2012-2013
11 novembre 2018, par Gerhard Krinner
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Abram, N. J., Mulvaney, R., Wolff, E., Triest, J., Kipfstuhl, S., Trusel, L. D., et al. (2013). Acceleration of snow melt in an Antarctic Peninsula ice core during the twentieth century. Nature Geoscience, 6(5), 404–411.
Abstract: Over the past 50 years, warming of the Antarctic Peninsula has been accompanied by accelerating glacier mass loss and the retreat and collapse of ice shelves. A key driver of ice loss is summer melting; however, it is not usually possible to specifically reconstruct the summer conditions that are critical for determining ice melt in Antarctic. Here we reconstruct changes in ice-melt intensity and mean temperature on the northern Antarctic Peninsula since AD 1000 based on the identification of visible melt layers in the James Ross Island ice core and local mean annual temperature estimates from the deuterium content of the ice. During the past millennium, the coolest conditions and lowest melt occurred from about AD 1410 to 1460, when mean temperature was 1.6 degrees C lower than that of 1981-2000. Since the late 1400s, there has been a nearly tenfold increase in melt intensity from 0.5 to 4.9%. The warming has occurred in progressive phases since about AD 1460, but intensification of melt is nonlinear, and has largely occurred since the mid-twentieth century. Summer melting is now at a level that is unprecedented over the past 1,000 years. We conclude that ice on the Antarctic Peninsula is now particularly susceptible to rapid increases in melting and loss in response to relatively small increases in mean temperature.
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Agosta, C., Favier, V., Krinner, G., Gallee, H., Fettweis, X., & Genthon, C. (2013). High-resolution modelling of the Antarctic surface mass balance, application for the twentieth, twenty first and twenty second centuries. Climate Dynamics, 41(11-12), 3247–3260.
Abstract: About 75 % of the Antarctic surface mass gain occurs over areas below 2,000 m asl, which cover 40 % of the grounded ice-sheet. As the topography is complex in many of these regions, surface mass balance modelling is highly dependent on horizontal resolution, and studying the impact of Antarctica on the future rise in sea level requires physical approaches. We have developed a computationally efficient, physical downscaling model for high-resolution (15 km) long-term surface mass balance (SMB) projections. Here, we present results of this model, called SMHiL (surface mass balance high-resolution downscaling), which was forced with the LMDZ4 atmospheric general circulation model to assess Antarctic SMB variability in the twenty first and the twenty second centuries under two different scenarios. The higher resolution of SMHiL better reproduces the geographical patterns of SMB and increase significantly the averaged SMB over the grounded ice-sheet for the end of the twentieth century. A comparison with more than 3200 quality-controlled field data shows that LMDZ4 and SMHiL reproduce the observed values equally well. Nevertheless, field data below 2,000 m asl are too scarce to efficiently show the added value of SMHiL and measuring the SMB in these undocumented areas should be a future scientific priority. Our results suggest that running LMDZ4 at a finer resolution (15 km) may give a future increase in SMB in Antarctica that is about 30 % higher than by using its standard resolution (60 km) due to the higher increase in precipitation in coastal areas at 15 km. However, a part (similar to 15 %) of these discrepancies could be an artefact from SMHiL since it neglects the foehn effect and likely overestimates the precipitation increase. Future changes in the Antarctic SMB at low elevations will result from the competition between higher snow accumulation and runoff. For this reason, developing downscaling models is crucial to represent processes in sufficient detail and correctly model the SMB in coastal areas.
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Alvarez-Solas, J., Robinson, A., Montoya, M., & Ritz, C. (2013). Iceberg discharges of the last glacial period driven by oceanic circulation changes. Proceedings Of The National Academy Of Sciences Of The United States Of America, 110(41), 16350–16354.
Abstract: Proxy data reveal the existence of episodes of increased deposition of ice-rafted detritus in the North Atlantic Ocean during the last glacial period interpreted as massive iceberg discharges from the Laurentide Ice Sheet. Although these have long been attributed to self-sustained ice sheet oscillations, growing evidence of the crucial role that the ocean plays both for past and future behavior of the cryosphere suggests a climatic control of these ice surges. Here, we present simulations of the last glacial period carried out with a hybrid ice sheet-ice shelf model forced by an oceanic warming index derived from proxy data that accounts for the impact of past ocean circulation changes on ocean temperatures. The model generates a time series of iceberg discharge that closely agrees with ice-rafted debris records over the past 80 ka, indicating that oceanic circulation variations were responsible for the enigmatic ice purges of the last ice age.
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Antoine, G., Jodeau, M., Camenen, B., Esteves, M., Nemery, J., & Lauters, F. (2013). Assessment of suspended sediment fluxes during Arc flushing flows, 2006-2011. Houille Blanche-Revue Internationale De L Eau, (4), 43–49.
Abstract: Dam flushing is conducted regularly on the Arc River (Northern French Alps) to remove sediments from dam reservoirs. This paper presents the analysis of five field campaigns carried out during dam flushing events in the upper Arc River basin (in June 2006, 2007, 2009, 2010 and 2011). Measurements of suspended sediment concentrations (SSC) and discharges were performed at several sampling sites along the river channel. To complete the discharge data set, a 1D numerical hydraulic model, MASCARET, was calibrated and used to estimate the instantaneous discharge at any position along the river system. Following the temporal integration of the instantaneous suspended sediment fluxes, a spatial distribution of the suspended load along the Arc and Isere Rivers was obtained. The classical error propagation method was used to quantify errors resulting from the method employed to estimate the integrated fluxes, by taking into account the relative errors introduced by both the measurements and the model. It appears that the response of the river's sediment transport differs from one year to another. The quantity of fine sediments removed from the reservoirs varied from 10,000 tons in 2007 to 40,000 tons in 2006. Over the 120 kilometers of river network, the suspended load could either increase significantly (30,000 tons in 2007) or else remain exactly the same (in 2009 and 2011). The mean relative standard deviation on the integrated fluxes is equal to 11 %. Taking into account these uncertainty values, it appears that the global tendencies are significant, but the local mass balances are more difficult to interpret.
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Argentini, S., Petenko, I., Viola, A., Mastrantonio, G., Pietroni, I., Casasanta, G., et al. (2013). The surface layer observed by a high-resolution sodar at DOME C, Antarctica. Annals Of Geophysics, 56(5).
Abstract: A one-year field experiment started on December 2011 at the French – Italian station of Concordia at Dome C, East Antarctic Plateau. The objective of the experiment was the study of the surface layer turbulent processes under stable/very stable stratifications, and the mechanisms leading to the formation of the warming events. A sodar was improved to achieve the vertical/temporal resolution needed to study these processes. The system, named surface layer sodar (SL-sodar), may operate both in high vertical resolution (low range) and low vertical resolution (high range) modes. SL-sodar observations were complemented with in situ turbulence and radiation measurements. A few preliminary results, concerning the standard summer diurnal cycle, a summer warming event, and unusually high frequency boundary layer atmospheric gravity waves are presented.
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Arnaud, J., Dutordoir, S., Nemery, J., & Belleudy, P. (2013). Influence of flow measurement uncertainty related to the calculation of TSS and particulate organic carbon fluxes: application to an alpine river (Isere, France). Houille Blanche-Revue Internationale De L Eau, (4), 37–42.
Abstract: Particulate Flux estimation (suspended sediment and associated substances such as organic matter or micropolutants) is a major issue in mountain rivers. A database of high-frequency (30 minutes) discharge, suspended solids (TSS) and particulate organic carbon (POC) on Isere River in Grenoble was used for the calculation of annual fluxes and during major floods over 6 years (2006-2011). Fluxes were calculated by summing the 30 minutes fluxes. The study presents a method for calculating uncertainty fluxes based on Monte Carlo simulations using the errors on the relationships between continuous measurement / experimental measurements (height / flow, TSS / Turbidity, TSS / POC). In particular, the effect of discharge for the calculation of uncertainty is analyzed for different error scenarios (0, 3, 10, 20, 30 % and using a mean daily discharge). The method allowed estimating uncertainties < 10 % for annual fluxes of TSS and POC whatever the scenario. However, uncertainties are more important for flood events (up to 30 % with the use of a mean daily discharge). It appears from the study that the influence of the error on the discharge is relatively less important than the errors on the parameters MES and POC. It is also shown that the mean daily discharge should be avoided for flux calculations and that high frequency database remains the best alternative for a good estimate of the fluxes.
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Arthern, R. J., Corr, H. F. J., Gillet-Chaulet, F., Hawley, R. L., & Morris, E. M. (2013). Inversion for the density-depth profile of polar firn using a stepped-frequency radar. Journal Of Geophysical Research-Earth Surface, 118(3), 1257–1263.
Abstract: Translating satellite measurements of ice sheet volume change into sea level contribution requires knowledge of the profile of density as a function of depth within the ice sheet and how this profile changes over time. This paper describes an interferometric method of inverting ground-penetrating radar returns for the profile of firn density as a function of depth. The method is an interferometric implementation of the common-midpoint approach, performed using a stepped-frequency, phase-sensitive ground-penetrating radar. By recording the phase difference of returns with a range of antenna separations, the different path lengths through the firn allow recovery of a smoothed representation of the density profile. This density model is characterized by three parameters: surface density and two decay lengths for porosity, each operating over a different density range. Our results suggest that the stepped-frequency radar used here can accurately recover differences in two-way traveltime and produce useful estimates of the density profile. In a test of the method performed at Summit station in Greenland, the recovered density-depth profile agreed with independent density measurements from an ice core and a neutron probe to within 6% root-mean-square error.
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Asmi, A., Coen, M. C., Ogren, J. A., Andrews, E., Sheridan, P., Jefferson, A., et al. (2013). Aerosol decadal trends – Part 2: In-situ aerosol particle number concentrations at GAW and ACTRIS stations. Atmospheric Chemistry And Physics, 13(2), 895–916.
Abstract: We have analysed the trends of total aerosol particle number concentrations (N) measured at long-term measurement stations involved either in the Global Atmosphere Watch (GAW) and/or EU infrastructure project ACTRIS. The sites are located in Europe, North America, Antarctica, and on Pacific Ocean islands. The majority of the sites showed clear decreasing trends both in the full-length time series, and in the intra-site comparison period of 2001-2010, especially during the winter months. Several potential driving processes for the observed trends were studied, and even though there are some similarities between N trends and air temperature changes, the most likely cause of many northern hemisphere trends was found to be decreases in the anthropogenic emissions of primary particles, SO2 or some co-emitted species. We could not find a consistent agreement between the trends of N and particle optical properties in the few stations with long time series of all of these properties. The trends of N and the proxies for cloud condensation nuclei (CCN) were generally consistent in the few European stations where the measurements were available. This work provides a useful comparison analysis for modelling studies of trends in aerosol number concentrations.
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Balarac, G., Le Sommer, J., Meunier, X., & Vollant, A. (2013). A dynamic regularized gradient model of the subgrid-scale scalar flux for large eddy simulations. Physics Of Fluids, 25(7), 075107.
Abstract: Accurate predictions of scalar fields advected by a turbulent flow is needed for various industrial and geophysical applications. In the framework of large-eddy simulation (LES), a subgrid-scale (SGS) model for the subgrid-scale scalar flux has to be used. The gradient model (GM), which is derived from a Taylor series expansions of the filtering operation, is a well-known approach to model SGS scalar fluxes. This model is known to lead to high correlation level with the SGS scalar flux. However, this type of model cannot be used in practical LES because it does not lead to enough global scalar variance transfer from the large to the small scales. In this work, a regularization of the GM is proposed based on a physical interpretation of this model. The impact of the resolved velocity field on the resolved scalar gradient is decomposed into compressional, stretching, and rotational effects. It is shown that rotational effect is not associated with transfers of variance across scales. Conversely, the compressional effect is shown to lead to forward transfer, whereas the stretching effect leads to back-scatter of scalar variance. The proposed regularization is to neglect the stretching effect in the model formulation. The accuracy of this regularized gradient model (RGM) is tested in comparison with direct numerical simulations and compared with other classic SGS models. The accuracy of the RGM is evaluated in term of structural and functional performances, i.e., the model ability to locally approximate the SGS unknown term and to reproduce its global effect on tracer variance, respectively. It is found that the RGM associated with a dynamic procedure exhibits good performances in comparison with the standard dynamic eddy diffusivity model and the standard gradient model. In particular, the dynamic regularized gradient model (DRGM) provides a better prediction of scalar variance transfers than the standard gradient model. The DRGM is then evaluated in a series of large-eddy simulations. This shows a substantial improvement for various scalar statistics predictions. (C) 2013 AIP Publishing LLC.
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Baruchel, J., Di Michiel, M., Lafford, T., Lhuissier, P., Meyssonnier, J., Henri, N. T., et al. (2013). Synchrotron X-ray imaging for crystal growth studies. Comptes Rendus Physique, 14(2-3), 208–220.
Abstract: The features associated with modern synchrotron radiation machines (intense and coherent beams) result in a substantial extension of X-ray imaging capabilities in terms of spatial and temporal resolution, phase contrast and 3D images. This allows crystal growth-related information to be obtained which is not available otherwise. After briefly describing the main synchrotron radiation based imaging techniques of interest, we give original examples illustrating the new capabilities for crystal growth: characterisation of crystals grown for applications, such as ice tri-crystals produced for mechanical deformation studies; SiC; crystalline silicon for solar photovoltaic cells; in situ and in real time studies of quasicrystal growth (AlPdMn); and ultrafast tomography for the study of the growth of dendrites in metallic alloys. (C) 2012 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
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Barzilai, R., Laronne, J. B., & Reid, I. (2013). Effect of changes in fine-grained matrix on bedload sediment transport in a gravel-bed river. Earth Surface Processes And Landforms, 38(5), 441–448.
Abstract: While clay and silt matrices of gravel-bed rivers have received attention from ecologists concerned variously with the deteriorating environments of benthic and hyporheic organisms, their impact on sediment entrainment and transport has been explored less. A recent increase of such a matrix in the bed of Nahal Eshtemoa, an ephemeral river of the northern Negev, has more than doubled the boundary shear stress needed to initiate bedload, from 7 N m-2 (*=0.027) during the flash floods of 19912001 to 15 N m-2 (*=0.059) during those of 20082009. The relation between bedload flux and boundary shear stress continues to be well-defined, but it is displaced. The matrix now contains a significant amount of silt and clay size material. The reasons for the increased entrainment threshold of bedload are explored. Large-scale laser scanning of the dry bed reveals a reduction in grain-scale morphological roughness, while artificial in situ tests of matrix integrity indicate considerable cohesion. The implications for adopting bed material sampling strategies that account for matrix development are assessed. Copyright (c) 2012 John Wiley & Sons, Ltd.
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Bazin, L., Landais, A., Lemieux-Dudon, B., Kele, H. T. M., Veres, D., Parrenin, F., et al. (2013). An optimized multi-proxy, multi-site Antarctic ice and gas orbital chronology (AICC2012): 120-800 ka. Climate Of The Past, 9(4), 1715–1731.
Abstract: An accurate and coherent chronological framework is essential for the interpretation of climatic and environmental records obtained from deep polar ice cores. Until now, one common ice core age scale had been developed based on an inverse dating method (Datice), combining glaciological modelling with absolute and stratigraphic markers between 4 ice cores covering the last 50 ka (thousands of years before present) (Lemieux-Dudon et al., 2010). In this paper, together with the companion paper of Veres et al. (2013), we present an extension of this work back to 800 ka for the NGRIP, TALDICE, EDML, Vostok and EDC ice cores using an improved version of the Datice tool. The AICC2012 (Antarctic Ice Core Chronology 2012) chronology includes numerous new gas and ice stratigraphic links as well as improved evaluation of background and associated variance scenarios. This paper concentrates on the long timescales between 120-800 ka. In this framework, new measurements of delta O-18(atm) over Marine Isotope Stage (MIS) 11-12 on EDC and a complete delta O-18(atm) record of the TALDICE ice cores permit us to derive additional orbital gas age constraints. The coherency of the different orbitally deduced ages (from delta O-18(atm), delta O-2/N-2 and air content) has been verified before implementation in AICC2012. The new chronology is now independent of other archives and shows only small differences, most of the time within the original uncertainty range calculated by Datice, when compared with the previous ice core reference age scale EDC3, the Dome F chronology, or using a comparison between speleothems and methane. For instance, the largest deviation between AICC2012 and EDC3 (5.4 ka) is obtained around MIS 12. Despite significant modifications of the chronological constraints around MIS 5, now independent of speleothem records in AICC2012, the date of Termination II is very close to the EDC3 one.
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Beguin, R., Philippe, P., & Faure, Y. H. (2013). Pore-Scale Flow Measurements at the Interface between a Sandy Layer and a Model Porous Medium: Application to Statistical Modeling of Contact Erosion. Journal Of Hydraulic Engineering-Asce, 139(1), 1–11.
Abstract: Contact erosion is potentially initiated at the interface between two soil layers by a groundwater flow within the coarser material. Once eroded by the flow, particles from the finer soil are transported through the pores of the coarser layer. Fluvial dikes are often exposed to this phenomenon. Small-scale experiments combining refractive index matching medium, planar laser-induced fluorescence, and particle image velocimetry were carried out to measure the flow characteristics in the vicinity of an interface between a model granular medium and a fine graded sandy layer. Longitudinal velocities and shear-stress distributions were obtained in Darcy flow conditions. They revealed a long tail toward large values, which reflects the spatial variability of the constrictions in the pores network. Taking into account these distributions can improve the modeling of contact erosion by going beyond the simple use of mean quantities, like Darcy velocity, as is usually proposed in the literature. This is done by successively considering the variability of the porous flow and also that of the critical shear stress. As a main consequence, the resulting global erosion rate is nownonzero for any value of the mean shear stress and there is no longer a stress threshold. Finally, the effect of paving at the sand surface can also be added to the statistical model and makes it possible to account very satisfactorily for previous contact erosion tests at the sample scale. DOI: 10.1061/(ASCE)HY.1943-7900.0000641. (C) 2013 American Society of Civil Engineers.
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Blard, P. H., Lave, J., Sylvestre, F., Placzek, C. J., Claude, C., Galy, V., et al. (2013). Cosmogenic He-3 production rate in the high tropical Andes (3800 m, 20 degrees S): Implications for the local last glacial maximum. Earth And Planetary Science Letters, 377, 260–275.
Abstract: To improve both precision and accuracy of cosmic-ray exposure dating methods, there is a crucial need of calibration sites constraining the production rate of cosmogenic isotopes. This is particularly true in the high tropical area, where existing scaling models present significant discrepancies. This study presents a new calibration site for cosmogenic He-3, located at 3800 m in the tropical Altiplano (19.9 degrees S, 67.6 degrees W), on the southern flank of the Tunupa volcano, in the vicinity of the Salar de Uyuni. It consists in a fluvio-glacial outwash that is stratigraphically bracketed by two successive lacustrine shorelines. These shorelines are well-dated by C-14 and U-series dating (n = 57), allowing determination of the age of the boulder field at the surface of the delta, at 15.3 +/- 0.5 ka (let). Eleven andesitic boulders were sampled on this fluvio-glacial surface and the cosmogenic He-3 contents of their pyroxene phenocrysts and amphiboles was analyzed. The nucleogenic contribution from Li-6 capture is well-constrained by determining the (U-Th-Sm)/He-4 age of these rocks. This correction is minimal (<2%) and does not represent a significant source of uncertainty. Cosmogenic He-3 (He-c) concentrations are characterized by a very high reproducibility: 10 samples overlap within 10- analytical error. This suggests that pre- or post-deposition processes have minimal impact on the 3Hec data. Furthermore, there is no correlation between the Li and the He-3(c) content of the mineral, indicating that He-3(c) production from cosmogenic thermal neutron is here negligible. Combined with the absolute dating of the delta, these He-3(c) data yield a local production rate of 999 +/- 38 (1 sigma) at g(-1) yr(-1), at 3800 m and 19.89 S. After scaling with the time dependent scaling scheme of (Stone, 2000), this result yields a rate of 121 +/- 5 (1 sigma) at g(-1) yr(-1) at high latitude and sea level. This new calibration is the highest among the existing global dataset It will thus permit the establishment of new robust glacial chronologies in the high Tropics, with an uncertainty lower than 5% at 1 sigma. Such precision may have important implications in paleoclimatology, notably because it will allow comparison with other well-dated paleoclimatic archives. Including this new data, the updated global He-3(c) production rate is 122 +/- 15 at g(-1) yr(-1), using the time-dependent scaling of Stone (2000). The new site-specific He-3(c) production rate is used here to refine glacier fluctuations on Cerro Tunupa, confirming that the local last glacial maximum was synchronous with the Lake Tauca highstand (15.5 ka). Data also suggest that a dramatic glacial retreat occurred at about 15 ka, few hundred years before the Lake Tauca regression, synchronously with the onset of the Bolling-Allerod. (C) 2013 Elsevier B.V. All rights reserved.
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Boulon, J., Sellegri, K., Katrib, Y., Wang, J., Miet, K., Langmann, B., et al. (2013). Sub-3 nm Particles Detection in a Large Photoreactor Background: Possible Implications for New Particles Formation Studies in a Smog Chamber. Aerosol Science And Technology, 47(2), 153–157.
Abstract: New particles formation experiments have been conducted in the experimental multiphasic atmospheric simulation chamber (CESAM) smog chamber. The nucleation events generated during the dark ozonolysis of sabinene were monitored using a neutral cluster and air ions spectrometer in the size range 0.8-42nm under simulated atmospheric conditions. The measurements show that a significant amount of pre-existing clusters exist in the 1-2nm size range even in very clean controlled conditions. Numerical modeling experiments indicate that a range of 9-67% of pre-existing clusters can explain, through activation by organic vapors, the number of new particles formed when sabinene is oxidized, with an increasing contributing fraction for increasing condensable vapor concentrations. These findings suggest that atmospheric simulation chambers backgrounds have to be carefully characterized for their sub-3-nm neutral cluster concentration content before nucleation parameterizations can be derived.
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Brankart, J. M. (2013). Impact of uncertainties in the horizontal density gradient upon low resolution global ocean modelling. Ocean Modelling, 66, 64–76.
Abstract: In this study, it is shown (i) that, as a result of the nonlinearity of the seawater equation of state, unresolved scales represent a major source of uncertainties in the computation of the large-scale horizontal density gradient from the large-scale temperature and salinity fields, and (ii) that the effect of these uncertain ties can be simulated using random processes to represent unresolved temperature and salinity fluctuations. The results of experiments performed with a low resolution global ocean model show that this parameterization has a considerable effect on the average large-scale circulation of the ocean, especially in the regions of intense mesoscale activity. The large-scale flow is less geostrophic, with more intense associated vertical velocities, and the average geographical position of the main temperature and salinity fronts is more consistent with observations. In particular, the simulations suggest that the stochastic effect of the unresolved temperature and salinity fluctuations on the large-scale density field may be sufficient to explain why the Gulf Stream pathway systematically overshoots in non-stochastic low resolution ocean models. (c) 2013 Elsevier Ltd. All rights reserved.
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Brutel-Vuilmet, C., Menegoz, M., & Krinner, G. (2013). An analysis of present and future seasonal Northern Hemisphere land snow cover simulated by CMIP5 coupled climate models. Cryosphere, 7(1), 67–80.
Abstract: The 20th century seasonal Northern Hemisphere (NH) land snow cover as simulated by available CMIP5 model output is compared to observations. On average, the models reproduce the observed snow cover extent very well, but the significant trend towards a reduced spring snow cover extent over the 1979-2005 period is underestimated (observed: (-3.4 +/- 1.1)% per decade; simulated: (-1.0 +/- 0.3)% per decade). We show that this is linked to the simulated Northern Hemisphere extratropical spring land warming trend over the same period, which is also underestimated, although the models, on average, correctly capture the observed global warming trend. There is a good linear correlation between the extent of hemispheric seasonal spring snow cover and boreal large-scale spring surface air temperature in the models, supported by available observations. This relationship also persists in the future and is independent of the particular anthropogenic climate forcing scenario. Similarly, the simulated linear relationship between the hemispheric seasonal spring snow cover extent and global mean annual mean surface air temperature is stable in time. However, the slope of this relationship is underestimated at present (observed: (-11.8 +/- 2.7)%degrees C-1; simulated: (-5.1 +/- 3.0)%degrees C-1) because the trend towards lower snow cover extent is underestimated, while the recent global warming trend is correctly represented.
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Calianno, M., Ruin, I., & Gourley, J. J. (2013). Supplementing flash flood reports with impact classifications. Journal Of Hydrology, 477, 1–16.
Abstract: In recent years, there has been an increase in flash flood impacts, even as our ability to forecast events and warn areas at risk increases. This increase results from a combination of extreme events and the exposure of vulnerable populations. The issues of exposure and vulnerability to flash floods are not trivial because environmental circumstances in such events are specific and complex enough to challenge the general understanding of natural risks. Therefore, it seems essential to consider physical processes of flash floods concurrently with the impacts they trigger. This paper takes a first step in addressing this need by creating and testing the coherence of an impact-focused database based on two pre-existing public and expert-based survey datasets: the Severe Hazards Analysis and Verification Experiment (SHAVE) and the US National Weather Service (NWS) Storm Data. The SHAVE initiative proposes a new method for collecting near-real-time high-resolution observations on both environmental circumstances and their disastrous consequences (material and human losses) to evaluate radar-based forecasting tools. Forecast verification tools and methods are needed to pursue improving the spatial and temporal accuracy of forecasts. Nevertheless by enhancing SHAVE and NWS datasets with socially and spatially relevant information, we aim at improving future forecast ability to predict the amount and types of impacts. This paper describes the procedures developed to classify and rank the impacts from the least to the most severe, then to verify the coherence and relevance of the impact-focused SHAVE dataset via cross-tabulation analysis of reported variables and GIS-sampled spatial characteristics. By crossing impact categories with socio-spatial characteristics, this analysis showed first benchmarks for the use of exposure layers in future flash flood impact forecasting models. The enhanced impact-focused datasets were used to test the capabilities of flash flood forecasting tools in predicting different categories of impacts for two extreme cases of flash flooding in Oklahoma, USA. Results showed a general tendency for the more severe impacts to be associated to higher mean exceedances over tool values. This means that, at least for these particular case studies, the tools were able to make a distinction between less severe and more severe impacts. Finally, a critical analysis of the NWS and SHAVE data collection methodologies was completed and challenges for future work were identified. (C) 2012 Elsevier B.V. All rights reserved.
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Capron, E., Landais, A., Buiron, D., Cauquoin, A., Chappellaz, J., Debret, M., et al. (2013). Glacial-interglacial dynamics of Antarctic firn columns: comparison between simulations and ice core air-delta N-15 measurements. Climate Of The Past, 9(3), 983–999.
Abstract: Correct estimation of the firn lock-in depth is essential for correctly linking gas and ice chronologies in ice core studies. Here, two approaches to constrain the firn depth evolution in Antarctica are presented over the last deglaciation: outputs of a firn densification model, and measurements of delta N-15 of N-2 in air trapped in ice core, assuming that delta N-15 is only affected by gravitational fractionation in the firn column. Since the firn densification process is largely governed by surface temperature and accumulation rate, we have investigated four ice cores drilled in coastal (Berkner Island, BI, and James Ross Island, JRI) and semi-coastal (TALDICE and EPICA Dronning Maud Land, EDML) Antarctic regions. Combined with available ice core air-delta N-15 measurements from the EPICA Dome C (EDC) site, the studied regions encompass a large range of surface accumulation rates and temperature conditions. Our delta N-15 profiles reveal a heterogeneous response of the firn structure to glacial-interglacial climatic changes. While firn densification simulations correctly predict TALDICE delta N-15 variations, they systematically fail to capture the large millennial-scale delta N-15 variations measured at BI and the delta N-15 glacial levels measured at JRI and EDML – a mismatch previously reported for central East Antarctic ice cores. New constraints of the EDML gas-ice depth offset during the Laschamp event (similar to 41 ka) and the last deglaciation do not favour the hypothesis of a large convective zone within the firn as the explanation of the glacial firn model-delta N-15 data mismatch for this site. While we could not conduct an in-depth study of the influence of impurities in snow for firnification from the existing datasets, our detailed comparison between the delta N-15 profiles and firn model simulations under different temperature and accumulation rate scenarios suggests that the role of accumulation rate may have been underestimated in the current description of firnification models.
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Carmagnola, C. M., Domine, F., Dumont, M., Wright, P., Strellis, B., Bergin, M., et al. (2013). Snow spectral albedo at Summit, Greenland: measurements and numerical simulations based on physical and chemical properties of the snowpack. Cryosphere, 7(4), 1139–1160.
Abstract: The broadband albedo of surface snow is determined both by the near-surface profile of the physical and chemical properties of the snowpack and by the spectral and angular characteristics of the incident solar radiation. Simultaneous measurements of the physical and chemical properties of snow were carried out at Summit Camp, Greenland (72 degrees 36' N, 38 degrees 25' W, 3210m a.s.l.) in May and June 2011, along with spectral albedo measurements. One of the main objectives of the field campaign was to test our ability to predict snow spectral albedo by comparing the measured albedo to the albedo calculated with a radiative transfer model, using measured snow physical and chemical properties. To achieve this goal, we made daily measurements of the snow spectral albedo in the range 350-2200 nm and recorded snow stratigraphic information down to roughly 80 cm. The snow specific surface area (SSA) was measured using the DUFISSS instrument (DUal Frequency Integrating Sphere for Snow SSA measurement, Gallet et al., 2009). Samples were also collected for chemical analyses including black carbon (BC) and dust, to evaluate the impact of light absorbing particulate matter in snow. This is one of the most comprehensive albedo-related data sets combining chemical analysis, snow physical properties and spectral albedo measurements obtained in a polar environment. The surface albedo was calculated from density, SSA, BC and dust profiles using the DISORT model (DIScrete Ordinate Radiative Transfer, Stamnes et al., 1988) and compared to the measured values. Results indicate that the energy absorbed by the snowpack through the whole spectrum considered can be inferred within 1.10 %. This accuracy is only slightly better than that which can be obtained considering pure snow, meaning that the impact of impurities on the snow albedo is small at Summit. In the near infrared, minor deviations in albedo up to 0.014 can be due to the accuracy of radiation and SSA measurements and to the surface roughness, whereas deviations up to 0.05 can be explained by the spatial heterogeneity of the snowpack at small scales, the assumption of spherical snow grains made for DISORT simulations and the vertical resolution of measurements of surface layer physical properties. At 1430 and around 1800 nm the discrepancies are larger and independent of the snow properties; we propose that they are due to errors in the ice refractive index at these wavelengths. This work contributes to the development of physically based albedo schemes in detailed snowpack models, and to the improvement of retrieval algorithms for estimating snow properties from remote sensing data.
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Causse, B., Spadini, L., Martins, J. F., Lenoir, T., Heyraud, A., & Delolme, C. (2013). Xanthan exopolysaccharide: Acid-base reactivity related to structure and conformation. A model for understanding the reactivity of degraded and colloidal soil organic matter. Chemical Geology, 359, 150–158.
Abstract: Bacterial cells and bacterial exopolysaccharides differ strongly in their respective colloidal and polymeric habits. This suggests different reactivities toward metals and protons, although recent studies point to the similarity of such substrates in terms of site density and metal complexation strength. The aim of this paper is to investigate the causes of this surprising uniqueness by studying the reactivity of a model exopolysaccharide, xanthan. NMR and molecular analysis were used to unambiguously determine the total density of reactive carboxylic sites of the xanthan molecule (1.65 mmol/g xanthan dry weight dw). This allowed the independent determination of the average protonation constant of xanthan (Xan), which is more acidic than its constitutive reactive ligands, glucuronic and pyruvic acids (Glc + Pyr) at high (0.5) and medium (0.1), but not at low (0.01) ionic strength. This shows that xanthan is intrinsically more acidic than its constitutive reactive ligands, and that electrostatic effects developed at low ionic strength can limit the relative lability of xanthan's protons. A cylindrical electrostatic model based on the molecular structure of xanthan identified a single intrinsic constant (pK(I = 0) = 1.95 +/- 0.1) as the only adjustable parameter needed to fit our experimental data. The density of xanthan reactive sites (1.65 mmol/g dw) is comparable to that of whole bacterial cells and with known highly reactive biopolymers such as humic substances, nucleic acids, proteins, and polysaccharides (similar to 1 to 5 mmol/g dw). The surprising similarity in site densities of these substrates is explained by the constitutive reactive site versus carbon concentration ratio, which is 1/23 in xanthan (one out of 23 carbons bears an acid-base reactive functional group), which is about the same in bacteria, and which increases to 1/6 in fulvic acids. These high ratios suggest the physical limits of stability for these polymers, and thus their high and similar site densities. (C) 2013 Elsevier B. V. All rights reserved.
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Cauvy-Fraunie, S., Condom, T., Rabatel, A., Villacis, M., Jacobsen, D., & Dangles, O. (2013). Technical Note: Glacial influence in tropical mountain hydrosystems evidenced by the diurnal cycle in water levels. Hydrology And Earth System Sciences, 17(12), 4803–4816.
Abstract: Worldwide, the rapid shrinking of glaciers in response to ongoing climate change is modifying the glacial meltwater contribution to hydrosystems in glacierized catchments. Determining the influence of glacial runoff to streams is therefore of critical importance to evaluate potential impact of glacier retreat on water quality and aquatic biota. This task has challenged both glacier hydrologists and ecologists over the last 20 yr due to both structural and functional complexity of the glacier-stream system interface. Here we propose quantifying the diurnal cycle amplitude of the stream-flow to determine the glacial influence in glacierized catchments. We performed water-level measurements using water pressure loggers over 10 months at 30 min time steps in 15 stream sites in 2 glacier-fed catchments in the Ecuadorian Andes (> 4000 m a.s.l.) where no perennial snow cover is observed outside the glaciers. For each stream site, we performed wavelet analyses on water-level time series, determined the scale-averaged wavelet power spectrum at 24 h scale and defined three metrics, namely the power, frequency and temporal clustering of the diurnal flow variation. The three metrics were then compared to the percentage of the glacier cover in the catchments, a metric of glacial influence widely used in the literature. As expected, we found that the diurnal variation power of glacier-fed streams decreased downstream with the addition of non-glacial tributaries. We also found that the diurnal variation power and the percentage of the glacier cover in the catchment were significantly positively correlated. Furthermore, we found that our method permits the detection of glacial signal in supposedly non-glacial sites, thereby revealing glacial meltwater resurgence. While we specifically focused on the tropical Andes in this paper, our approach to determine glacial influence may have potential applications in temperate and arctic glacierized catchments. The measure of diurnal water amplitude therefore appears as a powerful and cost-effective tool to understand the hydrological links between glaciers and hydrosystems better and assess the consequences of rapid glacier shrinking.
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Champollion, N., Picard, G., Arnaud, L., Lefebvre, E., & Fily, M. (2013). Hoar crystal development and disappearance at Dome C, Antarctica: observation by near-infrared photography and passive microwave satellite. Cryosphere, 7(4), 1247–1262.
Abstract: Hoar crystals episodically cover the snow surface in Antarctica and affect the roughness and reflective properties of the air-snow interface. However, little is known about their evolution and the processes responsible for their development and disappearance despite a probable influence on the surface mass balance and energy budget. To investigate hoar evolution, we use continuous observations of the surface by in situ near-infrared photography and by passive microwave remote sensing at Dome C in Antarctica. From the photography data, we retrieved a daily indicator of the presence/absence of hoar crystals using a texture analysis algorithm. The analysis of this 2 yr long time series shows that Dome C surface is covered almost half of the time by hoar. The development of hoar crystals takes a few days and seems to occur whatever the meteorological conditions. In contrast, the disappearance of hoar is rapid (a few hours) and coincident with either strong winds or with moderate winds associated with a change in wind direction from southwest (the prevailing direction) to southeast. From the microwave satellite data, we computed the polarisation ratio (i.e. horizontal over vertical polarised brightness temperatures), an indicator known to be sensitive to hoar in Greenland. Photography data and microwave polarisation ratio are correlated, i.e. high values of polarisation ratio which theoretically correspond to low snow density values near the surface are associated with the presence of hoar crystals in the photography data. Satellite data over nearly ten years (2002-2011) confirm that a strong decrease of the polarisation ratio (i.e. signature of hoar disappearance) is associated with an increase of wind speed or a change in wind direction from the prevailing direction. The photography data provides, in addition, evidence of interactions between hoar and snowfall. Further adding the combined influence of wind speed and wind direction results in a complex picture of the snow-atmosphere interactions in Antarctica which deserves further quantification and modelling.
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Chappellaz, J., Stowasser, C., Blunier, T., Baslev-Clausen, D., Brook, E. J., Dallmayr, R., et al. (2013). High-resolution glacial and deglacial record of atmospheric methane by continuous-flow and laser spectrometer analysis along the NEEM ice core. Climate Of The Past, 9(6), 2579–2593.
Abstract: The Greenland NEEM (North Greenland Eemian Ice Drilling) operation in 2010 provided the first opportunity to combine trace-gas measurements by laser spectroscopic instruments and continuous-flow analysis along a freshly drilled ice core in a field-based setting. We present the resulting atmospheric methane (CH4) record covering the time period from 107.7 to 9.5 ka b2k (thousand years before 2000 AD). Companion discrete CH4 measurements are required to transfer the laser spectroscopic data from a relative to an absolute scale. However, even on a relative scale, the high-resolution CH4 data set significantly improves our knowledge of past atmospheric methane concentration changes. New significant sub-millennial-scale features appear during interstadials and stadials, generally associated with similar changes in water isotopic ratios of the ice, a proxy for local temperature. In addition to the midpoint of Dansgaard-Oeschger (D/O) CH4 transitions usually used for cross-dating, sharp definition of the start and end of these events brings precise depth markers (with +/- 20 cm uncertainty) for further cross-dating with other palaeo- or ice core records, e. g. speleothems. The method also provides an estimate of CH4 rates of change. The onsets of D/O events in the methane signal show a more rapid rate of change than their endings. The rate of CH4 increase associated with the onsets of D/O events progressively declines from 1.7 to 0.6 ppbv yr(-1) in the course of marine isotope stage 3. The largest observed rate of increase takes place at the onset of D/O event #21 and reaches 2.5 ppbv yr(-1).
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Charbit, S., Dumas, C., Kageyama, M., Roche, D. M., & Ritz, C. (2013). Influence of ablation-related processes in the build-up of simulated Northern Hemisphere ice sheets during the last glacial cycle. Cryosphere, 7(2), 681–698.
Abstract: Since the original formulation of the positive-degree-day (PDD) method, different PDD calibrations have been proposed in the literature in response to the increasing number of observations. Although these formulations generally provide a satisfactory description of the present-day Greenland geometry, they have not all been tested for paleo ice sheets. Using the climate-ice sheet model CLIMBER-GRISLI coupled with different PDD models, we evaluate how the parameterisation of the ablation may affect the evolution of Northern Hemisphere ice sheets in the transient simulations of the last glacial cycle. Results from fully coupled simulations are compared to time-slice experiments carried out at different key periods of the last glacial period. We find large differences in the simulated ice sheets according to the chosen PDD model. These differences occur as soon as the onset of glaciation, therefore affecting the subsequent evolution of the ice system. To further investigate how the PDD method controls this evolution, special attention is given to the role of each PDD parameter. We show that glacial inception is critically dependent on the representation of the impact of the temperature variability from the daily to the inter-annual time scale, whose effect is modulated by the refreezing scheme. Finally, an additional set of sensitivity experiments has been carried out to assess the relative importance of melt processes with respect to initial ice sheet configuration in the construction and the evolution of past Northern Hemisphere ice sheets. Our analysis reveals that the impacts of the initial ice sheet condition may range from quite negligible to explaining about half of the LGM ice volume depending on the representation of stochastic temperature variations which remain the main driver of the evolution of the ice system. The main findings of this paper underline the need for conducting studies with high resolution climate models coupled to detailed snow models to better constrain the temporal and spatial variations of the PDD parameters. The development of such approaches could improve the calibration of the PDD formulation which is still widely used in climate-ice sheet studies.
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Chaudhuri, A., Sekhar, M., Descloitres, M., Godderis, Y., Ruiz, L., & Braun, J. J. (2013). Constraining complex aquifer geometry with geophysics (2-D ERT and MRS measurements) for stochastic modelling of groundwater flow. Journal Of Applied Geophysics, 98, 288–297.
Abstract: Stochastic modelling is a useful way of simulating complex hard-rock aquifers as hydrological properties (permeability, porosity etc.) can be described using random variables with known statistics. However, very few studies have assessed the influence of topological uncertainty (i.e. the variability of thickness of conductive zones in the aquifer), probably because it is not easy to retrieve accurate statistics of the aquifer geometry, especially in hard rock context. In this paper, we assessed the potential of using geophysical surveys to describe the geometry of a hard rock-aquifer in a stochastic modelling framework. The study site was a small experimental watershed in South India, where the aquifer consisted of a clayey to loamy-sandy zone (regolith) underlain by a conductive fissured rock layer (protolith) and the unweathered gneiss (bedrock) at the bottom. The spatial variability of the thickness of the regolith and fissured layers was estimated by electrical resistivity tomography (ERT) profiles, which were performed along a few cross sections in the watershed. For stochastic analysis using Monte Carlo simulation, the generated random layer thickness was made conditional to the available data from the geophysics. In order to simulate steady state flow in the irregular domain with variable geometry, we used an isoparametric finite element method to discretize the flow equation over an unstructured grid with irregular hexahedral elements. The results indicated that the spatial variability of the layer thickness had a significant effect on reducing the simulated effective steady seepage flux and that using the conditional simulations reduced the uncertainty of the simulated seepage flux. As a conclusion, combining information on the aquifer geometry obtained from geophysical surveys with stochastic modelling is a promising methodology to improve the simulation of groundwater flow in complex hard-rock aquifers. (C) 2013 Elsevier B.V. All rights reserved.
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Coen, M. C., Andrews, E., Asmi, A., Baltensperger, U., Bukowiecki, N., Day, D., et al. (2013). Aerosol decadal trends – Part 1: In-situ optical measurements at GAW and IMPROVE stations. Atmospheric Chemistry And Physics, 13(2), 869–894.
Abstract: Currently many ground-based atmospheric stations include in-situ measurements of aerosol physical and optical properties, resulting in more than 20 long-term (> 10 yr) aerosol measurement sites in the Northern Hemisphere and Antarctica. Most of these sites are located at remote locations and monitor the aerosol particle number concentration, wavelength-dependent light scattering, backscattering, and absorption coefficients. The existence of these multi-year datasets enables the analysis of long-term trends of these aerosol parameters, and of the derived light scattering Angstrom exponent and backscatter fraction. Since the aerosol variables are not normally distributed, three different methods (the seasonal Mann-Kendall test associated with the Sen's slope, the generalized least squares fit associated with an autoregressive bootstrap algorithm for confidence intervals, and the least-mean square fit applied to logarithms of the data) were applied to detect the long-term trends and their magnitudes. To allow a comparison among measurement sites, trends on the most recent 10 and 15 yr periods were calculated. No significant trends were found for the three continental European sites. Statistically significant trends were found for the two European marine sites but the signs of the trends varied with aerosol property and location. Statistically significant decreasing trends for both scattering and absorption coefficients (mean slope of -2.0% yr(-1)) were found for most North American stations, although positive trends were found for a few desert and high-altitude sites. The difference in the timing of emission reduction policy for the Europe and US continents is a likely explanation for the decreasing trends in aerosol optical parameters found for most American sites compared to the lack of trends observed in Europe. No significant trends in scattering coefficient were found for the Arctic or Antarctic stations, whereas the Arctic station had a negative trend in absorption coefficient. The high altitude Pacific island station of Mauna Loa presents positive trends for both scattering and absorption coefficients.
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Cole-Dai, J., Ferris, D. G., Lanciki, A. L., Savarino, J., Thiemens, M. H., & McConnell, J. R. (2013). Two likely stratospheric volcanic eruptions in the 1450s CE found in a bipolar, subannually dated 800 year ice core record. Journal Of Geophysical Research-Atmospheres, 118(14), 7459–7466.
Abstract: An 800 year volcanic record is constructed from high-resolution chemical analysis of recently obtained West Antarctica and central Greenland ice cores. The high accuracy and precision of the ice core chronologies are a result of dating by annual ice layer counting. Nineteen bipolar volcanic signals in this record represent large, explosive eruptions in the tropics with probable climatic impact. One of the two bipolar volcanic signals dated at 1453 and 1459 is probably left by the eruption of the submarine volcano Kuwae in the tropical Pacific, one of the largest volcanic eruptions in the last millennium. The discovery of the two signals in the 1450s casts doubt on the eruption year of 1452 or 1453 for Kuwae based on previous ice core records. The volcanic sulfate deposition patterns in this bipolar record suggest that the later signal is likely from the Kuwae eruption in 1458, although a firm attribution is not possible. Sulfur isotope composition in the volcanic sulfate in the central Greenland cores indicates that both eruptions in the 1450s injected sulfur gases into the stratosphere with probable impact on the global climate. These results are in agreement with tree ring records showing two short cold episodes during that decade. The bipolar volcanic record supports the hypothesis that unusually active volcanism in the thirteenth century contributed to the onset of the Little Ice Age and another active period in the mid fifteenth century may have helped to sustain the Little Ice Age.
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Contoux, C., Jost, A., Ramstein, G., Sepulchre, P., Krinner, G., & Schuster, M. (2013). Megalake Chad impact on climate and vegetation during the late Pliocene and the mid-Holocene. Climate Of The Past, 9(4), 1417–1430.
Abstract: Given the growing evidence for megalakes in the geological record, assessing their impact on climate and vegetation is important for the validation of palaeoclimate simulations and therefore the accuracy of model-data comparison in lacustrine environments. Megalake Chad (MLC) occurrences are documented not only for the mid-Holocene but also for the Mio-Pliocene (Schuster et al., 2009). At this time, the surface covered by water would have reached up to similar to 350 000 km(2) (Ghienne et al., 2002; Schuster et al., 2005; Leblanc et al., 2006), making it an important evaporation source, possibly modifying climate and vegetation in the Chad Basin. We investigated the impact of such a giant continental water area in two different climatic backgrounds within the Paleoclimate Model Intercomparison Project phase 3 (PMIP3): the late Pliocene (3.3 to 3 Ma, i. e. the mid-Piacenzian warm period) and the mid-Holocene (6 kyr BP). In all simulations including MLC, precipitation is drastically reduced above the lake surface because deep convection is inhibited by overlying colder air. Meanwhile, convective activity is enhanced around MLC because of the wind increase generated by the flat surface of the megalake, transporting colder and moister air towards the eastern shore of the lake. The effect of MLC on precipitation and temperature is not sufficient to widely impact vegetation patterns. Nevertheless, tropical savanna is present in the Chad Basin in all climatic configurations, even without MLC presence, showing that the climate itself is the driver of favourable environments for sustainable hominid habitats.
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Creutin, J. D., Borga, M., Gruntfest, E., Lutoff, C., Zoccatelli, D., & Ruin, I. (2013). A space and time framework for analyzing human anticipation of flash floods. Journal Of Hydrology, 482, 14–24.
Abstract: We propose a new simplified vision of flood dynamics through temporal and spatial scales to enhance understanding of potential and limits of human responses to flash floods. Based on data from a set of extreme flash floods that occurred in Europe, we analyze both the runoff response time of flash floods and the asynchronous character of flood peak time generation. We introduce the concept of “timeliness” to quantify the balance in time between the hydrological response of catchments to heavy storms and the human reactions. Timeliness is investigated across the range of spatial scales of concern for flash floods and for various types of anticipation actions. Results are reported for four extreme flash flood events occurred since 2002 in France, Italy and Slovenia. These results indicate that human actions adapt their pace to the physical context and are in a kind of “hurrying” process as the flood danger approaches in time. They show the importance of self-organization and 'unofficial' warning in response to flash flooding by individuals and groups. The work draws on flood hazard research findings from hydrology, meteorology and social science and put forward a framework for interdisciplinary collaboration between hydrologists and social scientists. (C) 2012 Elsevier B.V. All rights reserved.
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Crippa, M., DeCarlo, P. F., Slowik, J. G., Mohr, C., Heringa, M. F., Chirico, R., et al. (2013). Wintertime aerosol chemical composition and source apportionment of the organic fraction in the metropolitan area of Paris. Atmospheric Chemistry And Physics, 13(2), 961–981.
Abstract: The effect of a post-industrial megacity on local and regional air quality was assessed via a month-long field measurement campaign in the Paris metropolitan area during winter 2010. Here we present source apportionment results from three aerosol mass spectrometers and two aethalometers deployed at three measurement stations within the Paris region. Submicron aerosol composition is dominated by the organic fraction (30-36%) and nitrate (28-29%), with lower contributions from sulfate (14-16%), ammonium (12-14%) and black carbon (7-13%). Organic source apportionment was performed using positive matrix factorization, resulting in a set of organic factors corresponding both to primary emission sources and secondary production. The dominant primary sources are traffic (11-15% of organic mass), biomass burning (13-15%) and cooking (up to 35% during meal hours). Secondary organic aerosol contributes more than 50% to the total organic mass and includes a highly oxidized factor from indeterminate and/or diverse sources and a less oxidized factor related to wood burning emissions. Black carbon was apportioned to traffic and wood burning sources using a model based on wavelength-dependent light absorption of these two combustion sources. The time series of organic and black carbon factors from related sources were strongly correlated. The similarities in aerosol composition, total mass and temporal variation between the three sites suggest that particulate pollution in Paris is dominated by regional factors, and that the emissions from Paris itself have a relatively low impact on its surroundings.
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Crumeyrolle, S., Schwarzenboeck, A., Roger, J. C., Sellegri, K., Burkhart, J. F., Stohl, A., et al. (2013). Overview of aerosol properties associated with air masses sampled by the ATR-42 during the EUCAARI campaign (2008). Atmospheric Chemistry And Physics, 13(9), 4877–4893.
Abstract: Within the frame of the European Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) project, the Meteo-France aircraft ATR-42 performed 22 research flights over central Europe and the North Sea during the intensive observation period in May 2008. For the campaign, the ATR-42 was equipped to study the aerosol physical, chemical, hygroscopic and optical properties, as well as cloud microphysics. For the 22 research flights, retroplume analyses along the flight tracks were performed with FLEXPART in order to classify air masses into five sectors of origin, allowing for a qualitative evaluation of emission influence on the respective air parcel. This study shows that the extensive aerosol parameters (aerosol mass and number concentrations) show vertical decreasing gradients and in some air masses maximum mass concentrations (mainly organics) in an intermediate layer (1-3 km). The observed mass concentrations (in the boundary layer (BL): between 10 and 30 μg m(-3); lower free troposphere (LFT): 0.8 and 14 μg m(-3)) are high especially in comparison with the 2015 European norms for PM2.5 (25 μg m(-3)) and with previous airborne studies performed over England (Morgan et al., 2009; McMeeking et al., 2012). Particle number size distributions show a larger fraction of particles in the accumulation size range in the LFT compared to BL. The chemical composition of submicron aerosol particles is dominated by organics in the BL, while ammonium sulphate dominates the submicron aerosols in the LFT, especially in the aerosol particles originated from north-eastern Europe (similar to 80 %), also experiencing nucleation events along the transport. As a consequence, first the particle CCN acting ability, shown by the CCN/CN ratio, and second the average values of the scattering cross sections of optically active particles (i.e. scattering coefficient divided by the optical active particle concentration) are increased in the LFT compared to BL.
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Crumeyrolle, S., Weigel, R., Sellegri, K., Roberts, G., Gomes, L., Stohl, A., et al. (2013). Airborne investigation of the aerosols-cloud interactions in the vicinity and within a marine stratocumulus over the North Sea during EUCAARI (2008). Atmospheric Environment, 81, 288–303.
Abstract: Within the European Aerosol Cloud Climate and Air Quality Interactions (EUCAARI) project, the Meteo France research aircraft ATR-42 was operated from Rotterdam (Netherlands) airport during May 2008, to perform scientific flights dedicated to the investigation of aerosol-cloud interactions. The objective of this study is to illustrate the impact of cloud processing on the aerosol particle physical and chemical properties. The presented results are retrieved from measurements during flight operation with two consecutive flights, first from Rotterdam to Newcastle (United Kingdom) and subsequently reverse along the same waypoints back to Rotterdam using data measured with compact Time of Flight Aerosol Mass Spectrometer (cToF-AMS) and Scanning Mobility Particle Sizer (SMPS). Cloud-related measurements during these flights were performed over the North Sea within as well as in close vicinity of a marine stratocumulus cloud layer. Particle physical and chemical properties observed in the close vicinity, below and above the stratocumulus cloud, show strong differences: (1) the averaged aerosol size distributions, observed above and below the cloud layer, are of bimodal character with pronounced minima between Aitken and accumulation mode, very likely due to cloud processing. (2) the chemical composition of aerosol particles is strongly dependent on the position relative to the cloud layer (vicinity or below/above cloud). In general, the nitrate and organic relative mass fractions decrease with decreasing distance to the cloud, in the transit from cloud free conditions towards the cloud boundaries. This relative mass fraction decrease ranges from a factor of three to ten, thus leading to an increase of the sulfate and ammonium relative mass concentrations while approaching the cloud layer. (3), the chemical composition of cloud droplet residuals, analyzed downstream of a Counterflow virtual Impactor (CVI) inlet indicates increased fractions of mainly soluble chemical compounds such as nitrate and organics, compared to non cloud processed particles. Finally, a net overbalance of nitrate aerosol has been revealed by comparing cloud droplet residual and non cloud processed aerosol chemical compositions. Conclusively, this study highlights gaps concerning the sampling strategy that need to be addressed for the future missions. (C) 2013 Elsevier Ltd. All rights reserved.
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Dahl-Jensen, D., Albert, M. R., Aldahan, A., Azuma, N., Balslev-Clausen, D., Baumgartner, M., et al. (2013). Eemian interglacial reconstructed from a Greenland folded ice core. Nature, 493(7433), 489–494.
Abstract: Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling ('NEEM') ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 +/- 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 +/- 250 metres, reaching surface elevations 122,000 years ago of 130 +/- 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.
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de Angelis, M., Tison, J. L., Morel-Fourcade, M. C., & Susini, J. (2013). Micro-investigation of EPICA Dome C bottom ice: evidence of long term in situ processes involving acid-salt interactions, mineral dust, and organic matter. Quaternary Science Reviews, 78, 248–265.
Abstract: The EPICA Dome C ice core (EDC) reached a final depth of 3260 m, at a maximum height of about 15 m above the ice-bedrock interface in December 2004. We present here data gained from a detailed investigation of selected samples of the deeper part of the core located below 3200 m and referred to as bottom ice. This part of the core has been poorly investigated so far mainly because there are significant challenges in interpreting paleo-records that were very likely modified by long term in situ processes. Our study combines high resolution ion chromatography, high resolution synchrotron X-Ray micro-fluorescence (micro XRF), scanning, and transmission electron microscopy. Our aim was to identify the long term physico-chemical processes at work close to the bedrock, to determine how they have altered the initial registers, and, ultimately to extract information on the very ancient Antarctic environment. The ubiquitous presence of nanometer iron oxide crystals at the surface of wind-borne dust aggregates containing also large amount of organic matter raises the possibility that the consolidation of windborne dust clusters formed during ice recrystallization could be related to microbial iron reduction and, thus, to the progressive reactivation of dormant bacterial activity in warming ice. Inclusions of size and number density increasing with depth observed in the 12 last meters (3248-3260 m) contain liquid and solid species, among them marine biogenic acids, numerous wind-borne dust aggregates and clusters of large reversible calcium carbonate particles precipitated once the inclusion was formed and often covered by secondary gypsum. The refreezing of slush lenses is discussed as a potential cause of the formation of such heterogeneous and complex mixtures. In addition to the very fine micrometer size minerals windborne from extra-Antarctic continental sources and often accreted in large aggregates, single medium size particles (a few to ca 20 gm and among them organic debris) are commonly encountered. Their size, surface shape, and mineralogy suggest that aerosol transport from Antarctic ice-free areas played a significant role at the time EDC bottom ice was formed. Concentrations and concentration ratios of biogenic sulfur species also advocate for the strengthening of pen-Antarctic meteorological patterns that favor the inland penetration of disturbed flow carrying local material. Very large well preserved mineral particles several tens of micrometers in diameter, and biotope relics in deeper ice close to 3260 m likely come from the sub-glacial environment. (C) 2013 Elsevier Ltd. All rights reserved.
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Delmonte, B., Baroni, C., Andersson, P. S., Narcisi, B., Salvatore, M. C., Petit, J. R., et al. (2013). Modern and Holocene aeolian dust variability from Tabs Dome (Northern Victoria Land) to the interior of the Antarctic ice sheet. Quaternary Science Reviews, 64, 76–89.
Abstract: High-elevation sites from the inner part of the East Antarctic plateau sample windborne dust representative of large portions of the Southern hemisphere, and are sensitive to long-range atmospheric transport conditions to polar areas. On the periphery of the ice sheet, conversely, the aeolian transport of particles from high-elevation ice-free areas can locally represent a relatively important additional input of dust to the atmosphere, and the interplay of atmospheric dynamics, dust transport and deposition is strictly related to the regional atmospheric circulation behaviour both at present-day and in the past. The understanding of the spatial extent where local sources can influence the mineral dust budget on the ice sheet is fundamental for understanding the atmospheric dust cycle in Antarctica and for the interpretation of the dust history in marginal glaciological settings. In this work we investigate the spatial variability of dust flux and provenance during modern (pre-industrial) and Holocene times along a transect connecting Tabs Dome to the internal sites of the Antarctic plateau and we extend the existing documentation of the isotopic (Sr-Nd) fingerprint of dust-sized sediments from Victoria Land source areas. Dust flux, grain size and isotopic composition show a marked variability between Tabs Dome, Mid Point, D4 and Dome C/Vostok, suggesting that local sources play an important role on the periphery of the ice sheet. Microscope observations reveal that background mineral aerosol in the TALDICE core is composed by a mixture of dust, volcanic particles and micrometric-sized fragments of diatoms, these latter representing a small but pervasive component of Antarctic sediments. A set of samples from Victoria Land, mostly consisting of regolith and glacial deposits from high-elevation areas, was collected specially for this work and the isotopic composition of the dust-sized fraction of samples was analyzed. Results reveal a close relationship with the parent lithologies, but direct comparison between source samples and firn/ice core dust is problematical because of the ubiquitous volcanic contribution to the environmental particulate input in the Tabs Dome area. The frequency of events potentially suitable for peripheral dust transport to Tabs Dome appears relatively high for present-day conditions, according to back trajectories calculations, and the related air flow pattern well-defined from a seasonal and spatial perspective. Also, as expected from palaeo-data, these events appear extremely uncommon for internal sites. (C) 2012 Elsevier Ltd. All rights reserved.
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Descloitres, M., Chalikakis, K., Legchenko, A., Moussa, A. M., Genthon, P., Favreau, G., et al. (2013). Investigation of groundwater resources in the Komadugu Yobe Valley (Lake Chad Basin, Niger) using MRS and TDEM methods. Journal Of African Earth Sciences, 87, 71–85.
Abstract: Groundwater resources quantification and management is a key issue for agricultural development in the Komadugu Yobe (KY) River valley region in the semiarid part of the Lake Chad Basin. To improve the knowledge of available groundwater resources in this poorly-documented area, a geophysical survey across the river valley was conducted near the town of Diffa, southeast Niger. The goal was to estimate the hydrogeological properties of Quaternary formations to a depth of 100 m. Numerical modeling showed that sedimentary deposits composed of thin clayey, loamy, and sandy layers could not be accurately resolved in detail by non-invasive geophysical methods due to the limited spatial resolution of the methods. The use of the Time Domain Electromagnetic (TDEM) method alone was not sufficient to estimate aquifer parameters and the Magnetic Resonance Sounding method (MRS) was used to supplement the geophysical dataset. A twelve kilometer long profile (117 TDEM and 11 MR soundings) was surveyed across the valley to evaluate changes in hydrogeological properties of the Quaternary aquifer from the middle of the river valley to the surrounding sandy plain area. Our results show that the Quaternary aquifer below the KY valley differs from its surroundings and it can be described as follows: (a) the thickness of the KY aquifer ranges from 30 to 60 m and is limited at depth by electrically conductive clay layer, (b) at a depth of similar to 70-80 m, TDEM soundings show a continuous conductive substratum (resistivity is similar to 2 Omega m), attributed to Pliocene clayey formations, (c) the KY valley aquifer may have a larger volume of unconfined groundwater than the surrounding plain aquifer area (MRS water contents of 20-25% and similar to 15% respectively), (d) the KY aquifer transmissivity estimated from MRS data is higher than values derived from pumping tests conducted outside the river valley. This study confirms that the Komadugu Yobe valley aquifer represents a significant resource for future agricultural development. In addition, our study shows that the aquifer is not protected by shallow clayey layers; therefore, precautions against contamination should be taken to preserve the quality of this resource. (C) 2013 Elsevier Ltd. All rights reserved.
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Dias, D., & Kastner, R. (2013). Movements caused by the excavation of tunnels using face pressurized shields – Analysis of monitoring and numerical modeling results. Engineering Geology, 152(1), 17–25.
Abstract: In this paper, monitoring results of two cross tunnel sections are presented. This underground work has been realized for a subway in an urban area (Lyon, France). By comparison with measurements of other projects, it appears that the face instability and the annular gap identified after the shield release are the main sources of short-term settlements. These observations of vertical and horizontal movements during the tunnel excavation by a slurry pressurized tunnel boring machine are then compared with several numerical approaches. The 2D numerical approach uses the concept of volume loss and is applied to each excavation stage. It simulates approximately the observed movements but requires the use of empirical coefficients to represent in two dimensions the three-dimensional problem. The 3D approach considers more directly the physics of the problem and permits to take into account: the slurry pressure at the tunnel face, the shield conicity, the grout injection in the annular void and the grout consolidation. Three dimensional numerical calculations are the most accurate approaches to simulate all the physical processes occurring during tunneling. However this type of model assumes that all the parameters that control the movements induced by the excavation are well known. Due to the complexity of a tunneling boring machine, it is not necessarily the case. (C) 2012 Elsevier B.V. All rights reserved.
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Dieppois, B., Diedhiou, A., Durand, A., Fournier, M., Massei, N., Sebag, D., et al. (2013). Quasi-decadal signals of Sahel rainfall and West African monsoon since the mid-twentieth century. Journal Of Geophysical Research-Atmospheres, 118(22), 12587–12599.
Abstract: Sahel rainfall shows pronounced decadal variability and a negative trend between wet conditions in the 1950s-1960s and dry ones in the 1970s-1980s. Using continuous wavelet transform, the quasi-decadal variability (QDV) of rainfall reveals zonal contrasts. The highest QDV is identified in the 1950s-1960s over western Sahel and in the 1970s-1980s over eastern Sahel. The quasi-decadal atmospheric anomalies have been reconstructed using Fourier transform for the 1950s-1960s and the 1970s-1980s, respectively, and assessed by the composite analysis of the QDV phases for the periods before and after 1968. Over western Sahel, the rainfall QDV in the 1950s-1960s is related to the North Atlantic sea surface temperature (SST) variability, as highlighted by the wavelet coherence. A southward shift trend of the Intertropical Convergence Zone (ITCZ) is identified through an enhancement of northeasterly fluxes and moisture convergence over the western part of West Africa. A decrease (increase) of southern (northern) subtropical sinking motions seems to be involved. In the 1970s-1980s, a strengthening of cross-equatorial Atlantic SST and pressure gradients is related to an increase of monsoon flow from lower troposphere up to the midtroposphere and to the northward shift of the ITCZ, mainly over eastern Sahel. The Pacific SST influence is also identified, which involves changes in the global zonal circulation.
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do Carmo, A. I., Antonino, A. C. D., Martins, J. M. F., da Silva, V. L., Morel, M. C., & Gaudet, J. P. (2013). Leaching Transport Of Naphthalene In Urban Soils From The Metropolitan Region Of Recife, Pe. Revista Brasileira De Ciencia Do Solo, 37(5), 1415–1422.
Abstract: The main soil properties related to the leaching of naphthalene in soils are particle size, organic matter content and the mineralogical composition of silt and clay fractions. The aim of this study was to evaluate naphtalene reactive transfer in soil columns determinated by the dispersion and retardation of naphthalene in soil samples collected from the surroundings of fuel storage tanks in Recife, Brazil. Leaching experiments were carried out in columns containing disturbed soils and were modeled using the convection-dispersion equation, which was adjusted to the experimental data with the CXTFIT software 2.0. The results indicated that sandy soils offer less resistance to leaching of naphthalene and that the presence of goethite, together with organic matter appeared to be the main factors responsible for the retardation of naphtalene transport in the studied samples.
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Do Hur, S., Soyol-Erdene, T. O., Hwang, H. J., Han, C., Gabrielli, P., Barbante, C., et al. (2013). Climate-related variations in atmospheric Sb and Tl in the EPICA Dome C ice (East Antarctica) during the past 800,000 years. Global Biogeochemical Cycles, 27(3), 930–940.
Abstract: A record of antimony (Sb) and thallium (Tl) from the European Project for Ice Coring in Antarctica (EPICA) Dome C Antarctic ice core provides the characteristics of climate-related natural changes in concentrations and fluxes of these toxic elements over the time period back to Marine Isotope Stage 20.2, similar to 800kyrB.P. A strong variability in concentrations and fluxes are observed for both elements, with considerably higher values during glacial maxima and lower values during intermediate and warm periods. Rock and soil dust accounts for, on average, 58% of Sb and 76% of Tl in ice during glacial maxima. This contribution remains significant during warm periods, accounting for 21% for Sb and 27% for Tl. The contribution from volcanoes appears to be very important particularly for Tl when climatic conditions become warmer, with an estimated volcanic contribution of 72% for Tl during interglacials. The sea-salt contribution is significant for Sb, particularly during intermediate climatic periods, with an average contribution of 17%. This sea-salt contribution is most likely caused by greater production of sea salt from highly saline frost flowers and relatively more efficient transport of Sb-enriched sea-ice salt from source areas on the East Antarctic Plateau. Our ice core data, along with snow data recently reported from the Antarctic snow layers at Dome Fuji, shows that the present-day Sb flux (6.6ng/m(2)/yr) is approximately double the highest natural level (2.8ng/m(2)/yr) during glacial maxima throughout the last successive eight glacial/interglacial cycles. This result indicates that human activity has induced the greatest perturbation of the atmospheric cycle of Sb ever experienced over a period of similar to 800 kyr in the most remote area on Earth.
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Do, N. A., Dias, D., Oreste, P., & Djeran-Maigre, I. (2013). 2D numerical investigation of segmental tunnel lining behavior. Tunnelling And Underground Space Technology, 37, 115–127.
Abstract: The application field of shield tunneling has extended in recent years. Most shield-driven tunnels are supported by segmental concrete linings. Although many well documented experimental, numerical and analytical results exist in literature concerning the functioning of segmental tunnel linings, their behavior under the influence of joints is still not clear. This paper presents a numerical study that has been performed to investigate the factors that affect segmental tunnel lining behavior. Analyses have been carried out using a two-dimensional finite difference element model. The longitudinal joint between segments in a ring has been simulated through double node connections, with six degrees of freedom, represented by six springs. The proposed model allows the effect of not only the rotational stiffness but also the radial stiffness and the axial stiffness of the longitudinal joints to be taken into consideration. The numerical results show a significant reduction in the bending moment induced in the tunnel lining as the joint number increases. The tunnel behavior in terms of the bending moment considering the effect of joint distribution, when the lateral earth pressure factor K-0 is equal to 0.5, 1.5 and 2, is almost similar and differs when K0 is equal to unity. It has been seen that the influence of joint rotational stiffness, the reduction in joint rotation stiffness under the negative bending moment, the lateral earth pressure factor and Young's modulus of ground surrounding the tunnel should not be neglected. On the other hand, the results have also shown an insignificant influence of the axial and radial stiffness of the joints on segmental tunnel lining behavior. (c) 2013 Elsevier Ltd. All rights reserved.
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Domine, F., Bock, J., Voisin, D., & Donaldson, D. J. (2013). Can We Model Snow Photochemistry? Problems with the Current Approaches. Journal Of Physical Chemistry A, 117(23), 4733–4749.
Abstract: Snow is a very active photochemical reactor that considerably affects the composition and chemistry of the lower troposphere in polar regions. Snow photochemistry models have therefore been recently developed to describe these processes. In all those models, the chemically active medium is a brine formed at the surface of snow crystals by impurities whose presence cause surface melting. Reaction and photolysis rate coefficients are those measured in dilute liquid solutions. Here, we critically examine the basis for these models by considering the structure of ice crystal surfaces, the processes involved in the interactions between impurities and ice crystals, the location of impurities in snow, and the reactivity of impurities in the various media present in snow. We conclude that the brine formed by impurities can only be present in grooves at grain boundaries and cannot cover ice crystal surfaces because of insufficient ice wettability. It is then very likely that most reactions in snow do not take place in liquids, but rather either on an actual ice surface highly different from a liquid or in particulate matter contained in snow, such as organic particles that are thought to contain most snow chromophores. We discuss why some snow models appear to adequately reproduce some observations, concluding that they are insufficiently constrained and that the use of adjustable parameters allows acceptable fits. We discuss the complexity of developing a snow model without adjustable parameters and with a predictive value. We conclude that reaching this goal in the near future is a tremendous challenge. Modeling attempts focused on snow where the impact of organic particles is minimal, such as on the east Antarctic plateau, represents the best chance of midterm success.
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Donges, J., Montagnat, M., Bastie, P., & Grennerat, F. (2013). 3D diffraction imaging and orientation mapping in deformed ice crystals. Nuclear Instruments & Methods In Physics Research Section B-Beam Interactions With Materials And Atoms, 300, 6–10.
Abstract: A method to acquire three dimensional diffraction data and orientation mapping with the 2D imaging setup of Hasylab beamline G3 is presented. The method has been successfully applied to undeformed large grains extracted from ice samples and extended to the 2D determination of lattice misorientation and lattice orientation distribution within deformed grains. (c) 2013 Elsevier B.V. All rights reserved.
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Doron, M., Brasseur, P., Brankart, J. M., Losa, S. N., & Melet, A. (2013). Stochastic estimation of biogeochemical parameters from Globcolour ocean colour satellite data in a North Atlantic 3D ocean coupled physical-biogeochemical model. Journal Of Marine Systems, 117, 81–95.
Abstract: Biogeochemical parameters remain a major source of uncertainty in coupled physical-biogeochemical models of the ocean. In a previous study (Doron et al., 2011), a stochastic estimation method was developed to estimate a subset of biogeochemical model parameters from surface phytoplankton observations. The concept was tested in the context of idealised twin experiments performed with a 1/4 resolution model of the North Atlantic ocean. The method was based on ensemble simulations describing the model response to parameter uncertainty. The statistical estimation process relies on nonlinear transformations of the estimated space to cope with the non-Gaussian behaviour of the resulting joint probability distribution of the model state variables and parameters. In the present study, the same method is applied to real ocean colour observations, as delivered by the sensors SeaWiFS, MERIS and MODIS embarked on the satellites OrbView-2, Envisat and Aqua respectively. The main outcome of the present experiments is a set of regionalised biogeochemical parameters. The benefit is quantitatively assessed with an objective norm of the misfits, which automatically adapts to the different ecological regions. The chlorophyll concentration simulated by the model with this set of optimally derived parameters is closer to the observations than the reference simulation using uniform values of the parameters. In addition, the interannual and seasonal robustness of the estimated parameters is tested by repeating the same analysis using ocean colour observations from several months and several years. The results show the overall consistency of the ensemble of estimated parameters, which are also compared to the results of an independent study. (C) 2013 Elsevier B.V. All rights reserved.
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Drouet, A. S., Docquier, D., Durand, G., Hindmarsh, R., Pattyn, F., Gagliardini, O., et al. (2013). Grounding line transient response in marine ice sheet models. Cryosphere, 7(2), 395–406.
Abstract: Marine ice-sheet stability is mostly controlled by the dynamics of the grounding line, i.e. the junction between the grounded ice sheet and the floating ice shelf. Grounding line migration has been investigated within the framework of MISMIP (Marine Ice Sheet Model Intercomparison Project), which mainly aimed at investigating steady state solutions. Here we focus on transient behaviour, executing short-term simulations (200 yr) of a steady ice sheet perturbed by the release of the buttressing restraint exerted by the ice shelf on the grounded ice upstream. The transient grounding line behaviour of four different flowline ice-sheet models has been compared. The models differ in the physics implemented (full Stokes and shallow shelf approximation), the numerical approach, as well as the grounding line treatment. Their overall response to the loss of buttressing is found to be broadly consistent in terms of grounding line position, rate of surface elevation change and surface velocity. However, still small differences appear for these latter variables, and they can lead to large discrepancies (> 100 %) observed in terms of ice sheet contribution to sea level when cumulated over time. Despite the recent important improvements of marine ice-sheet models in their ability to compute steady state configurations, our results question the capacity of these models to compute short-term reliable sea-level rise projections.
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Ducrocq, V., Drobinski, P., Lambert, D., Molinie, G., & Llasat, C. (2013). Preface: Forecast and projection in climate scenario of Mediterranean intense events: uncertainties and propagation on environment (the MEDUP project). Natural Hazards And Earth System Sciences, 13(11), 3043–3047.
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Dufour, C. O., Le Sommer, J., Gehlen, M., Orr, J. C., Molines, J. M., Simeon, J., et al. (2013). Eddy compensation and controls of the enhanced sea-to-air CO2 flux during positive phases of the Southern Annular Mode. Global Biogeochemical Cycles, 27(3), 950–961.
Abstract: The current positive trend in the Southern Annular Mode (SAM) is thought to reduce the growth rate of the Southern Ocean CO2 sink because enhanced wind-driven upwelling of dissolved inorganic carbon (DIC) increases outgassing of natural CO2. However, no study to date has quantified the potentially large role of mesoscale eddies in compensating intensified upwelling nor the mixed-layer processes in terms of their effects on CO2 fluxes. Here we report on results from two new simulations in a regional 0.5 degrees eddying model of the Southern Ocean. The first simulation is forced with interannually varying atmospheric reanalysis and coupled to a biogeochemistry model run under constant preindustrial atmospheric CO2. The second simulation is like the first except that superimposed on the forcing is a poleward shifted and intensified westerlies wind anomaly consistent with the positive phase of the SAM. In response to the SAM, the Southern Ocean's sea-to-air CO2 flux is enhanced by 0.1PgCyr(-1) per standard deviation of the SAM, mostly from the Antarctic Zone (AZ), where enhanced surface DIC is only partly compensated by enhanced surface alkalinity. Increased mixed-layer DIC in the AZ results from a combination of increased upwelling below the mixed layer and increased vertical diffusion at the base of the mixed layer. Previous studies overlooked the latter. Thus, upward supply of DIC and alkalinity depends on associated vertical gradients just below the mixed layer, which are affected by interior ocean transport. Our eddying model study suggests that about one third of the SAM enhancement of the Ekman-induced northward DIC transport is compensated by southward transport from standing and transient eddies.
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Dufresne, J. L., Foujols, M. A., Denvil, S., Caubel, A., Marti, O., Aumont, O., et al. (2013). Climate change projections using the IPSL-CM5 Earth System Model: from CMIP3 to CMIP5. Climate Dynamics, 40(9-10), 2123–2165.
Abstract: We present the global general circulation model IPSL-CM5 developed to study the long-term response of the climate system to natural and anthropogenic forcings as part of the 5th Phase of the Coupled Model Intercomparison Project (CMIP5). This model includes an interactive carbon cycle, a representation of tropospheric and stratospheric chemistry, and a comprehensive representation of aerosols. As it represents the principal dynamical, physical, and bio-geochemical processes relevant to the climate system, it may be referred to as an Earth System Model. However, the IPSL-CM5 model may be used in a multitude of configurations associated with different boundary conditions and with a range of complexities in terms of processes and interactions. This paper presents an overview of the different model components and explains how they were coupled and used to simulate historical climate changes over the past 150 years and different scenarios of future climate change. A single version of the IPSL-CM5 model (IPSL-CM5A-LR) was used to provide climate projections associated with different socio-economic scenarios, including the different Representative Concentration Pathways considered by CMIP5 and several scenarios from the Special Report on Emission Scenarios considered by CMIP3. Results suggest that the magnitude of global warming projections primarily depends on the socio-economic scenario considered, that there is potential for an aggressive mitigation policy to limit global warming to about two degrees, and that the behavior of some components of the climate system such as the Arctic sea ice and the Atlantic Meridional Overturning Circulation may change drastically by the end of the twenty-first century in the case of a no climate policy scenario. Although the magnitude of regional temperature and precipitation changes depends fairly linearly on the magnitude of the projected global warming (and thus on the scenario considered), the geographical pattern of these changes is strikingly similar for the different scenarios. The representation of atmospheric physical processes in the model is shown to strongly influence the simulated climate variability and both the magnitude and pattern of the projected climate changes.
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El Haddad, I., D'Anna, B., Temime-Roussel, B., Nicolas, M., Boreave, A., Favez, O., et al. (2013). Towards a better understanding of the origins, chemical composition and aging of oxygenated organic aerosols: case study of a Mediterranean industrialized environment, Marseille. Atmospheric Chemistry And Physics, 13(15), 7875–7894.
Abstract: As part of the FORMES summer 2008 experiment, an Aerodyne compact time-of-flight aerosol mass spectrometer (cToF-AMS) was deployed at an urban background site in Marseille to investigate the sources and aging of organic aerosols (OA). France's second largest city and the largest port in the Mediterranean, Marseille, provides a locale that is influenced by significant urban industrialized emissions and an active photochemistry with very high ozone concentrations. Particle mass spectra were analyzed by positive matrix factorization (PMF2) and the results were in very good agreement with previous apportionments obtained using a chemical mass balance (CMB) approach coupled to organic markers and metals (El Haddad et al., 2011a). AMS/PMF2 was able to identify for the first time, to the best of our knowledge, the organic aerosol emitted by industrial processes. Even with significant industries in the region, industrial OA was estimated to contribute only similar to 5% of the total OA mass. Both source apportionment techniques suggest that oxygenated OA (OOA) constitutes the major fraction, contributing similar to 80% of OA mass. A novel approach combining AMS/PMF2 data with C-14 measurements was applied to identify and quantify the fossil and non-fossil precursors of this fraction and to explicitly assess the related uncertainties. Results show with high statistical confidence that, despite extensive urban and industrial emissions, OOA is overwhelmingly non-fossil, formed via the oxidation of biogenic precursors, including monoterpenes. AMS/PMF2 results strongly suggest that the variability observed in the OOA chemical composition is mainly driven in our case by the aerosol photochemical age. This paper presents the impact of photochemistry on the increase of OOA oxygenation levels, formation of humic-like substances (HULIS) and the evolution of alpha-pinene SOA (secondary OA) components.
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El Haddad, I., Marchand, N., D'Anna, B., Jaffrezo, J. L., & Wortham, H. (2013). Functional group composition of organic aerosol from combustion emissions and secondary processes at two contrasted urban environments. Atmospheric Environment, 75, 308–320.
Abstract: The quantification of major functional groups in atmospheric organic aerosol (OA) provides a constraint on the types of compounds emitted and formed in atmospheric conditions. This paper presents functional group composition of organic aerosol from two contrasted urban environments: Marseille during summer and Grenoble during winter. Functional groups were determined using a tandem mass spectrometry approach, enabling the quantification of carboxylic (RCOOH), carbonyl (RCOR'), and nitro (RNO2) functional groups. Using a multiple regression analysis, absolute concentrations of functional groups were combined with those of organic carbon derived from different sources in order to infer the functional group contents of different organic aerosol fractions. These fractions include fossil fuel combustion emissions, biomass burning emissions and secondary organic aerosol (SOA). Results clearly highlight the differences between functional group fingerprints of primary and secondary OA fractions. OA emitted from primary sources is found to be moderately functionalized, as about 20 carbons per 1000 bear one of the functional groups determined here, whereas SOA is much more functionalized, as in average 94 carbons per 1000 bear a functional group under study. Aging processes appear to increase both RCOOH and RCOR' functional group contents by nearly one order of magnitude. Conversely, RNO2 content is found to decrease with photochemical processes. Finally, our results also suggest that other functional groups significantly contribute to biomass smoke and SOA. In particular, for SOA, the overall oxygen content, assessed using aerosol mass spectrometer measurements by an 0:C ratio of 0.63, is significantly higher than the apparent 0:C* ratio of 0.17 estimated based on functional groups measured here. A thorough examination of our data suggests that this remaining unexplained oxygen content can be most probably assigned to alcohol (ROH), organic peroxides (ROOH), organonitrates (RONO2) and/or organosulfates (ROSO3H). (C) 2013 Elsevier Ltd. All rights reserved.
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Erbland, J., Vicars, W. C., Savarino, J., Morin, S., Frey, M. M., Frosini, D., et al. (2013). Air-snow transfer of nitrate on the East Antarctic Plateau – Part 1: Isotopic evidence for a photolytically driven dynamic equilibrium in summer. Atmospheric Chemistry And Physics, 13(13), 6403–6419.
Abstract: Here we report the measurement of the comprehensive isotopic composition (delta N-15, Delta O-17 and delta O-18) of nitrate at the air-snow interface at Dome C, Antarctica (DC, 75 degrees 06'S, 123 degrees 19'E), and in snow pits along a transect across the East Antarctic Ice Sheet (EAIS) between 66 degrees S and 78 degrees S. In most of the snow pits, nitrate loss (either by physical release or UV photolysis of nitrate) is observed and fractionation constants associated are calculated. Nitrate collected from snow pits on the plateau (snow accumulation rate below 50 kg m(-2)a(-1)) displays average fractionation constants of (-59 +/- 10) parts per thousand, (+2.0 +/- 1.0)parts per thousand and (+8.7 +/- 2.4)parts per thousand for delta N-15, Delta O-17 and delta O-18, respectively. In contrast, snow pits sampled on the coast show distinct isotopic signatures with average fractionation constants of (-16 +/- 14) parts per thousand, (-0.2 +/- 1.5)parts per thousand and (+3.1 +/- 5.8) parts per thousand, for delta N-15, Delta O-17 and delta O-18, respectively. Our observations corroborate that photolysis (associated with a N-15/N-14 fractionation constant of the order of -48 parts per thousand according to Frey et al. (2009)) is the dominant nitrate loss process on the East Antarctic Plateau, while on the coast the loss is less pronounced and could involve both physical release and photochemical processes. Year-round isotopic measurements at DC show a close relationship between the Delta O-17 of atmospheric nitrate and Delta O-17 of nitrate in skin layer snow, suggesting a photolytically driven isotopic equilibrium imposed by nitrate recycling at this interface. Atmospheric nitrate deposition may lead to fractionation of the nitrogen isotopes and explain the almost constant shift of the order of 25 parts per thousand between the delta N-15 values in the atmospheric and skin layer nitrate at DC. Asymptotic delta N-15(NO3-) values calculated for each snow pit are found to be correlated with the inverse of the snow accumulation rate (ln(delta N-15(as.) + 1) = (5.76 +/- 0.47) . (kg m(-2) a(-1)/A)+(0.01 +/- 0.02)), confirming the strong relationship between the snow accumulation rate and the degree of isotopic fractionation, consistent with previous observations by Freyer et al. (1996). Asymptotic Delta O-17(NO3-) values on the plateau are smaller than the values found in the skin layer most likely due to oxygen isotope exchange between the nitrate photoproducts and water molecules from the surrounding ice. However, the apparent fractionation in Delta O-17 is small, thus allowing the preservation of a portion of the atmospheric signal.
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Escudier, R., Bouffard, J., Pascual, A., Poulain, P. M., & Pujol, M. I. (2013). Improvement of coastal and mesoscale observation from space: Application to the northwestern Mediterranean Sea. Geophysical Research Letters, 40(10), 2148–2153.
Abstract: We present an innovative approach to the generation of remotely sensed high-resolution sea surface topography that improves coastal and mesoscale dynamic characterization. This new method is applied for the period 2002-2010 in the northwestern Mediterranean Sea, an area marked by a small Rossby radius. The spectral content of the new mapped data is closer to that of the along-track signal and displays higher levels of energy in the mesoscale bandwidth with the probability distribution of the new velocity fields 30% closer to drifter estimations. The fields yield levels of eddy kinetic energy 25% higher than standard altimetry products, especially over regions regularly impacted by mesoscale instabilities. Moreover, qualitative and quantitative comparisons with drifters, glider, and satellite sea surface temperature observations further confirm that the new altimetry product provides, in many cases, a better representation of mesoscale features (more than 25% improvement in correlation with glider data during an experiment).
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Evin, G., & Favre, A. C. (2013). Further developments of a transient Poisson-cluster model for rainfall. Stochastic Environmental Research And Risk Assessment, 27(4), 831–847.
Abstract: Stochastic point processes for rainfall are known to be able to preserve the temporal variability of rainfall on several levels of aggregation (e.g. hourly, daily), especially when the cluster approach is used. One major assumption in most of the applications todate is the stationarity of the rainfall properties in time, which must be reconsidered under a climate change hypothesis. Here, we propose new theoretical developments of a Poisson-based model with cluster, namely the Neyman-Scott Rectangular Pulses Model, which treats storm frequency as a nonstationary function. In this paper, storm frequency is modelled as a linear function of time in order to reproduce an increase (or decrease) of the mean annual precipitation. The basic theory is reconsidered and the moments are derived up to the third order. Then, a calibration method based on the generalized method of moments is proposed and discussed. An application to a rainfall time series from Uccle illustrates how this model can reproduce a trend for the average rainfall. This work opens new avenues for future developments on transient stochastic rainfall models and highlights the major challenges linked to this approach.
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Evrard, O., Poulenard, J., Nemery, J., Ayrault, S., Gratiot, N., Duvert, C., et al. (2013). Tracing sediment sources in a tropical highland catchment of central Mexico by using conventional and alternative fingerprinting methods. Hydrological Processes, 27(6), 911–922.
Abstract: Land degradation is intense in tropical regions where it causes for instance a decline in soil fertility and reservoir siltation. Two fingerprinting approaches (i.e. the conventional approach based on radionuclide and geochemical concentrations and the alternative diffuse reflectance infrared Fourier transform spectroscopy method) were conducted independently to outline the sources delivering sediment to the river network draining into the Cointzio reservoir, in Mexican tropical highlands. This study was conducted between May and October in 2009 in subcatchments representative of the different environments supplying sediment to the river network. Overall, Cointzio catchment is characterized by very altered soils and the dominance of Andisols and Acrisols. Both fingerprinting methods provided very similar results regarding the origin of sediment in Huertitas subcatchment (dominated by Acrisols) where the bulk of sediment was supplied by gullies. In contrast, in La Cortina subcatchment dominated by Andisols, the bulk of sediment was supplied by cropland. Sediment originating from Potrerillos subcatchment characterized by a mix of Acrisols and Andisols was supplied in variable proportions by both gullies and rangeland/cropland. In this latter subcatchment, results provided by both fingerprinting methods were very variable. Our results outline the need to take the organic carbon content of soils into account and the difficulty to use geochemical properties to fingerprint sediment in very altered volcanic catchments. However, combining our fingerprinting results with sediment export data provided a way of prioritizing the implementation of erosion control measures to mitigate sediment supply to the Cointzio reservoir supplying drinking water to Morelia city. Copyright (c) 2012 John Wiley & Sons, Ltd.
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Fain, X., Helmig, D., Hueber, J., Obrist, D., & Williams, M. W. (2013). Mercury dynamics in the Rocky Mountain, Colorado, snowpack. Biogeosciences, 10(6), 3793–3807.
Abstract: Gaseous elemental mercury (GEM) was monitored at the Niwot Ridge (NWT) Long-Term Ecological Research (LTER) site (Colorado, USA, 40 degrees N) from interstitial air extracted from the snowpack at depths ranging from the snow surface to 10 cm above the soil. A highly dynamic cycling of mercury (Hg) in this mid-latitude snowpack was observed. Patterns were driven by both GEM production in surface snow and GEM destruction in the deeper snowpack layers. Thorough mixing and vertical transport processes were observed through the snowpack. GEM was photochemically produced near the snow-air interface throughout the entire winter, leading to enhanced GEM levels in interstitial air of surface snow of up to 8 ng m(-3). During low-wind periods, GEM in surface snow layers remained significantly above ambient air levels at night as well, which may indicate a potential weak GEM production overnight. Analyses of vertical GEM gradients in the snowpack show that surface GEM enhancements efficiently propagated down the snowpack, with a temporal lag in peak GEM levels observed with increasing depth. Downward diffusion was responsible for much of these patterns, although vertical advection also contributed to vertical redistribution. Destruction of GEM in the lower snowpack layers was attributed to dark oxidation of GEM. Analysis of vertical GEM/CO2 flux ratios indicated that this GEM destruction occurred in the snow and not in the underlying soil. The strong, diurnal patterns of photochemical GEM production at the surface ultimately lead to re-emission losses of deposited Hg back to the atmosphere. The NWT data show that highest GEM surface production and re-emissions occur shortly after fresh snowfall, which possibly resupplies photoreducible Hg to the snowpack, and that photochemical GEM reduction is not radiation-limited as it is strong even on cloudy days.
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Favier, L., Daudon, D., & Donze, F. V. (2013). Rigid obstacle impacted by a supercritical cohesive granular flow using a 3D discrete element model. Cold Regions Science And Technology, 85, 232–241.
Abstract: This study examines the drag coefficient of an obstacle impacted by a 3D cohesive granular flow using a discrete element model. A specific numerical setup is used to carry out reproducible and controlled normal impact simulations, in which the upstream flow properties are fully controlled parameters. The micromechanical contact model involves the physical properties of friction, normal elastic plastic repulsion, dissipation, and a normal cohesion factor that induces bulk cohesion in the granular assembly. The effect of cohesion on the obstacle load is investigated through a micro-scale analysis. We show that increasing the cohesion leads to an increase of the obstacle drag, through a densification of the contact network, which enhances the transmission of contact forces to the obstacle. This experiment is extended to a wide range of supercritical flows, with Froude numbers between 1.5 and 11.2. The resulting drag coefficient curves are represented as power law functions of the Froude number. We then demonstrate the dependency of the power law exponent on the ratio between inertia and gravitational forces. Our results suggest that the assessment of drag coefficient critical values by conventional avalanche protection guidelines could be improved by a mechanical consideration of cohesion for certain snow types. (C) 2012 Elsevier B.V. All rights reserved.
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Favier, V., Agosta, C., Parouty, S., Durand, G., Delaygue, G., Gallee, H., et al. (2013). An updated and quality controlled surface mass balance dataset for Antarctica. Cryosphere, 7(2), 583–597.
Abstract: We present an updated and quality controlled surface mass balance (SMB) database for the Antarctic ice sheet. Importantly, the database includes formatted metadata, such as measurement technique, elevation, time covered, etc, which allows any user to filter out the data. Here, we discard data with limited spatial and temporal representativeness, too small measurement accuracy, or lack of quality control. Applied to the database, this filtering process gives four times more reliable data than when applied to previously available databases. New data with high spatial resolution are now available over long traverses, and at low elevation in some areas. However, the quality control led to a considerable reduction in the spatial density of data in several regions, particularly over West Antarctica. Over interior plateaus, where the SMB is low, the spatial density of measurements remains high. This quality controlled dataset was compared to results from ERA-Interim reanalysis to assess whether field data allow us to reconstruct an accurate description of the main SMB distribution features in Antarctica. We identified large areas where data gaps impede model validation: except for very few areas (e. g., Adelie Land), measurements in the elevation range between 200 m and 1000 m above sea level are not regularly distributed and do not allow a thorough validation of models in such regions with complex topography, where the highest scattering of SMB values is reported. Clearly, increasing the spatial density of field measurements at low elevations, in the Antarctic Peninsula and in West Antarctica is a scientific priority.
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Fettweis, X., Franco, B., Tedesco, M., van Angelen, J. H., Lenaerts, J. T. M., van den Broeke, M. R., et al. (2013). Estimating the Greenland ice sheet surface mass balance contribution to future sea level rise using the regional atmospheric climate model MAR. Cryosphere, 7(2), 469–489.
Abstract: To estimate the sea level rise (SLR) originating from changes in surface mass balance (SMB) of the Greenland ice sheet (GrIS), we present 21st century climate projections obtained with the regional climate model MAR (Modele Atmospherique Regional), forced by output of three CMIP5 (Coupled Model Intercomparison Project Phase 5) general circulation models (GCMs). Our results indicate that in a warmer climate, mass gain from increased winter snowfall over the GrIS does not compensate mass loss through increased meltwater run-off in summer. Despite the large spread in the projected near-surface warming, all the MAR projections show similar non-linear increase of GrIS surface melt volume because no change is projected in the general atmospheric circulation over Greenland. By coarsely estimating the GrIS SMB changes from GCM output, we show that the uncertainty from the GCM-based forcing represents about half of the projected SMB changes. In 2100, the CMIP5 ensemble mean projects a GrIS SMB decrease equivalent to a mean SLR of +4 +/- 2 cm and +9 +/- 4 cm for the RCP (Representative Concentration Pathways) 4.5 and RCP 8.5 scenarios respectively. These estimates do not consider the positive melt-elevation feedback, although sensitivity experiments using perturbed ice sheet topographies consistent with the projected SMB changes demonstrate that this is a significant feedback, and highlight the importance of coupling regional climate models to an ice sheet model. Such a coupling will allow the assessment of future response of both surface processes and ice-dynamic changes to rising temperatures, as well as their mutual feedbacks.
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Fettweis, X., Hanna, E., Lang, C., Belleflamme, A., Erpicum, M., & Gallee, H. (2013). “Important role of the mid-tropospheric atmospheric circulation in the recent surface melt increase over the Greenland ice sheet”. Cryosphere, 7(1), 241–248.
Abstract: Since 2007, there has been a series of surface melt records over the Greenland ice sheet (GrIS), continuing the trend towards increased melt observed since the end of the 1990s. The last two decades are characterized by an increase of negative phases of the North Atlantic Oscillation (NAO) favouring warmer and drier summers than normal over GrIS. In this context, we use a circulation type classification based on daily 500 hPa geopotential height to evaluate the role of atmospheric dynamics in this surface melt acceleration for the last two decades. Due to the lack of direct observations, the interannual melt variability is gauged here by the summer (June-July-August) mean temperature from reanalyses at 700 hPa over Greenland; analogous atmospheric circulations in the past show that similar to 70% of the 1993-2012 warming at 700 hPa over Greenland has been driven by changes in the atmospheric flow frequencies. Indeed, the occurrence of anticyclones centred over the GrIS at the surface and at 500 hPa has doubled since the end of 1990s, which induces more frequent southerly warm air advection along the western Greenland coast and over the neighbouring Canadian Arctic Archipelago (CAA). These changes in the NAO modes explain also why no significant warming has been observed these last summers over Svalbard, where northerly atmospheric flows are twice as frequent as before. Therefore, the recent warmer summers over GrIS and CAA cannot be considered as a long-term climate warming but are more a consequence of NAO variability affecting atmospheric heat transport. Although no global model from the CMIP5 database projects subsequent significant changes in NAO through this century, we cannot exclude the possibility that the observed NAO changes are due to global warming.
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Fischer, H., Severinghaus, J., Brook, E., Wolff, E., Albert, M., Alemany, O., et al. (2013). Where to find 1.5 million yr old ice for the IPICS “Oldest-Ice” ice core. Climate Of The Past, 9(6), 2489–2505.
Abstract: The recovery of a 1.5 million yr long ice core from Antarctica represents a keystone of our understanding of Quaternary climate, the progression of glaciation over this time period and the role of greenhouse gas cycles in this progression. Here we tackle the question of where such ice may still be found in the Antarctic ice sheet. We can show that such old ice is most likely to exist in the plateau area of the East Antarctic ice sheet (EAIS) without stratigraphic disturbance and should be able to be recovered after careful pre-site selection studies. Based on a simple ice and heat flow model and glaciological observations, we conclude that positions in the vicinity of major domes and saddle position on the East Antarctic Plateau will most likely have such old ice in store and represent the best study areas for dedicated reconnaissance studies in the near future. In contrast to previous ice core drill site selections, however, we strongly suggest significantly reduced ice thickness to avoid bottom melting. For example for the geothermal heat flux and accumulation conditions at Dome C, an ice thickness lower than but close to about 2500m would be required to find 1.5 Myr old ice (i.e., more than 700m less than at the current EPICA Dome C drill site). Within this constraint, the resolution of an Oldest-Ice record and the distance of such old ice to the bedrock should be maximized to avoid ice flow disturbances, for example, by finding locations with minimum geothermal heat flux. As the geothermal heat flux is largely unknown for the EAIS, this parameter has to be carefully determined beforehand. In addition, detailed bedrock topography and ice flow history has to be reconstructed for candidates of an Oldest-Ice ice coring site. Finally, we argue strongly for rapid access drilling before any full, deep ice coring activity commences to bring datable samples to the surface and to allow an age check of the oldest ice.
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Foley, A. M., Dalmonech, D., Friend, A. D., Aires, F., Archibald, A. T., Bartlein, P., et al. (2013). Evaluation of biospheric components in Earth system models using modern and palaeo-observations: the state-of-the-art. Biogeosciences, 10(12), 8305–8328.
Abstract: Earth system models (ESMs) are increasing in complexity by incorporating more processes than their predecessors, making them potentially important tools for studying the evolution of climate and associated biogeochemical cycles. However, their coupled behaviour has only recently been examined in any detail, and has yielded a very wide range of outcomes. For example, coupled climate-carbon cycle models that represent land-use change simulate total land carbon stores at 2100 that vary by as much as 600 Pg C, given the same emissions scenario. This large uncertainty is associated with differences in how key processes are simulated in different models, and illustrates the necessity of determining which models are most realistic using rigorous methods of model evaluation. Here we assess the state-of-the-art in evaluation of ESMs, with a particular emphasis on the simulation of the carbon cycle and associated biospheric processes. We examine some of the new advances and remaining uncertainties relating to (i) modern and palaeodata and (ii) metrics for evaluation. We note that the practice of averaging results from many models is unreliable and no substitute for proper evaluation of individual models. We discuss a range of strategies, such as the inclusion of pre-calibration, combined process- and system-level evaluation, and the use of emergent constraints, that can contribute to the development of more robust evaluation schemes. An increasingly data-rich environment offers more opportunities for model evaluation, but also presents a challenge. Improved knowledge of data uncertainties is still necessary to move the field of ESM evaluation away from a “beauty contest” towards the development of useful constraints on model outcomes.
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Fontana, C., Brasseur, P., & Brankart, J. M. (2013). Toward a multivariate reanalysis of the North Atlantic Ocean biogeochemistry during 1998-2006 based on the assimilation of SeaWiFS chlorophyll data. Ocean Science, 9(1), 37–56.
Abstract: Today, the routine assimilation of satellite data into operational models of ocean circulation is mature enough to enable the production of global reanalyses describing the ocean circulation variability during the past decades. The expansion of the “reanalysis” concept from ocean physics to biogeochemistry is a timely challenge that motivates the present study. The objective of this paper is to investigate the potential benefits of assimilating satellite-estimated chlorophyll data into a basin-scale three-dimensional coupled physical-biogeochemical model of the North Atlantic. The aim is on the one hand to improve forecasts of ocean biogeochemical properties and on the other hand to define a methodology for producing data-driven climatologies based on coupled physical-biogeochemical modeling. A simplified variant of the Kalman filter is used to assimilate ocean color data during a 9-year period. In this frame, two experiments are carried out, with and without anamorphic transformations of the state vector variables. Data assimilation efficiency is assessed with respect to the assimilated data set, nitrate of the World Ocean Atlas database and a derived climatology. Along the simulation period, the non-linear assimilation scheme clearly improves the surface analysis and forecast chlorophyll concentrations, especially in the North Atlantic bloom region. Nitrate concentration forecasts are also improved thanks to the assimilation of ocean color data while this improvement is limited to the upper layer of the water column, in agreement with recent related literature. This feature is explained by the weak correlation taken into account by the assimilation between surface phytoplankton and nitrate concentrations deeper than 50 meters. The assessment of the non-linear assimilation experiments indicates that the proposed methodology provides the skeleton of an assimilative system suitable for reanalyzing the ocean biogeochemistry based on ocean color data.
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Frey, M. M., Brough, N., France, J. L., Anderson, P. S., Traulle, O., King, M. D., et al. (2013). The diurnal variability of atmospheric nitrogen oxides (NO and NO2) above the Antarctic Plateau driven by atmospheric stability and snow emissions. Atmospheric Chemistry And Physics, 13(6), 3045–3062.
Abstract: Atmospheric nitrogen oxides (NO and NO2) were observed at Dome C, East Antarctica (75.1 degrees S, 123.3 degrees E, 3233 m), for a total of 50 days, from 10 December 2009 to 28 January 2010. Average (+/- 1 sigma) mixing ratios at 1.0m of NO and NO2, the latter measured for the first time on the East Antarctic Plateau, were 111 (+/- 89) and 98 (+/- 89) pptv, respectively. Atmospheric mixing ratios are on average comparable to those observed previously at South Pole, but in contrast show strong diurnal variability: a minimum around local noon and a maximum in the early evening coincide with the development and collapse of a convective boundary layer. The asymmetric diurnal cycle of NOx concentrations and likely any other chemical tracer with a photolytic surface source is driven by the turbulent diffusivity and height of the atmospheric boundary layer, with the former controlling the magnitude of the vertical flux and the latter the size of the volume into which snow emissions are transported. In particular, the average (+/- 1 sigma) NOx emission flux from 22 December 2009 to 28 January 2010, estimated from atmospheric concentration gradients, was 8.2 (+/- 7.4) x 10(12) molecule m(-2) s(-1) belongs to the largest values measured so far in the polar regions and explains the 3-fold increase in mixing ratios in the early evening when the boundary layer becomes very shallow. Dome C is likely not representative for the entire East Antarctic Plateau but illustrates the need of an accurate description of the boundary layer above snow in atmospheric chemistry models. A simple nitrate photolysis model matches the observed median diurnal NOx flux during the day but has significant low bias during the night. The difference is significant taking into account the total random error in flux observations and model uncertainties due to the variability of NO3- concentrations in snow and potential contributions from NO2- photolysis. This highlights uncertainties in the parameterization of the photolytic NOx source in natural snowpacks, such as the poorly constrained quantum yield of nitrate photolysis. A steadystate analysis of the NO2 : NO ratios indicates that peroxy (HO2 + RO2) or other radical concentrations in the boundary layer of Dome C are either higher than measured elsewhere in the polar regions or other processes leading to enhanced NO2 have to be invoked. These results confirm the existence of a strongly oxidising canopy enveloping the East Antarctic Plateau in summer.
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Gagliardini, O., Weiss, J., Duval, P., & Montagnat, M. (2013). On Duddu and Waisman (2012,2013) concerning continuum damage mechanics applied to crevassing and iceberg calving. Journal Of Glaciology, 59(216), 797–798.
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Gagliardini, O., Zwinger, T., Gillet-Chaulet, F., Durand, G., Favier, L., de Fleurian, B., et al. (2013). Capabilities and performance of Elmer/Ice, a new-generation ice sheet model. Geoscientific Model Development, 6(4), 1299–1318.
Abstract: The Fourth IPCC Assessment Report concluded that ice sheet flow models, in their current state, were unable to provide accurate forecast for the increase of polar ice sheet discharge and the associated contribution to sea level rise. Since then, the glaciological community has undertaken a huge effort to develop and improve a new generation of ice flow models, and as a result a significant number of new ice sheet models have emerged. Among them is the parallel finite-element model Elmer/Ice, based on the open-source multi-physics code Elmer. It was one of the first full-Stokes models used to make projections for the evolution of the whole Greenland ice sheet for the coming two centuries. Originally developed to solve local ice flow problems of high mechanical and physical complexity, Elmer/Ice has today reached the maturity to solve larger-scale problems, earning the status of an ice sheet model. Here, we summarise almost 10 yr of development performed by different groups. Elmer/Ice solves the full-Stokes equations, for isotropic but also anisotropic ice rheology, resolves the grounding line dynamics as a contact problem, and contains various basal friction laws. Derived fields, like the age of the ice, the strain rate or stress, can also be computed. Elmer/Ice includes two recently proposed inverse methods to infer badly known parameters. Elmer is a highly parallelised code thanks to recent developments and the implementation of a block preconditioned solver for the Stokes system. In this paper, all these components are presented in detail, as well as the numerical performance of the Stokes solver and developments planned for the future.
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Gallart, F., Perez-Gallego, N., Latron, J., Catari, G., Martinez-Carreras, N., & Nord, G. (2013). Short- and long-term studies of sediment dynamics in a small humid mountain Mediterranean basin with badlands. Geomorphology, 196, 242–251.
Abstract: Badland landscapes are the main sediment sources in the Vallcebre area (Eastern Pyrenees, Catalonia, Spain). Short-term studies (up to 3-years long) carried out between 1980 and 1994 were used to estimate the rates of both denudation on bare surfaces and sediment production at the plot scale, to analyse the seasonal dynamics of bedrock weathering and regolith behaviour, and to study the relationships between geomorphic activity and herbaceous plant colonisation. Since 1990, stream flow and suspended sediment loads have been monitored using three gauging stations equipped with infrared backscattering turbidimeters, ultrasonic beam attenuation solids sensors and automatic water samplers. The combination of the two different approaches has been useful for a better perception of the dynamics of the badland systems and to assess the long-term contribution of these areas to the basin sediment loads. Badland erosion at the event scale for a period of 15 years was simulated with the KINEROS2 model and allowed the long-term comparison between badland erosion and sediment yield at the small basin scale. Badlands are the main source of sediment in the basin for most of the events, but infrequent runoff events cause the removal of sediment stores and the activation of other sediment sources. The analysis of the uncertainty of sediment yield measurements for a range of record durations demonstrated that long records are needed for obtaining acceptable results due to the high interannual variability. Relatively low-cost short-term geomorphic observations may provide information useful for assessing the long-term sediment production in these basins with badland areas only if the observations are used to implement a model able to simulate long-term observations. (C) 2012 Elsevier B.V. All rights reserved.
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Gallee, H., Trouvilliez, A., Agosta, C., Genthon, C., Favier, V., & Naaim-Bouvet, F. (2013). Transport of Snow by the Wind: A Comparison Between Observations in Adelie Land, Antarctica, and Simulations Made with the Regional Climate Model MAR. Boundary-Layer Meteorology, 146(1), 133–147.
Abstract: For the first time a simulation of blowing snow events was validated in detail using one-month long observations (January 2010) made in Ad,lie Land, Antarctica. A regional climate model featuring a coupled atmosphere/blowing snow/snowpack model is forced laterally by meteorological re-analyses. The vertical grid spacing was 2 m from 2 to 20 m above the surface and the horizontal grid spacing was 5 km. The simulation was validated by comparing the occurrence of blowing snow events and other meteorological parameters at two automatic weather stations. The Nash test allowed us to compute efficiencies of the simulation. The regional climate model simulated the observed wind speed with a positive efficiency (0.69). Wind speeds higher than 12 m s (-1) were underestimated. Positive efficiency of the simulated wind speed was a prerequisite for validating the blowing snow model. Temperatures were simulated with a slightly negative efficiency (-0.16) due to overestimation of the amplitude of the diurnal cycle during one week, probably because the cloud cover was underestimated at that location during the period concerned. Snowfall events were correctly simulated by our model, as confirmed by field reports. Because observations suggested that our instrument (an acoustic sounder) tends to overestimate the blowing snow flux, data were not sufficiently accurate to allow the complete validation of snow drift values. However, the simulation of blowing snow occurrence was in good agreement with the observations made during the first 20 days of January 2010, despite the fact that the blowing snow flux may be underestimated by the regional climate model during pure blowing snow events. We found that blowing snow occurs in Ad,lie Land only when the 30-min wind speed value at 2 m a.g.l. is > 10 m s (-1). The validation for the last 10 days of January 2010 was less satisfactory because of complications introduced by surface melting and refreezing.
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Gardelle, J., Berthier, E., Arnaud, Y., & Kaab, A. (2013). Region-wide glacier mass balances over the Pamir-Karakoram-Himalaya during 1999-2011. Cryosphere, 7(4), 1263–1286.
Abstract: The recent evolution of Pamir-Karakoram-Himalaya (PKH) glaciers, widely acknowledged as valuable high-altitude as well as mid-latitude climatic indicators, remains poorly known. To estimate the region-wide glacier mass balance for 9 study sites spread from the Pamir to the Hengduan Shan (eastern Himalaya), we compared the 2000 Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM) to recent (2008-2011) DEMs derived from SPOT5 stereo imagery. During the last decade, the region-wide glacier mass balances were contrasted with moderate mass losses in the eastern and central Himalaya (-0.22 +/- 0.12m w.e. yr(-1) to -0.33 +/- 0.14m w.e. yr(-1)) and larger losses in the western Himalaya (-0.45 +/- 0.13m w.e. yr(-1)). Recently reported slight mass gain or balanced mass budget of glaciers in the central Karakoram is confirmed for a larger area (+0.10 +/- 0.16m w.e. yr(-1)) and also observed for glaciers in the western Pamir (+0.14 +/- 0.13m w.e. yr(-1)). Thus, the “Karakoram anomaly” should be renamed the “Pamir-Karakoram anomaly”, at least for the last decade. The overall mass balance of PKH glaciers, -0.14 +/- 0.08m w.e. yr(-1), is two to three times less negative than the global average for glaciers distinct from the Greenland and Antarctic ice sheets. Together with recent studies using ICESat and GRACE data, DEM differencing confirms a contrasted pattern of glacier mass change in the PKH during the first decade of the 21st century.
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Gardelle, J., Berthier, E., Arnaud, Y., & Kaab, A. (2013). Region-wide glacier mass balances over the Pamir-Karakoram-Himalaya during 1999-2011 (vol 7, pg 1263, 2013). Cryosphere, 7(6), 1885–1886.
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Gaultier, L., Verron, J., Brankart, J. M., Titaud, O., & Brasseur, P. (2013). On the inversion of submesoscale tracer fields to estimate the surface ocean circulation. Journal Of Marine Systems, 126, 33–42.
Abstract: In this paper, we demonstrate the feasibility of inverting the information contained in oceanic submesoscales, such as the ones evidenced in tracer observations of sea surface temperature (SST), to improve the description of mesoscale dynamics provided by altimetric observations. A small region of the Western Mediterranean Sea is chosen as a test case. From a SST snapshot of the region in July 2004, information is extracted to improve the velocity field as computed by geostrophy from the AVISO altimetric data at the same location and time. Image information is extracted from SST using a binarization of the SST gradients. Similarly, image information is extracted from the dynamic topography using finite size Lyapunov exponents (FSLE). The inverse problem is formulated in a Bayesian framework and expressed in terms of a cost function measuring the misfits between the two images. The large amount of information which is already available from ocean color satellites or which will be available from high-resolution altimetric satellites such as SWOT, is a strong motivation for this work. Moreover, the image data assimilation approach which is explored here, is a possible strategy for handling the huge amount of satellite data imprinted by small scale information. (c) 2012 Elsevier B.V. All rights reserved.
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Gauthier, F., Montagnat, M., Weiss, J., Allard, M., & Hetu, B. (2013). Ice cascade growth and decay: a thermodynamic approach. Journal Of Glaciology, 59(215), 507–523.
Abstract: The ice volume evolution of a frozen waterfall (or ice cascade) was studied using a thermodynamic model. The model was developed from meteorological data collected in the vicinity of the waterfall and validated from ice volume measurements estimated from terrestrial lidar images. The ice cascade forms over a 45 m high rock wall located in northern Gaspesie, Quebec, Canada. Two stages of formation were identified. During the first stage, the growth is mainly controlled by air convection around the flowing and free-falling water. The ice cascade growth rate increases with decreasing air temperature below 0 degrees C and when the water flow reaches its lowest level. During the second stage, the ice cascade covers the entire rock-wall surface, water flow is isolated from the outside environment and ice volume increases asymptotically. Heat is evacuated from the water flow through the ice cover by conduction. The growth is controlled mainly by the conductive heat loss through the ice cover but also by the longwave radiation emitted at the ice surface during the night. In spring, melting of the ice cascade is dependent on the air convection over the ice surface but also on the sensible heat carried by the increasing water flow and the solar radiation received during the day.
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Geng, L., Schauer, A. J., Kunasek, S. A., Sofen, E. D., Erbland, J., Savarino, J., et al. (2013). Analysis of oxygen-17 excess of nitrate and sulfate at sub-micromole levels using the pyrolysis method. Rapid Communications In Mass Spectrometry, 27(21), 2411–2419.
Abstract: RATIONALEThe oxygen-17 excess (O-17) of nitrate and sulfate contains valuable information regarding their atmospheric formation pathways. However, the current pyrolysis method to measure O-17 requires large sample amounts (>4 μmol for nitrate and >1 μmol for sulfate). We present a new approach employing a Gas Bench interface which cryofocuses O-2 produced from sample pyrolysis, enabling the analysis of sub-micromole size samples. METHODSSilver nitrate or sulfate at sub-micromole levels in a sample container was thermally decomposed to O-2 and byproducts in a modified Temperature Conversion/Elemental Analyzer (TC/EA). Byproducts (mainly NO2 for silver nitrate and SO2 for silver sulfate) were removed in a liquid nitrogen trap and the sample O-2 was carried by ultra-pure helium (He) gas to a Gas Bench II interface where it was cryofocused prior to entering an isotope ratio mass spectrometer. RESULTSAnalysis of the international nitrate reference material USGS35 (O-17=21.6) within the size range of 300-1000nmol O-2 gave a mean O-17 value of (21.6 +/- 0.69) parts per thousand (mean +/- 1 sigma). Three inter-laboratory calibrated sulfate reference materials, Sulf-, Sulf- and Sulf-epsilon, each within the size range of 180-1000nmol O-2, were analyzed and shown to possess mean O-17 values of (0.9 +/- 0.10) parts per thousand, (2.1 +/- 0.25) parts per thousand and (7.0 +/- 0.63) parts per thousand, respectively. CONCLUSIONSThe analyses of nitrate and sulfate reference materials at sub-micromole levels gave O-17 values consistent with their accepted values. This new approach of employing the Gas Bench to cryofocus O-2 after the pyrolysis of AgNO3 and Ag2SO4 particularly benefits the effort of measuring O-17 in sample types with a low abundance of nitrate and sulfate such as ice cores. Copyright (c) 2013 John Wiley & Sons, Ltd.
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Genthon, C., Six, D., Gallee, H., Grigioni, P., & Pellegrini, A. (2013). Two years of atmospheric boundary layer observations on a 45-m tower at Dome C on the Antarctic plateau. Journal Of Geophysical Research-Atmospheres, 118(8), 3218–3232.
Abstract: The lower atmospheric boundary layer at Dome C on the Antarctic plateau has been continuously monitored along a 45-m tower since 2009. Two years of observations (2009 and 2010) are presented. A strong diurnal cycle is observed near the surface in summer but almost disappears at the top of the tower, indicating that the summer nocturnal inversion is very shallow. Very steep inversions reaching almost 1 degrees C m(-1) on average along the tower are observed in winter. They are stronger and more frequent during the colder 2010 winter, reaching a maximum in a layer similar to 10-15 m above the surface. Winter temperature is characterized by strong synoptic variability. An extreme warm event occurred in July 2009. The temperature reached -30 degrees C, typical of midsummer weather. Meteorological analyses which agree with the observations near the surface confirm that heat is propagated downward from higher elevations. A high total water column indicates moist air masses aloft originating from the lower latitudes. The coldest temperatures and strongest inversions are associated with characteristic synoptic patterns and a particularly dry atmosphere. Measurement of moisture in the clean and cold Antarctic plateau atmosphere is a challenging task. Supersaturations are very likely but are not revealed by the observations. This is possibly an instrumental artifact that would affect other moisture measurements made in similar conditions. In spite of this, such observations offer a stringent test of the robustness of the polar boundary layer in meteorological and climate models, addressing a major concern raised in the IPCC 2007 report.
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Gianini, M. F. D., Piot, C., Herich, H., Besombes, J. L., Jaffrezo, J. L., & Hueglin, C. (2013). Source apportionment of PM10, organic carbon and elemental carbon at Swiss sites: An intercomparison of different approaches. Science Of The Total Environment, 454, 99–108.
Abstract: In this study, the results of source apportionment of particulate matter (PM10), organic carbon (OC), and elemental carbon (EC) – as obtained through different approaches at different types of sites (urban background, urban roadside, and two rural sites in Switzerland) – are compared. The methods included in this intercomparison are positive matrix factorisation modelling (PMF, applied to chemical composition data including trace elements, inorganic ions, OC, and EC), molecular marker chemical mass balance modelling (MM-CMB), and the aethalometer model (AeM). At all sites, the agreement of the obtained source contributions was reasonable for OC, EC, and PM10. Based on an annual average, and at most of the considered sites, secondary organic carbon (SOC) is the component with the largest contribution to total OC; the most important primary source of OC is wood combustion, followed by road traffic. Secondary aerosols predominate in PM10. All considered techniques identified road traffic as the dominant source of EC, while wood combustion emissions are of minor importance for this constituent. The intercomparison of different source apportionment approaches is helpful to identify the strengths and the weaknesses of the different methods. Application of PMF has limitations when source emissions have a strong temporal correlation, or when meteorology has a strong impact on PM variability. In these cases, the use of PMF can result in mixed source profiles and consequently in the under- or overestimation of the real-world sources. The application of CMB models can be hampered by the unavailability of source profiles and the non-representativeness of the available profiles for local source emissions. This study also underlines that chemical transformations of molecular markers in the atmosphere can lead to the underestimation of contributions from primary sources, in particular during the summer period or when emission sources are far away from the receptor sites. (C) 2013 Elsevier B.V. All rights reserved.
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Gilbert, A., & Vincent, C. (2013). Atmospheric temperature changes over the 20th century at very high elevations in the European Alps from englacial temperatures. Geophysical Research Letters, 40(10), 2102–2108.
Abstract: Given the paucity of observations, a great deal of uncertainty remains concerning temperature changes at very high altitudes over the last century. Englacial temperature measurements performed in boreholes provide a very good indicator of atmospheric temperatures for very high elevations although they are not directly related to air temperatures. Temperature profiles from seven deep boreholes drilled at three different sites between 4240 and 4300mabove sea level in the Mont Blanc area (French Alps) have been analyzed using a heat flow model and a Bayesian inverse modeling approach. Atmospheric temperature changes over the last century were estimated by simultaneous inversion of these temperature profiles. A mean warming rate of 0.14 degrees C/decade between 1900 and 2004 was found. This is similar to the observed regional low altitude trend in the northwestern Alps, suggesting that air temperature trends are not altitude dependent.
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Gimbert, F., Amitrano, D., & Weiss, J. (2013). Crossover from quasi-static to dense flow regime in compressed frictional granular media. Epl, 104(4).
Abstract: Being ubiquitous in a large variety of geomaterials, granular assemblies play a crucial role in the mechanical stability of engineering and geophysical structures. For these applications, an accurate knowledge of the processes at the origin of shear localization, i.e. faulting, in frictional granular assemblies submitted to compressive loading is needed. Here we tackle this problem by performing discrete-element numerical simulations. A thorough analysis of the evolution of multi-scale mechanical properties as approaching sample macroscopic instability is performed. Spatial correlations operating within the shear stress and strain fields are analyzed by means of a coarse-graining analysis. The divergence of correlation lengths is reported on both shear stress and strain fields as approaching the transition to sample instability. We thus show that the crossover from a quasi-static regime where the sample deforms infinitely slowly to a dense flow regime, where inertial forces play a significant role, can be interpreted as a critical phase transition. At this transition, no shear band of characteristic thickness can be defined. Copyright (C) EPLA, 2013
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Gosset, M., Viarre, J., Quantin, G., & Alcoba, M. (2013). Evaluation of several rainfall products used for hydrological applications over West Africa using two high-resolution gauge networks. Quarterly Journal Of The Royal Meteorological Society, 139(673), 923–940.
Abstract: The evaluation of rainfall products over the West African region will be an important component of the Megha-Tropiques (MT) Ground Validation (GV) plan. In this paper, two dense research gauge networks from Benin and Niger, integrated in the MT GV plan, are presented and are used to evaluate several currently available global or regional satellite-based rainfall products. Eight productsthe Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN), Climate Prediction Center Morphing method (CMORPH), Tropical Rainfall Measuring Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) 3B42 real-time and gauge-adjusted version, Global Satellite Mapping of Precipitation (GSMaP), Climate Prediction Center (CPC) African Rainfall Estimate (RFE), Estimation des Precipitation par SATellite (EPSAT), and Global Precipitation Climatology Project One Degree Daily estimate (GPCP-1DD)are compared to the ground reference. The comparisons are carried out at daily, 1 degrees resolution, over the rainy season (June-September), between the years 2003 and 2010. The work focuses on the ability of the various products to reproduce salient features of the rainfall regime that impact the hydrological response. The products are analysed on a multi-criteria basis, focusing in particular on the way they distribute the rainfall within the season and by rain rate class. Standard statistical diagnoses such as the correlation coefficient, bias, root mean square error and Nash skill score are computed and the inter-annual variability is documented. Two simplified hydrological models are used to illustrate how the nature and structure of the product error impact the model output in terms of runoff (calculated using the Soil Conservation Service method, SCS, in Niger) or outflow (calculated with the modele du Genie Rural a 4 parametres Journalier', GR4J model, in Benin). Copyright (c) 2013 Royal Meteorological Society
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Gourley, J. J., Hong, Y., Flamig, Z. L., Arthur, A., Clark, R., Calianno, M., et al. (2013). A Unified Flash Flood Database across the United States. Bulletin Of The American Meteorological Society, 94(6), 799–805.
Abstract: Despite flash flooding being one of the most deadly and costly weather-related natural hazards worldwide, individual datasets to characterize them in the United States are hampered by limited documentation and can be difficult to access. This study is the first of its kind to assemble, reprocess, describe, and disseminate a georeferenced U.S. database providing a long-term, detailed characterization of flash flooding in terms of spatiotemporal behavior and specificity of impacts. The database is composed of three primary sources: 1) the entire archive of automated discharge observations from the U.S. Geological Survey that has been reprocessed to describe individual flooding events, 2) flash-flooding reports collected by the National Weather Service from 2006 to the present, and 3) witness reports obtained directly from the public in the Severe Hazards Analysis and Verification Experiment during the summers 2008-10. Each observational data source has limitations; a major asset of the unified flash flood database is its collation of relevant information from a variety of sources that is now readily available to the community in common formats. It is anticipated that this database will be used for many diverse purposes, such as evaluating tools to predict flash flooding, characterizing seasonal and regional trends, and improving understanding of dominant flood-producing processes. We envision the initiation of this community database effort will attract and encompass future datasets.
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Grilli, R., Legrand, M., Kukui, A., Mejean, G., Preunkert, S., & Romanini, D. (2013). First investigations of IO, BrO, and NO2 summer atmospheric levels at a coastal East Antarctic site using mode-locked cavity enhanced absorption spectroscopy. Geophysical Research Letters, 40(4), 791–796.
Abstract: IO, BrO, and NO2 were measured for the first time at Dumont d'Urville (East Antarctic coast) during summer 2011/2012 by using a near-UV-Visible laser spectrometer based on mode-locked cavity-enhanced absorption spectroscopy. IO mixing ratios ranged from the 2 sigma detection limit (0.04 pptv) up to 0.15 pptv. BrO remained close or below the detection limit (2 pptv) of the instrument. Daily averaged NO2 values ranged between the detection limit (10 pptv) and 60 pptv being far higher than levels of a few pptv commonly observed in the remote marine boundary layer. Data are discussed and compared with those available for another coastal Antarctic station (Halley, West Antarctica). It is shown that the oxidative capacity of the atmospheric boundary layer at coastal Antarctic sites is quite different in nature from West to East Antarctica, with the halogen chemistry being promoted at West and the OH chemistry at East. Citation: Grilli, R., M. Legrand, A. Kukui, G. Mejean, S. Preunkert, and D. Romanini (2013), First investigations of IO, BrO, and NO2 summer atmospheric levels at a coastal East Antarctic site using mode-locked cavity enhanced absorption spectroscopy, Geophys. Res. Lett., 40, 791-796, doi: 10.1002/grl.50154.
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Guilhermet, J., Preunkert, S., Voisin, D., Baduel, C., & Legrand, M. (2013). Major 20th century changes of water-soluble humic-like substances (HULISWS) aerosol over Europe inferred from Alpine ice cores. Journal Of Geophysical Research-Atmospheres, 118(9), 3869–3878.
Abstract: Using a newly developed method dedicated to measurements of water-soluble humic-like substances (HULISWS) in atmospheric aerosol samples, the carbon mass quantification of HULISWS in an Alpine ice core is achieved for the first time. The method is based on the extraction of HULISWS with a weak anion-exchanger resin and the subsequent quantification of the extracted carbon fraction with a total organic carbon (TOC) analyzer. Measurements were performed along a Col du Dome (4250m above sea level, French Alps) ice core covering the 1920-2004 time period. The HULISWS concentrations exhibit a well-marked seasonal cycle with winter minima close to 7ppbC and summer maxima ranging between 10 and 50ppbC. Whereas the winter HULISWS concentrations remained unchanged over the twentieth century, the summer concentrations increased from 20ppbC prior to the Second World War to 35ppbC in the 1970-1990s. These different trends reflect the different types of HULISWS sources in winter and summer. HULISWS are mainly primarily emitted by domestic wood burning in winter and secondary in summer being produced from biogenic precursors. For unknown reason, the HULISWS signal is found to be unusual in ice samples corresponding to World War II.
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Guillevic, M., Bazin, L., Landais, A., Kindler, P., Orsi, A., Masson-Delmotte, V., et al. (2013). Spatial gradients of temperature, accumulation and delta O-18-ice in Greenland over a series of Dansgaard-Oeschger events. Climate Of The Past, 9(3), 1029–1051.
Abstract: Air and water stable isotope measurements from four Greenland deep ice cores (GRIP, GISP2, NGRIP and NEEM) are investigated over a series of Dansgaard Oeschger events (DO 8, 9 and 10), which are representative of glacial millennial scale variability. Combined with firn modeling, air isotope data allow us to quantify abrupt temperature increases for each drill site (1 sigma = 0.6 degrees C for NEEM, GRIP and GISP2, 1.5 degrees C for NGRIP). Our data show that the magnitude of stadial interstadial temperature increase is up to 2 degrees C larger in central and North Greenland than in northwest Greenland: i.e., for DO 8, a magnitude of +8.8 degrees C is inferred, which is significantly smaller than the +11.1 degrees C inferred at GISP2. The same spatial pattern is seen for accumulation increases. This pattern is coherent with climate simulations in response to reduced sea-ice extent in the Nordic seas. The temporal water isotope (delta O-18) temperature temperature relationship varies between 0.3 and 0.6 (+/- 0.08) %degrees C-1 and is systematically larger at NEEM, possibly due to limited changes in precipitation seasonality compared to GISP2, GRIP or NGRIP. The gas age ice age difference of warming events represented in water and air isotopes can only be modeled when assuming a 26 % (NGRIP) to 40 % (GRIP) lower accumulation than that derived from a Dansgaard Johnsen ice flow model.
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Hanna, E., Navarro, F. J., Pattyn, F., Domingues, C. M., Fettweis, X., Ivins, E. R., et al. (2013). Ice-sheet mass balance and climate change. Nature, 498(7452), 51–59.
Abstract: Since the 2007 Intergovernmental Panel on Climate Change Fourth Assessment Report, new observations of ice-sheet mass balance and improved computer simulations of ice-sheet response to continuing climate change have been published. Whereas Greenland is losing ice mass at an increasing pace, current Antarctic ice loss is likely to be less than some recently published estimates. It remains unclear whether East Antarctica has been gaining or losing ice mass over the past 20 years, and uncertainties in ice-mass change for West Antarctica and the Antarctic Peninsula remain large. We discuss the past six years of progress and examine the key problems that remain.
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Hazenberg, P., Torfs, P. J. J. F., Leijnse, H., Delrieu, G., & Uijlenhoet, R. (2013). Identification and uncertainty estimation of vertical reflectivity profiles using a Lagrangian approach to support quantitative precipitation measurements by weather radar. Journal Of Geophysical Research-Atmospheres, 118(18), 10243–10261.
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Hector, B., Seguis, L., Hinderer, J., Descloitres, M., Vouillamoz, J. M., Wubda, M., et al. (2013). Gravity effect of water storage changes in a weathered hard-rock aquifer in West Africa: results from joint absolute gravity, hydrological monitoring and geophysical prospection. Geophysical Journal International, 194(2), 737–750.
Abstract: Advances in groundwater storage monitoring are crucial for water resource management and hydrological processes understanding. The evaluation of water storage changes (WSC) often involve point measurements (observation wells, moisture probes, etc.), which may be inappropriate in heterogeneous media. Over the past few years, there has been an increasing interest in the use of gravimetry for hydrological studies. In the framework of the GHYRAF (Gravity and Hydrology in Africa) project, 3 yr of repeated absolute gravity measurements using a FG5-type gravimeter have been undertaken at Nalohou, a Sudanian site in northern Benin. Hydrological data are collected within the long-term observing system AMMA-Catch. Once corrected for solid earth tides, ocean loading, air pressure effects, polar motion contribution and non-local hydrology, seasonal gravity variations reach up to 11 μGal, equivalent to a WSC of 260-mm thick infinite layer of water. Absolute temporal gravity data are compared to WSC deduced from neutron probe and water-table variations through a direct modelling approach. First, we use neutronic measurements available for the whole vertical profile where WSC occur (the vadose zone and a shallow unconfined aquifer). The RMSD between observed and modelled gravity variations is 1.61 μGal, which falls within the error bars of the absolute gravity data. Second, to acknowledge for the spatial variability of aquifer properties, we use a 2-D model for specific yield (Sy) derived from resistivity mapping and Magnetic Resonance Soundings (MRS). The latter provides a water content (theta(MRS)) known to be higher than the specific yield. Hence, we scaled the 2-D model of theta(MRS) with a single factor (). WSC are calculated from water-table monitoring in the aquifer layer and neutronic measurements in the vadose layer. The value of is obtained with a Monte-Carlo sampling approach, minimizing the RMSD between modelled and observed gravity variations. This leads to alpha = Sy/theta(MRS) = 0.63 +/- 0.15, close to what is found in the literature on the basis of pumping tests experiments, with a RMSD value of 0.94 μGal. This hydrogeophysical experiment is a first step towards the use of time-lapse gravity data as an integrative tool to monitor interannual WSC even in complicated subsurface distribution.
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Hill-Falkenthal, J., Priyadarshi, A., Savarino, J., & Thiemens, M. (2013). Seasonal variations in S-35 and Delta O-17 of sulfate aerosols on the Antarctic plateau. Journal Of Geophysical Research-Atmospheres, 118(16), 9444–9455.
Abstract: The first reported seasonal O-17 anomaly in sulfate aerosols and measurements of radioactive (SO42-)-S-35 activities collected from Dome C, Antarctica, are reported. O-17 values exhibit minima during summer (as low as 0.91) when tropospheric oxidation patterns are dominated by OH/H2O2 mechanisms. Significant enrichment during autumn and spring is observed (up to 2.40) as ozone oxidation increases in the troposphere relative to summer and both stratospheric sources and long-range transport become more significant to the total sulfate budget. An unexpected decrease in O-17 is seen as winter progresses. This decline is concluded to potentially arise due to a reduction in vertical mixing in the troposphere or linked to variations in the long-range transport of sulfur species to Antarctica. (SO42-)-S-35 activities exhibit maxima during summer (up to 1219 atoms S-35/m(3)) that correlate with the peak in stratospheric flux and minima during winter (as low as 146 atoms S-35/m(3)) when the lack of solar radiation substantially reduces photochemical activity. It is shown that S-35 offers the potential to be used as an additional tracer to study stratospheric and tropospheric interactions and is used to estimate stratospheric input of sulfur (combination of SO2 and SO42-). Stratospheric sulfur input produces maxima during summer/autumn with an upper limit of 5.5ng/m(3) and minima during winter/spring with an upper limit of 1.1ng/m(3). From these results, it is concluded that the variation in O-17 is more reliant upon shifts in tropospheric oxidation mechanisms and long-range transport than on changes in the stratospheric flux.
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Hogarth, W. L., Lockington, D. A., Barry, D. A., Parlange, M. B., Haverkamp, R., & Parlange, J. Y. (2013). Infiltration in soils with a saturated surface. Water Resources Research, 49(5), 2683–2688.
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Hou, S., Chappellaz, J., Raynaud, D., Masson-Delmotte, V., Jouzel, J., Bousquet, P., et al. (2013). A new Himalayan ice core CH4 record: possible hints at the preindustrial latitudinal gradient. Climate Of The Past, 9(6), 2549–2554.
Abstract: Two ice cores recovered from the Himalayan East Rongbuk (ER) Glacier on the northeast saddle of Mt. Qomolangma (Everest) (28 degrees 01'N, 86 degrees 58'E, 6518m above sea level) give access to a tentative record of past Himalayan atmospheric mixing ratio of CH4 spanning the past 1200 yr. The major part of the record is affected by artifacts probably due to in situ production. After selecting what may represent the true atmospheric mixing ratio, an average of 749 +/- 25 ppbv of CH4 is estimated for the late preindustrial Holocene, which is similar to 36 +/- 17 (similar to 73 +/- 18) ppbv higher than the atmospheric levels recorded in the Greenland (Antarctic) ice cores. A comparison of these new data with model simulations of the CH4 latitudinal gradient suggests either that the models do not get a correct balance between high and low latitude CH4 sources, or that the filtered CH4 profile from the ER cores remains biased by small artifacts.
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Issoufou, H. B. A., Delzon, S., Laurent, J. P., Saadou, M., Mahamane, A., Cappelaere, B., et al. (2013). Change in water loss regulation after canopy clearcut of a dominant shrub in Sahelian agrosystems, Guiera senegalensis J. F. Gmel. Trees-Structure And Function, 27(4), 1011–1022.
Abstract: This paper analyzes the effect of the canopy age of Guiera senegalensis J.F. Gmel on water regulation processes and adaptative strategy to drought over a period of 2 years. The species is widespread in the agricultural Sahel. Before sowing, farmers cut back the shrubs to limit competition with crops. The stumps resprout after the millet harvest. Leaf water potential and stomatal conductance were measured in two fallows and in the two adjacent cultivated fields. Leaf transpiration rate and soil-to-leaf hydraulic conductance were deduced. The decrease in both stomatal and plant hydraulic conductance caused by seasonal drought was greater in mature shrubs than in current year resprouts. The decrease in predawn and midday leaf water potentials in response to seasonal drought was isohydrodynamic, and it was greater in mature shrubs, suggesting that current year resprouts are under less stress. In resprouts, the leaf transpiration rate stopped increasing beyond a hydraulic conductance threshold of 0.05 mol. m(-2) s(-1) MPa-1. Vulnerability to cavitation was determined on segments of stems in the laboratory. The leaf water potential value at which stomatal closure occurred was -2.99 +/- A 0.68 MPa, which corresponded to a 30 % loss in xylem conductivity. Thanks to its positive safety margin of 0.6 MPa, G. senegalensis can survive above this value. The observed strategy places G. senegalensis among the non-extreme xeric plants, leading us to suppose that this species will be vulnerable to the expected increase in regional drought.
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Jouanno, J., Marin, F., du Penhoat, Y., & Molines, J. M. (2013). Intraseasonal Modulation of the Surface Cooling in the Gulf of Guinea. Journal Of Physical Oceanography, 43(2), 382–401.
Abstract: A regional numerical model of the tropical Atlantic Ocean and observations are analyzed to investigate the intraseasonal fluctuations of the sea surface temperature at the equator in the Gulf of Guinea. Results indicate that the seasonal cooling in this region is significantly shaped by short-duration cooling events caused by wind-forced equatorial waves: mixed Rossby- gravity waves within the 12-20-day period band, inertia gravity waves with periods below 11 days, and equatorially trapped Kelvin waves with periods between 25 and 40 days. In these different ranges of frequencies, it is shown that the wave-induced horizontal oscillations of the northern front of the mean cold tongue dominate the variations of mixed layer temperature near the equator. But the model mixed layer heat budget also shows that the equatorial waves make a significant contribution to the mixed layer heat budget through modulation of the turbulent cooling, especially above the core of the Equatorial Undercurrent (EUC). The turbulent cooling variability is found to be mainly controlled by the intraseasonal modulation of the vertical shear in the upper ocean. This mechanism is maximum during periods of seasonal cooling, especially in boreal summer, when the surface South Equatorial Current is strongest and between 2 degrees S and the equator, where the presence of the EUC provides a background vertical shear in the upper ocean. It applies for the three types of intraseasonal waves. Inertia-gravity waves also modulate the turbulent heat flux at the equator through vertical displacement of the core of the EUC in response to equatorial divergence and convergence.
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Jourdain, N. C., Lengaigne, M., Vialard, J., Madec, G., Menkes, C. E., Vincent, E. M., et al. (2013). Observation-Based Estimates of Surface Cooling Inhibition by Heavy Rainfall under Tropical Cyclones. Journal Of Physical Oceanography, 43(1), 205–221.
Abstract: Tropical cyclones drive intense ocean vertical mixing that explains most of the surface cooling observed in their wake (the “cold wake”). In this paper, the authors investigate the influence of cyclonic rainfall on the cold wake at a global scale over the 2002-09 period. For each cyclone, the cold wake intensity and accumulated rainfall are obtained from satellite data and precyclone oceanic stratification from the Global Eddy-Permitting Ocean Reanalysis (GLORYS2). The impact of precipitation on the cold wake is estimated by assuming that cooling is entirely due to vertical mixing and that an extra amount of energy (corresponding to the energy used to mix the rain layer into the ocean) would be available for mixing the ocean column in the hypothetical case with no rain. The positive buoyancy flux of rainfall reduces the mixed layer depth after the cyclone passage, hence reducing cold water entrainment. The resulting reduction in cold wake amplitude is generally small (median of 0.07 K for a median 1 K cold wake) but not negligible (>19% for 10% of the cases). Despite similar cyclonic rainfall, the effect of rain on the cold wake is strongest in the Arabian Sea and weak in the Bay of Bengal. An analytical approach with a linearly stratified ocean allows attributing this difference to the presence of barrier layers in the Bay of Bengal. The authors also show that the cold wake is generally a “salty wake” because entrainment of subsurface saltier water overwhelms the dilution effect of rainfall. Finally, rainfall temperature has a negligible influence on the cold wake.
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Jouzel, J., Delaygue, G., Landais, A., Masson-Delmotte, V., Risi, C., & Vimeux, F. (2013). Water isotopes as tools to document oceanic sources of precipitation. Water Resources Research, 49(11), 7469–7486.
Abstract: The isotopic composition of precipitation, in deuterium, oxygen 18 and oxygen 17, depends on the climatic conditions prevailing in the oceanic regions where it originates, mainly the sea surface temperature and the relative humidity of air. This dependency applies to present-day precipitation but also to past records which are extracted, for example, from polar ice cores. In turn, coisotopic measurements of deuterium and oxygen 18 offer the possibility to retrieve information about the oceanic origin of modern precipitation as well as about past changes in sea surface temperature and relative humidity of air. This interpretation of isotopic measurements has largely relied on simple Rayleigh-type isotopic models and is complemented by Lagrangian back trajectory analysis of moisture sources. It is now complemented by isotopic General Circulation Models (IGCM) in which the origin of precipitation can be tagged. We shortly review published results documenting this link between the oceanic sources of precipitation and their isotopic composition. We then present experiments performed with two different IGCMs, the GISS model II and the LMDZ model. We focus our study on marine water vapor and its contribution to precipitation over Antarctica and over the Andean region of South America. We show how IGCM experiments allow us to relate climatic conditions prevailing in the oceanic source of precipitation to its isotopic composition. Such experiments support, at least qualitatively, the current interpretation of ice core isotopic data in terms of changes in sea surface temperature. Additionally, we discuss recent studies clearly showing the added value of oxygen 17 measurements.
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Kirchgeorg, T., Dreyer, A., Gabrieli, J., Kehrwald, N., Sigl, M., Schwikowski, M., et al. (2013). Temporal variations of perfluoroalkyl substances and polybrominated diphenyl ethers in alpine snow. Environmental Pollution, 178, 367–374.
Abstract: The occurrence and temporal variation of 18 perfluoroalkyl substances (PFASs) and 8 polybrominated diphenyl ethers (PBDEs) in the European Alps was investigated in a 10 m shallow firn core from Colle Gnifetti in the Monte Rosa Massif (4455 m above sea level). The firn core encompasses the years 1997-2007. Firn core sections were analyzed by liquid chromatography tandem mass spectrometry (PFASs) and gas chromatography mass spectrometry (PBDEs). We detected 12 PFASs and 8 PBDEs in the firn samples. Perfluorobutanoic acid (PFBA; 0.3-1.8 ng L-1) and perfluorooctanoic acid (PFOA; 0.2-0.6 ng L-1) were the major PFASs while BDE 99 (<MQL-4.5 ng L-1) and BDE 47 (n.d.-2.6 ng L-1) were the major PBDEs. This study demonstrates the occurrence of PFASs and PBDEs in the European Alps and provides the first evidence that PFASs compositions may be changing to PFBA-dominated compositions. (c) 2013 Elsevier Ltd. All rights reserved.
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Kirstetter, P. E., Andrieu, H., Boudevillain, B., & Delrieu, G. (2013). A Physically Based Identification of Vertical Profiles of Reflectivity from Volume Scan Radar Data. Journal Of Applied Meteorology And Climatology, 52(7), 1645–1663.
Abstract: The vertical profile of reflectivity (VPR) must be identified to correct estimations of rainfall rates by radar for the nonuniform beam filling associated with the vertical variation of radar reflectivity. A method for identifying VPRs from volumetric radar data is presented that takes into account the radar sampling. Physically based constraints on the vertical structure of rainfall are introduced with simple VPR models within a rainfall classification procedure defining more homogeneous precipitation patterns. The model parameters are identified in the framework of an extended Kalman filter to ensure their temporal consistency. The method is assessed using the dataset from a volume-scanning strategy for radar quantitative precipitation estimation designed in 2002 for the Bollene radar (France). The physical consistency of the retrieved VPR is evaluated. Positive results are obtained insofar as the physically based identified VPR (i) presents physically consistent shapes and characteristics considering beam effects, (ii) shows improved robustness in the difficult radar measurement context of the Cevennes-Vivarais region, and (iii) provides consistent physical insight into the rain field.
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Konrad, H., Bohleber, P., Wagenbach, D., Vincent, C., & Eisen, O. (2013). Determining the age distribution of Colle Gnifetti, Monte Rosa, Swiss Alps, by combining ice cores, ground-penetrating radar and a simple flow model. Journal Of Glaciology, 59(213), 179–189.
Abstract: Ice cores from cold Alpine glaciers may provide unique paleoclimate information from non-polar latitudes. We explore the three-dimensional internal age distribution of the small cold glacier saddle (Colle Gnifetti, Monte Rosa, Italy/Switzerland) to compare the age/depth relations from four local deep ice cores. Tracking isochronous reflection horizons detected by ground-penetrating radar (GPR) among the core locations reveals consistent dating up to 80 years BP. This approach is confined to recent ages, due to the lack of clear reflections below the firn/ice transition. We attempt to overcome this limitation by including a two-dimensional flow model adapted to the GPR-derived surface accumulation and ice thickness distribution. Modeled and GPR isochrones are compared, indicating agreement in shape but featuring a potential offset of 0-3.5 m. The modeled isochrones are interpolated to the core array with ages assigned according to the ice-core datings. The resulting age distribution is consistent up to 110 years BP, with age uncertainties increasing from 7 to >80 years in the lower half of the ice. This combination of methods is novel for Alpine sites and may be adapted for spatial extrapolation of ice properties other than age.
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Koshel, K. V., Sokolovskiy, M. A., & Verron, J. (2013). Three-vortex quasi-geostrophic dynamics in a two-layer fluid. Part2. Regular and chaotic advection around the perturbed steady states. Journal Of Fluid Mechanics, 717, 255–280.
Abstract: We study fluid-particle motion in the velocity field induced by a quasi-stationary point vortex structure consisting of one upper-layer vortex and two identical vortices in the bottom layer of a rotating two-layer fluid. The regular regimes are investigated, and the possibility of chaotic regimes (chaotic advection) under the effect of quite small non-stationary disturbances of stationary configurations has been shown. Examples of different scenarios are given for the origin and development of chaos. We analyse the role played by the stochastic layer in the processes of mixing and in the capture of fluid particles within a vortex area. We also study the influence of stratification on these effects. It is shown that regular and chaotic advection situations exhibit significant differences in the two layers.
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Kutuzov, S., Shahgedanova, M., Mikhalenko, V., Ginot, P., Lavrentiev, I., & Kemp, S. (2013). High-resolution provenance of desert dust deposited on Mt. Elbrus, Caucasus in 2009-2012 using snow pit and firn core records. Cryosphere, 7(5), 1481–1498.
Abstract: The first record of dust deposition events on Mt. Elbrus, Caucasus Mountains derived from a snow pit and a shallow firn core is presented for the 2009-2012 period. A combination of isotopic analysis, SEVIRI red-green-blue composite imagery, MODIS atmospheric optical depth fields derived using the Deep Blue algorithm, air mass trajectories derived using the HYSPLIT model and analyses of meteorological data enabled identification of dust source regions with high temporal (hours) and spatial (ca. 20-100 km) resolution. Seventeen dust deposition events were detected; fourteen occurred in March-June, one in February and two in October. Four events originated in the Sahara, predominantly in northeastern Libya and eastern Algeria. Thirteen events originated in the Middle East, in the Syrian Desert and northern Mesopotamia, from a mixture of natural and anthropogenic sources. Dust transportation from Sahara was associated with vigorous Saharan depressions, strong surface winds in the source region and mid-tropospheric southwesterly flow with daily winds speeds of 20-30 m s(-1) at 700 hPa level. Although these events were less frequent than those originating in the Middle East, they resulted in higher dust concentrations in snow. Dust transportation from the Middle East was associated with weaker depressions forming over the source region, high pressure centred over or extending towards the Caspian Sea and a weaker southerly or southeasterly flow towards the Caucasus Mountains with daily wind speeds of 12-18 m s(-1) at 700 hPa level. Higher concentrations of nitrates and ammonium characterised dust from the Middle East deposited on Mt. Elbrus in 2009 indicating contribution of anthropogenic sources. The modal values of particle size distributions ranged between 1.98 μm and 4.16 μm. Most samples were characterised by modal values of 2.0-2.8 μm with an average of 2.6 μm and there was no significant difference between dust from the Sahara and the Middle East.
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Lajevardi, S. H., Dias, D., & Racinais, J. (2013). Analysis of soil-welded steel mesh reinforcement interface interaction by pull-out tests. Geotextiles And Geomembranes, 40, 48–57.
Abstract: The interface friction coefficient is an important design parameter for reinforced soil structures where the friction between the soil and reinforcement elements is mobilized. The pull-out test is the most commonly adopted method to identify this friction coefficient. In this paper, 18 pull-out tests were conducted on two types of welded steel meshes (normal and dense mesh) embedded in a sand to investigate the soil/reinforcement interaction. The tests were conducted under vertical stresses ranging from 20 to 140 kPa on the reinforcement. The French standard (NF P 94-270, 2009) uses an analytical method to predict the friction coefficient. The comparison with the experimental results agrees reasonably well. Ju et al. (2004) developed a new test method based on a staged pull-out test for extensible reinforcements. In this study and for inextensible reinforcement, this new test method has also been used. In a usual pull-out test, only one vertical stress has been used. For a staged pull-out test, several vertical stresses are used. The results of staged pull-out tests are in good agreement with results of usual pull-out test. (C) 2013 Elsevier Ltd. All rights reserved.
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Landais, A., Dreyfus, G., Capron, E., Jouzel, J., Masson-Delmotte, V., Roche, D. M., et al. (2013). Two-phase change in CO2, Antarctic temperature and global climate during Termination II. Nature Geoscience, 6(12), 1062–1065.
Abstract: The end of the Last Glacial Maximum (Termination I), roughly 20 thousand years ago (ka), was marked by cooling in the Northern Hemisphere, a weakening of the Asian monsoon, a rise in atmospheric CO2 concentrations and warming over Antarctica. The sequence of events associated with the previous glacial-interglacial transition (Termination II), roughly 136 ka, is less well constrained. Here we present high-resolution records of atmospheric CO2 concentrations and isotopic composition of N2-an atmospheric temperature proxy-from air bubbles in the EPICA Dome C ice core that span Termination II. We find that atmospheric CO2 concentrations and Antarctic temperature started increasing in phase around 136 ka, but in a second phase of Termination II, from 130.5 to 129 ka, the rise in atmospheric CO2 concentrations lagged that of Antarctic temperature unequivocally. We suggest that during this second phase, the intensification of the low-latitude hydrological cycle resulted in the development of a CO2 sink, which counteracted the CO2 outgassing from the Southern Hemisphere oceans over this period.
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Larose, C., Dommergue, A., & Vogel, T. (2013). The Dynamic Arctic Snow Pack: An Unexplored Environment for Microbial Diversity and Activity. Biology, 2, 317–330.
Abstract: The Arctic environment is undergoing changes due to climate shifts, receiving contaminants from distant sources and experiencing increased human activity. Climate change may alter microbial functioning by increasing growth rates and substrate use due to increased temperature. This may lead to changes of process rates and shifts in the structure of microbial communities. Biodiversity may increase as the Arctic warms and population shifts occur as psychrophilic/psychrotolerant species disappear in favor of more mesophylic ones. In order to predict how ecological processes will evolve as a function of global change, it is essential to identify which populations participate in each process, how they vary physiologically, and how the relative abundance, activity and community structure will change under altered environmental conditions. This review covers aspects of the importance and implication of snowpack in microbial ecology emphasizing the diversity and activity of these critical members of cold zone ecosystems.
Keywords: Arctic; microbial ecology; biogeochemical cycling; snow
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Larose, C., Dommergue, A., & Vogel, T. M. (2013). Microbial nitrogen cycling in Arctic snowpacks. Environmental Research Letters, 8(3).
Abstract: Arctic snowpacks are often considered as chemical reactors for a variety of chemicals deposited through wet and dry events, but are overlooked as potential sites for microbial metabolism of reactive nitrogen species. The fate of deposited species is critical since warming leads to the transfer of contaminants to snowmelt-fed ecosystems. Here, we examined the role of microorganisms and the potential pathways involved in nitrogen cycling in the snow. Next generation sequencing data were used to follow functional gene abundances and a 16S rRNA (ribosomal ribonucleic acid) gene microarray was used to follow shifts in microbial community structure during a two-month spring-time field study at a high Arctic site, Svalbard, Norway (79 degrees N). We showed that despite the low temperatures and limited water supply, microbial communities inhabiting the snow cover demonstrated dynamic shifts in their functional potential to follow several different pathways of the nitrogen cycle. In addition, microbial specific phylogenetic probes tracked different nitrogen species over time. For example, probes for Roseomonas tracked nitrate concentrations closely and probes for Caulobacter tracked ammonium concentrations after a delay of one week. Nitrogen cycling was also shown to be a dominant process at the base of the snowpack.
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Larose, C., Prestat, E., Cecillon, S., Berger, S., Malandain, C., Lyon, D., et al. (2013). Interactions between Snow Chemistry, Mercury Inputs and Microbial Population Dynamics in an Arctic Snowpack. Plos One, 8(11).
Abstract: We investigated the interactions between snowpack chemistry, mercury (Hg) contamination and microbial community structure and function in Arctic snow. Snowpack chemistry (inorganic and organic ions) including mercury (Hg) speciation was studied in samples collected during a two-month field study in a high Arctic site, Svalbard, Norway (79 degrees N). Shifts in microbial community structure were determined by using a 16S rRNA gene phylogenetic microarray. We linked snowpack and meltwater chemistry to changes in microbial community structure by using co-inertia analyses (CIA) and explored changes in community function due to Hg contamination by q-PCR quantification of Hg-resistance genes in metagenomic samples. Based on the CIA, chemical and microbial data were linked (p=0.006) with bioavailable Hg (BioHg) and methylmercury (MeHg) contributing significantly to the ordination of samples. Mercury was shown to influence community function with increases in merA gene copy numbers at low BioHg levels. Our results show that snowpacks can be considered as dynamic habitats with microbial and chemical components responding rapidly to environmental changes.
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Leblois, E., & Creutin, J. D. (2013). Space-time simulation of intermittent rainfall with prescribed advection field: Adaptation of the turning band method. Water Resources Research, 49(6), 3375–3387.
Abstract: Space-time rainfall simulation is useful to study questions like, for instance, the propagation of rainfall-measurement uncertainty in hydrological modeling. This study adapts a classical Gaussian field simulation technique, the turning-band method, in order to produce sequences of rainfall fields satisfying three key features of actual precipitation systems: (i) the skewed point distribution and the space-time structure of nonzero rainfall (NZR); (ii) the average probability and the space-time structure of intermittency; and (iii) a prescribed advection field. The acronym of our simulator is SAMPO, for simulation of advected mesoscale precipitations and their occurrence. SAMPO assembles various theoretical developments available from the literature. The concept of backtrajectories introduces a priori any type of advection field in the heart of the turning band method (TBM). TBM outputs transformation into rainfall fields with a desired structure is controlled using Chebyshev-Hermite polynomial expansion. The intermittency taken as a binary process statistically independent of the NZR process allows the use of a common algorithm for both processes. The 3-D simulation with a space-time anisotropy captures important details of the precipitation kinematics summarized by the Taylor velocity of both NZR and intermittency. A case study based on high-resolution weather radar data serves as an example of model inference. Illustrative simulations revisit some classical questions about rainfall variography like the influence of advection or intermittency. They also show the combined role of Taylor's and advection velocities.
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Legout, C., Poulenard, J., Nemery, J., Navratil, O., Grangeon, T., Evrard, O., et al. (2013). Quantifying suspended sediment sources during runoff events in headwater catchments using spectrocolorimetry. Journal Of Soils And Sediments, 13(8), 1478–1492.
Abstract: Understanding hydro-sedimentary dynamics at the catchment scale requires high temporal resolution data on suspended sediments such as their origin, in addition to the common measurements of sediment concentrations and discharges. Some rapid and low-cost fingerprinting methods based on spectroscopy have recently been developed. We investigated how visible spectra could be used to predict the proportion of various source materials in suspended sediment samples, paying particular attention to the potential alteration of spectrocolorimetric signatures between soils and suspended sediments during transport. The 22-km(2) Galabre catchment, France, is composed of black marls, limestones, molasses, undifferentiated deposits and gypsum. Forty-eight source materials were sampled and 328 suspended sediment samples were collected at the outlet during 23 runoff events. Measurements were taken with a diffuse reflectance spectrophotometer on dried samples. As the erosion processes are particle size selective, five particle size fractions of source material were measured in order to assess the potential alteration of the fingerprint signatures. As the biogeochemical processes occurring in the river could also affect the signatures, source materials were immersed in the river for durations ranging from 1 to 63 days and subsequently measured. Finally, partial least-squares regression models were constructed on 81 artificial laboratory mixtures to predict the proportions of source materials. The spectrocolorimetric measurements discriminated the primary source materials but not the Quaternary deposits. As the gypsum was not conservative, only the black marls, molasses and limestones were used in the fingerprinting procedure. The construction of the partial least-squares regression models led to a median absolute error of 1.1%. This error increased to 3.9% when the models were applied to source samples with: (1) different particle sizes; (2) different durations spent in the river; or (3) different origins than those used for their construction. The effect of particle size on the fingerprinting procedure was larger than the effect of biogeochemical reactions or the spatial variability of the spectrocolorimetric signatures. Half of the 23 runoff events analysed exhibited huge variations in the source proportions from one sediment sample to another. The spectrocolorimetric fingerprinting approach was able to quantify routinely the proportion of primary source materials in all suspended sediment samples collected during runoff events. The high temporal resolution of the predicted proportions revealed that only analysing three or four suspended sediment samples during a runoff event could lead to a misunderstanding of the hydro-sedimentary processes for more than half of the investigated runoff events.
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Legrand, M., Preunkert, S., Jourdain, B., Guilhermet, J., Fain, X., Alekhina, I., et al. (2013). Water-soluble organic carbon in snow and ice deposited at Alpine, Greenland, and Antarctic sites: a critical review of available data and their atmospheric relevance. Climate Of The Past, 9(5), 2195–2211.
Abstract: While it is now recognized that organic matter dominates the present-day atmospheric aerosol load over continents, its sources remain poorly known. The studies of organic species or organic fractions trapped in ice cores may help to overcome this lack of knowledge. Available data on the dissolved (or total) organic carbon (DOC or TOC) content of snow and ice often appear largely inconsistent, and, until now, no critical review has been conducted to understand the causes of these inconsistencies. To draw a more consistent picture of the organic carbon amount present in solid precipitation that accumulates on cold glaciers, we here review available data and, when needed, complete the data set with analyses of selected samples. The different data sets are then discussed by considering the age (modern versus pre-industrial, Holocene versus Last glacial Maximum) and type (surface snow, firn, or ice) of investigated samples, the deployed method, and the applied contamination control. Finally, the OC (DOC or TOC) levels of Antarctic, Greenland, and Alpine ice cores are compared and discussed with respect to natural (biomass burning, vegetation emissions) and anthropogenic sources (fossil fuel combustion) contributing to atmospheric OC aerosol.
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Legrand, M., Preunkert, S., May, B., Guilhermet, J., Hoffman, H., & Wagenbach, D. (2013). Major 20th century changes of the content and chemical speciation of organic carbon archived in Alpine ice cores: Implications for the long-term change of organic aerosol over Europe. Journal Of Geophysical Research-Atmospheres, 118(9), 3879–3890.
Abstract: Dissolved organic carbon (DOC) and an extended array of organic compounds were investigated in an Alpine ice core covering the 1920-1988 time period. Based on this, a reconstruction was made of the long-term trends of water-soluble organic carbon (WSOC) aerosol in the European atmosphere. It is shown that light mono- and dicarboxylates, humic-like substances, and formaldehyde account together for more than half of the DOC content of ice. This extended chemical speciation of DOC is used to estimate the DOC fraction present in ice that is related to WSOC aerosol and its change over the past. It is suggested that after World War II, the WSOC levels have been enhanced by a factor of 2 and 3 in winter and summer, respectively. In summer, the fossil fuel contribution to the enhancement is estimated to be rather small, suggesting that it arises mainly from an increase in biogenic sources of WSOC.
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Legros, S., Doelsch, E., Feder, F., Moussard, G., Sansoulet, J., Gaudet, J. P., et al. (2013). Fate and behaviour of Cu and Zn from pig slurry spreading in a tropical water-soil-plant system. Agriculture Ecosystems & Environment, 164, 70–79.
Abstract: Pig slurry is commonly spread on crop fields as a means of managing this agricultural waste product. However, this practice has an impact on the environment, e.g. increasing soil copper (Cu) and zinc (Zn) concentrations. Many studies have assessed the fate of these elements, but some questions remain, especially with respect to tropical agrosystems which have yet to be studied in depth. The aim of this study was to determine the fate of Cu and Zn from pig slurry spreading while also focusing on describing the dynamics of these elements in a tropical system and accounting for the three compartments of the water-soil-plant system. We observed that all of the Zn accumulated within the 20-60 cm soil layer. Although the uncertainty calculated for these results was high, these findings were confirmed by the absence of uptake by the plant cover and of leaching via water flows. This pattern for Zn in a tropical setting differed from findings generally reported in temperate areas. The Zn accumulation mechanism in tropical soil seems to be a reversible sorption phenomenon, suggesting the possibility of long-term Zn leaching. The Cu mass derived from pig slurry spreading was stored in the 0-20 cm soil layer. This result obtained in a tropical environment was similar to that noted in temperate areas. This could be explained by Cu speciation in the pig slurry (insoluble copper sulfide), and was therefore relatively independent of the soil-climate system. (C) 2012 Elsevier B.V. All rights reserved.
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Lejeune, Y., Bertrand, J. M., Wagnon, P., & Morin, S. (2013). A physically based model of the year-round surface energy and mass balance of debris-covered glaciers. Journal Of Glaciology, 59(214), 327–344.
Abstract: Debris-covered glaciers respond to atmospheric conditions in different ways from debris-free glaciers, due to the presence of debris at the surface during the ablation season and at the snow/ice interface during the accumulation season. Understanding the response of debris-covered glaciers to a variety of meteorological conditions in a physically sound manner is essential to quantify meltwater discharge and to predict their response to climate change. To tackle this issue, we developed the Crocus-DEB model as an adaptation of the detailed snowpack model Crocus, to simulate the energy and mass balance of debris-covered glaciers, including periods when debris is covered by snow. Crocus-DEB was evaluated with data gathered during a field experiment using artificial debris covering the snowpack at Col de Porte, France, with very good results in terms of conductive heat flux, both at the surface and at the interface between the debris and the underlying dense snow taken as a surrogate for ice, with and without snow overlying the debris. The model was also evaluated using field data from the debris-covered glacier Changri Nup, Nepal, Himalaya. This paper introduces the design of the model, its performance and its ability to explore relationships between model parameters, meteorological conditions and the critical debris thickness.
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Libois, Q., Picard, G., France, J. L., Arnaud, L., Dumont, M., Carmagnola, C. M., et al. (2013). Influence of grain shape on light penetration in snow. Cryosphere, 7(6), 1803–1818.
Abstract: The energy budget and the photochemistry of a snowpack depend greatly on the penetration of solar radiation in snow. Below the snow surface, spectral irradiance decreases exponentially with depth with a decay constant called the asymptotic flux extinction coefficient. As with the albedo of the snowpack, the asymptotic flux extinction coefficient depends on snow grain shape. While representing snow by a collection of spherical particles has been successful in the numerical computation of albedo, such a description poorly explains the decrease of irradiance in snow with depth. Here we explore the limits of the spherical representation. Under the assumption of geometric optics and weak absorption by snow, the grain shape can be simply described by two parameters: the absorption enhancement parameter B and the geometric asymmetry factor g(G). Theoretical calculations show that the albedo depends on the ratio B/(1-g(G)) and the asymptotic flux extinction coefficient depends on the product B(1-g(G)). To understand the influence of grain shape, the values of B and g(G) are calculated for a variety of simple geometric shapes using ray tracing simulations. The results show that B and (1-g(G)) generally co-vary so that the asymptotic flux extinction coefficient exhibits larger sensitivity to the grain shape than albedo. In particular it is found that spherical grains propagate light deeper than any other investigated shape. In a second step, we developed a method to estimate B from optical measurements in snow. A multi-layer, two-stream, radiative transfer model, with explicit grain shape dependence, is used to retrieve values of the B parameter of snow by comparing the model to joint measurements of reflectance and irradiance profiles. Such measurements were performed in Antarctica and in the Alps yielding estimates of B between 0.8 and 2.0. In addition, values of B were estimated from various measurements found in the literature, leading to a wider range of values (1.0-9.9) which may be partially explained by the limited accuracy of the data. This work highlights the large variety of snow microstructure and experimentally demonstrates that spherical grains, with B = 1.25, are inappropriate to model irradiance profiles in snow, an important result that should be considered in further studies dedicated to subsurface absorption of short-wave radiation and snow photochemistry.
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Lucia, A., Recking, A., Martin-Duque, J. F., Storz-Peretz, Y., & Laronne, J. B. (2013). Continuous monitoring of bedload discharge in a small, steep sandy channel. Journal Of Hydrology, 497, 37–50.
Abstract: This paper reports on bedload flux and texture monitored in a natural, steep, sandy ephemeral channel draining a small gullied sandy watershed, the Barranca de los Pinos (1.32 ha), Spain. Bedload flux was continuously monitored with two independent Reid-type slot samplers; bedload texture was determined from the sediment collected in the samplers. Channel morphology was surveyed with a high spatial resolution with a Terrestrial Laser Scanner. The monitored instantaneous bedload fluxes are among the highest measured in natural rivers, characterized by high temporal and spatial variability related to the presence of bedforms, shallow bars and sand sheets, and to the reworking of the dry bed between and at the end of individual flow events. The grain size distribution of the bedload indicates equal mobility; but bedload texture fluctuates, depicting the transport of coarser bar surfaces and of finer-grained anabranch surfaces as well as of the overall bed subsurface. (C) 2013 Elsevier B.V. All rights reserved.
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Mahe, G., Lienou, G., Descroix, L., Bamba, F., Paturel, J. E., Laraque, A., et al. (2013). The rivers of Africa: witness of climate change and human impact on the environment. Hydrological Processes, 27(15), 2105–2114.
Abstract: In this paper, we study the impact of climate change on river regimes in several parts of Africa, and we look at the most probable causes of these changes either climatically or anthropogenically driven. We study time series of updated monthly and annual runoff of rivers of North Africa, West Africa (Sahelian and humid tropical regions) and Central Africa, including the largest river basins: Niger and Volta rivers in West Africa, and Congo and Ogooue rivers in Central Africa. The recent years are studied in the perspective of multi-decadal variability. In West Africa and in a part of Central Africa, the climate has changed since 1970, and rainfall has not returned to previous annual amounts, except in Equatorial Africa. The consequences of the long-lasting drought are, depending on the area concerned, the modification of seasonal regimes (Equatorial area), the groundwater table decrease (Tropical humid area) and the land cover degradation (Sahelian area). The increasing number of dams and of agricultural areas also plays a major role on the modification of river regimes. The population increase will continue to impact on the environment: land cover change, deforestation, agriculture and increasing number of dams will be associated with a reduction of water and sediment discharges to the sea, and major impacts on downstream ecosystems and coastal areas. It seems necessary to share with stakeholders a comprehensive approach of the water cycle from the basin to the sea, to prevent long-lasting damages to ecosystems and infrastructures. Copyright (c) 2013 John Wiley & Sons, Ltd.
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Mailhot, A., Lachance-Cloutier, S., Talbot, G., & Favre, A. C. (2013). Regional estimates of intense rainfall based on the Peak-Over-Threshold (POT) approach. Journal Of Hydrology, 476, 188–199.
Abstract: The Peak-Over-Threshold (POT) approach is an interesting alternative to the one based on Annual Maxima (AM) series since it gives the opportunity to take into consideration extreme events that would not be considered otherwise. It has also been recognized that the regional approach improves statistical inference when compared to the local approach, assuming that the region is statistically homogeneous. A regional POT approach was developed and applied to the network stations located in southern Quebec. POT series for 5-, 10-, 15-, 30-min and 1-, 2-, 6- and 12-h durations were constructed assuming a fixed exceedance rate. An analysis of local POT series showed that the intra-annual variability of the Generalized Pareto Distribution (GPD) parameters needs to be taken into consideration. Models of various complexities were defined combining local and regional representations as well as the intra-annual variability of GPD parameters. Regional likelihood was estimated and models were compared based on the Akaike Information Criterion (AIC). Models with regional shape and scale parameters and accounting for intra-annual variability were selected for all durations. Spatial covariates were also introduced through a simple model linking GPD parameters to latitude, longitude and altitude. The sensitivity of results to threshold values and selected models was also investigated. Interpolated maps of intense rainfall over the studied area are finally proposed. (C) 2012 Elsevier B.V. All rights reserved.
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Marinoni, A., Cristofanelli, P., Laj, P., Duchi, R., Putero, D., Calzolari, F., et al. (2013). High black carbon and ozone concentrations during pollution transport in the Himalayas: Five years of continuous observations at NCO-P global GAW station. Journal Of Environmental Sciences-China, 25(8), 1618–1625.
Abstract: To study the influence of polluted air-mass transport carrying ozone (O-3) and black carbon (BC) in the high Himalayas, since March 2006 the Nepal Climate Observatory at Pyramid (NCO-P) GAW-WMO global station (Nepal, 5079 m a.s.l.) is operative. During the first 5-year measurements, the O-3 and BC concentrations have shown a mean value of 48 +/- 12 ppb (+/- standard deviation) and 208 +/- 374 ng/m(3), respectively. Both O-3 and BC showed well defined seasonal cycles with maxima during pre-monsoon (O-3: 61.3 +/- 7.7 ppbV; BC: 444 +/- 433 ng/m(3)) and minima during the summer monsoon (O-3: 40.1 +/- 12.4 ppbV; BC: 64 +/- 101 ng/m(3)). The analysis of the days characterised by the presence of a significant BC increase with respect to the typical seasonal cycle identified 156 days affected by “acute” pollution events, corresponding to 9.1% of the entire data-set. Such events mostly occur in the pre-monsoon period, when the O-3 diurnal variability is strongly related to the transport of polluted air-mass rich on BC. On average, these “acute” pollution events were characterised by dramatic increases of BC (352%) and O-3 (29%) levels compared with the remaining days.
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Martins, J. M. F., Majdalani, S., Vitorge, E., Desaunay, A., Navel, A., Guine, V., et al. (2013). Role of macropore flow in the transport of Escherichia coli cells in undisturbed cores of a brown leached soil. Environmental Science-Processes & Impacts, 15(2), 347–356.
Abstract: The objective of this work was to evaluate the transport of Escherichia coli cells in undisturbed cores of a brown leached soil collected at La Cote St Andre (France). Two undisturbed soil cores subjected to repeated injections of bacterial cells and/or bromide tracer were used to investigate the effect of soil hydrodynamics and ionic strength on cell mobility. Under the tested experimental conditions, E. coli cells were shown to be transported at the water velocity (retardation factor close to 1) and their retention appeared almost insensitive to water flow and ionic strength variations, both factors being known to control bacterial transport in model saturated porous media. In contrast, E. coli breakthrough curves evolved significantly along with the repetition of the cell injections in each soil core, with a progressive acceleration of their transport. The evolution of E. coli cells BTCs was shown to be due to the evolution of the structure of soil hydraulic pathways caused by the repeated water infiltrations and drainage as may occur in the field. This evolution was demonstrated through mercury intrusion porosimetry (MIP) performed on soil aggregates before and after the repeated infiltrations of bacteria. MIP revealed a progressive and important reduction of the soil aggregate porosity, n, that decreased from approximately 0.5 to 0.3, along with a decrease of the soil percolating step from 27 to 2 μm. From this result a clear compaction of soil aggregates was evidenced that concerned preferentially the pores larger than 2 μm equivalent diameter, i.e. those allowing bacterial cell passage. Since no significant reduction of the global soil volume was observed at the core scale, this aggregate compaction was accompanied by macropore formation that became progressively the preferential hydraulic pathway in the soil cores, leading to transiently bi-modal bacterial BTCs. The evolution of the soil pore structure induced a modification of the main hydrodynamic processes, evolving from a matrix-dominant transfer of water and bacteria to a macropore-dominant transfer. This work points out the importance of using undisturbed natural soils to evaluate the mobility of bacteria in the field, since the evolving hydrodynamic properties of soils appeared to dominate most physicochemical factors.
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Marty, R., Zin, I., & Obled, C. (2013). Sensitivity of hydrological ensemble forecasts to different sources and temporal resolutions of probabilistic quantitative precipitation forecasts: flash flood case studies in the Cevennes-Vivarais region (Southern France). Hydrological Processes, 27(1), 33–44.
Abstract: This article analyses the performance of an integrated hydrological ensemble prediction system adapted to small to mid-sized catchments (100-600 km(2)) situated in the Cevennes-Vivarais region (Southern France) and characterized by short lag times (3-12 h). In this framework, flood forecasts need hourly Probabilistic Quantitative Precipitation Forecasts (PQPF) so as to provide early warning with 24-72 h of anticipation. Here, two sources of PQPF at daily and subdaily (6 h) meteorological time steps are considered: Ensemble Prediction Systems from the European Centre for Medium-range Weather Forecast (ECMWF) and analogy-based PQPF provided in real-time at the Laboratoire d'etude des Transferts en Hydrologie et Environnement. The two PQPF are firstly disaggregated to respect the required hydrological hourly time step, through either the use of a stochastic rainfall generator or the application of a multimodel approach. Then, disaggregated PQPF are used as input to a hydrological model, which is called TOPSIMPL, to provide hourly ensemble discharge forecasts up to 48 h ahead. Illustration and evaluation of ensemble discharge forecasts issued in near real-time conditions are given for some recently observed flash flood events. It is shown that hourly discharge forecasts are first-order conditioned by the accuracy of PQPF at daily or subdaily time step. Six-hour ensemble prediction systems correctly reproduce the rainfall temporal dynamics, whereas daily analogy-based PQPF are less underdispersive in terms of rainfall amounts. As a result, the merging of the two sources of PQPF substantially increases the performance of discharge forecasts, the contribution of a more sophisticated hourly rainfall generator becoming marginal. Copyright (C) 2012 John Wiley & Sons, Ltd.
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Mary, A., Dumont, M., Dedieu, J. P., Durand, Y., Sirguey, P., Milhem, H., et al. (2013). Intercomparison of retrieval algorithms for the specific surface area of snow from near-infrared satellite data in mountainous terrain, and comparison with the output of a semi-distributed snowpack model. Cryosphere, 7(2), 741–761.
Abstract: This study compares different methods to retrieve the specific surface area (SSA) of snow from satellite radiance measurements in mountainous terrain. It aims at addressing the effect on the retrieval of topographic corrections of reflectance, namely slope and aspect of terrain, multiple reflections on neighbouring slopes and accounting (or not) for the anisotropy of snow reflectance. Using MODerate resolution Imaging Spectrometer (MODIS) data for six different clear sky scenes spanning a wide range of snow conditions during the winter season 2008-2009 over a domain of 46 x 50 km in the French Alps, we compared SSA retrievals with and without topographic correction, with a spherical or non-spherical snow reflectance model and, in spherical case, with or without anisotropy corrections. The retrieved SSA values were compared to field measurements and to the results of the detailed snowpack model Crocus, fed by driving data from the SAFRAN meteorological analysis. It was found that the difference in terms of surface SSA between retrieved values and SAFRAN-Crocus output was minimal when the topographic correction was taken into account, when using a retrieval method assuming disconnected spherical snow grains. In this case, the root mean square deviation was 9.4 m(2) kg(-1) and the mean difference was 0.1 m(2) kg(-1), based on 3170 pairs of observation and simulated values. The added-value of the anisotropy correction was not significant in our case, which may be explained by the presence of mixed pixels and surface roughness. MODIS retrieved data show SSA variations with elevation and aspect which are physically consistent and in good agreement with SAFRAN-Crocus outputs. The variability of the MODIS retrieved SSA within the topographic classes of the model was found to be relatively small (3.9 m(2) kg(-1)). This indicates that semi-distributed snowpack simulations in mountainous terrain with a sufficiently large number of classes provides a representation of the snowpack variability consistent with the scale of MODIS 500 m pixels.
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May, B., Wagenbach, D., Hoffmann, H., Legrand, M., Preunkert, S., & Steier, P. (2013). Constraints on the major sources of dissolved organic carbon in Alpine ice cores from radiocarbon analysis over the bomb-peak period. Journal Of Geophysical Research-Atmospheres, 118(8), 3319–3327.
Abstract: Radiocarbon (C-14) has proven to be a powerful tool in distinguishing modern and fossil fuel sources contributing to organic aerosols. By applying this concept to ice core records of the dissolved organic carbon (DOC) fraction, we developed a setup dedicated to the extraction of DOC from Alpine ice core samples for C-14 microanalysis. With respect to the difficulties and limitations of this analytical method, it is shown that a total process blank mass of (6 +/- 3) μgC with a C-14 signature of (0.71 +/- 0.17) can be obtained, corresponding to a minimum sample size between 200 g for industrial and 800 g for pre-industrial ice. Radiocarbon analyses of eight DOC ice core samples from the high accumulation glacier Col du Dome (European Alps) were mainly performed over the bomb-peak period. These data, being associated with snow deposition over the summer half-years, show an overall mean fossil contribution of (25 +/- 9) %. Adaptation of the (DOC)-C-14 values to the atmospheric (CO2)-C-14 record revealed that the biogenic input to ice core DOC is associated with a fast recycling biospheric component, likely linked to a turnover time of less than 3 years.
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Mazzilli, N., Jourde, H., Jacob, T., Guinot, V., Le Moigne, N., Boucher, M., et al. (2013). On the inclusion of ground-based gravity measurements to the calibration process of a global rainfall-discharge reservoir model: case of the Durzon karst system (Larzac, southern France). Environmental Earth Sciences, 68(6), 1631–1646.
Abstract: This work examines the relevance of the inclusion of ground-based gravity data in the calibration process of a global rainfall-discharge reservoir model. The analysis is performed for the Durzon karst system (Larzac, France). The first part of the study focuses on the hydrological interpretation of the ground-based gravity measurements. The second part of the study investigates further the information content of the gravity data with respect to water storage dynamics modelling. The gravity-derived information is found unable to either reduce equifinality of the single-objective, discharge-based model calibration process or enhance model performance through assimilation.
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Mbaye, S., Sechet, P., Pignon, F., & Martins, J. M. F. (2013). Influence of hydrodynamics on the growth kinetics of glass-adhering Pseudomonas putida cells through a parallel plate flow chamber. Biomicrofluidics, 7(5).
Abstract: The objective of this work was to investigate the influence of hydrodynamics on the growth kinetics of surface-adhering Pseudomonas putida cells. The results showed in particular that under non substrate-limiting conditions, the early step of bacterial apparent growth rate is lower than those measured with suspended cells. Contrary to previously cited authors which explain this behavior to the different adhesive properties of the “daughter”-cells (which makes more probable the detachment of these daughter-cells), in our experimental conditions, that explanation does not hold and we show a clear dependence of growth kinetics with flow conditions, due to the formation of boundary layer concentration at low Reynolds number. These results revealed that using Monod law in the modeling of biofilm growth in fixed-biomass processes should be performed with care. (C) 2013 AIP Publishing LLC.
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Meinvielle, M., Brankart, J. M., Brasseur, P., Barnier, B., Dussin, R., & Verron, J. (2013). Optimal adjustment of the atmospheric forcing parameters of ocean models using sea surface temperature data assimilation. Ocean Science, 9(5), 867–883.
Abstract: In ocean general circulation models, near-surface atmospheric variables used to specify the atmospheric boundary condition remain one of the main sources of error. The objective of this research is to constrain the surface forcing function of an ocean model by sea surface temperature (SST) data assimilation. For that purpose, a set of corrections for ERAinterim (hereafter ERAi) reanalysis data is estimated for the period of 1989-2007, using a sequential assimilation method, with ensemble experiments to evaluate the impact of uncertain atmospheric forcing on the ocean state. The control vector of the assimilation method is extended to atmospheric variables to obtain monthly mean parameter corrections by assimilating monthly SST and sea surface salinity (SSS) climatological data in a low resolution global configuration of the NEMO model. In this context, the careful determination of the prior probability distribution of the parameters is an important matter. This paper demonstrates the importance of isolating the impact of forcing errors in the model to perform relevant ensemble experiments. The results obtained for every month of the period between 1989 and 2007 show that the estimated parameters produce the same kind of impact on the SST as the analysis itself. The objective is then to evaluate the long-term time series of the forcing parameters focusing on trends and mean error corrections of air-sea fluxes. Our corrections tend to equilibrate the net heat-flux balance at the global scale (highly positive in ERAi database), and to remove the potentially unrealistic negative trend (leading to ocean cooling) in the ERAi net heat flux over the whole time period. More specifically in the intertropical band, we reduce the warm bias of ERAi data by mostly modifying the latent heat flux by wind speed intensification. Consistently, when used to force the model, the corrected parameters lead to a better agreement between the mean SST produced by the model and mean SST observations over the period of 1989-2007 in the intertropical band.
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Melet, A., Gourdeau, L., Verron, J., & Djath, B. (2013). Solomon Sea circulation and water mass modifications: response at ENSO timescales. Ocean Dynamics, 63(1), 1–19.
Abstract: The South Pacific low latitude western boundary currents (LLWBCs) carry waters of subtropical origin through the Solomon Sea before joining the equatorial Pacific. Changes in their properties or transport are assumed to impact El Nino Southern Oscillation (ENSO) dynamics. At ENSO timescales, the LLWBCs transport tends to counterbalance the interior geostrophic one. When transiting through the complex geography of the Solomon Sea, the main LLWBC, the New Guinea Coastal Undercurrent, cannot follow a unique simple route to the equator. Instead, its routes and water mass properties are influenced by the circulation occurring in the Solomon Sea. In this study, the response of the Solomon Sea circulation to ENSO is investigated based on a numerical simulation. The transport anomalies entering the Solomon Sea from the south are confined to the top 250 m of the water column, where they represent 7.5 Sv (based on ENSO composites) for a mean transport of 10 Sv. The induced circulation anomalies in the Solomon Sea are not symmetric between the two ENSO states because of (1) a bathymetric control at Vitiaz Strait, which plays a stronger role during El Nino, and (2) an additional inflow through Solomon Strait during La Nina events. In terms of temperature and salinity, modifications are particularly notable for the thermocline water during El Nino conditions, with cooler and fresher waters compared to the climatological mean. The surface water at Vitiaz Strait and the upper thermocline water at Solomon Strait, feeding respectively the equatorial Pacific warm pool and the Equatorial Undercurrent, particularly affect the heat and salt fluxes. These fluxes can change by up to a factor of 2 between extreme El Nino and La Nina conditions.
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Menegoz, M., Gallee, H., & Jacobi, H. W. (2013). Precipitation and snow cover in the Himalaya: from reanalysis to regional climate simulations. Hydrology And Earth System Sciences, 17(10), 3921–3936.
Abstract: We applied a Regional Climate Model (RCM) to simulate precipitation and snow cover over the Himalaya, between March 2000 and December 2002. Due to its higher resolution, our model simulates a more realistic spatial variability of wind and precipitation than those of the reanalysis of the European Centre of Medium range Weather Forecast (ECMWF) used as lateral boundaries. In this region, we found very large discrepancies between the estimations of precipitation provided by reanalysis, rain gauges networks, satellite observations, and our RCM simulation. Our model clearly underestimates precipitation at the foothills of the Himalaya and in its eastern part. However, our simulation provides a first estimation of liquid and solid precipitation in high altitude areas, where satellite and rain gauge networks are not very reliable. During the two years of simulation, our model resembles the snow cover extent and duration quite accurately in these areas. Both snow accumulation and snow cover duration differ widely along the Himalaya: snowfall can occur during the whole year in western Himalaya, due to both summer monsoon and mid-latitude low pressure systems bringing moisture into this region. In Central Himalaya and on the Tibetan Plateau, a much more marked dry season occurs from October to March. Snow cover does not have a pronounced seasonal cycle in these regions, since it depends both on the quite variable duration of the monsoon and on the rare but possible occurrence of snowfall during the extramonsoon period.
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Menegoz, M., Krinner, G., Balkanski, Y., Cozic, A., Boucher, O., & Ciais, P. (2013). Boreal and temperate snow cover variations induced by black carbon emissions in the middle of the 21st century. Cryosphere, 7(2), 537–554.
Abstract: We used a coupled climate-chemistry model to quantify the impacts of aerosols on snow cover north of 30 degrees N both for the present-day and for the middle of the 21st century. Black carbon (BC) deposition over continents induces a reduction in the mean number of days with snow at the surface (MNDWS) that ranges from 0 to 10 days over large areas of Eurasia and Northern America for the present-day relative to the pre-industrial period. This is mainly due to BC deposition during the spring, a period of the year when the remaining of snow accumulated during the winter is exposed to both strong solar radiation and a large amount of aerosol deposition induced themselves by a high level of transport of particles from polluted areas. North of 30 degrees N, this deposition flux represents 222 Gg BC month(-1) on average from April to June in our simulation. A large reduction in BC emissions is expected in the future in all of the Representative Concentration Pathway (RCP) scenarios. In particular, considering the RCP8.5 in our simulation leads to a decrease in the spring BC deposition down to 110 Gg month-1 in the 2050s. However, despite the reduction of the aerosol impact on snow, the MNDWS is strongly reduced by 2050, with a decrease ranging from 10 to 100 days from present-day values over large parts of the Northern Hemisphere. This reduction is essentially due to temperature increase, which is quite strong in the RCP8.5 scenario in the absence of climate mitigation policies. Moreover, the projected sea-ice retreat in the next decades will open new routes for shipping in the Arctic. However, a large increase in shipping emissions in the Arctic by the mid-21st century does not lead to significant changes of BC deposition over snow-covered areas in our simulation. Therefore, the MNDWS is clearly not affected through snow darkening effects associated with these Arctic ship emissions. In an experiment without nudging toward atmospheric reanalyses, we simulated however some changes of the MNDWS considering such aerosol ship emissions. These changes are generally not statistically significant in boreal continents, except in Quebec and in the West Siberian plains, where they range between -5 and -10 days. They are induced both by radiative forcings of the aerosols when they are in the snow and in the atmosphere, and by all the atmospheric feedbacks. These experiments do not take into account the feedbacks induced by the interactions between ocean and atmosphere as they were conducted with prescribed sea surface temperatures. Climate change by the mid-21st century could also cause biomass burning activity (forest fires) to become more intense and occur earlier in the season. In an idealised scenario in which forest fires are 50% stronger and occur 2 weeks earlier and later than at present, we simulated an increase in spring BC deposition of 21 Gg BC month(-1) over continents located north of 30 degrees N. This BC deposition does not impact directly the snow cover through snow darkening effects. However, in an experiment considering all the aerosol forcings and atmospheric feedbacks, except those induced by the ocean-atmosphere interactions, enhanced fire activity induces a significant decrease of the MNDWS reaching a dozen of days in Quebec and in Eastern Siberia.
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Mollon, G., Dias, D., & Soubra, A. H. (2013). Probabilistic analyses of tunneling-induced ground movements. Acta Geotechnica, 8(2), 181–199.
Abstract: Tunneling-induced ground movements are investigated in this paper using both deterministic and probabilistic analyses. The deterministic model is based on three-dimensional (3D) numerical simulations using the commercial code FLAC(3D). This model attempts to reproduce some major phenomena during a typical slurry-shield tunnel excavation (ground movements due to the applied face pressure, the overcutting, the shield conicity, the annular void behind the shield, and the grout injection in this void). Moreover, the model provides useful information about the nature and magnitude of the soil movements at the ground surface. A probabilistic study is then undertaken in order to evaluate the impact of the variability of several input variables on the ground movements. An efficient probabilistic method called CSRSM is used to assess this uncertainty propagation. In a last section, the output variables of the model are linked to failure criteria. This allows one to determine probabilities of failure, depending on the probabilistic properties of the input variables and on the admissible threshold of each criterion.
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Mollon, G., Dias, D., & Soubra, A. H. (2013). Range of the Safe Retaining Pressures of a Pressurized Tunnel Face by a Probabilistic Approach. Journal Of Geotechnical And Geoenvironmental Engineering, 139(11), 1954–1967.
Abstract: A probabilistic analysis of the face stability of a pressurized tunnel is undertaken in this article. First, two existing velocity fields based on the limit analysis theory are briefly described. They allow one to compute the values of the critical pressures of collapse and blowout of a pressurized tunnel face in cases of both frictional and nonfrictional soils. These models, which have the great advantage of a low computational cost, are validated by comparison with a computationally expensive numerical model. Then, an efficient probabilistic method called collocation-based stochastic response surface methodology (CSRSM) is applied on these velocity fields to perform the uncertainty propagation. This method makes it possible to compute the probability of failure of the tunnel face against both collapse and blowout. In the case of a frictional soil, it appears that the blowout of the face is extremely unlikely and that the collapse is the only probable failure mode. On the contrary, in a purely cohesive soil, it appears that both failure modes are likely to appear and should be considered in the analysis. Finally, this paper presents a discussion concerning the application of the proposed probabilistic method for an economic and safe design of a pressurized shield. (C) 2013 American Society of Civil Engineers.
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Moreira, N., Miranda, T., Pinheiro, M., Fernandes, P., Dias, D., Costa, L., et al. (2013). Back analysis of geomechanical parameters in underground works using an Evolution Strategy algorithm. Tunnelling And Underground Space Technology, 33, 143–158.
Abstract: The purpose of this work is to evaluate the performance of an optimization algorithm from the field of evolutionary computation, namely an Evolution Strategy, in back analysis of geomechanical parameters in underground structures. This analysis was carried out through a parametric study of a synthetic case of a tunnel construction. Different combinations of parameters and measurements were carried out to test the performance of the algorithm. In order to have a comparison base for its performance also three classical optimization algorithms based on the gradient of the error function and a Genetic Algorithm were used. It was concluded that the Evolution Strategy algorithm presents interesting capabilities in terms of robustness and efficiency allowing the mitigation of some of the limitations of the classical algorithms. Moreover a back analysis study of geomechanical parameters using real monitoring data and a 3D numerical model of a hydraulic underground structure being built in the North of Portugal was performed using the Evolution Strategy algorithm, in order to reduce the uncertainties about the parameters evaluated by in situ and laboratory tests. It was verified that the low quantity of monitoring data available hinders the possibility to identify the parameters of interest. The existence of information of only one additional extensometer perpendicular to the existing one would allow this identification to succeed. (C) 2012 Elsevier Ltd. All rights reserved.
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Morera, S. B., Condom, T., Vauchel, P., Guyot, J. L., Galvez, C., & Crave, A. (2013). Pertinent spatio-temporal scale of observation to understand suspended sediment yield control factors in the Andean region: the case of the Santa River (Peru). Hydrology And Earth System Sciences, 17(11), 4641–4657.
Abstract: Hydro-sedimentology development is a great challenge in Peru due to limited data as well as sparse and confidential information. This study aimed to quantify and to understand the suspended sediment yield from the west-central Andes Mountains and to identify the main erosion-control factors and their relevance. The Tablachaca River (3132 km(2)) and the Santa River (6815 km(2)), located in two adjacent Andes catchments, showed similar statistical daily rainfall and discharge variability but large differences in specific suspended-sediment yield (SSY). In order to investigate the main erosion factors, daily water discharge and suspended sediment concentration (SSC) datasets of the Santa and Tablachaca rivers were analysed. Mining activity in specific lithologies was identified as the major factor that controls the high SSY of the Tablachaca (2204 t km(2) yr(-1)), which is four times greater than the Santa's SSY. These results show that the analysis of control factors of regional SSY at the Andes scale should be done carefully. Indeed, spatial data at kilometric scale and also daily water discharge and SSC time series are needed to define the main erosion factors along the entire Andean range.
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Morin, S., Domine, F., Dufour, A., Lejeune, Y., Lesaffre, B., Willemet, J. M., et al. (2013). Measurements and modeling of the vertical profile of specific surface area of an alpine snowpack. Advances In Water Resources, 55, 111–120.
Abstract: The specific surface area of snow (SSA) is a useful variable to describe the physical and chemical properties of snow, including a quantitative link to snow metamorphism and the optical properties of snow. Here we present a series of 16 weekly profiles of snow physical properties including SSA measured using the DUFISSS instrument spanning the period from January to April 2010 at the Col de Porte field site in the French Alps near Grenoble. Measured SSA values for dry snow ranged between ca. 5 and 80 m(2) kg(-1), and generally decreased over time in a given snow layer. Wet snow conditions encountered towards the end of the snow season show SSA values between 3 and 10 m(2) kg(-1). This unique dataset is compared with simulations carried out using the Crocus snowpack model, using two parameterizations of snow SSA: one simply derived from the internal computation of the optical radius in Crocus, and the other one determined from density and snow type. Both parameterizations perform rather satisfactorily qualitatively and quantitatively, compared to the performance in terms of snow density profile. Ample room for improvement exists, in particular through the implementation of SSA as a fully fledged prognostic variable in Crocus, which is currently in progress. (c) 2012 Elsevier Ltd. All rights reserved.
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Moussaoui, Y., Boumechhour, A., Jaffrezo, J. L., & Meklati, B. Y. (2013). The Chemical Composition Of Inorganic And Carbonaceous Materials In Pm10 From Urban And Rural Algerian Areas. Fresenius Environmental Bulletin, 22(5), 1357–1366.
Abstract: PM10 samples were collected at urban and rural sites in Algiers-Boumerdes, and three monitoring campaigns were done in summer and autumn 2007 and summer 2008. Quartz fiber filters of 47 mm diameter were used as collection substrates. The resulting samples were analysed for inorganic ions, elemental carbon (EC), organic carbon (OC), and water-soluble organic carbon (WSOC). Sulphate and particulate organic matter (POM) were the two most abundant constituents in all aerosol samples. Sulphate accounted for an average of 17.38-19.04% of the total urban aerosol and 20.80% of the total aerosol at the rural site, while the POM represent between 44.51 and 47.61% of the total mass aerosol identified in all sites studied, with a high contribution of POM recorded at the rural site. WSOC was a significant portion of OC, accounting for about one-third of OC fraction at the urban sites in all campaign samplings, and 56% of OC at the rural site in summer 2008. Sulphate (SO42-), ammonium (NH4+), and nitrate (NO3-) were the predominant ionic species in all sites studied.
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Nemery, J., Mano, V., Coynel, A., Etcheber, H., Moatar, F., Meybeck, M., et al. (2013). Carbon and suspended sediment transport in an impounded alpine river (Isere, France). Hydrological Processes, 27(17), 2498–2508.
Abstract: Carbon and total suspended sediment (TSS) loads were investigated from April 2006 to March 2008 in the mountainous watershed of the Isere River, French Alps (5570km(2)). The river bed has been highly impounded for hydroelectricity production during the last century. Hydraulic flushes are managed every year to prevent TSS storage within upstream dams. The Isere River has been instrumented for high-frequency monitoring of water, TSS by turbidity and carbon (organic, inorganic, dissolved and particulate) in order to evaluate the impact of natural floods and hydraulic flushes on annual loads. Annual TSS load which was estimated between 1.3 and 2.3 MT y(-1) (i.e. 233 to 413Tkm(-2) y(-1)) highlighted the high erodibility of the Isere watershed. Annual carbon load was estimated between 173 10(3)T y(-1) and 199 10(3)T y(-1) (i.e 31 to 36Tkm(-2) y(-1)). About 80% of the annual carbon loads were inorganic. The impact of hydraulic flushes on annual loads appeared limited (less than 3% for annual TSS load and about 1.5% for annual carbon load), whereas the most important natural flood event contributed to 20% of the annual TSS load and 10% of the annual carbon load. Copyright (c) 2012 John Wiley & Sons, Ltd.
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Nguyen, T. Q., Fontaine, H., Borde, Y., & Jacob, V. (2013). Identification and quantification of FOUP molecular contaminants inducing defects in integrated circuits manufacturing. Microelectronic Engineering, 105, 124–129.
Abstract: In the semiconductor industry, the control of contaminants is mandatory in order to prevent their detrimental impact on manufacturing yield. More specifically, it has been found that molecular contaminants coming from FOUPs could lead to defects on wafer. This paper presents two cases related with defects induced by molecular contamination, namely crystal growth and corrosion issues, respectively along two process sequences: (1) copper interconnect patterning and (2) Ionic Implantation of N-type or P-type dopants before Spacer Deposition. Three main ionic contaminants have been identified: HF, CH3COOH, HCOOH with levels varying from few ppbv to few tens of ppbv. In the first process sequence, dry Etching step has been identified as the source of contamination. In the second process sequence CF4 dry Stripping step generates HF. On the other hand, dry Stripping step and Implantation step generate CH3COOH, HCOOH. Organic contaminants have been characterized showing that the FOUP atmosphere represents a contaminated environment (about few tens of ppbv for total organics). A specific organic composition has been identified for in-process FOUPs but has not been related to a process or defects. (C) 2012 Elsevier B.V. All rights reserved.
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Nunez, M. A., Briancon, L., & Dias, D. (2013). Analyses of a pile-supported embankment over soft clay: Full-scale experiment, analytical and numerical approaches. Engineering Geology, 153, 53–67.
Abstract: The reinforcement of soils using rigid inclusions is a technique used to reduce settlements and to ensure the stability of an embankment built over soft soils. This technique reduces construction delays and is an economical and reliable solution, which has led to its widespread use. Thus, many design methods have been developed to assess the performance of these reinforced structures. These methods are mainly based on results from small scale models and numerical analyses. The reliability of these methods must be validated under in-situ conditions. This paper presents an analytical and numerical study of full-size experiments at the Chelles test site (France). The work presented in this paper is part of the ASIRI French National Research Project. The experiment consisted of a 5-m-high embankment built over soft alluvial ground improved by rigid vertical piles. The embankment is divided into four zones that illustrate the influence of the piles and the geosynthetic reinforcements on the soil's behavior. The performance of the embankment support system is assessed by monitoring data (total stresses, horizontal and vertical displacements). Several in-situ and laboratory soil investigations were performed using two axially loaded test piles. These tests verified the geotechnical hypothesis used for the numerical model and defined the soil-pile interaction parameters. Several analytical methods and numerical models were tested to assess the arching effect. Comparisons between the experimental data and these design methods are presented in terms of stress and the settlement efficacy of the improved system. The results show that these methods overestimate the stress efficacy but that the settlement efficacy is a reliable parameter to assess the overall performance of the rigid inclusion technique. (C) 2012 Elsevier B.V. All rights reserved.
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Panthou, G., Vischel, T., Lebel, T., Quantin, G., Pugin, A. C. F., Blanchet, J., et al. (2013). From pointwise testing to a regional vision: An integrated statistical approach to detect nonstationarity in extreme daily rainfall. Application to the Sahelian region. Journal Of Geophysical Research-Atmospheres, 118(15), 8222–8237.
Abstract: Global warming is expected to intensify the hydrologic cycle. Documenting whether significant changes in the extreme precipitation regimes have already happened is consequently one of the challenging topics in climatic research. The high natural variability of extreme precipitation often prevents from obtaining significant results when testing changes in the empirical distribution of extreme rainfall at regional scale. A regional integrated approach is proposed here as one possible answer to this complex methodological problem. Three methods are combined in order to detect regionally significant trends and/or breakpoints in series of annual maximum daily rainfall: (1) individual stationarity tests applied to the raw point series of maxima, (2) a maximum likelihood testing of time-dependent generalized extreme value (GEV) distributions fitted to these series, and (3) a heuristic testing of a regional time-dependent GEV distribution. This approach is applied to a set of 126 daily rain gauges covering the Sahel over the period 1950-1990. It is found that only a few stations are tested as nonstationary when applying classical tests on the raw series, while the two GEV-based models converge to show that the extreme rainfall series indeed underwent a negative breakpoint around 1970. The study evidences the limits of the widely used classical stationarity tests to detect trends in noisy series affected by sampling uncertainties, while using a parametric space and time-dependent GEV efficiently reduces this effect. Showing that the great Sahelian drought was accompanied by a significant decrease of extreme rainfall events is the other main result of this study.
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Parrenin, F., Masson-Delmotte, V., Kohler, P., Raynaud, D., Paillard, D., Schwander, J., et al. (2013). Synchronous Change of Atmospheric CO2 and Antarctic Temperature During the Last Deglacial Warming. Science, 339(6123), 1060–1063.
Abstract: Understanding the role of atmospheric CO2 during past climate changes requires clear knowledge of how it varies in time relative to temperature. Antarctic ice cores preserve highly resolved records of atmospheric CO2 and Antarctic temperature for the past 800,000 years. Here we propose a revised relative age scale for the concentration of atmospheric CO2 and Antarctic temperature for the last deglacial warming, using data from five Antarctic ice cores. We infer the phasing between CO2 concentration and Antarctic temperature at four times when their trends change abruptly. We find no significant asynchrony between them, indicating that Antarctic temperature did not begin to rise hundreds of years before the concentration of atmospheric CO2, as has been suggested by earlier studies.
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Pattyn, F., & Durand, G. (2013). Why marine ice sheet model predictions may diverge in estimating future sea level rise. Geophysical Research Letters, 40(16), 4316–4320.
Abstract: Despite major recent efforts, marine ice sheet models aiming at predicting future mass loss from ice sheets still suffer from uncertainties with respect to grounding line migration. A recent model intercomparison provided tools to test how models treat grounding line dynamics in a three-dimensional setting. Here we use these tools to address to what extent differences in mass loss occur according to the approximation to the Stokes equations, describing marine ice sheet flow, used. We find that models that neglect components of vertical shearing in the force budget wrongly estimate ice sheet mass loss by 50% over century time scales when compared to models that solve the full Stokes system of equations. Models that only include horizontal stresses also misrepresent velocities and ice shelf geometry, suggesting that interactions between the grounded ice sheet and the ocean will also be modeled incorrectly. Based on these findings, we strongly advise the use of high-order models to compute reliable projections of ice sheet contribution to sea level rise.
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Pattyn, F., Perichon, L., Durand, G., Favier, L., Gagliardini, O., Hindmarsh, R. C. A., et al. (2013). Grounding-line migration in plan-view marine ice-sheet models: results of the ice2sea MISMIP3d intercomparison. Journal Of Glaciology, 59(215), 410–422.
Abstract: Predictions of marine ice-sheet behaviour require models able to simulate grounding-line migration. We present results of an intercomparison experiment for plan-view marine ice-sheet models. Verification is effected by comparison with approximate analytical solutions for flux across the grounding line using simplified geometrical configurations (no lateral variations, no buttressing effects from lateral drag). Perturbation experiments specifying spatial variation in basal sliding parameters permitted the evolution of curved grounding lines, generating buttressing effects. The experiments showed regions of compression and extensional flow across the grounding line, thereby invalidating the boundary layer theory. Steady-state grounding-line positions were found to be dependent on the level of physical model approximation. Resolving grounding lines requires inclusion of membrane stresses, a sufficiently small grid size (<500 m), or subgrid interpolation of the grounding line. The latter still requires nominal grid sizes of <5 km. For larger grid spacings, appropriate parameterizations for ice flux may be imposed at the grounding line, but the short-time transient behaviour is then incorrect and different from models that do not incorporate grounding-line parameterizations. The numerical error associated with predicting grounding-line motion can be reduced significantly below the errors associated with parameter ignorance and uncertainties in future scenarios.
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Paul, F., Barrand, N. E., Baumann, S., Berthier, E., Bolch, T., Casey, K., et al. (2013). On the accuracy of glacier outlines derived from remote-sensing data. Annals Of Glaciology, 54(63), 171–182.
Abstract: Deriving glacier outlines from satellite data has become increasingly popular in the past decade. In particular when glacier outlines are used as a base for change assessment, it is important to know how accurate they are. Calculating the accuracy correctly is challenging, as appropriate reference data (e.g. from higher-resolution sensors) are seldom available. Moreover, after the required manual correction of the raw outlines (e.g. for debris cover), such a comparison would only reveal the accuracy of the analyst rather than of the algorithm applied. Here we compare outlines for clean and debris-covered glaciers, as derived from single and multiple digitizing by different or the same analysts on very high- (1 m) and medium-resolution (30 m) remote-sensing data, against each other and to glacier outlines derived from automated classification of Landsat Thematic Mapper data. Results show a high variability in the interpretation of debris-covered glacier parts, largely independent of the spatial resolution (area differences were up to 30%), and an overall good agreement for clean ice with sufficient contrast to the surrounding terrain (differences similar to 5%). The differences of the automatically derived outlines from a reference value are as small as the standard deviation of the manual digitizations from several analysts. Based on these results, we conclude that automated mapping of clean ice is preferable to manual digitization and recommend using the latter method only for required corrections of incorrectly mapped glacier parts (e.g. debris cover, shadow).
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Pellarin, T., Louvet, S., Gruhier, C., Quantin, G., & Legout, C. (2013). A simple and effective method for correcting soil moisture and precipitation estimates using AMSR-E measurements. Remote Sensing Of Environment, 136, 28–36.
Abstract: Recent studies have demonstrated the possibility to use surface soil moisture satellite products in order to correct precipitation estimates. The overall approaches are generally based on adjusting the precipitation rate into a water balance model in order to match observed and simulated soil moisture. In the present study, a recently proposed approach was improved and assessed over a 4-year period in West Africa. Three different satellite precipitation products (CMORPH, TRMM-3B42 and PERSIANN) were used in this study. The new algorithm was evaluated over three 0.25 x 0.25 degrees areas in Niger, Mali and Benin and precipitation estimates (before and after correction) were compared to observed ground-based precipitation measurements. An assessment was also conducted at the regional scale using 48 independent rainfall stations located in Togo, Niger and Burkina Faso. The proposed methodology showed that, after correction, the three satellite precipitation products converge to the ground-based precipitation measurements, demonstrating the robustness of the methodology. In addition, soil moisture estimates derived from the methodology were found to be in good agreement with ground-based soil moisture measurements (R-2 > 0.78 in Niger, R-2 > 0.54 in Mali and R-2 > 0.64 in Benin). (C) 2013 Elsevier Inc. All rights reserved.
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Perret, C., Morlot, T., & Favre, A. C. (2013). Dynamic rating curve assessment in hydrometric stations and calculation of the associated uncertainties: Quality and monitoring indicators. Houille Blanche-Revue Internationale De L Eau, (6), 24–30.
Abstract: Whether we talk about safety reasons, energy production or regulation, water resources management is one of EDF's main concerns. To meet these needs, since the fifties EDF-DTG operates a hydrometric network that includes more than 350 gauging stations. The data collected allows real time monitoring of rivers, as well as hydrological studies and the sizing of structures. Ensuring the quality of the stream flow data is a priority. A rating curve is an indirect method of estimating the discharge in rivers based on water level measurements. The value of discharge obtained thanks to the rating curve is not entirely accurate due to constant changes of the river bed morphology, to the precision of the gaugings (direct and punctual discharge measurements) and to the quality of the tracing. And as time goes on, the uncertainty of the estimated discharge from the rating curve “gets older” and increases: therefore the final level of uncertainty remains particularly difficult to assess. In order to get the most accurate stream flow data and to improve their reliability, EDF is exploring an original “dynamic” method to compute rating curves based on historical gaugings from a hydrometric station. A curve is computed for each new gauging and a model of uncertainty is adjusted for each of them. By taking into account the variability of the flow conditions and the life of the hydrometric station, this original dynamic method can answer important questions in the field of hydrometry such as “How many gauging a year have to be made so as to produce stream flow data with an average uncertainty of X?” and “When and in which range of water flow do we have to realize those gaugings?”.
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Petrenko, V. V., Martinerie, P., Novelli, P., Etheridge, D. M., Levin, I., Wang, Z., et al. (2013). A 60 yr record of atmospheric carbon monoxide reconstructed from Greenland firn air. Atmospheric Chemistry And Physics, 13(15), 7567–7585.
Abstract: We present the first reconstruction of the Northern Hemisphere (NH) high latitude atmospheric carbon monoxide (CO) mole fraction from Greenland firn air. Firn air samples were collected at three deep ice core sites in Greenland (NGRIP in 2001, Summit in 2006 and NEEM in 2008). CO records from the three sites agree well with each other as well as with recent atmospheric measurements, indicating that CO is well preserved in the firn at these sites. CO atmospheric history was reconstructed back to the year 1950 from the measurements using a combination of two forward models of gas transport in firn and an inverse model. The reconstructed history suggests that Arctic CO in 1950 was 140-150 nmol mol(-1), which is higher than today's values. CO mole fractions rose by 10-15 nmol mol(-1) from 1950 to the 1970s and peaked in the 1970s or early 1980s, followed by a approximate to 30 nmol mol(-1) decline to today's levels. We compare the CO history with the atmospheric histories of methane, light hydrocarbons, molecular hydrogen, CO stable isotopes and hydroxyl radicals (OH), as well as with published CO emission inventories and results of a historical run from a chemistry-transport model. We find that the reconstructed Greenland CO history cannot be reconciled with available emission inventories unless unrealistically large changes in OH are assumed. We argue that the available CO emission inventories strongly underestimate historical NH emissions, and fail to capture the emission decline starting in the late 1970s, which was most likely due to reduced emissions from road transportation in North America and Europe.
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Petzold, A., Ogren, J. A., Fiebig, M., Laj, P., Li, S. M., Baltensperger, U., et al. (2013). Recommendations for reporting “black carbon” measurements. Atmospheric Chemistry And Physics, 13(16), 8365–8379.
Abstract: Although black carbon (BC) is one of the key atmospheric particulate components driving climate change and air quality, there is no agreement on the terminology that considers all aspects of specific properties, definitions, measurement methods, and related uncertainties. As a result, there is much ambiguity in the scientific literature of measurements and numerical models that refer to BC with different names and based on different properties of the particles, with no clear definition of the terms. The authors present here a recommended terminology to clarify the terms used for BC in atmospheric research, with the goal of establishing unambiguous links between terms, targeted material properties and associated measurement techniques.
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Pfeffer, J., Champollion, C., Favreau, G., Cappelaere, B., Hinderer, J., Boucher, M., et al. (2013). Evaluating surface and subsurface water storage variations at small time and space scales from relative gravity measurements in semiarid Niger. Water Resources Research, 49(6), 3276–3291.
Abstract: The acquisition of reliable data sets representative of hydrological regimes and their variations is a critical concern for water resource assessment. For the subsurface, traditional approaches based on probe measurements, core analysis, and well data can be laborious, expensive, and highly intrusive, while only yielding sparse data sets. For this study, an innovative field survey, merging relative microgravimetry, magnetic resonance soundings, and hydrological measurements, was conducted to evaluate both surface and subsurface water storage variations in a semiarid Sahelian area. The instrumental setup was implemented in the lower part of a typical hillslope feeding to a temporary pond. Weekly measurements were carried out using relative spring gravimeters during 3 months of the rainy season in 2009 over a 350 x 500 m(2) network of 12 microgravity stations. Gravity variations of small to medium amplitude <= 220 nm s(-2)) were measured with accuracies better than 50 nm s(-2), revealing significant variations of the water storage at small time (from 1 week up to 3 months) and space (from a couple of meters up to a few hundred meters) scales. Consistent spatial organization of the water storage variations were detected, suggesting high infiltration at the outlet of a small gully. The comparison with hydrological measurements and magnetic resonance soundings involved that most of the microgravity variations came from the heterogeneity in the vadose zone. The results highlight the potential of time lapse microgravity surveys for detecting intraseasonal water storage variations and providing rich space-time data sets for process investigation or hydrological model calibration/evaluation.
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Philip, A., Meyssonnier, J., Kluender, R. T., & Baruchel, J. (2013). Three-dimensional rocking curve imaging to measure the effective distortion in the neighbourhood of a defect within a crystal: an ice example. Journal Of Applied Crystallography, 46, 842–848.
Abstract: Rocking curve imaging (RCI) is a quantitative version of monochromatic beam diffraction topography that involves using a two-dimensional detector, each pixel of which records its own 'local' rocking curve. From these local rocking curves one can reconstruct maps of particularly relevant quantities (e. g. integrated intensity, angular position of the centre of gravity, FWHM). Up to now RCI images have been exploited in the reflection case, giving a quantitative picture of the features present in a several-micrometre-thick subsurface layer. Recently, a three-dimensional Bragg diffraction imaging technique, which combines RCI with 'pinhole' and 'section' diffraction topography in the transmission case, was implemented. It allows three-dimensional images of defects to be obtained and measurement of three-dimensional distortions within a 50 x 50 x 50 mm elementary volume inside the crystal with angular misorientations down to 10(-5)-10(-6) rad. In the present paper, this three-dimensional-RCI (3D-RCI) technique is used to study one of the grains of a three-grained ice polycrystal. The inception of the deformation process is followed by reconstructing virtual slices in the crystal bulk. 3D-RCI capabilities allow the effective distortion in the bulk of the crystal to be investigated, and the predictions of diffraction theories to be checked, well beyond what has been possible up to now.
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| Picard, G., Brucker, L., Roy, A., Dupont, F., Fily, M., Royer, A., et al. (2013). Simulation of the microwave emission of multi-layered snowpacks using the Dense Media Radiative transfer theory: the DMRT-ML model. Geoscientific Model Development, 6(4), 1061–1078. | |