersion. A first diagnostic is the simulated runoff spatial distribution, it provides a useful information about areas with a high risk of submersion. Then an indicator of the flood severity is given by simulated discharges presented with respect to return periods. The latter has to be used together with information about the vulnerability of road-river cross-sections. (C) 2016 Elsevier B.V. All rights reserved.
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Vincent, C., & Moreau, L. (2016). Sliding velocity fluctuations and subglacial hydrology over the last two decades on Argentiere glacier, Mont Blanc area. Journal Of Glaciology, 62(235), 805–815.
Abstract: The subglacial observatory beneath the Argentiere glacier provides a rare opportunity to study the interactions between glacier sliding velocity and subglacial runoff. The sliding velocity has been monitored in this cavity almost continuously since 1997 and the resulting data indicate a decrease in annual sliding velocities over the last two decades. We found close relationships between annual surface velocity, sliding velocity and ice thickness. These relationships indicate that the ice-flow velocity changes do not depend on subglacial water runoff changes at the annual timescale. The seasonal magnitudes of sliding also show a decrease over the last two decades. At the seasonal timescale, sliding velocity increases before or simultaneously with the large runoff increase in May, indicating a distributed drainage system. Conversely, at the end of the melt season, sliding velocity continues to decrease after the runoff returns to low winter values. The simultaneous increases of runoff and sliding velocity occur mainly before the spring transition. Later, sliding velocity generally appears not to be related to water inputs coming from the surface, except for some large accelerations after midAugust that are always associated with periods of rapidly increasing water inputs to the subglacial drainage system.
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Vincent, C., Wagnon, P., Shea, J. M., Immerzeel, W. W., Kraaijenbrink, P., Shrestha, D., et al. (2016). Reduced melt on debris-covered glaciers: investigations from Changri Nup Glacier, Nepal. Cryosphere, 10(4), 1845–1858.
Abstract: Approximately 25% of the glacierized area in the Everest region is covered by debris, yet the surface mass balance of debris-covered portions of these glaciers has not been measured directly. In this study, ground-based measurements of surface elevation and ice depth are combined with terrestrial photogrammetry, unmanned aerial vehicle (UAV) and satellite elevation models to derive the surface mass balance of the debris-covered tongue of Changri Nup Glacier, located in the Everest region. Over the debris-covered tongue, the mean elevation change between 2011 and 2015 is -0.93 m year(-1) or 0.84 m water equivalent per year (w.e.a(-1)). The mean emergence velocity over this region, estimated from the total ice flux through a cross section immediately above the debris-covered zone, is +0.37 m w.e.a(-1). The debris-covered portion of the glacier thus has an areaaveraged mass balance of -1.21 +/- 0.2 m w.e.a(-1) between 5240 and 5525 m above sea level (m a.s.l.). Surface mass balances observed on nearby debris-free glaciers suggest that the ablation is strongly reduced (by ca. 1.8 m w.e.a(-1) /by the debris cover. The insulating effect of the debris cover has a larger effect on total mass loss than the enhanced ice ablation due to supraglacial ponds and exposed ice cliffs. This finding contradicts earlier geodetic studies and should be considered for modelling the future evolution of debris-covered glaciers.
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Vouillamoz, J. M., Valois, R., Lun, S., Caron, D., & Arnout, L. (2016). Can groundwater secure drinking-water supply and supplementary irrigation in new settlements of North-West Cambodia? Hydrogeology Journal, 24(1), 195–209.
Abstract: Since the end of the Cambodian Civil War in 1998, the population of the Oddar Meanchey province has drastically increased despite the lack of adequate infrastructure, including basic amenities such as drinking-water supply. To improve the access to drinking water, governmental and aid agencies have focussed on drilling shallow boreholes. The use of groundwater for irrigation is also a growing concern to cope with the occasional late arrival of the rainy season or to produce food during the dry season. Since the groundwater resource in the province has not been documented, a 4-year study was undertaken (2011-2014), aiming to estimate the capability of groundwater to supply domestic needs and supplementary irrigation for rice production. Aquifer properties were estimated by combined use of hydrogeological techniques with the geophysical magnetic resonance sounding method. Groundwater storage and recharge were estimated based on new developments in the application of the geophysical method for quantifying specific yield. The median groundwater storage of the targeted sandstone aquifer is 173 mm, the recharge is diffuse and annually ranges from 10 to 70 mm, and the transmissivity is low to medium. Simulations of pumping indicate that the aquifer can easily supply 100 L of drinking water per capita daily, even considering the estimated population in 2030. However, the shallow aquifer can generally not deliver enough water to irrigate paddy fields of several hectares during a 2-month delay in the onset of the monsoon.
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Wang, T., Lin, X., Liu, Y. W., Dantec-Nedelec, S., & Ottle, C. (2016). Causes of uncertainty in China's net primary production over the 21st century projected by the CMIP5 Earth system models. International Journal Of Climatology, 36(5), 2323–2334.
Abstract: Net primary production is the initial step of the carbon cycle in which atmospheric CO2 is fixed by plants. The responses of net primary production (NPP) to climate change and CO2 are key processes that have the potential to significantly affect the climate-carbon feedback and future atmospheric CO2 levels. Understanding future NPP changes is important for China that became the world's largest CO2 emitter since 2006. Here, we analysed NPP changes in China under the four emission scenarios from 11 Earth system models participating in the Coupled Model Intercomparison Project Phase 5. We find a general increase of NPP over the 21st century under the four emission scenarios, with the large percentage increase in northwestern China and Qinghai-Tibetan Plateau. However, there is a large model spread in the increase of NPP at both country and local scales. We present a statistical approach to assess various processes to explain this large spread, and find that the large spread at the country level is predominantly attributed to inter-model difference in parameterization of CO2 fertilization effect within each emission scenario. But the parameterization of CO2 fertilization effect not always dominates over the model spread across China. When it comes to the local scale, the model spread can be significantly contributed by inter-model difference in parameterization of NPP responses to precipitation along with precipitation projection in northwestern China. Our findings provide the reasons for divergent responses of future NPP through process decomposition and are the first to pinpoint that the model process dominating over the uncertainty exhibits regional dependence.
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Wang, W. L., Rinke, A., Moore, J. C., Ji, D. Y., Cui, X. F., Peng, S. S., et al. (2016). Evaluation of air-soil temperature relationships simulated by land surface models during winter across the permafrost region. Cryosphere, 10(4), 1721–1737.
Abstract: A realistic simulation of snow cover and its thermal properties are important for accurate modelling of permafrost. We analyse simulated relationships between air and near-surface (20 cm) soil temperatures in the Northern Hemisphere permafrost region during winter, with a particular focus on snow insulation effects in nine land surface models, and compare them with observations from 268 Russian stations. There are large cross-model differences in the simulated differences between near-surface soil and air temperatures (Delta T; 3 to 14 degrees C), in the sensitivity of soil-to-air temperature (0.13 to 0.96 degrees C degrees C-1), and in the relationship between Delta T and snow depth. The observed relationship between Delta T and snow depth can be used as a metric to evaluate the effects of each model's representation of snow insulation, hence guide improvements to the model's conceptual structure and process parameterisations. Models with better performance apply multilayer snow schemes and consider complex snow processes. Some models show poor performance in representing snow insulation due to underestimation of snow depth and/or overestimation of snow conductivity. Generally, models identified as most acceptable with respect to snow insulation simulate reasonable areas of near-surface permafrost (13.19 to 15.77 million km(2)). However, there is not a simple relationship between the sophistication of the snow insulation in the acceptable models and the simulated area of Northern Hemisphere near-surface permafrost, because several other factors, such as soil depth used in the models, the treatment of soil organic matter content, hydrology and vegetation cover, also affect the simulated permafrost distribution.
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Weiss, J., Pellissier, V., Marsan, D., Arnaud, L., & Renard, F. (2016). Cohesion versus friction in controlling the long-term strength of a self-healing experimental fault. Journal Of Geophysical Research-Solid Earth, 121(12), 8523–8547.
Abstract: The Coulomb's failure criterion, which postulates that failure occurs along a fault plane when the applied shear stress overcomes a resistance made of two parts of different nature, cohesion, and friction, remains the standard conceptual framework of faulting mechanics. More recently, rate-and-state friction laws became the main modeling tool of the seismic cycle. These laws implicitly assume that only frictional resistance sets the fault strength and its evolution. We therefore raise the question of the role of cohesion and related healing/sealing mechanisms on fault mechanics. We designed an original laboratory experiment based on a model material, an ice thin layer sitting on top of a water tank and mechanically deformed at various rates with a circular Couette-like geometry. This allowed sliding along the fault plane over arbitrarily large slip distances, and analyzing the competition between faulting and cohesion-healing rates, whereas frictional resistance, resulting from the fault roughness, is limited in magnitude. We show that cohesion-healing plays an essential role in the time evolution of the interface strength under constant loading rate. In particular, the magnitudes of shear stress fluctuations are observed to be temperature and velocity weakening. The present experiment shares several properties similar to that of active faults during seismic cycles, suggesting that cohesive healing could control some of these features: scale invariance properties, Gutenberg-Richter law of rupture event size distribution, Omori's law of energy dissipation, at least after major ruptures, time asymmetry in energy dissipation, and periods of creep under high shear stress alternating with major earthquake-like ruptures.
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Wendling, V., Legout, C., Gratiot, N., Michallet, H., & Grangeon, T. (2016). Dynamics of soil aggregate size in turbulent flow: Respective effect of soil type and suspended concentration. Catena, 141, 66–72.
Abstract: The fate of eroded soil particles impacts soil loss, river engineering and aquatic ecosystems. However, little is known about soil aggregate dynamics within the flow just after their detachment from the soil matrix. The relationship between particle size and turbulence has already been studied but few studies analysed the associated effect of particle concentration. The disaggregation/flocculation of three soils, two badland materials and a well developed calcareous brown soil, was studied by using a grid-stirred tank. An isotropic and homogeneous turbulence was generated to focus on the effects of suspended concentration on particle sizes. Increasing the suspended concentration in the range 1-10 g L-1 leads to a decrease of the proportion of the medium size particles and of an increase of the proportion of the smallest particles, as a consequence to enhanced abrasion. The soil aggregates with the largest organic content had the highest strength but were still subject to disaggregation within the turbulent flow, the resulting particle size depending on the suspended concentration. This study demonstrates that soil aggregate characteristics are influenced by concentration, this behaviour being dependent upon the soil type. (C) 2016 Elsevier B.V. All rights reserved.
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Westermann, S., Langer, M., Boike, J., Heikenfeld, M., Peter, M., Etzelmuller, B., et al. (2016). Simulating the thermal regime and thaw processes of ice-rich permafrost ground with the land-surface model CryoGrid 3. Geoscientific Model Development, 9(2), 523–546.
Abstract: Thawing of permafrost in a warming climate is governed by a complex interplay of different processes of which only conductive heat transfer is taken into account in most model studies. However, observations in many permafrost landscapes demonstrate that lateral and vertical movement of water can have a pronounced influence on the thaw trajectories, creating distinct landforms, such as thermokarst ponds and lakes, even in areas where permafrost is otherwise thermally stable. Novel process parameterizations are required to include such phenomena in future projections of permafrost thaw and subsequent climatic-triggered feedbacks. In this study, we present a new land-surface scheme designed for permafrost applications, CryoGrid 3, which constitutes a flexible platform to explore new parameterizations for a range of permafrost processes. We document the model physics and employed parameterizations for the basis module CryoGrid 3, and compare model results with in situ observations of surface energy balance, surface temperatures, and ground thermal regime from the Samoylov permafrost observatory in NE Siberia. The comparison suggests that CryoGrid 3 can not only model the evolution of the ground thermal regime in the last decade, but also consistently reproduce the chain of energy transfer processes from the atmosphere to the ground. In addition, we demonstrate a simple 1-D parameterization for thaw processes in permafrost areas rich in ground ice, which can phenomenologically reproduce both formation of thermokarst ponds and subsidence of the ground following thawing of ice-rich subsurface layers. Long-term simulation from 1901 to 2100 driven by reanalysis data and climate model output demonstrate that the hydrological regime can both accelerate and delay permafrost thawing. If meltwater from thawed ice-rich layers can drain, the ground subsides, as well as the formation of a talik, are delayed. If the meltwater pools at the surface, a pond is formed that enhances heat transfer in the ground and leads to the formation of a talik. The model results suggest that the trajectories of future permafrost thaw are strongly influenced by the cryostratigraphy, as determined by the late Quaternary history of a site.
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Wilhelm, B., Nomade, J., Crouzet, C., Litty, C., Sabatier, P., Belle, S., et al. (2016). Quantified sensitivity of small lake sediments to record historic earthquakes: Implications for paleoseismology. Journal Of Geophysical Research-Earth Surface, 121(1), 2–16.
Abstract: Seismic hazard assessment is a critical but challenging issue for modern societies. A key parameter to be estimated is the recurrence interval of damaging earthquakes. This requires the establishment of earthquake records long enough to be relevant, i.e., far longer than historical observations. We study how lake sediments can be used for this purpose and explore conditions that enable lake sediments to record earthquakes. This was achieved (i) through the compilation of eight lake-sediment sequences from the European Alps to reconstruct chronicles of mass movement deposits and (ii) through the comparison of these chronicles with the well-documented earthquake history. This allowed 24 occurrences of mass movements to be identified, of which 21 were most probably triggered by an earthquake. However, the number of earthquake-induced deposits varies between lakes of a same region, suggesting variable thresholds of the lake sequences to record earthquake shaking. These thresholds have been quantified by linking the mass movement occurrences in a single lake to both intensity and distance of the triggering earthquakes. This method offers a quantitative approach to estimate locations and intensities of past earthquake epicenters. Finally, we explored which lake characteristics could explain the various sensitivities. Our results suggest that sedimentation rate should be larger than 0.5mmyr(-1) so that a given lake records earthquakes in moderately active seismotectonic regions. We also postulate that an increasing sedimentation rate may imply an increasing sensitivity to earthquake shaking. Hence, further paleoseismological studies should control carefully that no significant change in sedimentation rates occurs within a record, which could falsify the assessment of earthquake recurrence intervals.
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Wilhelm, B., Vogel, H., Crouzet, C., Etienne, D., & Anselmetti, F. S. (2016). Frequency and intensity of palaeofloods at the interface of Atlantic and Mediterranean climate domains. Climate Of The Past, 12(2), 299–316.
Abstract: The long-term response of the flood activity to both Atlantic and Mediterranean climatic influences was explored by studying a lake sequence (Lake Foreant) of the Western European Alps. High-resolution sedimentological and geochemical analysis revealed 171 event layers, 168 of which result from past flood events over the last millennium. The layer thickness was used as a proxy of intensity of past floods. Because the Foreant palaeoflood record is in agreement with the documented variability of historical floods resulting from local and mesoscale, summer-to-autumn convective events, it is assumed to highlight changes in flood frequency and intensity related to such events typical of both Atlantic (local events) and Mediterranean (mesoscale events) climatic influences. Comparing the Foreant record with other Atlantic-influenced and Mediterranean-influenced regional flood records highlights a common feature in all flood patterns that is a higher flood frequency during the cold period of the Little Ice Age (LIA, AD 1300-1900). In contrast, high-intensity flood events are apparent during both the cold LIA and the warm Medieval Climate Anomaly (MCA, AD 9501250). However, there is a tendency towards higher frequencies of high-intensity flood events during the warm MCA. The MCA extremes could mean that under the global warming scenario, we might see an increase in intensity (not in frequency). However, the flood frequency and intensity in the course of the 20th century warming trend did not change significantly. Uncertainties in future evolution of flood intensity lie in the interpretation of the lack of 20th century extremes (transition or stable?) and the different climate forcing factors between the two periods (greenhouse gases vs. solar and/or volcanic eruptions).
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Wolff, E. W., Bigler, M., Curran, M. A. J., Dibb, J. E., Frey, M. M., Legrand, M., et al. (2016). Comment on “Low time resolution analysis of polar ice cores cannot detect impulsive nitrate events” by DF Smart et al. Journal Of Geophysical Research-Space Physics, 121(3), 1920–1924.
Abstract: Smart et al. (2014) suggested that the detection of nitrate spikes in polar ice cores from solar energetic particle (SEP) events could be achieved if an analytical system with sufficiently high resolution was used. Here we show that the spikes they associate with SEP events are not reliably recorded in cores from the same location, even when the resolution is clearly adequate. We explain the processes that limit the effective resolution of ice cores. Liquid conductivity data suggest that the observed spikes are associated with sodium or another nonacidic cation, making it likely that they result from deposition of sea salt or similar aerosol that has scavenged nitrate, rather than from a primary input of nitrate in the troposphere. We consider that there is no evidence at present to support the identification of any spikes in nitrate as representing SEP events. Although such events undoubtedly create nitrate in the atmosphere, we see no plausible route to using nitrate spikes to document the statistics of such events.
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Yousefi, S., Pourghasemi, H. R., Hooke, J., Navratil, O., & Kidova, A. (2016). Changes in morphometric meander parameters identified on the Karoon River, Iran, using remote sensing data. Geomorphology, 271, 55–64.
Abstract: River meander dynamics and mobility are important indicators of environmental change related to climate changes and anthropogenic activities at local and river basin scales. The aim of the present study is to identify morphological changes of the Maroon River in Iran using high accuracy maps and Landsat satellite images by analyses during the time period 1989-2008. In this study, 20 meandering reaches were analyzed over a 128-km-long river reach located in the middle part of the Karoon River, Iran. Morphometric indicators such as: river width (W), meander neck length (L), axis length (A), radius of curvature (R), water flow length (S), and sinuosity of meander (C) were extracted for the identified meanders. The results of a paired t-test showed that river width (W) and meander neck length (L) have significantly changed during the study period (1989-2008), with an increase of +3.5 m for Wand a decrease of 274 m for L Spearman correlation analysis has shown that meander parameter changes are highly correlated to each other. The parameters that do not have significant correlation together are C with W and L, Wand L, and L with S and A. During the period of the study, the flow length and river sinuosity decreased for the whole river reach, by 4.77 km and 0.11, respectively. Analysis of land use/land cover categories (1989 and 2008) using the support vector machine (SVM) and kernel function method served as one of the tools for interpretation of the meander parameter changes. These changes can be attributed not only to LU/LC (riparian vegetation to agriculture area ratio) but also to dam construction in the upstream part of the river that leads to major hydrological regime and sediment transfer alteration. Sediment extraction may also be an important factor. (C) 2016 Elsevier B.V. All rights reserved.
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Yue, C., Ciais, P., Zhu, D., Wang, T., Peng, S. S., & Piao, S. L. (2016). How have past fire disturbances contributed to the current carbon balance of boreal ecosystems? Biogeosciences, 13(3), 675–690.
Abstract: Boreal fires have immediate effects on regional carbon budgets by emitting CO2 into the atmosphere at the time of burning, but they also have legacy effects by initiating a long-term carbon sink during post-fire vegetation recovery. Quantifying these different effects on the current-day pan-boreal (44-84 degrees N) carbon balance and quantifying relative contributions of legacy sinks by past fires is important for understanding and predicting the carbon dynamics in this region. Here we used the global dynamic vegetation model ORCHIDEE-SPITFIRE (Organising Carbon and Hydrology In Dynamic Ecosystems – SPread and InTensity of FIRE) to attribute the contributions by fires in different decades between 1850 and 2009 to the carbon balance of 2000-2009, taking into account the atmospheric CO2 change and climate change since 1850. The fire module of ORCHIDEE-SPITFIRE was turned off for each decade in turn and was also turned off before and after the decade in question in order to model the legacy carbon trajectory by fires in each past decade. We found that, unsurprisingly, fires that occurred in 2000-2009 are a carbon source (-0.17 Pg C yr(-1)) for the carbon balance of 2000-2009, whereas fires in all decades before 2000 contribute carbon sinks with a collective contribution of 0.23 Pg C yr(-1). This leaves a net fire sink effect of 0.06 Pg Cyr(-1), or 6.3% of the simulated regional carbon sink (0.95 Pg C yr(-1)). Further, fires with an age of 10-40 years (i.e., those that occurred during 1960-1999) contribute more than half of the total sink effect of fires. The small net sink effect of fires indicates that current-day fire emissions are roughly balanced out by legacy sinks. The future role of fires in the regional carbon balance remains uncertain and will depend on whether changes in fires and associated carbon emissions will exceed the enhanced sink effects of previous fires, both being strongly affected by global change.
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Zanatta, M., Gysel, M., Bukowiecki, N., Müller, T., Weingartner, E., Areskoug, H., et al. (2016). A European aerosol phenomenology-5: Climatology of black carbon optical properties at 9 regional background sites across Europe. Atmospheric Environment, 145, 346–364.
Abstract: A reliable assessment of the optical properties of atmospheric black carbon is of crucial importance for an accurate estimation of radiative forcing. In this study we investigated the spatio-temporal variability of the mass absorption cross-section (MAC) of atmospheric black carbon, defined as light absorption coefficient (σap) divided by elemental carbon mass concentration (mEC). σap and mEC have been monitored at supersites of the ACTRIS network for a minimum period of one year. The 9 rural background sites considered in this study cover southern Scandinavia, central Europe and the Mediterranean. σap was determined using filter based absorption photometers and mEC using a thermal-optical technique. Homogeneity of the data-set was ensured by harmonization of all involved methods and instruments during extensive intercomparison exercises at the European Center for Aerosol Calibration (ECAC). Annual mean values of σap at a wavelength of 637 nm vary between 0.66 and 1.3 Mm-1 in southern Scandinavia, 3.7-11 Mm-1 in Central Europe and the British Isles, and 2.3-2.8 Mm-1 in the Mediterranean. Annual mean values of mEC vary between 0.084 and 0.23 μg m-3 in southern Scandinavia, 0.28-1.1 in Central Europe and the British Isles, and 0.22-0.26 in the Mediterranean. Both σap and mEC in southern Scandinavia and Central Europe have a distinct seasonality with maxima during the cold season and minima during summer, whereas at the Mediterranean sites an opposite trend was observed. Annual mean MAC values were quite similar across all sites and the seasonal variability was small at most sites. Consequently, a MAC value of 10.0 m2 g-1 (geometric standard deviation = 1.33) at a wavelength of 637 nm can be considered to be representative of the mixed boundary layer at European background sites, where BC is expected to be internally mixed to a large extent. The observed spatial variability is rather small compared to the variability of values in previous literature, indicating that the harmonization efforts resulted in substantially increased precision of the reported MAC. However, absolute uncertainties of the reported MAC values remain as high as ± 30-70% due to the lack of appropriate reference methods and calibration materials. The mass ratio between elemental carbon and non-light-absorbing matter was used as a proxy for the thickness of coatings around the BC cores, in order to assess the influence of the mixing state on the MAC of BC. Indeed, the MAC was found to increase with increasing values of the coating thickness proxy. This provides evidence that coatings do increase the MAC of atmospheric BC to some extent, which is commonly referred to as lensing effect.
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Zannoni, N., Gros, V., Lanza, M., Sarda, R., Bonsang, B., Kalogridis, C., et al. (2016). OH reactivity and concentrations of biogenic volatile organic compounds in a Mediterranean forest of downy oak trees. Atmospheric Chemistry And Physics, 16(3), 1619–1636.
Abstract: Total OH reactivity, defined as the total loss frequency of the hydroxyl radical in the atmosphere, has proved to be an excellent tool to identify the total loading of reactive species in ambient air. High levels of unknown reactivity were found in several forests worldwide and were often higher than at urban sites. Our study presents atmospheric mixing ratios of biogenic compounds and total OH reactivity measured during late spring 2014 at the forest of downy oak trees of the Observatoire de Haute Provence (OHP), France. Air masses were sampled at two heights: 2 m, i.e., inside the canopy, and 10 m, i.e., above the canopy, where the mean canopy height is 5 m. We found that the OH reactivity at the site mainly depended on the main primary biogenic species emitted by the forest, which was isoprene and to a lesser extent by its degradation products and long-lived atmospheric compounds (up to 26% during daytime). During daytime, no significant missing OH reactivity was reported at the site, either inside or above the canopy. However, during two nights we determined a missing fraction of OH reactivity up to 50 %, possibly due to unmeasured oxidation products. We confirmed that no significant oxidation of the primary species occurred within the canopy; primary compounds emitted by the forest were fast transported to the atmosphere. Finally, the OH reactivity at this site was maximum 69 s(-1), which is a high value for a forest characterized by a temperate climate. Observations in various and diverse forests in the Mediterranean region are therefore needed to better constrain the impact of reactive gases over this area.
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Zatko, M., Erbland, J., Savarino, J., Geng, L., Easley, L., Schauer, A., et al. (2016). The magnitude of the snow-sourced reactive nitrogen flux to the boundary layer in the Uintah Basin, Utah, USA. Atmospheric Chemistry And Physics, 16(21), 13837–13851.
Abstract: Reactive nitrogen(N-r = NO, NO2, HONO) and volatile organic carbon emissions from oil and gas extraction activities play a major role in wintertime ground-level ozone exceedance events of up to 140 ppb in the Uintah Basin in eastern Utah. Such events occur only when the ground is snow covered, due to the impacts of snow on the stability and depth of the boundary layer and ultraviolet actinic flux at the surface. Recycling of reactive nitrogen from the photolysis of snow nitrate has been observed in polar and mid-latitude snow, but snow-sourced reactive nitrogen fluxes in mid-latitude regions have not yet been quantified in the field. Here we present vertical profiles of snow nitrate concentration and nitrogen isotopes (delta N-15) collected during the Uintah Basin Winter Ozone Study 2014 (UBWOS 2014), along with observations of insoluble light-absorbing impurities, radiation equivalent mean ice grain radii, and snow density that determine snow optical properties. We use the snow optical properties and nitrate concentrations to calculate ultraviolet actinic flux in snow and the production of N-r from the photolysis of snow nitrate. The observed delta N-15(NO3-) is used to constrain modeled fractional loss of snow nitrate in a snow chemistry column model, and thus the source of N-r to the overlying boundary layer. Snow-surface delta N-15(NO3-) measurements range from -5 to 10% and suggest that the local nitrate burden in the Uintah Basin is dominated by primary emissions from anthropogenic sources, except during fresh snowfall events, where remote NOx sources from beyond the basin are dominant. Modeled daily averaged snow-sourced N-r fluxes range from 5.6 to 71 x 10(7) molec cm(-2) s(-1) over the course of the field campaign, with a maximum noontime value of 3.1 x 10(9) molec cm(-2) s(-1). The top-down emission estimate of primary, anthropogenic NOx in Uintah and Duchesne counties is at least 300 times higher than the estimated snow NOx emissions presented in this study. Our results suggest that snow-sourced reactive nitrogen fluxes are minor contributors to the N-r boundary layer budget in the highly polluted Uintah Basin boundary layer during winter 2014.
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Zhu, D., Peng, S., Ciais, P., Zech, R., Krinner, G., Zimov, S., et al. (2016). Simulating soil organic carbon in yedoma deposits during the Last Glacial Maximum in a land surface model. Geophysical Research Letters, 43(10), 5133–5142.
Abstract: Substantial quantities of organic carbon (OC) are stored in the thick, ice-rich, and organic-rich sediments called yedoma deposits, distributed in eastern Siberia and Alaska today. Quantifying yedoma carbon stocks during the glacial period is important for understanding how much carbon could have been decomposed during the last deglaciation. Yet processes that yield the formation of thick frozen OC in yedoma deposits aremissing in global carbon cycle models. Here we incorporate sedimentation parameterizations into the Organizing Carbon and Hydrology In Dynamic Ecosystems (ORCHIDEE-MICT) land surface model, which leads to reasonable results in OC vertical distribution and regional budgets, compared with site-specific observations and inventories for today's nondegraded yedoma region. Simulated total soil OC stock for the northern permafrost region during the Last Glacial Maximum (LGM) is 1536-1592 Pg C, of which 390-446 Pg C is within today's yedoma region. This result is an underestimation since we did not account for the potentially much larger yedoma area during the LGM than the present day.
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Zwiebel, J., Van Baelen, J., Anquetin, S., Pointin, Y., & Boudevillain, B. (2016). Impacts of orography and rain intensity on rainfall structure. The case of the HyMeX IOP7a event. Quarterly Journal Of The Royal Meteorological Society, 142, 310–319.
Abstract: The first Special Observation Period of the HyMeX campaign took place during autumn 2012. A specific observational network was set up in the south of France with the aim of better understanding the role and impact of complex terrain on the horizontal and vertical structure of rainfall and the associated microphysical processes. The event, referred to as IOP7a, which occurred on 26 September 2012, is studied in this article. A first analysis, based on two reference parameters of the drop-size distribution (DSD), shows that the topography of the region has an impact on the rainfall structure at the fine scale and that rainfall intensity also has an influence on the DSD parameters. A more detailed study, based on the evolution of the shape of the DSD and of the liquid water content (LWC) within different rainfall intensities, reveals that the coalescence mechanism is of significant importance for different rainfall regimes and for different topographic areas. As we get closer to the ground, the modification of the DSD shows that other microphysical and dynamical processes (such as evaporation, break-up, turbulence in the presence of vertical wind) might compete with the coalescence mechanism or enhance it. The respective strengths of these processes depend on the local environment; coalescence seems enhanced by a layer with higher LWC as the local terrain becomes more complex, and evaporation appears more important above flatter terrain. Thus, both the orography and the rainfall intensity can impact the rainfall structure and the associated microphysical and dynamical processes.
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