2005-2009
11 novembre 2018, par Gerhard Krinner
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Ali, A., & Lebel, T. (2009). The Sahelian standardized rainfall index revisited. International Journal Of Climatology, 29(12), 1705–1714.
Abstract: The Standardized Precipitation index (SPI) IS usually defined as the arithmetic mean of the normalized precipitation recorded at several stations over it re-ion of interest where (lie standard deviation computed at each Station over it period of reference is Used as the normalizing factor It is common to use this index in order to diagnose whether the Sahehan region can be considered as wet or dry for a given year There ire several key factors interfering with [lie relevance of the SPI as I measure of how ramy is it season over the Sahel The strong spatial variability of the Sahehan rainfall at the annual scale. the uneven distribution of the ramguage network and the mean interannual climatological gradients across the region are the most important of these factors, and their influence IS studied in detail here Using an optimal Interpolation algorithm 10 compute the SPI, [lie effects Of various sampling schemes are first Studied showing that the SPI computed its a single mean value over the whole Sahehan region is relatively robust Willi respect to these effects. However, the central key question remains that computing I single mean SPI over the Sahehan region hides the strong underlying][12 Spatial variability of this index For instance, 2006 was I significantly dry season over the Sahel as it whole, but working at the 0 5 degrees x 0.5 degrees resolution shows that in fact only 28% of the area was significantly dry, while 15% of the Sahel was significantly wet From conditional empirical distributions I distribution function is proposed to determine the spatial distributions of the 0 5 degrees x 0 5 degrees SPI Values for a given mean regional SPI value Studying In detail the space-time pattern of the SPI over the 1950-2006 period. also shows that recent years are characterized by a greater interannual,11 variability than the previous 40 years. and by,I contrast between the western Sahel remaining dry and the eastern Sahel returning to wetter conditions. Copyright (C) 2008 Royal Meteorological Society
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Anquetin, S., Ducrocq, V., Braud, I., & Creutin, J. D. (2009). Hydrometeorological modelling for flash flood areas: the case of the 2002 Gard event in France. Journal Of Flood Risk Management, 2(2), 101–110.
Abstract: In the context of flash flood forecasting, this paper proposes a few advances in our understanding of the hydrometeorological processes and their associated modelling requirements that may be useful to introduce within an operational forecasting chain. The study is focused on the September 2002 storm that produced more than 600 mm of rainfall in < 24 h and triggered a series of flash floods in the South of France. This catastrophic event took 23 human lives in 16 distinct subcatchments. This paper proposes a combined detailed analysis of the meteorological event and hydrological simulations of the response of four small-ungauged catchments. The meteorological analyses are based on observations and results of simulation of rain fields obtained with the MesoNH model. These analyses explained the steadiness of the storms that led to a locally intense precipitation: the role of the orography and favourable synoptic conditions. The hydrological model is set up without any calibration and the soil parameter specification is based on an existing soil database. Radar rainfall estimations are used. Simulated specific peak discharges are found to be in agreement with estimations from a postevent in situ investigation. Based on the model results, a cartography of the dominant process is proposed for the four selected catchments.
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Baduel, C., Voisin, D., & Jaffrezo, J. L. (2009). Comparison of analytical methods for Humic Like Substances (HULIS) measurements in atmospheric particles. Atmos. Chem. Phys., 9(16), 5949–5962.
Abstract: Humic-Like Substances (HULIS) are a major contributor to the organic carbon in atmospheric aerosol. It would be necessary to standardize an analytical method that could be easily and routinely used for HULIS measurements. We present one of the first comparisons of two of the main methods in use to extract HULIS, using I) a weak anion exchanger (DEAE) and II) the combination of two separation steps, one according to polarity (on C-18) and the second according to acidity (with a strong anion exchanger SAX). The quantification is performed with a TOC analyzer, complemented by an investigation of the chemical structure of the extracted fractions by UV-Visible spectroscopy. The analytical performances of each method are determined and compared for humic substances standards. These methods are further applied to determine the water extractable HULIS (HULISWS) and the 0.1M NaOH alkaline extractable HULIS (HULIST) in atmospheric aerosol collected in an Alpine Valley during winter time. This comparison, although on a limited batch of samples shows that the simpler DEAE isolation procedure leads to higher recoveries and better reproducibility than the C18-SAX procedure, and might therefore be preferable.
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Ben Daoud, A., Sauquet, E., Lang, M., Obled, C., & Bontron, G. (2009). Precipitation forecasting through an analog sorting technique : state of the art and further investigations. Houille Blanche-Revue Internationale De L Eau, (6), 60–65.
Abstract: Precipitation forecasting based on an adaptation of model outputs through an analog sorting technique has been improved for around ten years. The method runs operationally in several French institutions. First, this short paper presents the state of the art of this approach and the more recent developments. Second, a sensitivity analysis to the choice of the database from which the variables that characterise the past meteorological situations are extracted is performed. Two available archives are tested (ERA-40 and NCEP/NCAR re-analyses). The results show that despite the performances obtained with the ERA-40 database are slightly better especially for heavy rainfall events, the sensitivity is weak. Finally, further ways for improvement that could be investigated are suggested.
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Berne, A., Delrieu, G., & Boudevillain, B. (2009). Variability of the spatial structure of intense Mediterranean precipitation. Advances In Water Resources, 32(7), 1031–1042.
Abstract: Intense Mediterranean precipitation can generate devastating flash floods. A better understanding of the spatial structure of intense rainfall is critical to better identify catchments that will produce strong hydrological responses. We focus on two intense Mediterranean rain events of different types that occured in 2002. Radar and rain gauge measurements are combined to have a data set with a high spatial (1 x 1 km(2)) and temporal (5 min) resolution. Two thresholds are determined using the quantiles of the rain rate values, corresponding to the precipitating system at large and to the intense rain cells. A method based on indicator variograms associated with the thresholds is proposed in order to automatically quantify the spatial structure at each time step during the entire rain events. Therefore, its variability within intense rain events can be investigated. The spatial structure is found to be homogeneous over periods that can be related to the dynamics of the events. Moreover, a decreasing time resolution (i.e., increasing accumulation period) of the rain rate data will stretch the spatial structure because of the advection of rain cells by the wind. These quantitative characteristics of the spatial structure of intense Mediterranean rainfall will be useful to improve our understanding of the dynamics of flash floods. (C) 2008 Published by Elsevier Ltd.
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Bhattacharya, S. K., Savarino, J., & Luz, B. (2009). Mass-Dependent Isotopic Fractionation in Ozone Produced by Electrolysis. Anal. Chem., 81(13), 5226–5232.
Abstract: During the electrolysis of water in an acidified medium, ozone is produced, in association with oxygen, at the anode. This ozone is found to be depleted in heavy isotopes (O-18 and O-17), with respect to the source water, following a strict mass-dependent rule. Our experiments also suggest that the isotopes are distributed at the apex and base positions of the bent ozone molecule in a random fashion, without obeying the zero-point energy constraint. Endowed with these characteristics, the electrolytic ozone provides a source of reference that has a known internal heavy isotope distribution for spectroscopic studies. In addition, this ozone, when subjected to photolytic decomposition, can be used as a source of atomic oxygen with mass-dependent isotope ratios that can be varied by simply changing the water composition. Such an oxygen source is important for studying isotope effects in gas-phase recombination/exchange reactions such as COO + O* -> [COOO*] -> COO* + O.
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Bodin, X., Thibert, E., Fabre, D., Ribolini, A., Schoeneich, P., Francou, B., et al. (2009). Two Decades of Responses (1986-2006) to Climate by the Laurichard Rock Glacier, French Alps. Permafrost Periglacial Process., 20(4), 331–344.
Abstract: The Laurichard active rock glacier is the permafrost-related landform with the longest record of monitoring in France, including an annual geodetic survey, repeated geoelectrical campaigns from 1979 onwards and continuous recording of ground temperature since 2003. These data were used to examine changes in creep rates and internal structure from 1986 to 2006. The control that climatic variables exert on rock glacier kinematics was investigated over three time scales. Between the 1980s and the early 2000s, the main observed changes were a general increase in surface velocity and a decrease in internal resistivity. At a multi-year scale, the high correlation between surface movement and snow thickness in the preceding December appears to confirm the importance of snow cover conditions in early winter through their influence oil the ground thermal regime. A comparison of surface velocities, regional climatic datasets and ground sub-surface temperatures over six years suggests a strong relation between rock glacier deformation and ground temperature, as well as a role for liquid water due to melt of thick snow cover. Finally, Unusual surface lowering that accompanied peak velocities in 2004 may be due to a general thaw of the top of the permafrost, probably caused both by two successive snowy winters and by high energy inputs during the warm summer of 2003. Copyright (C) 2009 John Wiley & Sons, Ltd.
Keywords: rock glacier; surface kinematics; DC resistivity; climate controls; French Alps
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Bonelli, S., Charbit, S., Kageyama, M., Woillez, M. N., Ramstein, G., Dumas, C., et al. (2009). Investigating the evolution of major Northern Hemisphere ice sheets during the last glacial-interglacial cycle. Clim. Past., 5(3), 329–345.
Abstract: A 2.5-dimensional climate model of intermediate complexity, CLIMBER-2, fully coupled with the GREMLINS 3-D thermo-mechanical ice sheet model is used to simulate the evolution of major Northern Hemisphere ice sheets during the last glacial-interglacial cycle and to investigate the ice sheets responses to both insolation and atmospheric CO2 concentration. This model reproduces the main phases of advance and retreat of Northern Hemisphere ice sheets during the last glacial cycle, although the amplitude of these variations is less pronounced than those based on sea level reconstructions. At the last glacial maximum, the simulated ice volume is 52.5 x 10(15) m(3) and the spatial distribution of both the American and Eurasian ice complexes is in reasonable agreement with observations, with the exception of the marine parts of these former ice sheets. A set of sensitivity studies has also been performed to assess the sensitivity of the Northern Hemisphere ice sheets to both insolation and atmospheric CO2. Our results suggest that the decrease of summer insolation is the main factor responsible for the early build up of the North American ice sheet around 120 kyr BP, in agreement with benthic foraminifera delta O-18 signals. In contrast, low insolation and low atmospheric CO2 concentration are both necessary to trigger a long-lasting glaciation over Eurasia.
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Bonnifait, L., Delrieu, G., Le Lay, M., Boudevillain, B., Masson, A., Belleudy, P., et al. (2009). Distributed hydrologic and hydraulic modelling with radar rainfall input: Reconstruction of the 8-9 September 2002 catastrophic flood event in the Gard region, France. Advances In Water Resources, 32(7), 1077–1089.
Abstract: On 8-9 September 2002, an extreme rainfall event caused by a stationary mesoscale convective system (MCS) occurred in the Gard region, France. Distributed hydrologic and hydraulic modelling has been carried out to assess and compare the various sources of data collected operationally and during the post-event field surveys. Distributed hydrological modelling was performed with n-TOPMODELs and assessed for ungauged basins with the discharge estimates of the post-event surveys. A careful examination of the occurrence in time and space of the flash floods over the head watersheds indicates that flooding was controlled by the trajectory of the convective part of the MCS. Stationarity of the MCS over the Gardon watershed (1858 km(2) at Remoulins) for 28 h was responsible for the exceptional magnitude of the flood at this scale. The flood dynamics were characterized by an extensive inundation of the Gardonnenque plain upstream of the Gardon Gorges resulting in a significant peak flow reduction downstream. One-dimensional unsteady-flow hydraulic modelling was found to be required to reproduce these dynamics. Hydraulic modelling also proved to be potentially useful for the critical analysis and extrapolation of operational discharge rating curves. (C) 2009 Elsevier Ltd. All rights reserved.
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Bottenheim, J. W., Netcheva, S., Morin, S., & Nghiem, S. V. (2009). Ozone in the boundary layer air over the Arctic Ocean: measurements during the TARA transpolar drift 2006-2008. Atmos. Chem. Phys., 9(14), 4545–4557.
Abstract: A full year of measurements of surface ozone over the Arctic Ocean far removed from land is presented (81 degrees N-88 degrees N latitude). The data were obtained during the drift of the French schooner TARA between September 2006 and January 2008, while frozen in the Arctic Ocean. The data confirm that long periods of virtually total absence of ozone occur in the spring (mid March to mid June) after Polar sunrise. At other times of the year, ozone concentrations are comparable to other oceanic observations with winter mole fractions of ca. 30-40 nmol mol(-1) and summer minima of ca. 20 nmol mol(-1). Contrary to earlier observations from ozone sonde data obtained at Arctic coastal observatories, the ambient temperature was well above -20 degrees C during most ODEs (ozone depletion episodes). Backwards trajectory calculations suggest that during these ODEs the air had previously been in contact with the frozen ocean surface for several days and originated largely from the Siberian coast where several large open flaw leads and polynyas developed in the spring of 2007.
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Boucher, M., Favreau, G., Descloitres, M., Vouillamoz, J. M., Massuel, S., Nazoumou, Y., et al. (2009). Contribution of geophysical surveys to groundwater modelling of a porous aquifer in semiarid Niger: An overview. Comptes Rendus Geoscience, 341(10-11), 800–809.
Abstract: Subsurface geophysical surveys were carried out using a large range of methods in an unconfined sandstone aquifer in semiarid south-western Niger for improving both the conceptual model of water flow through the unsaturated zone and the parameterization of numerical a groundwater model of the aquifer. Methods included: electromagnetic mapping, electrical resistivity tomography (ERT), resistivity logging, time domain electromagnetic sounding (TDEM), and magnetic resonance sounding (MRS). Analyses of electrical conductivities, complemented by geochemical measurements, allowed us to identify preferential pathways for infiltration and drainage beneath gullies and alluvial fans. The mean water content estimated by MRS (13%) was used for computing the regional groundwater recharge from long-term change in the water table. The ranges in permeability and water content obtained with MRS allowed a reduction of the degree of freedom of aquifer parameters used in groundwater modelling. To cite this article: M. Boucher et al., C R. Geoscience 341 (2009). (C) 2009 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
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Boucher, M., Favreau, G., Vouillamoz, J. M., Nazoumou, Y., & Legchenko, A. (2009). Estimating specific yield and transmissivity with magnetic resonance sounding in an unconfined sandstone aquifer (Niger). Hydrogeology Journal, 17(7), 1805–1815.
Abstract: The unconfined aquifer of the Continental Terminal in Niger was investigated by magnetic resonance sounding (MRS) and by 14 pumping tests in order to improve calibration of MRS outputs at field scale. The reliability of the standard relationship used for estimating aquifer transmissivity by MRS was checked; it was found that the parametric factor can be estimated with an uncertainty a parts per thousand currency sign150% by a single point of calibration. The MRS water content (theta (MRS)) was shown to be positively correlated with the specific yield (Sy), and theta (MRS) always displayed higher values than Sy. A conceptual model was subsequently developed, based on estimated changes of the total porosity, Sy, and the specific retention Sr as a function of the median grain size. The resulting relationship between theta (MRS) and Sy showed a reasonably good fit with the experimental dataset, considering the inherent heterogeneity of the aquifer matrix (residual error is similar to 60%). Interpreted in terms of aquifer parameters, MRS data suggest a log-normal distribution of the permeability and a one-sided Gaussian distribution of Sy. These results demonstrate the efficiency of the MRS method for fast and low-cost prospection of hydraulic parameters for large unconfined aquifers.
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Bouilloud, L., Delrieu, G., Boudevillain, B., Borga, M., & Zanon, F. (2009). Radar rainfall estimation for the post-event analysis of a Slovenian flash-flood case: application of the Mountain Reference Technique at C-band frequency. Hydrology And Earth System Sciences, 13(7), 1349–1360.
Abstract: This article is dedicated to radar rainfall estimation for the post-event analysis of a flash flood that occurred on 18 September 2007 in Slovenia. The utility of the Mountain Reference Technique is demonstrated to quantify rain attenuation effects that affect C-band radar measurements in heavy rain. Maximum path-integrated attenuation between 15 and 20 dB were estimated thanks to mountain returns for path-averaged rain rates between 10 and 15 mm h(-1) over a 120-km path. Assuming the reflectivity-attenuation relationship to be known, the proposed technique allows for estimating an effective radar calibration correction factor to be accounted for in the parameterization of the attenuation correction. Screening effects are quantified using a geometrical calculation based on a digitized terrain model of the region. The vertical structure of the reflectivity is modeled with a normalized apparent vertical profile of reflectivity. Implementation of the radar data processing indicates that: (1) the combined correction for radar calibration and attenuation effects allows for obtaining satisfactory radar rain estimates (Nash criterion of 0.8 at the event time scale); (2) due to the attenuation equation instability, it is however compulsory to limit the maximum path-integrated attenuation to be corrected to about 10 dB; (3) the results also prove to be sensitive on the parameterization of reflectivity-attenuation-rainrate relationships.
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Bouilloud, L., Martin, E., Habets, F., Boone, A., Le Moigne, P., Livet, J., et al. (2009). Road Surface Condition Forecasting in France. Journal Of Applied Meteorology And Climatology, 48(12), 2513–2527.
Abstract: A numerical model designed to simulate the evolution of a snow layer on a road surface was forced by meteorological forecasts so as to assess its potential for use within an operational suite for road management in winter. The suite is intended for use throughout France, even in areas where no observations of surface conditions are available. It relies on short-term meteorological forecasts and long-term simulations of surface conditions using spatialized meteorological data to provide the initial conditions. The prediction of road surface conditions (road surface temperature and presence of snow on the road) was tested at an experimental site using data from a comprehensive experimental field campaign. The results were satisfactory, with detection of the majority of snow and negative road surface temperature events. The model was then extended to all of France with an 8-km grid resolution, using forcing data from a real-time meteorological analysis system. Many events with snow on the roads were simulated for the 2004/05 winter. Results for road surface temperature were checked against road station data from several highways, and results for the presence of snow on the road were checked against measurements from the Meteo-France weather station network.
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Branger, F., Tournebize, J., Carluer, N., Kao, C., Braud, I., & Vauclin, M. (2009). A simplified modelling approach for pesticide transport in a tile-drained field: The PESTDRAIN model. Agricultural Water Management, 96(3), 415–428.
Abstract: The paper presents a simplified model called PESTDRAIN. it simulates pesticide transport in a subsurface tile-drained field. It computes surface runoff and tile-drainage flow rates, along with the associated pesticide concentrations, with a variable event-driven time step. PESTDRAIN consists of three coupled modules: SIDRA, SIRUP and SILASOL. SIDRA and SIRUP are the water flow simulation modules in the saturated and unsaturated zones, respectively. SIDRA follows a simplified physically based approach while SIRUP follows a conceptual capacitive approach. SILASOL is the solute transport module for both the saturated and unsaturated zones and is based on transfer functions. it includes simple representations of adsorption and degradation of pesticides. PESTDRAIN was tested on field data sets collected for three drainage seasons at the La Jailliere experimental site in north-western France, for the wheat herbicides isoproturon (IPU) and diflufenican (DFF). After model calibration, relative errors for drainage and surface runoff flows over the season were 14% and 4%, respectively, and the Nash-Sutcliffe efficiency coefficient (N(eff)) value for drainage discharge was 0.58. A fair reproduction of a high temporal resolution IPU concentration data set in drainage discharge was also obtained (N(eff) = 0.28). For the validation data sets, PESTDRAIN was able to simulate accurately drainage discharge with Nash-Sutcliffe efficiency coefficients of 0.57 and 0.69. The global N(eff) was 0.44 for all flow-weighted average weekly concentrations in drainage. Relative errors for the pesticide losses were 2.5% and 35% (IPU), and 60% (DFF). For surface runoff the results were not as accurate, but they remained correct in terms of time location and order of magnitude. Although further validation is necessary with more field data, PESTDRAIN appears as a promising tool for agricultural water management. (C) 2008 Elsevier B.V, All rights reserved.
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Brankart, J. M., Ubelmann, C., Testut, C. E., Cosme, E., Brasseur, P., & Verron, J. (2009). Efficient Parameterization of the Observation Error Covariance Matrix for Square Root or Ensemble Kalman Filters: Application to Ocean Altimetry. Monthly Weather Review, 137(6), 1908–1927.
Abstract: In the Kalman filter standard algorithm, the computational complexity of the observational update is proportional to the cube of the number y of observations (leading behavior for large y). In realistic atmospheric or oceanic applications, involving an increasing quantity of available observations, this often leads to a prohibitive cost and to the necessity of simplifying the problem by aggregating or dropping observations. If the filter error covariance matrices are in square root form, as in square root or ensemble Kalman filters, the standard algorithm can be transformed to be linear in y, providing that the observation error covariance matrix is diagonal. This is a significant drawback of this transformed algorithm and often leads to an assumption of uncorrelated observation errors for the sake of numerical efficiency. In this paper, it is shown that the linearity of the transformed algorithm in y can be preserved for other forms of the observation error covariance matrix. In particular, quite general correlation structures (with analytic asymptotic expressions) can be simulated simply by augmenting the observation vector with differences of the original observations, such as their discrete gradients. Errors in ocean altimetric observations are spatially correlated, as for instance orbit or atmospheric errors along the satellite track. Adequately parameterizing these correlations can directly improve the quality of observational updates and the accuracy of the associated error estimates. In this paper, the example of the North Brazil Current circulation is used to demonstrate the importance of this effect, which is especially significant in that region of moderate ratio between signal amplitude and observation noise, and to show that the efficient parameterization that is proposed for the observation error correlations is appropriate to take it into account. Adding explicit gradient observations also receives a physical justification. This parameterization is thus proved to be useful to ocean data assimilation systems that are based on square root or ensemble Kalman filters, as soon as the number of observations becomes penalizing, and if a sophisticated parameterization of the observation error correlations is required.
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Brasseur, P., Gruber, N., Barciela, R., Brander, K., Doron, M., El Moussaoui, A., et al. (2009). Integrating Biogeochemistry and Ecology Into Ocean Data Assimilation Systems. Oceanography, 22(3), 206–215.
Abstract: Monitoring and predicting the biogeochemical state of the ocean and marine ecosystems is an important application of operational oceanography that needs to be expanded. The accurate depiction of the ocean's physical environment enabled by Global Ocean Data Assimilation Experiment (GODAE) systems, in both real-time and reanalysis modes, is already valuable for various for various applications, such as the fishing industry and fisheries management. However, most of these applications require accurate estimates of both physical and biogeochemical ocean conditions over a wide range of spatial and temporal scales. In this paper, we discuss recent developments that enable coupling new biogeochemical models and assimilation components with the existing GODAE systems, and we examine the potential of such systems in several areas of interest: phytoplankton biomass monitoring in the open ocean, ocean carbon cycle monitoring and assessment, marine ecosystem management at seasonal and longer time scales, and downscaling in coastal areas. A number of key requirements and research priorities are then identified for the future, GODAE systems will need to improve their representation of physical variables that are not yet considered essential, such as upper-ocean vertical fluxes that are critically important to biological activity. Further, the observing systems will need to be expanded in terms of in situ platforms (with intensified deployments of sensors for O-2 and chlorophyll, and inclusion of new sensors for nutrients, zooplankton, micronekton biomass, and others), satellite missions (e.g., hyperspectral instruments for ocean color, lidar systems for mixed-layer depths, and wide-swath altimeters for coastal sea level), and improved methods to assimilate these new measurements.
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Braud, I., Bariac, T., Biron, P., & Vauclin, M. (2009). Isotopic composition of bare soil evaporated water vapor. Part II: Modeling of RUBIC IV experimental results. Journal Of Hydrology, 369(1-2), 17–29.
Abstract: Stable water isotopes such as oxygen 18, are tracers of water movement within the soil-vegetation-atmosphere system. They provide useful information for a better understanding of evaporation and water vapor transport within soils. In part I of this paper, we presented a novel control experimental set-up under non steady conditions, dedicated to the measurement of the evaporation flux and corresponding isotopic composition from six bare soil columns. Data analysis raised several questions about the soil depth controlling the isotopic composition of the evaporated water vapor, suggesting different behavior before and after the appearance of back diffusion. Experimental data also suggested a time variable value of the kinetic fractionation factor. The present paper presents the modeling of the experimental results using the coupled heat, water and stable isotope transfer model SiSPAT_Isotope. Model results were used for investigating the above questions more in details. For this purpose, model parameters were calibrated for each soil column in order to reproduce the data. Then model results were inverted to estimate the kinetic fractionation factor. the results show that the hypothesis that the kinetic fractionation factor varies in time is plausible but the uncertainty is too large to derive firm conclusions. The largest uncertainty is found when the soil relative humidity is lower than one but water vapor is still negligible. When back diffusion has occurred. model results are the most robust and confirm that the isotopic composition of the evaporated water vapor is controlled by the soil isotopic composition of the liquid water at the peak. In this case, the retrieved kinetic fractionation factor is close to 18.9 parts per thousand, corresponding to laminar flow. (C) 2009 Elsevier B.V. All rights reserved.
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Braud, I., Biron, P., Bariac, T., Richard, P., Canale, L., Gaudet, J. P., et al. (2009). Isotopic composition of bare soil evaporated water vapor. Part I: RUBIC IV experimental setup and results. Journal Of Hydrology, 369(1-2), 1–16.
Abstract: Stable water isotopes such as oxygen 18, are natural tracers of water movement within the soil-vegetation-atmosphere continuum. They provide useful information for a better understanding of evaporation and water vapor transport within soils. This paper presents a novel controlled experimental setup. It is dedicated to detailed measurements of the evaporation fluxes from bare soil columns, as well as to the corresponding isotopic composition of the water vapor, under non-steady state conditions. The experiment allowed an accurate determination of these quantities. The formulae encountered in the literature were used to estimate the isotopic composition of the evaporated water vapor. None of them was able to correctly reproduce the measured isotopic composition of water. The data were then used to estimate the value of the isotopic composition of the soil liquid water. which should be used to get the right results for the isotopic composition of the evaporated water vapor. Results suggest that, when liquid transfer is dominant within the soil, the isotopic composition of evaporation was controlled by the isotopic composition of the liquid water within very thin soil surface layers. When there is a peak in the isotopic profile, i.e. when water vapor is dominant close to the surface, the isotopic composition of the evaporated water seems to be governed by the isotopic composition of the soil liquid water at the peak. The data were also used to estimate the kinetic fractionation factor. The results suggest that the latter is not constant in time. The values seem to depend on the shape of the isotopic profile. In both cases, the uncertainty on the results is very large. The estimation of the kinetic fractionation factor is Studied more in details using the modeling results presented in Part II of a companion paper where the data set is modeled using the SiSPAT_Isotope model. (C) 2009 Elsevier B.V. All rights reserved.
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Braun, J. J., Descloitres, M., Riotte, J., Deschamps, P., Violette, A., Marechal, J. C., et al. (2009). Contemporary versus long-term weathering rates in Tropics: Mule Hole, South India. Geochimica Et Cosmochimica Acta, 73(13), A157.
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Bravo-Espinosa, M., Mendoza, M. E., Medina-Orozco, L., Prat, C., Garcia-Oliva, F., & Lopez-Granados, E. (2009). Runoff, Soil Loss, And Nutrient Depletion Under Traditional And Alternative Cropping Systems In The Transmexican Volcanic Belt, Central Mexico. Land Degradation & Development, 20(6), 640–653.
Abstract: In the Transmexican Volcanic Belt a traditional fallow system is practiced, called “ano y vez” (AV), which does not benefit soil conservation due to its low level of nutrient recycling and because soil protection is poor during the cultivation year. The objective of the present work was to measure runoff and soil and nutrient losses during three annual cycles (2002-2004) in Central Mexico under AV rotation and two alternative systems: improved traditional (IT) and traditional organic (TO). Soil losses in the three systems were moderate (<1.2 Mg ha(-1) y(-1)) except during 2002, in which significant soil losses were recorded in IT and TO due to the scarcity of plant cover (<20 per cent) that was present throughout the rainy season. During the resting period of the AV system (2003), the annual runoff increased from 19 to about 600 per cent, compared to IT and TO without grazing. The difference in runoff was attributed to an 18 per cent increase in bulk density of soil surface (0-5 cm) caused by cattle trampling while grazing. Nutrient losses in the three treatments were mainly of N, Ca(2+), Mg(2+), Na(+), and K(+). These results suggest that AV has a higher topsoil degradation effect during the resting year than during the cultivation period. The study shows that incorporating the maize/beans-black oat rotation and residue cover causes a low runoff response that is important in reducing soil degradation. A spatial analysis is presented of erosion at watershed level for two soil management systems assessed. Copyright (C) 2009 John Wiley & Sons, Ltd.
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Bulat, S., Alekhin, I., & Petit, J. R. (2009). Life detection strategy for subglacial Lake Vostok, Antarctica: Lessons for Jovian moon Europa. Geochim. Cosmochim. Acta, 73(13), A173.
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Bulat, S. A., Alekhina, I. A., Lipenkov, V. Y., Lukin, V. V., Marie, D., & Petit, J. R. (2009). Cell concentrations of microorganisms in glacial and lake ice of the Vostok ice core, East Antarctica. Microbiology, 78(6), 808–810.
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Burn, L. J., Rosman, K. J. R., Candelone, J. P., Vallelonga, P., Burton, G. R., Smith, A. M., et al. (2009). An ultra-clean technique for accurately analysing Pb isotopes and heavy metals at high spatial resolution in ice cores with sub-pg g(-1) Pb concentrations. Anal. Chim. Acta, 634(2), 228–236.
Abstract: Measurements of Pb isotope ratios in ice containing sub-pg g(-1) concentrations are easily compromised by contamination, particularly where limited sample is available. Improved techniques are essential if Antarctic ice cores are to be analysed with sufficient spatial resolution to reveal seasonal variations due to climate. This was achieved here by using stainless steel chisels and saws and strict protocols in an ultra-clean cold room to decontaminate and section ice cores. Artificial ice cores, prepared from high purity water were used to develop and refine the procedures and quantify blanks. Ba and In, two other important elements present at pg g-1 and fg g(-1) concentrations in Polar ice, were also measured. The final blank amounted to 0.2 +/- 0.2 pg of Pb with Pb-206/Pb-207 and Pb-208/Pb-207 ratios of 1.16 +/- 0.12 and 2.35 +/- 0.16, respectively, 1.5 +/- 0.4 pg of Ba and 0.6 +/- 2.0 fg of In, most of which probably originates from abrasion of the steel saws by the ice. The procedure was demonstrated on a Holocene Antarctic ice core section and was shown to contribute blanks of only similar to 5%, similar to 14% and similar to 0.8% to monthly resolved samples with respective Pb, Ba and In concentrations of 0.12 pg g(-1), 0.3 pg g(-1) and 2.3 fg g(-1). Uncertainties in the Pb isotopic ratio measurements were degraded by only similar to 0.2%. (C) 2009 Elsevier B.V. All rights reserved.
Keywords: Ice cores; Antarctica; Mass spectrometry; Lead isotopes; Barium; Indium
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Camara, M., Diedhiou, A., & Gaye, A. T. (2009). Interannual variability of Atlantic hurricane activity and some features of West African climate. International Journal Of The Physical Sciences, 4(12), 806–817.
Abstract: The aim of this study is to describe over the Atlantic Ocean and West Africa, the large scale differences between an inactive hurricane period (1991 – 1994) and an active hurricane period (1998 – 2001), before and during the peak of the cyclone season (May – June – July; MJJ and August- September – October; ASO). Over West Africa, the monsoon flow at low level extends more northward during the active period. This period is also characterized by the northward shift and the westward extension over the Atlantic Ocean of the African Easterly Jet (AEJ) and by the existence of a strong Tropical Easterly Jet (TEJ). Moreover, intense low-level cyclonic vortices propagate in an unstable and weak sheared environment. These favorable conditions were generally present over Africa and over the North Atlantic Ocean during MJJ. The relationships between Atlantic cyclonic activity and the West African climate are strong during active cyclone years. African Easterly Waves (AEWs) are more intense and more frequent during the active period, both in their Sahelian and Gulf of Guinea tracks, while over the ocean, their activity and number decrease. During MJJ, the AEWs are more active over West Africa during active than inactive years.
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Camp, S., Ple, O., & Gourc, J. P. (2009). Proposed Protocol for Characterizing a Clay Layer Subjected to Bending. Geotechnical Testing Journal, 32(3), 273–279.
Abstract: Solid waste landfills have a cover barrier including a compacted clay liner which is the major element of the safety of the site. However, this barrier encounters many problems, in particular those related to its implementation and to the mechanical loading, such as potential differential settlements within the waste, after closing the cell. This phenomenon can induce bending strains in the clay layer and so create damage. The originality of this work is that the study includes the comparison of results from different types of standard tests, inducing compressive and tensile stresses. Compression tests, under low confinement, have been performed in the laboratory to characterize the clay behavior and to compare results with four-point bending tests. Particular attention is paid to experimental results by focusing the analysis on failure initiation. For the bending test, the limit value of the extension strain of the clay layer without damage is characterized. To optimize the landfill cap cover, particularly in terms of deformability, an improvement of the mechanical clay layer capability is proposed by addition of fiber reinforcement.
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Cappelaere, B., Descroix, L., Lebel, T., Boulain, N., Ramier, D., Laurent, J. P., et al. (2009). The AMMA-CATCH experiment in the cultivated Sahelian area of south-west Niger – Investigating water cycle response to a fluctuating climate and changing environment. Journal Of Hydrology, 375(1-2), 34–51.
Abstract: Among the three sites distributed along the West African latitudinal gradient in the AMMA-CATCH observation system, the experimental setup in the Niamey area of south-west Niger samples the cultivated Sahel environment, for hydrological, vegetation and land surface processes. The objective is to investigate relationships between climate, land cover, and the water cycle, in a rapidly changing semiarid environment. This paper first presents the main characteristics of the area, where previous research, including the EPSAT and HAPEX-Sahel experiments, had evidenced a widespread decadal increase in water resources, concurrently with severe drought conditions. The specifics of AMMA-CATCH research and data acquisition at this site, over the long-term (similar to 2001-2010) and enhanced (similar to 2005-2008) observation periods, are introduced. Objectives and observation strategy are explained, and the main characteristics of instrument deployment are detailed. A very large number of parameters – covering rainfall, vegetation ecophysiology, phenology and production, surface fluxes of energy, water vapour and CO2, runoff and sediment, pond water, soil moisture, and groundwater – were monitored at local to meso scales in a nested structure of sites. The current state of knowledge is summarized, connecting processes and patterns of variation for rainfall, vegetation/land cover, and the terrestrial hydrologic cycle. The central role of land use and of its spectacular change in recent decades is highlighted. This paper provides substantial background information that sets the context for papers relating to the south-west Niger site in this AMMA-CATCH special issue. (C) 2009 Elsevier B.V. All rights reserved.
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Castebrunet, H., Martinerie, P., Genthon, C., & Cosme, E. (2009). A three-dimensional model study of methanesulphonic acid to non sea salt sulphate ratio at mid and high-southern latitudes. Atmos. Chem. Phys., 9(24), 9449–9469.
Abstract: The Antarctic and sub-Antarctic methanesulphonic acid (MSA) to non sea salt sulphate (nssSO(4)) ratio is simulated with the Laboratoire de M,t,orologie Dynamique Atmospheric General Circulation Model including an atmospheric sulphur chemistry module. Spatial variations of the MSA/nssSO(4) ratio in different regions have been suggested to be mostly dependent on temperature or sulphur source contributions. Its past variations in ice cores have been interpreted as related to the DMS precursor source location. Our model results are compared with available field measurements in the Antarctic and sub-Antarctic regions. This suggests that the MSA/nssSO(4) ratio in the extra-tropical south hemisphere is mostly dependent on the relative importance of various DMS oxidation pathways. In order to evaluate the effect of a rapid conversion of dimethyl sulphoxide (DMSO) into MSA, not implemented in the model, the MSA+DMSO to nssSO(4) ratio is also discussed. Using this modified ratio, the model mostly captures the seasonal variations of MSA/nssSO(4) at mid and high-southern latitudes. In addition, the model qualitatively reproduces the bell shaped meridional variations of the ratio, which is highly dependent on the adopted relative reaction rates for the DMS+OH addition and abstraction pathways, and on the assumed reaction products of the MSIA+OH reaction. MSA/nssSO(4) ratio in Antarctic snow is fairly well reproduced except at the most inland sites characterized with very low snow accumulation rates. Our results also suggest that atmospheric chemistry plays an important role in the observed decrease of the ratio in snow between coastal regions and central Antarctica. The still insufficient understanding of the DMS oxidation scheme limits our ability to model the MSA/nssSO(4) ratio. Specifically, reaction products of the MSIA+OH reaction should be better quantified, and the impact of a fast DMSO conversion to MSA in spring to fall over Antarctica should be evaluated. A better understanding of BrO source processes is needed in order to include DMS + BrO chemistry in global models. Completing the observations of DMS, BrO and MSA at Halley Bay with DMSO measurements would better constrain the role of BrO in DMS oxidation. Direct measurements of MSA and nssSO(4) dry deposition velocities on Antarctic snow would improve our ability to model MSA and nssSO(4) in ice cores.
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Castro, L., Dommergue, A., Ferrari, C., & Maron, L. (2009). A DFT study of the reactions of O-3 with Hg degrees or Br-. Atmos. Environ., 43(35), 5708–5711.
Abstract: In the mid 1980s the study of ozone reactivity gained a significant interest with the discoveries of the stratospheric ozone hole (Farman et al., 1985) and of the ozone depletion events in the polar boundary layer (Oltmans et al., 1989). In the stratosphere, the mechanism involves heterogeneous reactions on polar stratospheric clouds that lead to chlorine activation (Solomon et al., 1986). In contrast, tropospheric ozone depletion occurring during polar springtime rather involves reactive bromine species. They are released during a series of photochemical and heterogeneous reactions often called the bromine explosion (see the review of Simpson et al., 2007). In this reaction sequence, an essential step is the generation of photolyzable Br-2, the precursor of two Br atoms, via the multiphasic reaction (1): HOBr + Br- + H+ -> H2O + Br-2 (1) The production of reactive HOBr could occur with the oxidation of BrO by HO2. (C) 2009 Elsevier Ltd. All rights reserved.
Keywords: Mercury; Ozone; Bromide; DFT modelling
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Chalikakis, K., Nielsen, M. R., Legchenko, A., & Hagensen, T. F. (2009). Investigation of sedimentary aquifers in Denmark using the magnetic resonance sounding method (MRS). Comptes Rendus Geoscience, 341(10-11), 918–927.
Abstract: In order to evaluate the applicability of the magnetic resonance sounding method (MRS) in the Danish context and its accuracy in estimating the hydraulic conductivity of different sedimentary aquifers, three MRS campaigns were conducted in Denmark between 2003 and 2007. In total, 12 sites were investigated. At two sites, it was impossible to measure with MRS due to the high level of ambient electromagnetic noise. For the present study, MRS results from 10 investigated sites throughout Denmark are used. A simplified classification of the grain-size within the investigated aquifers was established, based on a geological description of available borehole data. MRS-obtained parameters, water content (theta(MRS)) and decay time (T(1)), as well as normalised values of the MRS hydraulic conductivity estimator (K(MRS)(n)), from 23 MRS stations were compared with the aquifer class defined by geological description of adjacent boreholes. The comparison revealed a qualitative correlation. Moreover, K(MRS)(n) results revealed variations in the hydraulic conductivity within the investigated areas. The observed variations may suggest preferential locations for water-supply boreholes. Further MRS measurements close to boreholes with detailed grain-size analyses and pumping tests are required for a more quantitative estimation. To cite this article: K. Chalikakis et al., C R. Geoscience 341 (2009). (C) 2009 Published by Elsevier Masson SAS on behalf of Academie des sciences.
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Clement, R., Descloitres, M., Gunther, T., Ribolzi, O., & Legchenko, A. (2009). Influence of shallow infiltration on time-lapse ERT: Experience of advanced interpretation. Comptes Rendus Geoscience, 341(10-11), 886–898.
Abstract: Previous time-lapse Electrical Resistivity Tomography (ERT) studies have experienced difficulties in reconstructing reliable calculated resistivity changes in the subsurface. Increases or decreases of resistivity appear in the calculated ERT image where no changes were noted in the subsurface, leading to erroneous hydrological interpretations of the geophysical results. In this article, we investigate how a variation of actual resistivity with time and at shallow depth can influence time-lapse ERT results and produce resistivity artefacts at depth. We use I and 2-D numerical modelling to simulate infiltration scenarios. Using a standard time-lapse inversion, we demonstrate the resistivity artefact production according to the electrode spacing parameter. We used an advanced inversion methodology with a decoupling line at shallow depth to attenuate or remove resistivity artefacts. We also applied this methodology to a field data set obtained in a semi-arid environment in Burkina Faso, West Africa. Here, time-lapse ERT shows several resistivity artefacts of calculated resistivity if a standard inversion is used. We demonstrate the importance of a dense sampling of shallow resistivity variations at shallow depth. Advanced interpretation allows us to significantly attenuate or remove the resistivity artefact production at intermediate depth and produce reliable interpretation of hydrological processes. To cite I this article: R. Clement et al., C. R. Geoscience 341 (2009). (C) 2009 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
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Cole-Dai, J., Ferris, D., Lanciki, A., Savarino, J., Baroni, M., & Thiemens, M. H. (2009). Cold decade (AD 1810-1819) caused by Tambora (1815) and another (1809) stratospheric volcanic eruption. Geophys. Res. Lett., 36, 6 pp.
Abstract: Climate records indicate that the decade of AD 1810-1819 including "the year without a summer'' (1816) is probably the coldest during the past 500 years or longer, and the cause of the climatic extreme has been attributed primarily to the 1815 cataclysmic Tambora eruption in Indonesia. But the cold temperatures in the early part of the decade and the timing of the Tambora eruption call into question the real climatic impact of volcanic eruptions. Here we present new evidence, based on sulfur isotope anomaly (Delta S-33), a unique indicator of volcanic sulfuric acid produced in the stratosphere and preserved in polar snow, and on the precise timing of the volcanic deposition in both polar regions, that another large eruption in 1809 of a volcano is also stratospheric and occurred in the tropics. The Tambora eruption and the undocumented 1809 eruption are together responsible for the unusually cold decade. Citation: Cole-Dai, J., D. Ferris, A. Lanciki, J. Savarino, M. Baroni, and M. H. Thiemens (2009), Cold decade (AD 1810-1819) caused by Tambora (1815) and another (1809) stratospheric volcanic eruption, Geophys. Res. Lett., 36, L22703, doi:10.1029/2009GL040882.
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Colleoni, F., Krinner, G., & Jakobsson, M. (2009). Sensitivity of the Late Saalian (140 kyrs BP) and LGM (21 kyrs BP) Eurasian ice sheet surface mass balance to vegetation feedbacks. Geophys. Res. Lett., 36, 5 pp.
Abstract: This work uses an atmospheric general circulation model (AGCM) asynchronously coupled to an equilibrium vegetation model to investigate whether vegetation feedbacks could be one of the reasons why the Late Saalian ice sheet (140 kyrs BP) in Eurasia was substantially larger than the Last Glacial Maximum(LGM, 21 kyrs BP) Eurasian ice sheet. The modeled vegetation changes induce a regional cooling for the Late Saalian while they cause a slight regional warming for LGM. As a result, ablation along the margins of the Late Saalian ice sheet is significantly reduced, leading to an increased surface mass balance, while there are no significant mass balance changes observed from vegetation feedbacks at LGM. Citation: Colleoni, F., G. Krinner, and M. Jakobsson (2009), Sensitivity of the Late Saalian (140 kyrs BP) and LGM (21 kyrs BP) Eurasian ice sheet surface mass balance to vegetation feedbacks, Geophys. Res. Lett., 36, L08704, doi:10.1029/2009GL037200.
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Colleoni, F., Krinner, G., Jakobsson, M., Peyaud, V., & Ritz, C. (2009). Influence of regional parameters on the surface mass balance of the Eurasian ice sheet during the peak Saalian (140 kya). Glob. Planet. Change, 68(1-2), 132–148.
Abstract: Recent geologically-based reconstructions of the Eurasian ice sheet show that during the peak Saalian (approximate to 140 kya) the ice sheet was larger over Eurasia than during the Last Glacial Maximum (LGM) at approximate to 21 kya. To address this problem we use the LMDZ4 atmospheric general circulation model to evaluate the impact on the Saalian ice sheet's surface mass balance (SMB) from proglacial lakes. dust deposition on snow, vegetation and sea surface temperatures (SST) since geological records suggest that these environmental parameters were different during the two glacial periods. Seven model simulations have been carried out. Dust deposition decreases the mean SMB by intensifying surface melt during summer while proglacial lakes cool the summer climate and reduce surface melt on the ice sheet. A simulation including both proglacial lakes and dust shows that the presence of the former parameter reduces the impact of the latter, in particular, during summer. A switch from needle-leaf to tundra vegetation affects the regional climate but not enough to significantly influence the SMB of the nearby ice margin. However, a steady-state vegetation in equilibrium with the climate should be computed to improve the boundary conditions for further evaluations of the vegetation impact on the ice sheet's SMB. Finally, changes of the SST broadly affect the regional climate with significant consequences for the SMB. (C) 2009 Elsevier B.V. All rights reserved.
Keywords: surface mass balance; Eurasian ice sheet; Saalian; proglacial lakes; dust; SST; vegetation
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Creutin, J. D., Borga, M., Lutoff, C., Scolobig, A., Ruin, I., & Creton-Cazanave, L. (2009). Catchment dynamics and social response during flash floods: the potential of radar rainfall monitoring for warning procedures. Meteorological Applications, 16(1), 115–125.
Abstract: The objective of this paper is to examine how Current techniques for flash-flood monitoring and forecasting can meet the needs of the Population at risk to evaluate the flood severity and anticipate its danger. To this end, the social response time for different social actions in the course of two well studied flash flood events which Occurred ill France and Italy is identified. The event management activities are broadly characterized into three types according to their main objective (information, organisation and protection). The activities are also classified into three other types according, to the scale and nature of the human group involved (individuals, communities and institutions). The conclusions reached relate to (1) the characterisation of the social responses according to watershed scale and to the information available, and (2) to the appropriateness of the existing surveillance and forecasting tools to Support the social responses. Results suggest that representing the dynamics of the social response with just one number representing the average time for warning a population is an oversimplification. It appears that the social response time exhibits a parallel with the hydrological response time by diminishing in time with decreasing size of the relevant watershed. A second result is that the human groups have different capabilities of anticipation, apparently based oil the nature of information they use. Comparing watershed response times and social response times shows clearly that at scales of less than 100 km(2), a number of actions were taken with response times comparable to the catchment response time. The implications for adapting the warning processes to social scales (individual or organisational scales) are considerable. At small scales, and for the implied anticipation times. the reliable and high-resolution description of the actual rainfall field becomes the major Source of information for decision-making processes Such as deciding between evacuations or advising to stay home. This points to the need to improve the accuracy and quality control of real time radar rainfall data, especially for extreme flash flood-generating storms. Copyright (C) 2009 Royal Meteorological Society
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Cummings, J., Bertino, L., Brasseur, P., Fukumori, I., Kamachi, M., Martin, M. J., et al. (2009). Ocean Data Assimilation Systems For Godae. Oceanography, 22(3), 96–109.
Abstract: Ocean data assimilation has matured to the point that observations are now routinely combined with model forecasts to produce a variety of ocean products. Approaches to ocean data assimilation vary widely both in terms of the sophistication of the method and the observations assimilated, and also in terms of specification of the forecast error covariances, model biases, observation errors, and quality-control procedures. In this paper, we describe some of the ocean data assimilation systems that have been developed within the Global Ocean Data Assimilation Experiment (GODAE) community. We discuss assimilation methods, observations assimilated, and techniques used to specify error covariances. In addition, we describe practical implementation aspects and present analysis performance results for some of the analysis systems. Finally, we describe plans for improving the assimilation systems in the post-GODAE time period beyond 2008.
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de Rosnay, P., Drusch, M., Boone, A., Balsamo, G., Decharme, B., Harris, P., et al. (2009). AMMA Land Surface Model Intercomparison Experiment coupled to the Community Microwave Emission Model: ALMIP-MEM. Journal Of Geophysical Research-Atmospheres, 114.
Abstract: This paper presents the African Monsoon Multidisciplinary Analysis (AMMA) Land Surface Models Intercomparison Project (ALMIP) for Microwave Emission Models (ALMIP-MEM). ALMIP-MEM comprises an ensemble of simulations of C-band brightness temperatures over West Africa for 2006. Simulations have been performed for an incidence angle of 55 degrees, and results are evaluated against C-band satellite data from the Advanced Microwave Scanning Radiometer on Earth Observing System (AMSR-E). The ensemble encompasses 96 simulations, for 8 Land Surface Models (LSMs) coupled to 12 configurations of the Community Microwave Emission Model (CMEM). CMEM has a modular structure which permits combination of several parameterizations with different vegetation opacity and soil dielectric models. ALMIP-MEM provides the first intercomparison of state-of-theart land surface and microwave emission models at regional scale. Quantitative estimates of the relative importance of land surface modeling and radiative transfer modeling for the monitoring of low-frequency passive microwave emission on land surfaces are obtained. This is of high interest for the various users of coupled land surface microwave emission models. Results show that both LSMs and microwave model components strongly influence the simulated top of atmosphere (TOA) brightness temperatures. For most of the LSMs, the Kirdyashev opacity model is the most suitable to simulate TOA brightness temperature in best agreement with the AMSR-E data. When this best microwave modeling configuration is used, all the LSMs are able to reproduce the main temporal and spatial variability of measured brightness temperature. Averaged among the LSMs, correlation is 0.67 and averaged normalized standard deviation is 0.98.
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Debret, M., Sebag, D., Crosta, X., Massei, N., Petit, J. R., Chapron, E., et al. (2009). Evidence from wavelet analysis for a mid-Holocene transition in global climate forcing. Quat. Sci. Rev., 28(25-26), 2675–2688.
Abstract: A strong mid-Holocene transition has been identified by wavelet analyses in several sea ice cover records from the circum-Antarctic area, ice core records (Taylor dome, Byrd) and tropical marine records. The results are compared with those previously published in a synthesis of North Atlantic records and with 4 new records from the Norwegian and Icelandic seas and from a coastal site in Ireland. These new records confirm the previous pattern for the North Atlantic area, extend this pattern nearly to the Arctic Circle, and include a continental record. We further tested the possibility of extending this scheme using continental records from South America. The Holocene pattern proposed here confirms the importance of external forcing during the Early Holocene (solar activity: 1000 years cyclicity and 2500 years during the entire Holocene), even if the signal is disturbed by meltwater fluxes. The second part of the Holocene is then marked by the gradual appearance of internal forcing (thermohaline circulation around 1500 years), associated with a stabilisation of the signal. Coupling between ocean and atmosphere seems to play a fundamental role in the observed frequencies which vary accordingly in the Atlantic, circum-Antarctic and Pacific areas. The North Atlantic area seems to be the instigator of thermohaline circulation as shown by its sensitivity to meltwater discharges during the Early Holocene, even though each sector is independent with regards to its frequency content (around 1600 years for Atlantic Area; around 1250 years for Antarctica). The Holocene methane pattern, still under debate [Ruddiman, W.F., 2003a. Orbital insolation, ice volume and greenhouse gases. Quaternary Science Review 22,1597-1629; Ruddiman, W.F., 2003b. The anthropogenic greenhouse era began thousands of years ago. Climatic Change 61, 261-293]. could be explained by a more efficient thermohaline circulation around the mid-Holocene with an anthropogenic effect initiated at similar to 2500 BP as shown by the inter-hemispheric gradient. (C) 2009 Elsevier Ltd. All rights reserved.
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Delrieu, G., Boudevillain, B., Nicol, J., Chapon, B., Kirstetter, P. E., Andrieu, H., et al. (2009). Bollene-2002 Experiment: Radar Quantitative Precipitation Estimation in the Cevennes-Vivarais Region, France. Journal Of Applied Meteorology And Climatology, 48(7), 1422–1447.
Abstract: The Bollene-2002 Experiment was aimed at developing the use of a radar volume-scanning strategy for conducting radar rainfall estimations in the mountainous regions of France. A developmental radar processing system, called Traitements Regionalises et Adaptatifs de Donnees Radar pour l'Hydrologie (Regionalized and Adaptive Radar Data Processing for Hydrological Applications), has been built and several algorithms were specifically produced as part of this project. These algorithms include 1) a clutter identification technique based on the pulse-to-pulse variability of reflectivity Z for noncoherent radar, 2) a coupled procedure for determining a rain partition between convective and widespread rainfall R and the associated normalized vertical profiles of reflectivity, and 3) a method for calculating reflectivity at ground level from reflectivities measured aloft. Several radar processing strategies, including nonadaptive, time-adaptive, and space-time-adaptive variants, have been implemented to assess the performance of these new algorithms. Reference rainfall data were derived from a careful analysis of rain gauge datasets furnished by the Cevennes-Vivarais Mediterranean Hydrometeorological Observatory. The assessment criteria for five intense and long-lasting Mediterranean rain events have proven that good quantitative precipitation estimates can be obtained from radar data alone within 100-km range by using well-sited, well-maintained radar systems and sophisticated, physically based data-processing systems. The basic requirements entail performing accurate electronic calibration and stability verification, determining the radar detection domain, achieving efficient clutter elimination, and capturing the vertical structure(s) of reflectivity for the target event. Radar performance was shown to depend on type of rainfall, with better results obtained with deep convective rain systems (Nash coefficients of roughly 0.90 for point radar-rain gauge comparisons at the event time step), as opposed to shallow convective and frontal rain systems ( Nash coefficients in the 0.6-0.8 range). In comparison with time-adaptive strategies, the space-time-adaptive strategy yields a very significant reduction in the radar-rain gauge bias while the level of scatter remains basically unchanged. Because the Z-R relationships have not been optimized in this study, results are attributed to an improved processing of spatial variations in the vertical profile of reflectivity. The two main recommendations for future work consist of a
dapting the rain separation method for radar network operations and documenting Z-R relationships conditional on rainfall type. |
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Delrieu, G., Braud, I., Berne, A., Borga, M., Boudevillain, B., Fabry, F., et al. (2009). Weather radar and hydrology Preface. Advances In Water Resources, 32(7), 969–974.
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Depraetere, C., Gosset, M., Ploix, S., & Laurent, H. (2009). The organization and kinematics of tropical rainfall systems ground tracked at mesoscale with gages: First results from the campaigns 1999-2006 on the Upper Oueme Valley (Benin). Journal Of Hydrology, 375(1-2), 143–160.
Abstract: A dense network of rain gages, set up in the Upper Oueme Valley in Benin is used to study the spatial organization and the kinematics of the convective systems that cross the region. The study area is situated under Soudanian climate and set up as part of the AMMA-CATCH (African Monsoon Multidisciplinary Analysis – Couplage de l'Atmosphere Tropicale et du Cycle Hydrologique) observing system. Previous works focusing on the rainy events that occur in the Sahelian region of Niamey have shown that most of the rainfall in that region is provided by Organized Convective System that cover several thousand km(2) and usually propagate with a strong westward component. It was shown also that the time evolution of these Sahelian rainy events usually exhibits a convective peak followed by longer lasting and weaker stratiform rainfall. The aim of the present study is to analyze the spatial organization and kinematics of the rainy events occurring further south under the distinct, much more humid, Soudanian climate. These events have been poorly documented so far and the extent to which the Soudanian rainfall events behave like their Sahelian counterparts remains unclear. Seven years of rainfall data gathered over the AMMA-CATCH Benin site are studied. A new method called the 'Average Synchronized Hyetograph' (ASH) is proposed to analyze the kinematics of the rain patterns. The method also allows the assessment of the spatial organization of the system. A classification of the rainy events is proposed. It is based on assessing if (i) the rain patterns show a global propagation velocity and direction and (ii) if the time evolution of the rain rate within the network is typical of organized tropical Mesoscale Convective Systems (MCS) with a well-defined convective peak. The present study shows that about 55% of the events have a signature typical of those of MCS. Conversely, about 27% of rainfall events do not show evidence of being associated with MCS or even propagating. The kinematic properties of the events classified as MCS appear to be globally consistent with what was observed for the Sahelian zone from satellite tracking; the dominant direction is south-southwest but with a large departure from this average trend; their velocity ranges mostly between 20 and 50 km/h but a significant number of events are faster than that. MCS remains the dominant type of events during the whole rainy season but the period between mid July and mid August. The classification of rainfall events resulting from ground tracking has been compared with METEOSAT satellite tracking for the years 2003 and 2004. There is a significant correspondence of classes of rainfall events between the two tracking methods despite differences of scales and criterions in their definition. (C) 2009 Elsevier B.V. All rights reserved.
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Descroix, L., Mahe, G., Lebel, T., Favreau, G., Galle, S., Gautier, E., et al. (2009). Spatio-temporal variability of hydrological regimes around the boundaries between Sahelian and Sudanian areas of West Africa: A synthesis. Journal Of Hydrology, 375(1-2), 90–102.
Abstract: Abundant information is available on West African drought and its hydrological and environmental impacts. Land-use and climatic changes have greatly modified the conditions of Sudanian and Sahelian hydrology, impacting the regime and discharge of the main rivers. Human pressure on the environment (significant increase in crops and disappearance of natural bushes and landscapes, for example) has led to severe soil crusting and desertification throughout Sahelian regions. Despite recent increases in rainfall, the drought has not ended, resulting in two different hydrological evolutions. In the Sudanian areas, stream flows have been reduced, sometimes as much as twice the rainfall reduction rate. In the Sahelian regions, runoff coefficients have increased to such a degree that discharges are increasing, in spite of the reduced rainfall. The main goal of this paper is to synthesize the recent advances in the Sahelian and Sudano-Sahelian West African hydrology. The other objectives are two fold: First, to discuss the “Sahelian Paradox” (the increase in runoff in most of the Sahel during the drought, at least during the 1968-1995 period, as described in the 1980s) and paradox of groundwater highlighted in the square degree of Niamey (the rise in water table levels in some endorheic areas during the same drought, evidenced in the 1990s), and second, to attempt to define the application of their respective geographical areas. The land-use changes act as a general factor of hydrological evolution of soils and basins, while some spatial factors explain the great variability in the response to environmental evolution, such as endorheism, geological context, latitudinal climate gradient, and local hydrodynamic behaviour of environment. This paper is literature-based, and incorporates current research advances in the field, as well as a prospective focused on resources and socio-economic impacts. (C) 2009 Elsevier B.V. All rights reserved.
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Domine, F., Taillandier, A. S., Cabanes, A., Douglas, T. A., & Sturm, M. (2009). Three examples where the specific surface area of snow increased over time. Cryosphere, 3(1), 31–39.
Abstract: Snow on the ground impacts climate through its high albedo and affects atmospheric composition through its ability to adsorb chemical compounds. The quantification of these effects requires the knowledge of the specific surface area (SSA) of snow and its rate of change. All relevant studies indicate that snow SSA decreases over time. Here, we report for the first time three cases where the SSA of snow increased over time. These are (1) the transformation of a melt- freeze crust into depth hoar, producing an increase in SSA from 3.4 to 8.8 m(2) kg(-1). (2) The mobilization of surface snow by wind, which reduced the size of snow crystals by sublimation and fragmented them. This formed a surface snow layer with a SSA of 61 m(2) kg(-1) from layers whose SSAs were originally 42 and 50 m(2) kg(-1). (3) The sieving of blowing snow by a snow layer, which allowed the smallest crystals to penetrate into open spaces in the snow, leading to an SSA increase from 32 to 61 m(2) kg(-1). We discuss that other mechanisms for SSA increase are possible. Overall, SSA increases are probably not rare. They lead to enhanced uptake of chemical compounds and to increases in snow albedo, and their inclusion in relevant chemical and climate models deserves consideration.
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Dumont, M., Arnaud, Y., Six, D., & Corripio, J. G. (2009). Retrieval of glacier surface albedo using terrestrial photography. Houille Blanche-Rev. Int., (2), 102–108.
Abstract: The use of terrestrial photography to determine snow surface albedo has been developed by J. Corripio in 2002. This method allow's an easy determination of spatio-temporal variability of this parameter which is decisive in glacier mass balance. Two digital cameras have been settled for this intent on the Saint Sorlin glacier (Alps, France) in order to complement meteorological and glaciological monitoring instruments. Corripio method has been applied on Saint Sorlin glacier and improved using a database of Bidirectional Reflectance Distribution Function (BRDF) of snow in order to take into account anisotropy of snow radiative transfer. This database has been built using BRDFs of different types a snow measured in laboratory. Spatial and temporal evolution of glacier albedo has been derived during summer 2006 using terrestrial photography and surface albedo measurement on one Point of the glacier; these data show that the original method improved with reflected radiation anisotropy processing allows coherent retrieval of albedo values. Nevertheless, the original method requires an albedo reference point (measured) on glacier A new method based oil Corripio original method has been also developed in order to avoid the necessity of an albedo reference point. This method includes several improvements, spectral processing of incident and reflected radiation, narrow-to-broadband conversion, anisotropy treatment and so allows absolute retrieval of surface albedo value without the necessity of all albedo reference point. This study described the results obtained during summer 2006 with the original method but also the principles of this new method.
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Durand, G., Gagliardini, O., de Fleurian, B., Zwinger, T., & Le Meur, E. (2009). Marine ice sheet dynamics: Hysteresis and neutral equilibrium. J. Geophys. Res.-Earth Surf., 114, 10 pp.
Abstract: [1] The stability of marine ice sheets and outlet glaciers is mostly controlled by the dynamics of their grounding line, i.e., where the bottom contact of the ice changes from bedrock or till to ocean water. The last report of the Intergovernmental Panel on Climate Change has clearly underlined the poor ability of models to capture the dynamics of outlet glaciers. Here we present computations of grounding line dynamics on the basis of numerical solutions of the full Stokes equations for ice velocity, coupled with the evolution of the air ice- and sea ice-free interfaces. The grounding line position is determined by solving the contact problem between the ice and a rigid bedrock using the finite element code Elmer. Results of the simulations show that marine ice sheets are unstable on upsloping beds and undergo hysteresis under perturbation of ice viscosity, confirming conclusions from boundary layer theory. The present approach also indicates that a 2-D unconfined marine ice sheet sliding over a downsloping bedrock does not exhibit neutral equilibrium. It is shown that mesh resolution around the grounding line is a crucial issue. A very fine grid size (< 100 m spacing) is needed in order to achieve consistent results.
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Durand, G., Gagliardini, O., Zwinger, T., Le Meur, E., & Hindmarsh, R. C. A. (2009). Full Stokes modeling of marine ice sheets: influence of the grid size. Ann. Glaciol., 50(52), 109–114.
Abstract: Using the finite-element code Elmer, we show that the full Stokes modeling of the ice-sheet/ice-shelf transition we propose can give consistent predictions of grounding-line migration. Like other marine ice-sheet models our approach is highly sensitive to the chosen mesh resolution. However, with a grid size down to <5 km in the vicinity of the grounding line, predictions start to be robust because: (1) whatever the grid size (<5 km) the steady-state grounding-line position is sensibly the same (6 km standard deviation), and (2) with a grid-size refinement in the vicinity of the grounding line (200m), the steady-state solution is independent of the applied perturbation in fluidity, provided this perturbation remains monotonic.
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El Haddad, I., Marchand, N., Dron, J., Temime-Roussel, B., Quivet, E., Wortham, H., et al. (2009). Comprehensive primary particulate organic characterization of vehicular exhaust emissions in France. Atmos. Environ., 43(39), 6190–6198.
Abstract: A Study to characterize primary particulate matter (PM2.5 and PM10) from the French vehicular fleet was conducted during winter 2008, in a tunnel in Marseille, France. The carbonaceous fraction represents 70% of the aerosol mass and elemental carbon fraction (EC) represent 60% of the carbonaceous fraction. The organic carbon OC was characterized in term of its water soluble fraction, functionalization rate and HULIS content. Seventy trace organic compounds including alkanes, polycyclic aromatic hydrocarbons (PAH), petroleum biomarkers and carboxylic acids were also quantified. in order to determine an organic emission profile for chemical mass balance modeling studies Such source profiles were still missing in Europe and particularly in France. The profile obtained in this study is consistent with profiles determined in tunnel or dynamometer studies performed in other countries during the last ten years. These results suggest that organic compounds profiles from vehicular exhaust emissions are not significantly influenced by the geographic area and are thus suitable for use in aerosol source apportionment modeling applied across extensive regions. The chernical profile determined here is very similar to those obtained for diesel emissions with high concentrations of EC relative to CC (EC/OC = 1.8) and low concentrations of the higher molecular weight PAH. These results are consistent with the high proportion of diesel vehicles in the French fleet (49%). (C) 2009 Elsevier Ltd All rights reserved.
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Ezersky, M., Legchenko, A., Camerlynck, C., & Al-Zoubi, A. (2009). Identification of sinkhole development mechanism based on a combined geophysical study in Nahal Hever South area (Dead Sea coast of Israel). Environmental Geology, 58(5), 1123–1141.
Abstract: Seismic refraction, magnetic resonance sounding (MRS), and the transient electromagnetic (TEM) method were applied to investigate the geological and hydrogeological conditions in the Nahal Hever South sinkhole development area at the Dead Sea (DS) coast of Israel. Microgravity and MRS results reliably reveal large karst cavity in the central part of investigated area. The map of the seismic velocity shows that sinkholes in Nahal Hever can be divided into two major groups: sinkholes close to the salt edge and sinkholes over compact salt formations between a few tens to a hundred meters from the major cavern. The present study shows that the formation of sinkholes of the first group is caused by soil collapsing into the cavern. In the area occupied by sinkholes of the second group, karst was not detected either by MRS or by seismic diffraction methods. TEM results reveal shallow clay layer saturated with DS brine underlain sinkholes of this group. It allows suggestion that the water drainage and intensive water circulation during rain events wash out fine rock particles from the unsaturated zone into the pre-existing cavern, initiating the formation of sinkholes of the second group. Karst development takes place at a very low bulk resistivity (< 1 Omega m) of the DS aquifer, attesting to the fact that pores are filled with a highly saline solution. Refilling of the karstic cavities with collapsing and flushed soil slows down sinkhole development in the area. The sinkhole formation cycle at the site is estimated at 10 years. Sinkhole development throughout the studied area is triggered by a drop in the level of the DS, which reduces the head of the confined aquifer and the strength of the overlain sediments.
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Fain, X., Ferrari, C. P., Dommergue, A., Albert, M. R., Battle, M., Severinghaus, J., et al. (2009). Polar firn air reveals large-scale impact of anthropogenic mercury emissions during the 1970s. Proc. Natl. Acad. Sci. U. S. A., 106(38), 16114–16119.
Abstract: Mercury (Hg) is an extremely toxic pollutant, and its biogeochemical cycle has been perturbed by anthropogenic emissions during recent centuries. In the atmosphere, gaseous elemental mercury (GEM; Hg degrees) is the predominant form of mercury (up to 95%). Here we report the evolution of atmospheric levels of GEM in mid- to high-northern latitudes inferred from the interstitial air of firn (perennial snowpack) at Summit, Greenland. GEM concentrations increased rapidly after World War II from approximate to 1.5 ng m(-3) reaching a maximum of approximate to 3 ng m(-3) around 1970 and decreased until stabilizing at approximate to 1.7 ng m(-3) around 1995. This reconstruction reproduces real-time measurements available from the Arctic since 1995 and exhibits the same general trend observed in Europe since 1990. Anthropogenic emissions caused a two-fold rise in boreal atmospheric GEM concentrations before the 1970s, which likely contributed to higher deposition of mercury in both industrialized and remotes areas. Once deposited, this toxin becomes available for methylation and, subsequently, the contamination of ecosystems. Implementation of air pollution regulations, however, enabled a large-scale decline in atmospheric mercury levels during the 1980s. The results shown here suggest that potential increases in emissions in the coming decades could have a similar large-scale impact on atmospheric Hg levels.
Keywords: atmosphere; Greenland; past century; pollution
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Favier, V., Falvey, M., Rabatel, A., Praderio, E., & Lopez, D. (2009). Interpreting discrepancies between discharge and precipitation in high-altitude area of Chile's Norte Chico region (26-32 degrees S). Water Resour. Res., 45, 20 pp.
Abstract: The water resources of high-altitude areas of Chile's semiarid Norte Chico region (26-32 degrees S) are studied using surface hydrological observations (from 59 rain gauges and 38 hydrological stations), remotely sensed data, and output from atmospheric prediction models. At high elevations, the observed discharge is very high in comparison with precipitation. Runoff coefficients exceed 100% in many of the highest watersheds. A glacier inventory performed with aerial photographs and ASTER images was combined with information from past studies, suggesting that glacier retreat could contribute between 5% and 10% of the discharge at 3000 m in the most glacierized catchment of the region. Snow extent was studied using MOD10A2 data. Results show that snow is present during 4 months at above 3000 m, suggesting that snow processes are crucial. The mean annual sublimation (similar to 80 mm a(-1) at 4000 m) was estimated from the regional circulation model (WRF) and data from past studies. Finally, spatial distribution of precipitation was derived from available surface data and the global forecast system (GFS) atmospheric prediction model. Results suggest that annual precipitation is three to five times higher near the peak of the Andes than in the lowlands to the west. The GFS model suggests that daily precipitation rates in the mountains are similar to those in the coastal region, but precipitation events are more frequent and tend to last longer. Underestimation of summer precipitation may also explain part of the excess in discharge. Simple calculations show that consideration of GFS precipitation distributions, sublimation, and glacier melt leads to a better hydrological balance.
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Fowler, D., Pilegaard, K., Sutton, M. A., Ambus, P., Raivonen, M., Duyzer, J., et al. (2009). Atmospheric composition change: Ecosystems-Atmosphere interactions. Atmos. Environ., 43(33), 5193–5267.
Abstract: Ecosystems and the atmosphere: This review describes the state of understanding the processes involved in the exchange of trace gases and aerosols between the earth's surface and the atmosphere. The gases covered include NO, NO2, HONO, HNO3, NH3, SO2, DMS, Biogenic VOC, O-3, CH4, N2O and particles in the size range 1 nm-10 μm including organic and inorganic chemical species. The main focus of the review is on the exchange between terrestrial ecosystems, both managed and natural and the atmosphere, although some new developments in ocean-atmosphere exchange are included. The material presented is biased towards the last decade, but includes earlier work, where more recent developments are limited or absent. New methodologies and instrumentation have enabled, if not driven technical advances in measurement. These developments have advanced the process understanding and upscaling of fluxes, especially for particles, VOC and NH3. Examples of these applications include mass spectrometric methods, such as Aerosol Mass Spectrometry (AMS) adapted for field measurement of atmosphere-surface fluxes using micrometeorological methods for chemically resolved aerosols. Also briefly described are some advances in theory and techniques in micrometeorology. For some of the compounds there have been paradigm shifts in approach and application of both techniques and assessment. These include flux measurements over marine surfaces and urban areas using micrometeorological methods and the up-scaling of flux measurements using aircraft and satellite remote sensing. The application of a flux-based approach in assessment of O-3 effects on vegetation at regional scales is an important policy linked development secured through improved quantification of fluxes. The coupling of monitoring, modelling and intensive flux measurement at a continental scale within the NitroEurope network represents a quantum development in the application of research teams to address the underpinning science of reactive nitrogen in the cycling between ecosystems and the atmosphere in Europe. Some important developments of the science have been applied to assist in addressing policy questions, which have been the main driver of the research agenda, while other developments in understanding have not been applied to their wider field especially in chemistry-transport models through deficiencies in obtaining appropriate data to enable application or inertia within the modelling community. The paper identifies applications, gaps and research questions that have remained intractable at least since 2000 within the specialized sections of the paper, and where possible these have been focussed on research questions for the coming decade. (C) 2009 Published by Elsevier Ltd.
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Frappart, F., Hiernaux, P., Guichard, F., Mougin, E., Kergoat, L., Arjounin, M., et al. (2009). Rainfall regime across the Sahel band in the Gourma region, Mali. Journal Of Hydrology, 375(1-2), 128–142.
Abstract: The Sahel is characterized by low and highly variable rainfall, which strongly affects the hydrology and the climate of the region and creates severe constraints for agriculture and water management. This study provides the first characterization of the rainfall regime for the Gourma region located in Mali, Century Sahel (14.5-17.5 degrees N and 2-1 degrees S). The rainfall regime is described using two datasets: the daily long term raingauge records covering the period 1950-2007, and the high frequency raingauge records collected under the African Monsoon Multidisciplinary Analysis (AMMA) project between 2005 and 2008. The first rainfall dataset was used to analyse the interannual variability and the spatial distribution of the precipitation. The second dataset is used to analyse the diurnal cycle of precipitation and the nature of the rainfall. This study is complementary to previous analyses conducted in Sahelian areas located further south, where the influence of the continental Sahara heat low is expected to be less pronounced in summer. Rainfall regimes in the Gourma region present a succession of wet (1950-1969) and dry decades (1970-2007). The decrease of summer cumulative rainfall is explained by a reduction in the number of the rainy days in southern Gourma, and a decrease in both the number of rainy days and the daily rainfall in northern and central Gourma. This meridional difference may be related to the relative distances of the zones from the intertropical discontinuity, which is closer to the northern stations. The length of the rainy season has varied since the 1950s with two episodes of shorter rainy seasons: during the drought of the 1980s and also since 2000. However, this second episode is characterized by an increase in the daily rainfall, which suggests an intensification of rainfall events in the more recent years. High-frequency data reveal that a large fraction of the rainfall is produced by intense rain events mostly occurring in late evenings and early mornings during the core of the rainy season (July-September). Conversely, rainfall amounts are less around noon, and this mid-day damping is more pronounced in northern Gourma. All these characteristics have strong implications for agriculture and water resources management. (C) 2009 Elsevier B.V. All rights reserved.
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Frey, M. M., Savarino, J., Morin, S., Erbland, J., & Martins, J. M. F. (2009). Photolysis imprint in the nitrate stable isotope signal in snow and atmosphere of East Antarctica and implications for reactive nitrogen cycling. Atmos. Chem. Phys., 9(22), 8681–8696.
Abstract: The nitrogen (delta N-15) and triple oxygen (delta O-17 and delta O-18) isotopic composition of nitrate (NO3-) was measured year-round in the atmosphere and snow pits at Dome C, Antarctica (DC, 75.1 degrees S, 123.3 degrees E), and in surface snow on a transect between DC and the coast. Comparison to the isotopic signal in atmospheric NO3- shows that snow NO3- is significantly enriched in delta N-15 by > 200 parts per thousand and depleted in delta O-18 by < 40 parts per thousand. Post-depositional fractionation in delta O-17(NO3-) is small, potentially allowing reconstruction of past shifts in tropospheric oxidation pathways from ice cores. Assuming a Rayleigh-type process we find fractionation constants epsilon of -60 +/- 15 parts per thousand, 8 +/- 2 parts per thousand and 1 +/- 1 parts per thousand, for delta N-15, delta O-18 and delta O-17, respectively. A photolysis model yields an upper limit for the photolytic fractionation constant (15)epsilon of delta N-15, consistent with lab and field measurements, and demonstrates a high sensitivity of (15)epsilon to the incident actinic flux spectrum. The photolytic (15)epsilon is process-specific and therefore applies to any snow covered location. Previously published (15)epsilon values are not representative for conditions at the Earth surface, but apply only to the UV lamp used in the reported experiment (Blunier et al., 2005; Jacobi et al., 2006). Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals. Conversely, N-15 enrichment of the NO3- fraction in the snow implies N-15 depletion of emissions. Indeed, delta N-15 in atmospheric NO3- shows a strong decrease from background levels (4 +/- 7 parts per thousand) to -35 parts per thousand in spring followed by recovery during summer, consistent with significant snowpack emissions of reactive nitrogen. Field and lab evidence therefore suggest that photolysis is an important process driving fractionation and associated NO3- loss from snow. The delta O-17 signature confirms previous coastal measurements that the peak of atmospheric NO3- in spring is of stratospheric origin. After sunrise photolysis drives then redistribution of NO3- from the snowpack photic zone to the atmosphere and a snow surface skin layer, thereby concentrating NO3- at the surface. Little NO3- appears to be exported off the EAIS plateau, still snow emissions from as far as 600 km inland can contribute to the coastal NO3- budget.
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Gabrielli, P., Planchon, F., Barbante, C., Boutron, C. F., Petit, J. R., Bulat, S., et al. (2009). Ultra-low rare earth element content in accreted ice from sub-glacial Lake Vostok, Antarctica. Geochim. Cosmochim. Acta, 73(20), 5959–5974.
Abstract: This paper reports the first rare earth element (REE) concentrations in accreted ice refrozen from sub-glacial Lake Vostok (East Antarctica). REE were determined in various sections of the Vostok ice core in order to geochemically characterize its impurities. Samples were obtained from accreted ice and, for comparison, from the upper glacier ice of atmospheric origin (undisturbed, disturbed and glacial. our ice). REE concentrations ranged between 0.8-56 pg g(-1) for Ce and 0.0035-0.24 pg g(-1) for Lu in glacier ice, and between <0.1-24 pg g(-1) for Ce and < 0.0004-0.02 pg g(-1) for Lu in accreted ice. Interestingly, the REE concentrations in the upper accreted ice (AC(1); characterized by visible aggregates containing a mixture of very. ne terrigenous particles) and in the deeper accreted ice (AC(2); characterized by transparent ice) are lower than those in fresh water and seawater, respectively. We suggest that such ultra-low concentrations are unlikely to be representative of the real REE content in Lake Vostok, but instead may reflect phase exclusion processes occurring at the ice/water interface during refreezing. In particular, the uneven spatial distribution (on the order of a few cm) and the large range of REE concentrations observed in AC(1) are consistent with the occurrence/absence of the aggregates in adjacent ice, and point to the presence of solid-phase concentration/exclusion processes occurring within separate pockets of frazil ice during AC(1) formation. Interestingly, if the LREE enrichment found in AC(1) was not produced by chemical fractionation occurring in Lake Vostok water, this may reflect a contribution of bedrock material, possibly in combination with aeolian dust released into the lake by melting of the glacier ice. Collectively, this valuable information provides new insight into the accreted ice formation processes, the bedrock geology of East Antarctica as well as the water chemistry and circulation of Lake Vostok. Published by Elsevier Ltd.
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Gallet, J. C., Domine, F., Zender, C. S., & Picard, G. (2009). Measurement of the specific surface area of snow using infrared reflectance in an integrating sphere at 1310 and 1550 nm. Cryosphere, 3(2), 167–182.
Abstract: Even though the specific surface area (SSA) and the snow area index (SAI) of snow are crucial variables to determine the chemical and climatic impact of the snow cover, few data are available on the subject. We propose here a novel method to measure snow SSA and SAI. It is based on the measurement of the hemispherical infrared reflectance of snow samples using the DUFISSS instrument (DUal Frequency Integrating Sphere for Snow SSA measurement). DUFISSS uses the 1310 or 1550 nm radiation of laser diodes, an integrating sphere 15 cm in diameter, and InGaAs photodiodes. For SSA < 60 m(2) kg(-1), we use the 1310 nm radiation, reflectance is between 15 and 50% and the accuracy of SSA determination is 10%. For SSA > 60 m(2) kg(-1), snow is usually of low density (typically 30 to 100 kg m(-3)), resulting in insufficient optical depth and 1310 nm radiation reaches the bottom of the sample, causing artifacts. The 1550 nm radiation is therefore used for SSA > 60 m(2) kg(-1). Reflectance is then in the range 5 to 12% and the accuracy on SSA is 12%. We propose empirical equations to determine SSA from reflectance at both wavelengths, with that for 1310 nm taking into account the snow density. DUFISSS has been used to measure the SSA of snow and the SAI of snowpacks in polar and Alpine regions.
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Gascoin, S., Ducharne, A., Ribstein, P., Lejeune, Y., & Wagnon, P. (2009). Dependence of bare soil albedo on soil moisture on the moraine of the Zongo glacier (Bolivia): Implications for land surface modeling. J. Geophys. Res.-Atmos., 114, 11 pp.
Abstract: Although the dependence of bare soil albedo on soil moisture is a familiar observation, it is not commonly represented in climate modeling. We investigate the impact of this dependence in a land surface model using meteorological data collected on the moraine of a Bolivian glacier. The relationship which is implemented to simulate albedo variations with soil moisture is deduced from a previous field study. The model is set up at the scale of the meteorological station plot to have the most accurate control on the model calibration and validation. A snow parameter is modified to account for the fact that the model was designed for larger cell sizes. Water content measurements are used to calibrate the parameter controlling the vertical water fluxes within the soil surface layer. This allows us to enhance the model's ability to capture the fast changes in surface soil moisture. The comparison of simulated ground heat flux and outgoing longwave radiations with observations shows that the model performs well despite the fact that all other parameters are set a priori on the basis of local properties of the surface. The results show that the dependence of bare soil albedo on soil moisture, which causes an increase in the net radiation, importantly influences the turbulent fluxes at the annual and monthly time scales. The mean annual evaporation is increased by 12%. As a consequence, this parameterization modifies the computed runoff, which is reduced by more than 5% during the rainy season.
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Gascoin, S., Ducharne, A., Ribstein, P., Perroy, E., & Wagnon, P. (2009). Sensitivity of bare soil albedo to surface soil moisture on the moraine of the Zongo glacier (Bolivia). Geophys. Res. Lett., 36, 5 pp.
Abstract: The dependence of bare soil albedo on soil water content is investigated using in situ data collected on the moraine of an Andean glacier (Bolivia). This study demonstrates a high negative correlation between the two variables that is best approximated by an exponential function, in agreement with previous studies. More importantly, the average snow-free albedo value during the rainy season is 40% lower than during the dry season (0.16 vs. 0.26). These results are relevant for climate and land surface modeling applications, where bare soil albedo is often considered as a constant parameter. Citation: Gascoin, S., A. Ducharne, P. Ribstein, E. Perroy, and P. Wagnon (2009), Sensitivity of bare soil albedo to surface soil moisture on the moraine of the Zongo glacier (Bolivia), Geophys. Res. Lett., 36, L02405, doi:10.1029/2008GL036377.
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Gaudart, J., Toure, O., Dessay, N., Dicko, A. L., Ranque, S., Forest, L., et al. (2009). Modelling malaria incidence with environmental dependency in a locality of Sudanese savannah area, Mali. Malaria Journal, 8.
Abstract: Background: The risk of Plasmodium falciparum infection is variable over space and time and this variability is related to environmental variability. Environmental factors affect the biological cycle of both vector and parasite. Despite this strong relationship, environmental effects have rarely been included in malaria transmission models. Remote sensing data on environment were incorporated into a temporal model of the transmission, to forecast the evolution of malaria epidemiology, in a locality of Sudanese savannah area. Methods: A dynamic cohort was constituted in June 1996 and followed up until June 2001 in the locality of Bancoumana, Mali. The 15-day composite vegetation index (NDVI), issued from satellite imagery series (NOAA) from July 1981 to December 2006, was used as remote sensing data. The statistical relationship between NDVI and incidence of P. falciparum infection was assessed by ARIMA analysis. ROC analysis provided an NDVI value for the prediction of an increase in incidence of parasitaemia. Malaria transmission was modelled using an SIRS-type model, adapted to Bancoumana's data. Environmental factors influenced vector mortality and aggressiveness, as well as length of the gonotrophic cycle. NDVI observations from 1981 to 2001 were used for the simulation of the extrinsic variable of a hidden Markov chain model. Observations from 2002 to 2006 served as external validation. Results: The seasonal pattern of P. falciparum incidence was significantly explained by NDVI, with a delay of 15 days (p = 0.001). An NDVI threshold of 0.361 (p = 0.007) provided a Diagnostic Odd Ratio (DOR) of 2.64 (CI95% [1.26;5.52]). The deterministic transmission model, with stochastic environmental factor, predicted an endemoepidemic pattern of malaria infection. The incidences of parasitaemia were adequately modelled, using the observed NDVI as well as the NDVI simulations. Transmission pattern have been modelled and observed values were adequately predicted. The error parameters have shown the smallest values for a monthly model of environmental changes. Conclusion: Remote-sensed data were coupled with field study data in order to drive a malaria transmission model. Several studies have shown that the NDVI presents significant correlations with climate variables, such as precipitations particularly in Sudanese savannah environments. Nonlinear model combining environmental variables, predisposition factors and transmission pattern can be used for community level risk evaluation.
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Genthon, C., Krinner, G., & Castebrunet, H. (2009). Antarctic precipitation and climate-change predictions: horizontal resolution and margin vs plateau issues. Ann. Glaciol., 50(50), 55–60.
Abstract: All climate models participating in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, as made available by the Program for Climate Model Diagnosis and Intercomparison (PCMDI) as the Coupled Model Intercomparison Project 3 (CMIP3) archive, predict a significant surface warming of Antarctica by the end of the 21st century under a moderate (SRESA1B) greenhouse-gas scenario. All models but one predict a concurrent precipitation increase but with a large scatter of results. The models with finer horizontal resolution tend to predict a larger precipitation increase. Because modeled Antarctic surface mass balance is known to be sensitive to horizontal resolution, extrapolating predictions from the different models with respect to model resolution may provide simple yet better multi-model estimates of Antarctic precipitation change than mere averaging or even more complex approaches. Using such extrapolation, a conservative estimate of the predicted precipitation increase at the end of the 21st century is +30 kg m(-2) a(-1) on the grounded ice sheet, corresponding to a >1 mm a(-1) sea-level rise. About three-quarters of this rise originates from the marginal regions of the Antarctic ice sheet with surface elevation below 2250 m. This is where field programs are most urgently needed to better understand and monitor accumulation at the surface of Antarctica, and to improve and verify prediction models.
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Genthon, C., Magand, O., Krinner, G., & Fily, M. (2009). Do climate models underestimate snow accumulation on the Antarctic plateau? A re-evaluation of/from in situ observations in East Wilkes and Victoria Lands. Ann. Glaciol., 50, 61–65.
Abstract: It has been suggested that meteorological and climate models underestimate snow accumulation on the Antarctic plateau, because accumulation (or surface mass balance (SMB)) is dominated by clear-sky precipitation while this process is not properly taken into account in the models. Here, we show that differences between model and field SMB data are much reduced when the in situ SMB reports used to evaluate the models are filtered through quality-control criteria and less reliable reports are subsequently left out. We thus argue that, although not necessarily unsupported, model biases and their interpretations in terms of clear-sky vs synoptic precipitation on the Antarctic plateau may have been overstated in the past. To avoid such misleading issues, it is important that in situ SMB reports of insufficient or unassessed reliability are discarded, even at the cost of a strong reduction in spatial sampling and coverage.
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Gerbaux, M., Hall, N., Dessay, N., & Zin, I. (2009). The sensitivity of Sahelian runoff to climate change. Hydrological Sciences Journal-Journal Des Sciences Hydrologiques, 54(1), 5–16.
Abstract: A simple method is used to study the response of runoff in the Sahel to climate change. The statistical characteristics of rainfall are calculated over the western part of the Sahel for the period 1961-1990, using the BADOPLU network. Daily rainfall is simulated using a Markov process with Weibull distribution for rainfall depths. Runoff is modelled using a conceptual SCS model and the curve numbers are calculated for West Africa. Climate change is provided by simulations using the Arpege GCM (Scenario A1B), and a perturbation method is used on the parameters which describe the rainfall. Changes in rainfall are assumed to occur through increases in frequency, not intensity. Using Arpege, runoff is mainly found to increase, in depth and in number of events, by the end of the 21st century. Changes in evaporation and land use are not included in the analysis. The impact of this 21st century potential climate change (rainfall) on the runoff is found to be of the same magnitude as the impact of changes in land use.
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Girard, L., Weiss, J., Molines, J. M., Barnier, B., & Bouillon, S. (2009). Evaluation of high-resolution sea ice models on the basis of statistical and scaling properties of Arctic sea ice drift and deformation. Journal Of Geophysical Research-Oceans, 114.
Abstract: Sea ice drift and deformation from models are evaluated on the basis of statistical and scaling properties. These properties are derived from two observation data sets: the RADARSAT Geophysical Processor System (RGPS) and buoy trajectories from the International Arctic Buoy Program (IABP). Two simulations obtained with the Louvain-la-Neuve Ice Model (LIM) coupled to a high-resolution ocean model and a simulation obtained with the Los Alamos Sea Ice Model (CICE) were analyzed. Model ice drift compares well with observations in terms of large-scale velocity field and distributions of velocity fluctuations although a significant bias on the mean ice speed is noted. On the other hand, the statistical properties of ice deformation are not well simulated by the models: (1) The distributions of strain rates are incorrect: RGPS distributions of strain rates are power law tailed, i.e., exhibit “wild randomness,” whereas models distributions remain in the Gaussian attraction basin, i.e., exhibit “mild randomness.” (2) The models are unable to reproduce the spatial and temporal correlations of the deformation fields: In the observations, ice deformation follows spatial and temporal scaling laws that express the heterogeneity and the intermittency of deformation. These relations do not appear in simulated ice deformation. Mean deformation in models is almost scale independent. The statistical properties of ice deformation are a signature of the ice mechanical behavior. The present work therefore suggests that the mechanical framework currently used by models is inappropriate. A different modeling framework based on elastic interactions could improve the representation of the statistical and scaling properties of ice deformation.
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Godart, A., Anquetin, S., & Leblois, E. (2009). Rainfall regimes associated with banded convection in the C,vennes-Vivarais area. Meteorology And Atmospheric Physics, 103(1-4), 25–34.
Abstract: It is well known that relief plays an important part in the triggering and enhancement of rainfall. Our study is focussed on the southeastern part of France where the topography and the specific meteorological conditions lead to an important hydrometeorological potential risk. From a climatologic point of view, we consider that these rainfall events arise from two kinds of convection: a deep convection essentially governed by synoptic conditions and where the relief has little direct impact and a shallow convection which is strongly controlled by the air circulation within the relief that leads to banded rain patterns. This study aims at understanding and analysing the atmospheric variables that control banded convection in this region. The methodology is first based on the exploration of the meteorological and rainfall data bases in order to identify the meteorological characteristics associated with this convection. Our results show that banded convection events can be characterized by specific vertical profiles of the dynamics variables (wind velocity, shear) and thermodynamic variables (humidity and wet bulb potential temperature profiles, stratification). We thus propose a generic sounding that is used as an input to the MesoNH meteorological model. Preliminary simulation results show a banded organisation of rainfall, which confirms our selection and allow for sensibility studies and further investigations of meteorological characteristics associated with this particular precipitation pattern.
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Gonzalez-Toril, E., Amils, R., Delmas, R. J., Petit, J. R., Komarek, J., & Elster, J. (2009). Bacterial diversity of autotrophic enriched cultures from remote, glacial Antarctic, Alpine and Andean aerosol, snow and soil samples. Biogeosciences, 6(1), 33–44.
Abstract: Four different communities and one culture of autotrophic microbial assemblages were obtained by incubation of samples collected from high elevation snow in the Alps (Mt. Blanc area) and the Andes (Nevado Illimani summit, Bolivia), from Antarctic aerosol (French station Dumont d'Urville) and a maritime Antarctic soil (King George Island, South Shetlands, Uruguay Station Artigas), in a minimal mineral (oligotrophic) media. Molecular analysis of more than 200 16S rRNA gene sequences showed that all cultured cells belong to the Bacteria domain. Phylogenetic comparison with the currently available rDNA database allowed sequences belonging to Proteobacteria (Alpha-, Beta- and Gamma-proteobacteria), Actinobacteria and Bacteroidetes phyla to be identified. The Andes snow culture was the richest in bacterial diversity (eight microorganisms identified) and the marine Antarctic soil the poorest (only one). Snow samples from Col du Midi (Alps) and the Andes shared the highest number of identified microorganisms (Agrobacterium, Limnobacter, Aquiflexus and two uncultured Alphaproteobacteria clones). These two sampling sites also shared four sequences with the Antarctic aerosol sample (Limnobacter, Pseudonocardia and an uncultured Alphaproteobacteria clone). The only microorganism identified in the Antarctica soil (Brevundimonas sp.) was also detected in the Antarctic aerosol. Most of the identified microorganisms had been detected previously in cold environments, marine sediments soils and rocks. Air current dispersal is the best model to explain the presence of very specific microorganisms, like those identified in this work, in environments very distant and very different from each other.
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Guerin, R., Baltassat, J. M., Boucher, M., Chalikakis, K., Galibert, P. Y., Girard, J. F., et al. (2009). Geophysical characterisation of karstic networks – Application to the Ouysse system (Poumeyssen, France). Comptes Rendus Geoscience, 341(10-11), 810–817.
Abstract: In the framework of the management of karstic aquifers, geophysical reconnaissance can be used to locate conduits and caves, and to characterise the surrounding limestone matrix. Suitable characterisation of heterogeneities in the karstic environment is, however, challenging for ground-based geophysical methods. The present article describes the results, and evaluates the response and accuracy of combined geophysical measurements carried out at the Poumeyssen test site in France, involving electrical resistivity imaging (ERI), magnetic resonance sounding (MRS), “mise-a-la-masse” electrical mapping, and seismic tomography. This site provides the opportunity to study a relatively wide, shallow, water-filled conduit whose location and shape are known from topographic work carried out by cave divers. Seismic and MRS provided the exact location and width of the conduit, to within a I few meters. The seismic and electrical data suggest that the limestone medium surrounding the conduit is not homogeneous. To cite this article: R. Guerin et al., C. R. Geoscience 341 (2009). (C) 2009 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
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Guyot, A., Cohard, J. M., Anquetin, S., Galle, S., & Lloyd, C. R. (2009). Combined analysis of energy and water balances to estimate latent heat flux of a sudanian small catchment. Journal Of Hydrology, 375(1-2), 227–240.
Abstract: Actual evapotranspiration is one of the major components of both energy and water budgets, but is often difficult to monitor over long period with sufficient accuracy. Within the framework of the “AMMA-CATCH” program, a project dedicated to the study of the West African Monsoon, a large aperture scintillometer has been installed in a small catchment (12 km(2)), located in the North of Benin. a region exposed to sudanian climate. The present study is an attempt to estimate the latent heat flux over this small but heterogeneous catchment based on scintillation and ground observations. The analysis covers the end of the dry season (lasting from February to April 2006). During this period two isolated rainfall events occurred, giving a unique opportunity to study energy and water budgets simultaneously. The comparison between the average sensible heat flux derived from scintillometer observations and the one obtained with conventional eddy correlation shows a relatively good agreement, where the scattering is mainly explained by differences in footprint associated with both instruments. A relevant hourly residual latent heat flux is then obtained through the energy balance equation, with careful attention brought to the net radiation, and the ground heat fluxes. The residual of the energy budget equation is compared to soil water losses from vadose zone and water table, in order to evaluate whether this estimation is consistent with the water budget of the ground. Daily soil water depletion within the first meter of the surface shows a similar dynamic as the one calculated from the energy balance equation, but exhibits a constant 1 mm/day lag. The excess of actual evapotranspiration is supposed to be explained by water table losses and root extraction by trees. Finally, this study shows how combined energy and water budget analysis can help to better understand water transfers at the watershed scale. (C) 2009 Elsevier B.V. All rights reserved.
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Hernandez, J. G., Boillat, J. L., Jordan, F., & Hingray, B. (2009). Hydrometeorological forecast on the Rhone River Catchment upstream of Lake Geneva. Houille Blanche-Revue Internationale De L Eau, (5), 61–70.
Abstract: The main goal of the 3(rd) Rhone Correction project is to improve the flood protection in the Upper Rhone River basin. In this context, the MINERVE project aims contributing to a better flow control during flood events, taking advantage from the multireservoirs system existing in the watershed. For this purpose, a hydrometeorological forecast model has been developed as well as a decision support tool for the hydropower plants preventive management.
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Hinderer, J., de Linage, C., Boy, J. P., Gegout, P., Masson, F., Rogister, Y., et al. (2009). The GHYRAF (Gravity and Hydrology in Africa) experiment: Description and first results. Journal Of Geodynamics, 48(3-5), 172–181.
Abstract: This paper is the first presentation of a project called GHYRAF (Gravity and Hydrology in Africa) devoted to the detailed comparison between models and multidisciplinary observations (ground and satellite gravity, geodesy, hydrology, meteorology) of the variations of water storage in Africa from the Sahara and part to the monsoon equatorial part. We describe the various actions planned in this project. We first detail the actions planned in gravimetry which consist in two main surface gravity experiments: on the one hand the periodic repetition of absolute gravity measurements along a north-south monsoonal gradient of rainfall in West Africa, going from Tamanrasset (20 mm/year) in southern Algeria to Djougou (1200 mm/year) in central Benin; on the other hand the continuous measurements at Djougou (Benin) with a superconducting gravimeter to monitor with a higher sampling rate the gravity changes related to an extreme hydrological cycle. Another section describes the actions planned in GPS which will maintain and develop the present-day existing network in West Africa. The third type of actions deals with hydrology and we review the three sites that will be investigated in this joint hydrogeophysics project namely Wankama (near Niamey) and Bagara (near Diffa) in the Niger Sahelian zone and Nalohou (near Djougou) in the Benin monsoon zone. We also address the question of the ground truth of satellite-derived missions: in this context the GHYRAF project will lead to test the hydrology models by comparison both with in situ and satellite data such as GRACE, as well as to an important increase of our knowledge of the seasonal water cycle in Africa. We finally present preliminary results in GPS based on the analysis of the vertical motion of the Djougou site. The resulting absolute gravity changes related to the 2008 monsoon are finally given. (C) 2009 Elsevier Ltd. All rights reserved.
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Hindmarsh, R. C. A., Vieli, G. J. M. C. L., & Parrenin, F. (2009). A large-scale numerical model for computing isochrone geometry. Ann. Glaciol., 50(51), 130–140.
Abstract: A finite-difference model for the calculation of radar layer geometries in large ice masses is presented. Balance velocities are used as coefficients in the age equation and in the heat equation. Solution of the heat equation allows prediction of sliding areas and computation of basal melt rates. Vertical distributions of velocity are parameterized using shape functions. These can be set uniformly, or allowed to vary in space according to the distribution of sliding. The vertical coordinate can either be uniformly distributed within the thickness of the ice, or be uniformly distributed within the flux. The finite-difference scheme results in a large set of linear equations. These are solved using a nested factorization preconditioned conjugate gradient scheme. The convergence properties of some other iteration solution schemes are studied. The output is computations of age and temperature assuming steady state, in large ice masses at high resolution. Age calculations are used to generate isochrones which show the best fit to observed layers. Comparisons with analytical solutions are made, and the influence of the order of the finite-difference approximation and the choice of vertical coordinate on solution accuracy is considered.
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Hong, S. M., Lee, K., Hou, S. G., Hur, S. D., Ren, J. W., Burn, L. J., et al. (2009). An 800-Year Record of Atmospheric As, Mo, Sn, and Sb in Central Asia in High-Altitude Ice Cores from Mt. Qomolangma (Everest), Himalayas. Environ. Sci. Technol., 43(21), 8060–8065.
Abstract: As, Mo, Sn, and Sb have been determined by inductively coupled plasma sector field mass spectrometry (ICP-SFMS) in 143 depth intervals of high-altitude ice cores from Mt. Everest covering an 800-year time period from 1205 to 2002 AD. The results clearly demonstrate the long-term historical record of atmospheric transport and deposition of As, Mo, Sn, and Sb that has prevailed at high altitudes in the central Himalayas. Natural contributions, mainly from mineral dust, have dominated the atmospheric cycles of As, Mo, Sn, and to some extent Sb during the 700 years prior to the 20th century. Compared to those of the pre-1900 period, pronounced increases of both concentrations and crustal enrichment factors are observed since the 1970s, with the highest increase factor for Sri and the lowest for As. Such increases are attributed to anthropogenic emissions of these elements, largely from stationary fossil fuel combustion and nonferrous metals production, particularly in India. Our central Himalayan ice core record provides an explicit recognition of rising atmospheric As, Mo, Sn, and Sb pollution in response to rapid economic growth in central Asia.
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Huiban, Y., Noirez, S., Prinzhofer, A., Girard, J. P., & Chappellaz, J. (2009). Chemical and isotopic analysis of hydrocarbon gas at trace levels Methodology and results. Chem. Geol., 265(3-4), 363–368.
Abstract: Isotopic mass spectrometry coupled online with gas chromatography (GC-C-IRMS) permits measurement of relative proportions of gaseous hydrocarbon (CH4 to C4H10) and CO2, and determination of carbon isotope ratio of hydrocarbon molecules. Access to these parameters provides valuable information about the source and the genesis of naturally-occurring gas, as well as on post-formation physico-chemical processes which might have taken place in the geological environment. In particular, it is possible to distinguish hydrocarbon gas of bacterial origin from that of thermogenic origin based on proportion and carbon isotope ratio of methane as measured by GC-C-IRMS. However, in samples containing very low amounts of hydrocarbons (from I ppm to 1000 ppm), accurate measurement of isotope ratios is often impossible due to the limitations of conventional GC-C-IRMS techniques using direct sample introduction. A technique was developed to overcome this limitation. It is based on a novel approach allowing pre-concentration of hydrocarbons prior to GC-C-IRMS analysis. The pre-concentration step consists in selective trapping of hydrocarbon molecules on a cold adsorbent phase, and removal of non-adsorbed gases (N-2, O-2, Ar,...). In a second step, pre-concentrated alkanes are desorbed, and released in an inert carrier gas, focused through a capillary and introduced into the GC-C-IRMS for chromatographic separation and measurement of concentration and carbon isotope composition of each individual carbon molecule. In order to achieve sufficient accuracy, several operating conditions are of prime importance, including sufficient signal intensity, well defined peak shape and low signal/noise ratio. Accurate measurements can be performed on samples as small as 10 cm(3) of bulk gas in standard conditions, with concentrations as low as 1 ppm of methane, 0.5 ppm of ethane and 0.3 ppm of propane and butane. Total analytical uncertainty on delta C-13 measurements ranges from +/-0.2 parts per thousand to +/-1.5 parts per thousand, depending on the hydrocarbon molecule. (C) 2009 Elsevier B.V. All rights reserved.
Keywords: Carbon isotopes; Pre-concentration; Hydrocarbon traces; Cryofocusation
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Isaksen, I. S. A., Granier, C., Myhre, G., Berntsen, T. K., Dalsoren, S. B., Gauss, M., et al. (2009). Atmospheric composition change: Climate-Chemistry interactions. Atmos. Environ., 43(33), 5138–5192.
Abstract: Chemically active climate compounds are either primary compounds like methane (CH4), removed by oxidation in the atmosphere, or secondary compounds like ozone (O-3), sulfate and organic aerosols, both formed and removed in the atmosphere. Man-induced climate-chemistry interaction is a two-way process: Emissions of pollutants change the atmospheric composition contributing to climate change through the aforementioned climate components, and climate change, through changes in temperature, dynamics, the hydrological cycle, atmospheric stability, and biosphere-atmosphere interactions, affects the atmospheric composition and oxidation processes in the troposphere. Here we present progress in our understanding of processes of importance for climate-chemistry interactions, and their contributions to changes in atmospheric composition and climate forcing. A key factor is the oxidation potential involving compounds like O-3 and the hydroxyl radical (OH). Reported studies represent both current and future changes. Reported results include new estimates of radiative forcing based on extensive model studies of chemically active climate compounds like O-3, and of particles inducing both direct and indirect effects. Through EU projects like ACCENT, QUANTIFY, and the AeroCom project, extensive studies on regional and sector-wise differences in the impact on atmospheric distribution are performed. Studies have shown that land-based emissions have a different effect on climate than ship and aircraft emissions, and different measures are needed to reduce the climate impact. Several areas where climate change can affect the tropospheric oxidation process and the chemical composition are identified. This can take place through enhanced stratospheric-tropospheric exchange of ozone, more frequent periods with stable conditions favoring pollution build up over industrial areas, enhanced temperature induced biogenic emissions, methane releases from permafrost thawing, and enhanced concentration through reduced biospheric uptake. During the last 5-10 years, new observational data have been made available and used for model validation and the study of atmospheric processes. Although there are significant uncertainties in the modeling of composition changes, access to new observational data has improved modeling capability. Emission scenarios for the coming decades have a large uncertainty range, in particular with respect to regional trends, leading to a significant uncertainty range in estimated regional composition changes and climate impact. (C) 2009 Elsevier Ltd. All rights reserved.
Keywords: Atmosphere climate chemistry; Feedbacks modelling
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Jenk, T. M., Szidat, S., Bolius, D., Sigl, M., Gaggeler, H. W., Wacker, L., et al. (2009). A novel radiocarbon dating technique applied to an ice core from the Alps indicating late Pleistocene ages. J. Geophys. Res.-Atmos., 114, 8 pp.
Abstract: Ice cores retrieved from high-altitude glaciers are important archives of past climatic and atmospheric conditions in midlatitude and tropical regions. Because of the specific flow behavior of ice, their age-depth relationship is nonlinear, preventing the application of common dating methods such as annual layer counting in the deepest and oldest part. Here we present a new approach and technique, allowing dating of any such ice core at arbitrary depth for the age range between similar to 500 years B. P. and the late Pleistocene. This new, complementary dating tool has great potential for numerous ice core related paleoclimate studies since it allows improvement and extension of existing and future chronologies. Using small to ultrasmall sample size (100 μg > carbon content > 5 μg) accelerator mass spectrometry, we take advantage of the ice-included, water-insoluble organic carbon fraction of carbonaceous aerosols for radiocarbon (C-14) dating. Analysis and dating of the bottom ice of the Colle Gnifetti glacier (Swiss-Italian Alps, 45 degrees 55'50 '' N, 7 degrees 52'33 '' E, 4455 m asl) has been successful in a first application, and the results revealed the core to cover most of the Holocene at the least with indication for late Pleistocene ice present at the very bottom.
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Jitaru, P., Gabrielli, P., Marteel, A., Plane, J. M. C., Planchon, F. A. M., Gauchard, P. A., et al. (2009). Atmospheric depletion of mercury over Antarctica during glacial periods. Nat. Geosci., 2(7), 505–508.
Abstract: Mercury is a globally dispersed toxic metal that affects even remote polar areas. During seasonal atmospheric mercury depletion events in polar areas, mercury is removed from the atmosphere(1,2) and subsequently deposited in the surface snows(3). However, it is unknown whether these events, which have been documented for the past two decades, have occurred in the past. Here we show that over the past 670,000 years, atmospheric mercury deposition in surface snows was greater during the coldest climatic stages, coincident with the highest atmospheric dust loads. A probable explanation for this increased scavenging is that the oxidation of gaseous mercury by sea-salt-derived halogens occurred in the cold atmosphere. The oxidized mercury compounds were then transferred to the abundant mineral dust particles and deposited on the snowpack, leading to the depletion of gaseous mercury in the Antarctic atmosphere. We conclude that polar regions acted as a mercury sink during the coldest climatic stages, and that substantial polar deposition of atmospheric mercury is therefore not an exclusively recent phenomenon.
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Jost, A., Fauquette, S., Kageyama, M., Krinner, G., Ramstein, G., Sue, J. P., et al. (2009). High resolution climate and vegetation simulations of the Late Pliocene, a model-data comparison over western Europe and the Mediterranean region. Clim. Past., 5(4), 585–606.
Abstract: Here we perform a detailed comparison between climate model results and climate reconstructions in western Europe and the Mediterranean area for the mid-Piacenzian warm interval (ca 3 Myr ago) of the Late Pliocene epoch. This region is particularly well suited for such a comparison as several quantitative climate estimates from local pollen records are available. They show evidence for temperatures significantly warmer than today over the whole area, mean annual precipitation higher in northwestern Europe and equivalent to modern values in its southwestern part. To improve our comparison, we have performed high resolution simulations of the mid-Piacenzian climate using the LMDz atmospheric general circulation model (AGCM) with a stretched grid which allows a finer resolution over Europe. In a first step, we applied the PRISM2 (Pliocene Research, Interpretation, and Synoptic Mapping) boundary conditions except that we used modern terrestrial vegetation. Second, we simulated the vegetation for this period by forcing the ORCHIDEE (Organizing Carbon and Hydrology in Dynamic Ecosystems) dynamic global vegetation model (DGVM) with the climatic outputs from the AGCM. We then supplied this simulated terrestrial vegetation cover as an additional boundary condition in a second AGCM run. This gives us the opportunity to investigate the model's sensitivity to the simulated vegetation changes in a global warming context. Model results and data show a great consistency for mean annual temperatures, indicating increases by up to 4 degrees C in the study area, and some disparities, in particular in the northern Mediterranean sector, as regards winter and summer temperatures. Similar continental mean annual precipitation and moisture patterns are predicted by the model, which broadly underestimates the wetter conditions indicated by the data in northwestern Europe. The biogeophysical effects due to the changes in vegetation simulated by ORCHIDEE are weak, both in terms of the hydrological cycle and of the temperatures, at the regional scale of the European and Mediterranean mid-latitudes. In particular, they do not contribute to improve the model-data comparison. Their main influence concerns seasonal temperatures, with a decrease of the temperatures of the warmest month, and an overall reduction of the intensity of the continental hydrological cycle.
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Jouanno, J., Sheinbaum, J., Barnier, B., & Molines, J. M. (2009). The mesoscale variability in the Caribbean Sea. Part II: Energy sources. Ocean Modelling, 26(3-4), 226–239.
Abstract: The processes which drive the production and the growth of the strong mesoscale eddy field in the Caribbean Sea are examined using a general circulation model. Diagnostics of the simulations suggest that: (1) The mean currents in the Caribbean Sea are intrinsically unstable. The nature of the instability and its strength vary spatially due to strong differences of current structure among basins. (2) The greatest and most energetic eddies of the Caribbean Sea originate in the Venezuela Basin by mixed barotropic-baroclinic instability of an intense jet, formed with waters mostly froth the surface return flow of the Meridional Overturning Circulation and the North Equatorial Current which converge and accelerate through the Grenada Passage. The vertical shear of this inflow is enhanced by an eastward undercurrent, which flows along the south American Coast between 100 and 250 to depth. The shallow eddies (less than 200 m depth) formed in the vicinity of the Grenada Passage get rapidly deeper (down to 1000 to depth) and stronger by their interaction with the deep interior flow of the Subtropical Gyre, which enters through passages north of St. Lucia. These main eastern Caribbean inflows merge and form the southern Caribbean Current, whose baroclinic instability is responsible for the westward growth and strengthening of these eddies from the Venezuela to the Colombia Basin. (3) Eddies of lesser strength are produced in other regions of the Caribbean Sea. Their generation and growth is also linked with instability of the local currents. First, cyclones are formed in the cyclonic shear of the northern Caribbean Current, but appear to be rapidly dissipated or absorbed by the large anticyclones coming from the southern Caribbean. Second, eddies in the Cayman Sea, which impact the Yucatan region, are locally produced and enhanced by barotropic instability of the deep Cayman Current. (4) The role of the North Brazil Current (NBC) rings is mostly to act as a finite perturbation for the instability of the mean flow. Their presence near the Lesser Antilles is ubiquitous and they appear to be linked with most of the Caribbean eddies. There are some evidences that the frequency at which they form near the Grenada Passage is influenced by the frequency at which the NBC rings impinge the Lesser Antilles. But large Caribbean eddies also form without a close influence of any ring, and comparison between simulations shows that mean eddy kinetic energy and eddy population in the Caribbean Sea are not substantially different in absence or presence of NBC rings: their presence is not a necessary condition for the generation and growth of the Caribbean eddies. (C) 2008 Elsevier Ltd. All rights reserved.
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Kelle, L., Gratiot, N., & de Thoisy, B. (2009). Olive ridley turtle Lepidochelys olivacea in French Guiana: back from the brink of regional extirpation? Oryx, 43(2), 243–246.
Abstract: The estimated number of olive ridley marine turtles Lepidochelys olivacea nesting annually in 2002-2007 in French Guiana was 1,716-3,257, the highest ever recorded in the country and similar to nesting numbers recorded in neighbouring Suriname c. 40 years ago, where the species has now severely declined. A shift of nesting females from Suriname to French Guiana beaches and improvement of nationwide marine turtle monitoring appear to be the most plausible explanations for the current high level of nesting recorded in French Guiana. The species' nesting status in French Guiana therefore appears less critical than previously documented but ongoing threats suggest the need to reinforce regional conservation efforts in the West Atlantic.
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Khvorostyanov, D. V., Krinner, G., Ciais, P., Heimann, M., & Zimov, S. A. (2009). Reply to L. Kutzbach. Tellus Ser. B-Chem. Phys. Meteorol., 61(3), 579–580.
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Kirstetter, P. E., Delrieu, G., & Andrieu, H. (2009). Radar quantitative precipitation estimation : inference of the vertical profile of reflectivity, radar-raingauge error model. Houille Blanche-Revue Internationale De L Eau, (6), 150–156.
Abstract: This paper summarizes our PhD research conducted at Laboratoire d'etude des Transferts en Hydrologie et Environnement. The Cevennes-Vivarais Mediterranean Hydrometeorological Observatory is a research initiative aimed at improving the rainfall observation. A number of innovative quantitative precipitation estimation (QPE) algorithms aimed at a space-time adaptive radar data processing were developed using the data-set of the Bollene 2002 Experiment. The last involved the operational weather radar of Bollene, which is part of the ARAMIS network. Our contribution in the present thesis deals with the vertical heterogeneity of rainfall. We consider the vertical profile of reflectivity inversion method proposed by Andrieu et Creutin (1995). It is adapted to the case of time-varying geographical supports designed with preliminary rain typing, Characterizing the error structure of radar quantitative precipitation estimates is recognized as a major issue. We assess the radar QPE with respect to reference rain estimates derived from rain gauge networks. A geostatistical framework is proposed for the establishment of such reference estimates. Assuming the residual between radar and reference value to be a random variable, we describe radar error by mean of probability distributions. The model consists in an additive random error, described by a random distribution. Some elements of space-time error structure are given. A new approach to determine the vertical profile of reflectivity (VPR) is proposed. It is based on a simplified microphysics in synergy with radar observations to simulate the VPR. A physically-based model of reflectivity is coupled with the radar sampling model to describe a VPR with a reduced number of physical parameters. The new identified VPRs are thus less refined but more valid, since the microphysical model imposes a physically coherent frame which prevents the drawbacks of the statistical constraints met by the initial method to fit the VPR to the observations.
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Koven, C., Friedlingstein, P., Ciais, P., Khvorostyanov, D., Krinner, G., & Tarnocai, C. (2009). On the formation of high-latitude soil carbon stocks: Effects of cryoturbation and insulation by organic matter in a land surface model. Geophys. Res. Lett., 36, L21501.
Abstract: We modify the soil component of the ORCHIDEE terrestrial carbon cycle model to include vertically-discretized soil carbon. With this model, we investigate the feedback of considering thermal insulation by soil carbon, which modifies the soil thermal regime by lowering the thermal conductivity and increasing the heat capacity of a carbon-rich soil, on the total carbon stocks the model builds up. In addition, we demonstrate the effects of diffusive vertical mixing of soil organic matter by cryoturbation on the total carbon stocks that the model builds up in mineral soils in equilibrium with a steady climate. We show that including these two effects together leads to up to 30% higher soil carbon stocks in the top meter of permafrost soils, as well as large stocks of carbon below 1m in the upper permafrost soil layers. The vertical profile of partitioning of carbon between different lability pools is also affected, as the slower pools are more deeply mixed; also the time to reach equilibrium lengthens considerably. These effects are largest in the coldest regions such as Eastern Siberia. The inclusion of cryoturbative mixing and insulation by soil carbon leads to better agreement with estimates of high-latitude soil carbon stocks, where substantial amounts of carbon are found in permafrost regions, to depths of three meters; however we do not include peat, Yedoma, or alluvial deposition processes here, so the total carbon stocks are still lower than observed. Citation: Koven, C., P. Friedlingstein, P. Ciais, D. Khvorostyanov, G. Krinner, and C. Tarnocai (2009), On the formation of high-latitude soil carbon stocks: Effects of cryoturbation and insulation by organic matter in a land surface model, Geophys. Res. Lett., 36, L21501, doi:10.1029/2009GL040150.
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Lacroix, P., Legresy, B., Remy, F., Blarel, F., Picard, G., & Brucker, L. (2009). Rapid change of snow surface properties at Vostok, East Antarctica, revealed by altimetry and radiometry. Remote Sens. Environ., 113(12), 2633–2641.
Abstract: We present results of snow surface properties using the EWSAT dual frequency altimeter at S (3.2 GHz) and Ku (13.6 GHz) bands and the AMSR-E microwave radiometer at frequencies ranging between 6 and 36 GHz in the Vostok region, East Antarctica. The altimetric time series observed between 2002 and 2008 show variations at 3 different time scales (daily, seasonal and inter-annual), that correlate directly with variations in the snow surface properties. In this study we focus on the analysis of the rapid daily event, occurring on February 14th 2005, that created a jump of the backscatter coefficient of up to 5.3 dB at the S band and 2.5 dB at the Ku band. The ratio of V/H-polarization brightness temperature slowly decreased in December and January 2005, and suddenly increased on February 14th 2005. The origin of this rapid event is investigated using AWS data from Vostok station, altimetric and radiometric data simultaneously. Both snow surface density and roughness are found to vary during this event. This event is shown to be synchronous with strong wind occuring during a period of anomalous wind direction, and the presence of surface hoar. These particular conditions certainly modified the snow surface roughness and thus impacted the altimetric signal. We finally investigate the impact of this event on the calculation of the regional ice-sheet mass-balance using different corrections of height with echo shape variations. It is shown to be negligible only if the full echo shape correction (Legresy et al., 2006) is used. (C) 2009 Elsevier Inc. All rights reserved.
Keywords: Ice-sheets; Microwave; Altimetry; Radiometry; Snow surface properties; Mass-balance
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Laj, P., Bonasoni, P., Roger, J. C., Sellegri, K., Venzac, H., Villani, P., et al. (2009). Aerosol radiative forcing estimated from in situ measurements at the NCO-P station (5100m, Nepal). Geochim. Cosmochim. Acta, 73(13), A715.
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Laj, P., Klausen, J., Bilde, M., Plass-Duelmer, C., Pappalardo, G., Clerbaux, C., et al. (2009). Measuring atmospheric composition change. Atmos. Environ., 43(33), 5351–5414.
Abstract: Scientific findings from the last decades have clearly highlighted the need for a more comprehensive approach to atmospheric change processes. In fact, observation of atmospheric composition variables has been an important activity of atmospheric research that has developed instrumental tools (advanced analytical techniques) and platforms (instrumented passenger aircrafts, ground-based in situ and remote sensing stations, earth observation satellite instruments) providing essential information on the composition of the atmosphere. The variability of the atmospheric system and the extreme complexity of the atmospheric cycles for short-lived gaseous and aerosol species have led to the development of complex models to interpret observations, test our theoretical understanding of atmospheric chemistry and predict future atmospheric composition. The validation of numerical models requires accurate information concerning the variability of atmospheric composition for targeted species via comparison with observations and measurements. In this paper, we provide an overview of recent advances in instrumentation and methodologies for measuring atmospheric composition changes from space, aircraft and the surface as well as recent improvements in laboratory techniques that permitted scientific advance in the field of atmospheric chemistry. Emphasis is given to the most promising and innovative technologies that will become operational in the near future to improve knowledge of atmospheric composition. Our current observation capacity, however, is not satisfactory to understand and predict future atmospheric composition changes, in relation to predicted climate warming. Based on the limitation of the current European observing system, we address the major gaps in a second part of the paper to explain why further developments in current observation strategies are still needed to strengthen and optimise an observing system not only capable of responding to the requirements of atmospheric services but also to newly open scientific questions. (C) 2009 Elsevier Ltd. All rights reserved.
Keywords: Atmosphere; Instrumentation; Observation; Air quality; Climate
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Langlais, C., Barnier, B., Molines, J. M., Fraunie, P., Jacob, D., & Kotlarski, S. (2009). Evaluation of a dynamically downscaled atmospheric reanalyse in the prospect of forcing long term simulations of the ocean circulation in the Gulf of Lions. Ocean Modelling, 30(4), 270–286.
Abstract: The paper evaluates atmospheric reanalysis as possible forcing of model simulations of the ocean circulation inter-annual variability in the Gulf of Lions in the Western Mediterranean Sea between 1990 and 2000. The sensitivity of the coastal atmospheric patterns to the model resolution is investigated using the REMO regional climate model (18 km, I h), and the recent global atmospheric reanalysis ERA40 (125 km, 6 h). At scales from a few years to a few days, both atmospheric data sets exhibit a very similar weather, and agreement between REMO and ERA40 is especially good on the seasonal cycle and at the daily variability scale. At smaller scales, REMO reproduces more realistic spatio-temporal patterns in the ocean forcing: specific wind systems, particular atmospheric behaviour on the shelf, diurnal cycle, sea-breeze. Ocean twin experiments (1990-1993) clearly underline REMO skills to drive dominant oceanic processes in this microtidal area. Finer wind patterns induce a more realistic circulation and hydrology of the shelf water: unique shelf circulation, upwelling, temperature and salinity exchanges at the shelf break. The hourly sampling of REMO introduces a diurnal forcing which enhances the behaviour of the ocean mixed layer. In addition, the more numerous wind extremes modify the exchanges at the shelf break: favouring the export of dense shelf water, enhancing the mesoscale variability and the interactions of the along slope current with the bathymetry. (C) 2009 Elsevier Ltd. All rights reserved.
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Langlois, A., Brucker, L., Kohn, J., Royer, A., Derksen, C., Cliche, P., et al. (2009). Simulation of Snow Water Equivalent (SWE) Using Thermodynamic Snow Models in Quebec, Canada. J. Hydrometeorol., 10(6), 1447–1463.
Abstract: Snowcover plays a key role in the climate system by influencing the transfer of energy and mass between the soil and the atmosphere. In particular, snow water equivalent (SWE) is of primary importance for climatological and hydrological processes and is a good indicator of climate variability and change. Efforts to quantity SWE over land from spaceborne passive microwave measurements have been conducted since the 1980s, but a more suitable method has yet to be developed for hemispheric-scale studies. Tools such as snow thermodynamic models allow for a better understanding of the snow cover and can potentially significantly improve existing snow products at the regional scale. In this study, the use of three snow models [SNOWPACK, CROCUS, and Snow Thermal Model (SNTHERM)] driven by local and reanalysis meteorological data for the simulation of SWE is investigated temporally through three winter seasons and spatially over intensively sampled sites across northern Quebec. Results show that the SWE simulations are in agreement with ground measurements through three complete winter seasons (2004/05, 2005/06, and 2007/08) in southern Quebec, with higher error for 2007/08. The correlation coefficients between measured and predicted SWE values ranged between 0.72 and 0.99 for the three models and three seasons evaluated in southern Quebec. In subarctic regions, predicted SWE driven with the North American Regional Reanalysis (NARR) data fall within the range of measured regional variability. NARR data allow snow models to be used regionally, and this paper represents a first step for the regionalization of thermodynamic multilayered snow models driven by reanalysis data for improved global SWE evolution retrievals.
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Lauvernet, C., Brankart, J. M., Castruccio, F., Broquet, G., Brasseur, P., & Verron, J. (2009). A truncated Gaussian filter for data assimilation with inequality constraints: Application to the hydrostatic stability condition in ocean models. Ocean Modelling, 27(1-2), 1–17.
Abstract: In many data assimilation problems, the state variables are subjected to inequality constraints. These constraints often contain valuable information that must be taken into account in the estimation process. However, with linear estimation methods (like the Kalman filter), there is no way to incorporate optimally that kind of additional information. In this study, it is shown that an optimal filter dealing with inequality constraints can be formulated under the assumption that the probability distributions are truncated Gaussian distributions. The statistical tools needed to implement this truncated Gaussian filter are described. It is also shown how the filter can be adapted to work in a reduced dimension space, and flow it can be simplified following several additional hypotheses. As an application, the truncated Gaussian assumption is shown to be adequate to deal with the condition of hydrostatic stability in ocean analyses. First, a detailed evaluation of the method is made using a one-dimensional z-coordinate model of the mixed layer: particular attention is paid to the parameterization of the probability distribution, the accuracy of the estimation and the sensitivity to the observation system. In a second step, the method is applied to a three-dimensional hybrid coordinate ocean model (HYCOM) of the Bay of Biscay (at a 1/15 degrees resolution), to show that it is efficient enough to be applied to real size problems. These examples also demonstrate that the algorithm can deal with the hydrostatic stability condition in isopycnic coordinates as well as in z-coordinates. (C) 2008 Elsevier Ltd. All rights reserved.
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Lebel, T., & Ali, A. (2009). Recent trends in the Central and Western Sahel rainfall regime (1990-2007). Journal Of Hydrology, 375(1-2), 52–64.
Abstract: One motivation for setting up the CATCH (Couplage de l'Atmosphere Tropicale et du Cycle Hydrologique) project at the end of the 1990s, was to contribute to documenting the Sahelian rainfall variability at the interannual scale and to provide a fine monitoring of possible long-term trends of the rainfall regime. This paper is a first attempt at characterising the Sahelian rainfall regime of the two last decades (19902007) by comparison to the rainfall regime of the previous decades, namely the 20-year wet period 19501969 and the 20-year dry period 1970-1989. While the rainfall deficit remained unabated in the Western Sahel (1990-2007 mean equal to the 1970-1989 mean, both being lower than the 1950-1969 mean), the Central Sahel progressively recorded wetter years from the end of the 1990s, but this recovery is limited (1990-2007 average larger by 10% than the 1970-1989 average, but still lower than the 1950-1989 average). There are also significant differences between the Western Sahel and the Central Sahel when looking at the interannual variability pattern and at the seasonal cycle. The low-frequency rainfall patterns are similar between the Western Sahel and the Central Sahel, but the interannual year-to-year variability is weakly related to each other. in the Central Sahel, the major modification of the seasonal cycle in the most recent decades was the disappearance of the well marked August peak observed during the wet period. In the Western Sahel the rainfall deficit is more or less evenly distributed all along the rainy season. The second part of the paper makes use of the CATCH-Niger recording rain gauge network in order to compare several ways of defining rainy events. The statistical properties of these various populations of rainy events are compared. It is shown that a simple CPP model allows for retrieving the statistical characteristics of point rainy events from daily rainfall series. It is also confirmed that in this area, the interannual rainfall variability is primarily linked to the year-to-year fluctuation of the number of large mesoscale rainfall events. (C) 2008 Elsevier B.V. All rights reserved.
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Lebel, T., Cappelaere, B., Galle, S., Hanan, N., Kergoat, L., Levis, S., et al. (2009). AMMA-CATCH studies in the Sahelian region of West-Africa: An overview. Journal Of Hydrology, 375(1-2), 3–13.
Abstract: The African Monsoon Multidisciplinary Analysis (AMMA) is an international and interdisciplinary experiment designed to investigate the interactions between atmospheric, oceanic and terrestrial systems and their joint controls on tropical monsoon dynamics in West Africa. This special issue reports results from a group of AMMA studies regrouped in the component “Couplage de l'Atmosphere Tropicale et du Cycle Hydrologique” (CATCH). AMMA-CATCH studies focus on measuring and understanding land surface properties and processes in West Africa, the role of terrestrial systems in altering boundary layer dynamics, and thus the potential that surface hydrology and biology, and human land use practices, may directly or indirectly affect monsoon dynamics and rainfall in the region. AMMA-CATCH studies focus on three intensively instrumented mesoscale sites in Mali, Niger and Benin that sample across the 1001300 mm/annum rainfall gradient of the Sahel, Sudan and North-Guinean bioclimatic zones. Studies report on: (i) surface-boundary layer interactions that may influence atmospheric convergence and convective processes and thus rainfall type, timing and amount; (ii) vegetation dynamics at seasonal to decadal time-scales that may respond to, and alter, atmospheric processes; (iii) surface-atmosphere fluxes of heat, water and carbon dioxide that directly influence the atmosphere; (iv) soil moisture variability in space and time that provide the proximate control on vegetation activity, evapotranspiration and energy balance; and (v) local and mesoscale modeling of hydrology and land surface-atmosphere exchanges to assess their role in the hydrological, atmospheric and rainfall dynamics of West Africa. The AMMA-CATCH research reported in this issue will be extended in future years as measurements and analysis continue and are concluded within the context of both CATCH and the wider AMMA study. This body of research will contribute to an improved understanding of the functioning of the coupled West African system, and enhance our ability to model and predict rainfall, vegetation and biogeochemical dynamics across time-scales (day, year, decade, and century), and in response to changing climate and land use. Such information is vital for policy makers and managers in planning for future economic development, sustainability and livelihoods of the growing populations of the region. (C) 2009 Elsevier B.V. All rights reserved.
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Lebensohn, R. A., Montagnat, M., Mansuy, P., Duval, P., Meysonnier, J., & Philip, A. (2009). Modeling viscoplastic behavior and heterogeneous intracrystalline deformation of columnar ice polycrystals. Acta Mater., 57(5), 1405–1415.
Abstract: A Full-field formulation based on fast Fourier transforms (FFT) has been adapted and used to predict the micromechanical fields that develop in two-dimensional columnar Ih ice polycrystals deforming in compression by dislocation creep. The predicted intragranular mechanical fields are in qualitative good agreement with experimental observations, in particular those involving the formation of shear and kink bands. These localized bands are associated with the large internal stresses that develop during, creep in such anisotropic material, and their location, intensity, morphology and extension are found to depend strongly on the crystallographic orientation of the grains and on their interaction with neighboring crystals. The predictions of the model are also discussed in relation to the deformation of columnar sea and lake ice, its well as with the mechanical behavior of granular ice of glaciers and polar ice sheets. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
Keywords: Ice; Creep; Microstructure; Shear bands; Micromechanical modeling
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Lebyodkin, M. A., Lebedkina, T. A., Chmelik, F., Lamark, T. T., Estrin, Y., Fressengeas, C., et al. (2009). Intrinsic structure of acoustic emission events during jerky flow in an Al alloy. Phys. Rev. B, 79(17), 6 pp.
Abstract: Scaling behavior is found in acoustic-emission events associated with stress drops observed in velocity-driven plastic deformation of an Al alloy, which exhibits jerky plastic flow. The occurrence of scaling proves that these acoustic-emission events, which are commonly regarded as “elementary” ones, have a small-scale self-organized structure comprising a group of peaks correlated in time. This structure reveals details of the temporal variation in elementary plastic events at a microsecond scale, which are hardly accessible by other measurement techniques.
Keywords: acoustic emission; aluminium alloys; plastic flow; self-assembly
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Legchenko, A., Ezersky, M., Camerlynck, C., Al-Zoubi, A., & Chalikakis, K. (2009). Joint use of TEM and MRS methods in a complex geological setting. Comptes Rendus Geoscience, 341(10-11), 908–917.
Abstract: Transient Electromagnetic (TEM), known also as Time Domain Electromagnetic (TDEM) and Magnetic Resonance Sounding (MRS) methods were applied jointly to investigate variations in lithology and groundwater salinity in the Nahal Hever South area (Dead Sea coast of Israel). The subsurface in this area is highly heterogeneous and composed of intercalated sand and clay layers over a salt formation, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100-225 g/l. TEM is known as an efficient tool for investigating electrically conductive targets like saline water, but it is sensitive to both the salinity of groundwater and the porosity of rocks. MRS, however, is sensitive primarily to groundwater volume, but it also allows delineating of lithological variations in water-saturated formations. MRS is much less sensitive to variations in groundwater salinity in comparison with TEM. We show that MRS enables us to resolve the fundamental uncertainty in TEM interpretation caused by the equivalence between groundwater resistivity and lithology. Combining TEM and MRS, we found that the sandy Dead Sea aquifer filled with Dead Sea brine is characterized by a bulk resistivity of rho(x) > 0.4 Omega m, whereas zones with silt and clay in the subsurface are characterized by a bulk resistivity of rho(x) < 0.4 Omega m. These observations are confirmed by calibration of the TEM method performed near 18 boreholes. To cite this article: A. Legchenko et al., C R. Geoscience 341 (2009). (C) 2009 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.
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Legout, A., Legout, C., Nys, C., & Dambrine, E. (2009). Preferential flow and slow convective chloride transport through the soil of a forested landscape (Fougeres, France). Geoderma, 151(3-4), 179–190.
Abstract: This study aims to assess the water flow and non-reactive solute transfers occurring in a glossic acidic soil under a beech forest in Brittany (Fougeres, France). The specific objectives were to study the water and chloride transfers in this soil, to understand the spatial and temporal variability of these transfers and to produce a data set in this forest site for future modelling. For this, we carried out a field tracer experiment and sprinkled chloride enriched solution over two areas of soil (2 x 66 m(2)) in March, 2006. Subsequently, we monitored the composition of the soil solutions collected by zero tension plate lysimeters and ceramic cup lysimeters installed at depths between 0 and 240 cm, over a period of 18 months. We prove that preferential flow through rapid-mobile porosity and slow transfers by convective flux though slow-mobile porosity coexist in the soils of the experimental plot, and that the time scales brought into play ranged from a few days to a yearly scale. The transfer velocities ranged between 2.38 mm day(-1) for the slowest convective flux and 600 mm day(-1) for the fastest preferential flows. We also prove that the rapid-mobile porosity represents only a small proportion of the soil volume (the mean of all depths, except 10 cm, was about 11%) but the quantity of solute transferred, which by-passes a large part of the rooting zone, may be important (around 17% of the tracer mass applied). The rapid transfer is mainly governed by the soil moisture combined with precipitation intensities and the slow transfer mainly by the cumulative percolation flux. Both transfers are also characterised by wide spatial and temporal variability. The wide transfer variability may be explained by the hydrodynamic dispersion related to the heterogeneity of the slow and rapid porosities, combined with the impact of the 006 growing season, which slowed down the tracer displacement. Lastly, the experiment proves that the zero tension plate lysimeters mainly collect rapid drainage water, as preferential flows, while the ceramic cup lysimeters mainly collect slow-mobile water mixed with rapid drainage water. (C) 2009 Elsevier B.V. All rights reserved.
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Legout, C., Molenat, J., & Hamon, Y. (2009). Experimental and Modeling Investigation of Unsaturated Solute Transport with Water-Table Fluctuation. Vadose Zone Journal, 8(1), 21–31.
Abstract: We studied the transport of a nonreactive tracer in an unconsolidated weathered granite column under steady-state flow and transient flow induced by water-table fluctuation in the bottom half of the column. Experiments reproducing one-dimensional vertical flow and tracer transport were performed at two infiltration rates, 12 and 22 mm h(-1). Breakthrough data were used (i) to compare tracer dispersion under steady-state and transient flow, (ii) to analyze the mechanisms responsible for tracer transport under both flow conditions, and (iii) to test the ability of existing transport models to reproduce observed breakthrough curves (BTCs). The BTCs under steady-state flow are typical of physical nonequilibrium transport in a dual-porosity medium. The shapes of BTCs under transient flow are less common, showing multiple peaks. The spreading of the tracer was one order of magnitude greater under transient flow than under steady-state flow. We attribute the multiple-peak shape of BTCs and the greater spreading under transient flow to rapid convection in a rapid mobile domain. Diffusion and advection could account for tracer exchanges between the slow mobile domain, which is active during unsaturated water flow, and the rapid mobile domain, which becomes active as soon as the pore space is fully saturated. Water flow was simulated with the Richards equation using HYDRUS-1D. A mobile-immobile transport model coupled to the Richards equation reproduced the BTCs satisfactorily under steady-state flow at both infiltration rates; however, it was unable to reproduce the multiple peaks of tracer concentrationse under a fluctuating water-table regime.
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Legrand, M., Preunkert, S., Jourdain, B., Gallee, H., Goutail, F., Weller, R., et al. (2009). Year-round record of surface ozone at coastal (Dumont d'Urville) and inland (Concordia) sites in East Antarctica. J. Geophys. Res.-Atmos., 114, 12 pp.
Abstract: Surface ozone is measured since 2004 at the coastal East Antarctic station of Dumont d'Urville (DDU) and since 2007 at the Concordia station located on the high East Antarctic plateau. Ozone levels at Concordia reach a maximum of 35 ppbv in July and a minimum of 21 ppbv in February. From November to January, sudden increases of the ozone level, up to 15-20 ppbv above average, often take place. They are attributed to local photochemical ozone production as previously seen at the South Pole. The detailed examination of the diurnal ozone record in summer at Concordia suggests a local photochemical ozone production of around 0.2 ppbv h(-1) during the morning. The ozone record at DDU exhibits a maximum of 35 ppbv in July and a minimum of 18 ppbv in January. Mixing ratios at DDU are always higher than those at Neumayer (NM), another coastal Antarctic station. A noticeable difference in the ozone records at the two coastal sites lies in the larger ozone depletion events occurring from July to September at NM compared to DDU, likely due to stronger BrO episodes in relation with a larger sea ice coverage offshore that site. A second difference is the large day-to-day fluctuations which are observed from November to January at DDU but not at NM. That is attributed to a stronger impact at DDU than at NM of air masses coming from the Antarctic plateau. The consequences of such a high oxidizing property of the atmosphere over East Antarctica are discussed with regard to the dimethylsulfide (DMS) chemistry.
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Leroux, S., & Hall, N. M. J. (2009). On the Relationship between African Easterly Waves and the African Easterly Jet. Journal Of The Atmospheric Sciences, 66(8), 2303–2316.
Abstract: This idealized modeling study investigates how convectively triggered African easterly waves (AEWs) are influenced by the intraseasonal variability of the African easterly jet (AEJ). A set of 10-day averaged zonally varying basic states is constructed with the NCEP-2 reanalysis (1979-2006). A primitive equation model is used to simulate linear AEWs on each of these basic states using the same idealized convective heating localized over the Darfur mountains as an initial trigger. It is shown that the transient response depends strongly on the basic state. With the same trigger, many configurations of the AEJ fail to produce a wave disturbance, while others produce strong easterly wave structures. Necessary conditions for the development of strong waves can be characterized by a strong jet, a strong vertical shear, or a strong and extended potential vorticity reversal. In strong-wave cases the jet is extended to the south and west, and the jet core is aligned with the maximum of surface westerlies, maximizing the vertical shear. The pattern that is optimal for generating easterly waves also closely resembles the dominant mode of variation of the AEJ revealed by an empirical orthogonal function (EOF) analysis of the set of basic states.
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Lique, C., Treguier, A. M., Scheinert, M., & Penduff, T. (2009). A model-based study of ice and freshwater transport variability along both sides of Greenland. Climate Dynamics, 33(5), 685–705.
Abstract: We investigate some aspects of the variability of the Arctic freshwater content during the 1965-2002 period using the DRAKKAR eddy admitting global ocean/sea-ice model (12 km resolution in the Arctic). A comparison with recent mooring sections shows that the model realistically represents the major advective exchanges with the Arctic basin, through Bering, Fram and Davis Straits, and the Barents Sea. This allows the separate contributions of the inflows and outflows across each section to be quantified. In the model, the Arctic freshwater content variability is explained by the sea-ice flux at Fram and the combined variations of ocean freshwater inflow (at Bering) and outflow (at Fram and Davis). At all routes, except trough Fram Strait, the freshwater transport variability is mainly accounted for by the liquid component, with small contributions from the sea-ice flux. The ocean freshwater transport variability through both Davis and Fram is controlled by the variability of the export branch (Baffin Island Current and East Greenland Current, respectively), the variability of the inflow branches playing a minor role. We examine the respective role of velocity and salinity fluctuations in the variability of the ocean freshwater transport. Fram and Davis Straits offer a striking contrast in this regard. Freshwater transport variations across Davis Strait are completely determined by the variations of the total volume flux (0.91 correlation). On the other hand, the freshwater transport through Fram Strait depends both on variations of volume transport and salinity. As a result, there is no significant correlation between the variability of freshwater flux at Fram and Davis, although the volume transports on each side of Greenland are strongly anti-correlated (-0.84). Contrary to Davis Strait, the salinity of water carried by the East Greenland Current through Fram Strait varies strongly due to the ice-ocean flux north of Greenland.
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Lombard, A., Garric, G., & Penduff, T. (2009). Regional patterns of observed sea level change: insights from a 1/4A degrees global ocean/sea-ice hindcast. Ocean Dynamics, 59(3), 433–449.
Abstract: A global eddy-admitting ocean/sea-ice simulation driven over 1958-2004 by daily atmospheric forcing is used to evaluate spatial patterns of sea level change between 1993 and 2001. In the present study, no data assimilation is performed. The model is based on the Nucleus for European Models of the Ocean code at the 1/4A degrees resolution, and the simulation was performed without data assimilation by the DRAKKAR project. We show that this simulation correctly reproduces the observed regional sea level trend patterns computed using satellite altimetry data over 1993-2001. Generally, we find that regional sea level change is best simulated in the tropical band and northern oceans, whereas the Southern Ocean is poorly simulated. We examine the respective contributions of steric and bottom pressure changes to the total regional sea level changes. For the steric component, we analyze separately the contributions of temperature and salinity changes as well as upper and lower ocean contributions. Generally, the model results show that most regional sea level changes arise from temperature changes in the upper 750 m of the ocean. However, contributions of salinity changes and deep steric changes can be locally important. We also propose a map of ocean bottom pressure changes. Finally, we assess the robustness of such a model by comparing this simulation with a second simulation performed by MERCATOR-Ocean based on the same core model, but differing by its short length of integration (1992-2001) and its surface forcing data set. The long simulation presents better performance over 1993-2001 than the short simulation, especially in the Southern Ocean where a long adjustment time seems to be needed.
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Louchet, F., & Duval, P. (2009). Andrade creep revisited. Int. J. Mater. Res., 100(10), 1433–1439.
Abstract: Transient creep of many materials at high temperatures obeys Andrade's law, in which creep strain is proportional to the cube root of time. The present paper aims at revisiting in terms of criticality the different explanations proposed so far. In agreement with Mott's statistical theory, and using the concept of load shedding and mechanical cascades introduced in Cottrell's microscopic model, we show that Andrade creep is obtained assuming only long range back stresses and delayed obstacle overcoming, both of them involving a large number of interacting sites responsible for stress fluctuations. The time exponent is 1/3 if work hardening is linear, and larger otherwise. Andrade's creep appears as a power law approach of the linear (or chi) creep regime.
Keywords: Andrade creep; Criticality; Power law; Recovery; Transient
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Machado, S. L., Carvalho, M. F., Gourc, J. P., Vilar, O. M., & do Nascimento, J. C. F. (2009). Methane generation in tropical landfills: Simplified methods and field results. Waste Management, 29(1), 153–161.
Abstract: This paper deals with the use of simplified methods to predict methane generation in tropical landfills. Methane recovery data obtained on site as part of a research program being carried Out at the Metropolitan Landfill, Salvador, Brazil, is analyzed and used to obtain field methane generation over time. Laboratory data from MSW samples of different ages are presented and discussed: and simplified procedures to estimate the methane generation potential, L(o), and the constant related to the biodegradation rate, k are applied. The first order decay method is used to fit field and laboratory results. It is demonstrated that despite the assumptions and the simplicity of the adopted laboratory procedures, the values L(o) and k obtained are very close to those measured in the field, thus making this kind of analysis very attractive for first approach purposes. (C) 2008 Elsevier Ltd. All rights reserved.
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Mano, V., Nemery, J., Belleudy, P., & Poirel, A. (2009). Assessment of suspended sediment transport in four alpine watersheds (France): influence of the climatic regime. Hydrological Processes, 23(5), 777–792.
Abstract: High-frequency water discharge and suspended sediment concentration (SSC) databases were collected for 3 years oil four contrasted watersheds: the Asse and the Bleone (two Mediterranean rainfall regime watersheds) and the Romanche and the Ferrand (two rainfall-snowmelt regime watersheds). SSCs were calculated from turbidity recordings (1-h time step). converted into SSC values. The rating curve was calculated by means of simultaneous SSC measurement taken by water sampling and turbidity recording, Violent storms during springtime and autumn were responsible for suspended sediment transport oil the Asse and the Bleone rivers. Oil the Ferrand and the Romanche. a large share of suspended sediment transport was also caused by local storms. but 30% of annual fluxes results front snowmelt or icemelt Which Occurred from April to October. Oil each Watershed. SSC Up to 50 g l(-1) were observed. Annual Specific fluxes ranged from 450 to 800 t km(-2) year(-1) and 40-80% of annual suspended sediment fluxes Occurred within 2% of the time. These general indicators clearly demonstrate the intensity Of Suspended sediment transport oil these types of watersheds. Suspended sediment fluxes proved to be highly variable at the annual scale (inter-annual variability of specific fluxes) as well as at the event scale (through a hysteresis loop in the SSC/Q relationship) on these watersheds. In both cases, water discharge and precipitations were the Main processes involved in Suspended sediment production and transport. The temporal and spatial variability of hydro-meteorological processes on the watershed provides a better understanding of suspended sediment dynamics. Copyright (C) 2009 John Wiley & Sons. Ltd.
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Manus, C., Anquetin, S., Braud, I., Vandervaere, J. P., Creutin, J. D., Viallet, P., et al. (2009). A modeling approach to assess the hydrological response of small mediterranean catchments to the variability of soil characteristics in a context of extreme events. Hydrology And Earth System Sciences, 13(2), 79–97.
Abstract: This paper presents a modeling study aiming at quantifying the possible impact of soil characteristics on the hydrological response of small ungauged catchments in a context of extreme events. The study focuses on the September 2002 event in the Gard region (South-Eastern France), which led to catastrophic flash-floods. The proposed modeling approach is able to take into account rainfall variability and soil profiles variability. Its spatial discretization is determined using Digital Elevation Model (DEM) and a soil map. The model computes infiltration, ponding and vertical soil water distribution, as well as river discharge. In order to be applicable to ungauged catchments, the model is set up without any calibration and the soil parameter specification is based on an existing soil database. The model verification is based on a regional evaluation using 17 estimated discharges obtained from an extensive post-flood investigation. Thus, this approach provides a spatial view of the hydrological response across a large range of scales. To perform the simulations, radar rainfall estimations are used at a 1 km(2) and 5 min resolution. To specify the soil hydraulic properties, two types of pedotransfer function (PTF) are compared. It is shown that the PTF including information about soil structure reflects better the spatial variability that can be encountered in the field. The study is focused on four small ungauged catchments of less than 10 km(2) , which experienced casualties. Simulated specific peak discharges are found to be in agreement with estimations from a post-event in situ investigation. Examining the dynamics of simulated infiltration and saturation degrees, two different behaviors are shown which correspond to different runoff production mechanisms that could be encountered within catchments of less than 10 km(2). They produce simulated runoff coefficients that evolve in time and highlight the variability of the infiltration capacity of the various soil types. Therefore, we propose a cartography distinguishing between areas prone to saturation excess and areas prone only to infiltration excess mechanisms. The questions raised by this modeling study will be useful to improve field observations, aiming at better understanding runoff generation for these extreme events and examine the possibility for early warning, even in very small ungauged catchments.
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Marchi, L., Borga, M., Preciso, E., Sangati, M., Gaume, E., Bain, V., et al. (2009). Comprehensive post-event survey of a flash flood in Western Slovenia: observation strategy and lessons learned. Hydrological Processes, 23(26), 3761–3770.
Abstract: Spatial and temporal scales of occurrence of flash floods, combined with the space and time scales of conventional measurement networks of rain and discharge, make these events particularly difficult to observe. The effective documentation of flash floods requires post-flood survey strategies encompassing accurate radar rainfall estimation, field observations of the geomorphic processes associated with the flood, indirect reconstruction of peak discharges and interviews of eyewitnesses. This paper describes the methods applied and the results achieved in the survey of a flash flood that occurred on 18th September 2007 in the Selska Sora watershed (Western Slovenia). Hydrometeorological analyses of the storm are based on radar reflectivity observations. The documentation of the flash flood reveals high peak flood discharges and a complex flood response. Peak discharges were estimated at 22 cross sections, with drainage areas ranging from 0.2 to 147 km(2). Among the lessons learned from the field study of the Selska Sora flash flood, there are three key conclusions that can inform similar studies. Firstly, geomorphological surveys are an important prerequisite for flood discharge reconstruction in mountainous watersheds affected by debris flow and intense sediment transport. Secondly, the accounts of eyewitnesses of the flood provide a unique contribution to event reconstruction. Finally, it is necessary to have quality controlled weather radar data, which may permit coupling field observations with rainfall-runoff modelling. Copyright (C) 2009 John Wiley & Sons, Ltd.
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Marechal, J. C., Varma, M. R. R., Riotte, J., Vouillamoz, J. M., Kumar, M. S. M., Ruiz, L., et al. (2009). Indirect and direct recharges in a tropical forested watershed: Mule Hole, India. Journal Of Hydrology, 364(3-4), 272–284.
Abstract: It is commonly accepted that forest plays role to modify the water cycle at the watershed scale. However, the impact of forest on aquifer recharge is still discussed: some studies indicate that infiltration is facilitated under forest while other studies suggest a decrease of recharge. This paper presents an estimate of recharge rates to groundwater in a humid forested watershed of India. Recharge estimates are based on the joint use of several methods: chloride mass balance, water table fluctuation, geophysics, groundwater chemistry and flow analysis. Two components of the recharge (direct and indirect) are estimated over 3 years of monitoring (2003-2006). The direct and localized recharges resulting from rainfall over the entire watershed surface area is estimated to 45 mm/yr while the indirect recharge occurring from the stream during flood events is estimated to 30 mm/yr for a 2 km-long stream. Calculated recharge rates, rainfall and runoff measurements are then combined in a water budget to estimate yearly evapotranspiration which ranges from 80% to 90% of the rainfall, i.e. 1050 mm/y as an average. This unexpected high value for a deciduous forest is nevertheless in agreement with the forest worldwide relationship between rainfall and evapotranspiration. The large evapotranspiration from the forest cover contributes to decrease the recharge rate which leads to a lowering of the water table. This is the reason why the stream is highly ephemeral. (c) 2008 Elsevier B.V. All rights reserved.
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Marteel, A., Gaspari, V., Boutron, C. F., Barbante, C., Gabrielli, P., Cescon, P., et al. (2009). Climate-related variations in crustal trace elements in Dome C (East Antarctica) ice during the past 672 kyr. Clim. Change, 92(1-2), 191–211.
Abstract: Cr, Fe, Rb, Ba and U were determined by inductively coupled plasma sector field mass spectrometry (ICP-SFMS) in various sections of the 3,270 m deep ice core recently drilled at Dome C on the high East Antarctic plateau as part of the EPICA program. The sections were dated from 263 kyr bp (depth of 2,368 m) to 672 kyr bp (depth of 3,062 m). When combined with the data previously obtained by Gabrielli and co-workers for the upper 2,193 m of the core, it gives a detailed record for these elements during a 672-kyr period from the Holocene back to Marine Isotopic Stage (MIS) 16.2. Concentrations and fallout fluxes of all elements are found to be highly variable with low values during the successive interglacial periods and much higher values during the coldest periods of the last eight climatic cycles. Crustal enrichment factors indicates that rock and soil dust is the dominant source for Fe, Rb, Ba and U whatever the period and for Cr during the glacial maxima. The relationship between Cr, Fe, Rb, Ba and U concentrations and the deuterium content of the ice appears to be similar before and after the Mid-Brunhes Event (MBE, around 430 kyr bp). Mean concentration values observed during the successive interglacials from the Holocene to MIS 15.5 appear to vary from one interglacial to another at least for part of the elements. Concentrations observed during the successive glacial maxima suggest a decreasing trend from the most recent glacial maxima (MIS 2.2 and 4.2) to the oldest glacial maxima such as MIS 14.2, 14.4 and 16.2, which could be linked with changes in the size distribution of dust particles transported from mid-latitude areas to the East Antarctic ice cap.
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Martin, C., Gudmundsson, G. H., Pritchard, H. D., & Gagliardini, O. (2009). On the effects of anisotropic rheology on ice flow, internal structure, and the age-depth relationship at ice divides. J. Geophys. Res.-Earth Surf., 114, 18 pp.
Abstract: We use numerical modeling with a full-system Stokes solver to elucidate the effects of nonlinear rheology and strain-induced anisotropy on ice flow at ice divides. We find that anisotropic rheology profoundly affects the shape of both isochrone layering and surface topography. Anisotropic effects cause the formation of a downward curving fold, i.e., a syncline, in isochrones in the lower central area beneath the ice divide. When the resulting syncline is superimposed on the well-known Raymond anticline, a double-peaked Raymond bump is formed. Furthermore, to each side of the Raymond bump, flanking synclines are formed. In addition, anisotropic effects are found to give rise to a subtle concavity in the surface profile to both sides of the summit. The lower center syncline, the flanking synclines, and the near-summit surface concavity have all previously been observed in nature, but hitherto no explanation for the genesis of these features has been given. We compare modeling results with radiograms collected from Fuchs Ice Piedmont and Kealey Ice Rise, Antarctica. Good overall agreement is found. In particular, we are able to reproduce all observed qualitative features of surface geometry and internal layering by including, and only by including, the effects of induced nonlinear rheological anisotropy on flow. Rheological anisotropy has the potential to profoundly affect the age distribution with depth, and caution must be exercised when estimating age of ice from ice cores with an isotropic model. The occurrence of linear features parallel to the ridge of ice divides, often seen in satellite imagery, is indicative of long-term stability rather than signs of ongoing ice divide migration as previously suggested. Such ice divides are ideal locations for extracting ice cores.
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Martinerie, P., Nourtier-Mazauric, E., Barnola, J. M., Sturges, W. T., Worton, D. R., Atlas, E., et al. (2009). Long-lived halocarbon trends and budgets from atmospheric chemistry modelling constrained with measurements in polar firn. Atmos. Chem. Phys., 9(12), 3911–3934.
Abstract: The budgets of seven halogenated gases (CFC-11, CFC-12, CFC-113, CFC-114, CFC-115, CCl4 and SF6) are studied by comparing measurements in polar firn air from two Arctic and three Antarctic sites, and simulation results of two numerical models: a 2-D atmospheric chemistry model and a 1-D firn diffusion model. The first one is used to calculate atmospheric concentrations from emission trends based on industrial inventories; the calculated concentration trends are used by the second one to produce depth concentration profiles in the firn. The 2-D atmospheric model is validated in the boundary layer by comparison with atmospheric station measurements, and vertically for CFC-12 by comparison with balloon and FTIR measurements. Firn air measurements provide constraints on historical atmospheric concentrations over the last century. Age distributions in the firn are discussed using a Green function approach. Finally, our results are used as input to a radiative model in order to evaluate the radiative forcing of our target gases. Multi-species and multi-site firn air studies allow to better constrain atmospheric trends. The low concentrations of all studied gases at the bottom of the firn, and their consistency with our model results confirm that their natural sources are small. Our results indicate that the emissions, sinks and trends of CFC-11, CFC-12, CFC-113, CFC-115 and SF6 are well constrained, whereas it is not the case for CFC-114 and CCl4. Significant emission-dependent changes in the lifetimes of halocarbons destroyed in the stratosphere were obtained. Those result from the time needed for their transport from the surface where they are emitted to the stratosphere where they are destroyed. Efforts should be made to update and reduce the large uncertainties on CFC lifetimes.
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Menegoz, M., Melia, D. S. Y., Legrand, M., Teyssedre, H., Michou, M., Peuch, V. H., et al. (2009). Equilibrium of sinks and sources of sulphate over Europe: comparison between a six-year simulation and EMEP observations. Atmos. Chem. Phys., 9(13), 4505–4519.
Abstract: Sulphate distributions were simulated with a global chemistry transport model. A chemical scheme describing the sulphur cycle and the parameterisations of the main sinks for sulphate aerosols were included in the model. A six-year simulation was conducted from the years 2000 to 2005, driven by the ECMWF operational analyses. Emissions come from an inventory representative of the year 2000. This paper focuses on the analysis of the sulphate sinks and sources over Europe for the entire period of simulation. The Sulphate burden shows a marked annual cycle, which is the result of the annual variations of the aqueous and gaseous chemistry. Regionally, the monthly mean aerosol burden can vary by a factor of 2 from one year to another, because of different weather conditions, driving chemistry, transport and wet deposition of sulphate aerosols. Sulphate ground concentrations, scavenging fluxes and precipitation modelled were compared with observations. The model represents quite well sulphate fields over Europe, but has a general tendency to overestimate sulphate ground concentrations, in particular over Northern Europe. We assume that it is linked to the representation of the scavenging fluxes, which are underestimated. We suggest that uncertainties in modelled precipitation explain only partially the underestimation of the scavenging fluxes in the model.
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Mezghani, A., & Hingray, B. (2009). A combined downscaling-disaggregation weather generator for stochastic generation of multisite hourly weather variables over complex terrain: Development and multi-scale validation for the Upper Rhone River basin. Journal Of Hydrology, 377(3-4), 245–260.
Abstract: This paper presents a combined downscaling and disaggregation weather generator developed for multisite generation of hourly precipitation and temperature time series over complex terrain. Daily regional weather variables are first generated from Generalized Linear Models based on daily atmospheric circulation indices from NCEP reanalysis. They are then disaggregated to the required spatial and temporal scales using a K-nearest neighbour approach. The weather generator is applied to the Upper Rhone River basin in the Swiss Alps. It successfully reproduces standard statistics for temperature as well as total and liquid precipitation at the temporal and spatial resolutions required for hydrological modelling of the system (3 h, 100 km(2)) and at lower resolutions down to those relevant at the river basin scale (3 days, similar to 5500 km(2)). In addition, it reproduces the monthly distributions of the 1 degrees C isotherm elevation and of maximum precipitation amounts while preserving the spatial heterogeneity of weather variables and their spatial and temporal correlations. The weather generator can also be used to produce weather scenarios for the studied basin in a future climate. (C) 2009 Elsevier B.V. All rights reserved.
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Molinie, G., Escobar, J., & Gazen, D. (2009). A stochastic lightning-flash scheme for 3D explicitly resolving cloud models. Quarterly Journal Of The Royal Meteorological Society, 135(638), 113–124.
Abstract: The paper presents a stochastic lightning-flash scheme designed for mesoscale cloud-resolving models (MCRMs). The lightning-flash scheme is implemented on-line in an MCRM. It is fully parallellized and vectorized. A lightning flash is schematized as two single conducting channels (single tracks) propagating in opposite directions from the lightning ignition point, and of branch patterns propagating from the single tracks. On the base of scale similarities between lightning flashes and discharges in dielectrics at centimetre scales, a stochastic scheme has been designed to compute branch trajectories. A fractal relationship is used to limit the branch number. Charge neutralization operates along the single tracks and branch trajectories to threshold the cloud charge density. The scheme has been implemented in the French meteorological community model MesoNH. Two kinds of tests were designed to assess the scheme's capabilities. A first set consists of single-lightning simulations, which demonstrate that, thanks to branches, the simulated lightning flashes are (i) able to reach sparse electric charges and (ii) are fractal objects. The second set consists of comprehensive 3D-thundercloud life-cycle simulations. A simple non-inductive charging process is activated in order to assess the sensitivity of thundercloud electrical behaviour to lightning patterns. It is shown that, paradoxically, lightning flashes with quasi-plane branch propagations (i.e. fractal dimension close to 2) lead to more steady electrical behaviour than those completely filling volumes (i.e. fractal dimension close to 3). Copyright (C) 2009 Royal Meteorological Society
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Monks, P. S., Granier, C., Fuzzi, S., Stohl, A., Williams, M. L., Akimoto, H., et al. (2009). Atmospheric composition change – global and regional air quality. Atmos. Environ., 43(33), 5268–5350.
Abstract: Air quality transcends all scales with in the atmosphere from the local to the global with handovers and feedbacks at each scale interaction. Air quality has manifold effects on health, ecosystems heritage and, climate. In this review the state of scientific understanding in relation to global and regional air quality is outlined. The review discusses air quality, in terms of emissions, processing and transport of trace gases and aerosols. New insights into the characterization of both natural and anthropogenic emissions are reviewed looking at both natural (e.g. dust and lightning) as well as plant emissions. Trends in anthropogenic emissions both by region and globally are discussed as well as biomass burning emissions. In terms of chemical processing the major air quality elements of ozone, non-methane hydrocarbons, nitrogen oxides and aerosols are covered. A number of topics are presented as a way of integrating the process view into the atmospheric context; these include the atmospheric oxidation efficiency, halogen and HOx chemistry, nighttime chemistry, tropical chemistry, heat waves, megacities, biomass burning and the regional hot spot of the Mediterranean. New findings with respect to the transport of pollutants across the scales are discussed, in particular the move to quantify the impact of long-range transport on regional air quality. Gaps and research questions that remain intractable are identified. The review concludes with a focus of research and policy questions for the coming decade. In particular, the policy challenges for concerted air quality and climate change policy (co-benefit) are discussed. (C) 2009 Elsevier Ltd. All rights reserved.
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Morin, S., Savarino, J., Frey, M. M., Domine, F., Jacobi, H. W., Kaleschke, L., et al. (2009). Comprehensive isotopic composition of atmospheric nitrate in the Atlantic Ocean boundary layer from 65 degrees S to 79 degrees N. J. Geophys. Res.-Atmos., 114, 19 pp.
Abstract: The comprehensive isotopic composition of atmospheric nitrate (i.e., the simultaneous measurement of all its stable isotope ratios: N-15/N-14, O-17/O-16 and O-18/O-16) has been determined for aerosol samples collected in the marine boundary layer (MBL) over the Atlantic Ocean from 65 degrees S (Weddell Sea) to 79 degrees N (Svalbard), along a ship-borne latitudinal transect. In nonpolar areas, the delta N-15 of nitrate mostly deriving from anthropogenically emitted NOx is found to be significantly different (from 0 to 6%) from nitrate sampled in locations influenced by natural NOx sources (-4 +/- 2)%. The effects on delta N-15(NO3-) of different NOx sources and nitrate removal processes associated with its atmospheric transport are discussed. Measurements of the oxygen isotope anomaly (Delta O-17 = delta O-17 – 0.52 x delta O-18) of nitrate suggest that nocturnal processes involving the nitrate radical play a major role in terms of NOx sinks. Different Delta O-17 between aerosol size fractions indicate different proportions between nitrate formation pathways as a function of the size and composition of the particles. Extremely low delta N-15 values (down to -40%) are found in air masses exposed to snow-covered areas, showing that snowpack emissions of NOx from upwind regions can have a significant impact on the local surface budget of reactive nitrogen, in conjunction with interactions with active halogen chemistry. The implications of the results are discussed in light of the potential use of the stable isotopic composition of nitrate to infer atmospherically relevant information from nitrate preserved in ice cores.
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Mougin, E., Hiernaux, P., Kergoat, L., Grippa, M., de Rosnay, P., Timouk, F., et al. (2009). The AMMA-CATCH Gourma observatory site in Mali: Relating climatic variations to changes in vegetation, surface hydrology, fluxes and natural resources. Journal Of Hydrology, 375(1-2), 14–33.
Abstract: The Gourma site in Mali is one of the three instrumented meso-scale sites deployed in West-Africa as part of the African Monsoon Multi-disciplinary Analysis (AMMA) project. Located both in the Sahelian zone sensu stricto, and in the Saharo-Sahelian transition zone, the Gourma meso-scale window is the northernmost site of the AMMA-CATCH observatory reached by the West African Monsoon. The experimental strategy includes deployment of a variety of instruments, from local to meso-scale, dedicated to monitoring and documentation of the major variables characterizing the climate forcing, and the spatio-temporal variability of surface processes and state variables such as vegetation mass, leaf area index (LAI), soil moisture and surface fluxes. This paper describes the Gourma site, its associated instrumental network and the research activities that have been carried out since 1984. In the AMMA project, emphasis is put on the relations between climate, vegetation and surface fluxes. However, the Gourma site is also important for development and validation of satellite products, mainly due to the existence of large and relatively homogeneous surfaces. The social dimension of the water resource uses and governance is also briefly analyzed, relying on field enquiry and interviews. The climate of the Gourma region is semi-arid, daytime air temperatures are always high and annual rainfall amounts exhibit strong inter-annual and seasonal variations. Measurements sites organized along a north-south transect reveal sharp gradients in surface albedo, net radiation, vegetation production, and distribution of plant functional types. However, at any point along the gradient, surface energy budget, soil moisture and vegetation growth contrast between two main types of soil surfaces and hydrologic systems. On the one hand, sandy soils with high water infiltration rates and limited run-off support almost continuous herbaceous vegetation with scattered woody plants. On the other hand, water infiltration is poor on shallow soils, and vegetation is sparse and discontinuous, with more concentrated run-off that ends in pools or low lands within structured endorheic watersheds. Land surface in the Gourma is characterized by rapid response to climate variability, strong intra-seasonal, seasonal and inter-annual variations in vegetation growth, soil moisture and energy balance. Despite the multi-decadal drought, which still persists, ponds and lakes have increased, the grass cover has largely recovered, and there are signs of increased tree cover at least in the low lands. (C) 2009 Elsevier B.V. All rights reserved.
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Moulin, L., Gaume, E., & Obled, C. (2009). Uncertainties on mean areal precipitation: assessment and impact on streamflow simulations. Hydrology And Earth System Sciences, 13(2), 99–114.
Abstract: This paper investigates the influence of mean areal rainfall estimation errors on a specific case study: the use of lumped conceptual rainfall-runoff models to simulate the flood hydrographs of three small to medium-sized catchments of the upper Loire river. This area (3200 km(2)) is densely covered by an operational network of stream and rain gauges. It is frequently exposed to flash floods and the improvement of flood forecasting models is then a crucial concern. Particular attention has been drawn to the development of an error model for rainfall estimation consistent with data in order to produce realistic streamflow simulation uncertainty ranges. The proposed error model combines geostatistical tools based on kriging and an autoregressive model to account for temporal dependence of errors. It has been calibrated and partly validated for hourly mean areal precipitation rates. Simulated error scenarios were propagated into two calibrated rainfall-runoff models using Monte Carlo simulations. Three catchments with areas ranging from 60 to 3200 km(2) were tested to reveal any possible links between the sensitivity of the model outputs to rainfall estimation errors and the size of the catchment. The results show that a large part of the rainfall-runoff (RR) modelling errors can be explained by the uncertainties on rainfall estimates, especially in the case of smaller catchments. These errors are a major factor limiting accuracy and sharpness of rainfall-runoff simulations, and thus their operational use for flood forecasting.
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Mubarak, I., Mailhol, J. C., Angulo-Jaramillo, R., Bouarfa, S., & Ruelle, P. (2009). Effect of temporal variability in soil hydraulic properties on simulated water transfer under high-frequency drip irrigation. Agricultural Water Management, 96(11), 1547–1559.
Abstract: The effect of changes in the hydraulic properties of a loamy topsoil on water transfer under daily drip irrigation was studied over a cropping cycle. Soil water contents were measured continuously with neutron probes and capacitance sensors placed in access tubes (EnviroSMART) and were compared to predications made by the Hydrus-2D model. Three different sets of hydraulic parameters measured before and after irrigation started, were used. Our results demonstrated that, based on the assumptions used in this study, the accuracy of the Hydrus predictions is good. Graphical and statistical comparisons of simulated and measured soil water contents and consequently the total water storage revealed a similar trend throughout the monitoring period for the all three different sets of parameters. The soil hydraulic properties determined after irrigation started were found to be much more representative of the majority of the irrigation season, as confirmed by the accuracy of the simulation results with high values of the index of agreement and with values of RMSE similar in magnitude to the error associated with field measurements (0.020 cm(3) cm(-3)). The highest RMSE values (about 0.04 cm(3) cm(-3)) Were found when the model used input soil parameters measured before irrigation started. Generally, changes in topsoil hydraulic properties over time had no significant effect on soil moisture distribution in our agro-pedo-climatic context. One possible explanation is that daily water application was conducted at the same time as maximal root water uptake. This meant the soil did not need to store total daily crop water requirements and consequently that the water redistribution phase represented a very short stage in the irrigation cycle. It is probable that irrigating in the daytime when crop evapotranspiration is highest could prevent the effects of a temporal change and other problems connected with the soil. Moreover, water will be always available for the crop. Further experiments are needed to justify the results and to study the effects of low frequency drip irrigation on soil hydraulic characterization and consequently on soil water transfer in order to improve irrigation scheduling practices. (C) 2009 Elsevier B.V. All rights reserved.
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Nicholson, L., Marin, J., Lopez, D., Rabatel, A., Bown, F., & Rivera, A. (2009). Glacier inventory of the upper Huasco valley, Norte Chico, Chile: glacier characteristics, glacier change and comparison with central Chile. Ann. Glaciol., 50(53), 111–118.
Abstract: Results of a new glacier inventory of the upper Huasco valley, which lies within the arid Norte Chico zone of the Chilean Andes, are presented for 2004. Despite the high altitude, the glaciation in this region is limited in extent and is not classical mountain glaciation, which poses difficulties in completing standard inventory attribute tables. Small cornice-style ridgeline features constitute a large number of the non-transient ice bodies identified, and glaciers with surface areas <0.1 km(2) comprise 18% of the glacierized area and 3% of the water resource stored as glacier ice within the Huasco valley. Rock glaciers are an important component of the cryosphere, comprising 12% of the total water volume stored in glacial features. Changes in glacier area over the last 50 years are in line with those for glaciers in central Chile despite the contrasting climate conditions. Projections of glacier area change based on glacier hypsometry and zero isotherm shifts predicted using the PRECIS regional model temperature change for IPCC scenario B2 conditions suggest that the survival of 65% of glacier area and 77% of active rock-glacier area will be threatened under forecast conditions for the end of the 21st century.
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Nogaro, G., Mermillod-Blondin, F., Valett, M. H., Francois-Carcaillet, F., Gaudet, J. P., Lafont, M., et al. (2009). Ecosystem engineering at the sediment-water interface: bioturbation and consumer-substrate interaction. Oecologia, 161(1), 125–138.
Abstract: In soft-bottom sediments, consumers may influence ecosystem function more via engineering that alters abiotic resources than through trophic influences. Understanding the influence of bioturbation on physical, chemical, and biological processes of the water-sediment interface requires investigating top-down (consumer) and bottom-up (resource) forces. The objective of the present study was to determine how consumer bioturbation mode and sediment properties interact to dictate the hydrologic function of experimental filtration systems clogged by the deposition of fine sediments. Three fine-grained sediments characterized by different organic matter (OM) and pollutant content were used to assess the influence of resource type: sediment of urban origin highly loaded with OM and pollutants, river sediments rich in OM, and river sediments poor in OM content. The effects of consumer bioturbation (chironomid larvae vs. tubificid worms) on sediment reworking, changes in hydraulic head and hydraulic conductivity, and water fluxes through the water-sediment interface were measured. Invertebrate influences in reducing the clogging process depended not only on the mode of bioturbation (construction of biogenic structures, burrowing and feeding activities, etc.) but also on the interaction between the bioturbation process and the sediments of the clogging layer. We present a conceptual model that highlights the importance of sediment influences on bioturbation and argues for the integration of bottom-up influence on consumer engineering activities.
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Nord, G., Esteves, M., Lapetite, J. M., & Hauet, A. (2009). Effect of particle density and inflow concentration of suspended sediment on bedload transport in rill flow. Earth Surface Processes And Landforms, 34(2), 253–263.
Abstract: Laboratory flume experiments were carried out to evaluate the effect of particle density on bedload transport of sand-sized particles and the effect of a suspended load of clay particles (kaolinite) on bedload transport of sand-sized particles in rill flow conditions. Three materials in the range 400-600 μm were selected to simulate bedload transport of primary particles and aggregates: sand (2650 kg/m(3)), Crushed brick (2450 kg/m(3)) and anthracite (1300-1700 kg/m(3)). In the two first experiments, two different methods were applied to determine bedload transport capacity of coarse particles for various conditions of flow discharge (from 2 to 15 L/min) and slope (2.2, 3 and 4%). In the third experiment, clear water was replaced with kaolinite-water mixture and bedload transport capacity of crushed brick particles was determined for a 4% slope and different concentrations of kaolinite (0, 7, 41 and 84 g/L). The results showed that bedload transport increased significantly with the decrease in particle density but the effect of particle density on transport rates was much less important than flow discharge. Velocity measurements of clear flow, flow mixed with coarse particles and coarse particles confirmed the existence of a differentiation between suspended load and bedload. In these experimental conditions, suspended load of kaolinite did not affect bedload rates of crushed brick particles. Three transport capacity formulae were tested against observed bedload rates. A calibration of the Foster formula revealed that the shear stress exponent should be greater than 1.5. The Low and the Covers unit stream power (USP) equations were then evaluated. The Low equation was preferred for the prediction of bedload rates of primary particles but it was not recommended in the case of aggregates of low density because of the limited experimental conditions applied to derive this equation. Copyright (C) 2008 John Wiley & Sons, Ltd.
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Obled, C., Zin, I., & Hingray, B. (2009). Optimal space and time scales for parsimonious rainfall-runoff models. Houille Blanche-Revue Internationale De L Eau, (5), 81–87.
Abstract: The proposed methodology allows for the identification of time and space scales to be satisfied for the development of a parsimonious rainfall-runoff model. The maximum acceptable time step (MATS) is first determined so that the dynamical hydrological response of the basin can be properly represented. A characteristic time of the basin hydrological response is used therefore. The MATS allows next to estimate the maximum acceptable space scale with respect to the geostatistical properties of the precipitation cumulated over the MATS. As a result, it is possible to define if the basin can be represented for the MATS by a lumped hydrological model or if not, the minimum number of sub-basins needed for a reasonable representation of the precipitation spatial variability.
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Ourmieres, Y., Brasseur, P., Levy, M., Brankart, J. M., & Verron, J. (2009). On the key role of nutrient data to constrain a coupled physical-biogeochemical assimilative model of the North Atlantic Ocean. Journal Of Marine Systems, 75(1-2), 100–115.
Abstract: A sequential assimilative system has been implemented into a coupled physical-biogeochemical model (CPBM)of the North Atlantic basin at eddy-permitting resolution (1/4 degrees), with the long-term goal of estimating the basin scale patterns of the oceanic primary production and their seasonal variability. The assimilation system, which is based on the SEEK filter [Brasseur, P., Verron, J., 2006. The SEEK filter method for data assimilation in oceanography: a synthesis. Ocean Dynamics. doi: 10.1007/s10236-006-0080-3], has been adapted to this CPBM in order to control the physical and biogeochemical components of the coupled model separately or in combination. The assimilated data are the satellite Topex/Poseidon and ERS altimetric data, the AVHRR Sea Surface Temperature observations, and the Levitus climatology for salinity, temperature and nitrate. In the present study, different assimilation experiments are conducted to assess the relative usefulness of the assimilated data to improve the representation of the primary production by the CPBM. Consistently with the results obtained by Berline et al. [Berline, L, Brankart, J-M., Brasseur, P., Ourmieres, Y., Verron, J., 2007. Improving the physics of a coupled physical-biogeochemical model of the North Atlantic through data assimilation: impact on the ecosystem. J. Mar. Syst. 64 (1-4),153-172] with a comparable assimilative model, it is shown that the assimilation of physical data alone can improve the representation of the mixed layer depth, but the impact on the ecosystem is rather weak. In some situations, the physical data assimilation can even worsen the ecosystem response for areas where the prior nutrient distribution is significantly incorrect. However, these experiments also show that the combined assimilation of physical and nutrient data has a positive impact on the phytoplankton patterns by comparison with SeaWiFS ocean colour data, demonstrating the good complementarity between SST, altimetry and in situ nutrient data. These results suggest that more intensive in situ measurements of biogeochemical nutrients are urgently needed at basin scale to initiate a permanent monitoring of oceanic ecosystems. (c) 2008 Elsevier B.V. All rights reserved.
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Payet, N., Findeling, A., Chopart, J. L., Feder, F., Nicolini, E., Saint Macary, H., et al. (2009). Modelling the fate of nitrogen following pig slurry application on a tropical cropped acid soil on the island of Reunion (France). Agriculture Ecosystems & Environment, 134(3-4), 218–233.
Abstract: A comprehensive field study was conducted to determine the fate of nitrogen in pig (Sus scrofa) slurry applied to an acid tropical andic soil of Reunion with the aim of estimating drainage and nitrogen leaching below the root zone. Water movement and nitrate dynamics were monitored during two successive cropping seasons on a plot (PSP) treated with liquid manure, with an input of 264 kg N ha(-1) the first year and 185 kg N ha(-1) the second year, in comparison to levels recorded in a unfertilized control plot (CP). The field was cropped with rainfed maize (Zea mays L.)The process-based WAVE field-scale model was used to simulate water flow and nitrogen transport in the unsaturated zone and highlight the main processes controlling water and N fate. A calibration procedure was performed one year, while the prediction capability of the model was assessed during another cropping year. A sensitivity analysis was performed to address some critical parameters. Due to the high hydraulic conductivities measured in this andic soil, the drainage risk became high when the rain intensity was above 30 mm d(-1) and the soil humidity was close to saturation. The time between the first slurry application on PSP and the nitrate onset in the drainage water at 135 cm depth (about 15 months) was attributed to nitrate adsorption on the soil particles (the retardation factor was estimated at 2.6 in the surface layer and 1.5 in deeper layers) and to the fact that the water stored in the 0-135 cm soil layer was slowly displaced. The nitrate migrated in this andic soil at rate of about 50 mm per 100 mm of infiltrated water. The main features of the experimental values of state variables (water content, water pressure head, NO(3)(-) concentration, natural mineralization and nitrification of the pig slurry ammonium at different depths and dates) as well as the water fluxes across boundaries were generally correctly reproduced by WAVE for both plots. The calibrated modelled budget error arising from net mineralization was +15 and +9 kg N ha(-1) for CP and PSP, respectively. For the model evaluation, it was estimated at -9 and -13 kg N ha(-1), respectively, which was considered as very acceptable. WAVE required refinements in some processes and parameters but was still found to be robust enough to work in conditions for which it was not primarily designed. (C) 2009 Elsevier B.V. All rights reserved.
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Pellarin, T., Laurent, J. P., Cappelaere, B., Decharme, B., Descroix, L., & Ramier, D. (2009). Hydrological modelling and associated microwave emission of a semi-arid region in South-western Niger. Journal Of Hydrology, 375(1-2), 262–272.
Abstract: Studying surface-atmosphere feedback is often limited by the accuracy of the land surface observations (particularly soil moisture estimates) or the performance of the land surface models. To further our understanding of soil moisture effects on land-atmosphere fluxes, improvements in soil moisture mapping over large regions are necessary. The aim of this study was to obtain accurate soil moisture mapping over a 120 by 100 km(2) area in West Africa using the considerable amount of measurements available from local and regional scales, recorded during the African monsoon multidisciplinary analysis (AMMA) experiment. The modelling strategy was based on the use of a land surface model (LSM), employed to provide high-resolution soil moisture mapping over the studied area. A microwave emission model was then used to simulate associated microwave brightness temperatures (TB) to compare with the Advanced microwave scanning radiometer (AMSR) at the same spatial (25 by 20 km(2)) and temporal resolution (daily). Discrepancies between observed and simulated TB were analysed and used to calibrate the LSM and the microwave emission models to match the specific hydrology and soil microwave behaviour of the studied area. Nevertheless, a positive bias of the near-surface soil moisture remained and the land surface model was still unable to reproduce the rapid dynamic of the near-surface soil moisture observed at the local and regional scales in this climatic context. To solve this problem, a secondary surface soil layer was added to match in situ soil moisture measurements as well as satellite microwave measurements. Additionally, the choice of the soil permittivity model was found to be of prior importance in order to perform suitable microwave brightness temperatures. Finally, a soil moisture retrieval algorithm based on AMSR and meteosat second generation (MSG) measurements was proposed in order to improve the quality of the soil moisture estimates over the studied area (the root mean square error decreases from 5.4 % vol. to 2.8 % vol). (C) 2009 Elsevier B.V. All rights reserved.
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Pellarin, T., Tran, T., Cohard, J. M., Galle, S., Laurent, J. P., de Rosnay, P., et al. (2009). Soil moisture mapping over West Africa with a 30-min temporal resolution using AMSR-E observations and a satellite-based rainfall product. Hydrology And Earth System Sciences, 13(10), 1887–1896.
Abstract: An original and simple method to map surface soil moisture over large areas has been developed to obtain data with a high temporal and spatial resolution for the study of possible feedback mechanisms between soil moisture and convection in West Africa. A rainfall estimation product based on Meteosat geostationary satellite measurements is first used together with a simple Antecedent Precipitation Index (API) model to produce soil moisture maps at a spatial resolution of 10x10 km(2) and a temporal resolution of 30-min. However, given the uncertainty of the satellite-based rainfall estimation product, the resulting soil moisture maps are not sufficiently accurate. For this reason, a technique based on assimilating AMSR-E C-band measurements into a microwave emission model was developed in which the estimated rainfall rates between two successive AMSR-E brightness temperature (TB) measurements are adjusted by multiplying them by a factor between 0 and 7 that minimizes the difference between simulated and observed TBs. Ground-based soil moisture measurements obtained at three sites in Niger, Mali and Benin were used to assess the method which was found to improve the soil moisture estimates on all three sites.
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Petit, J. R., & Delmonte, B. (2009). A model for large glacial-interglacial climate-induced changes in dust and sea salt concentrations in deep ice cores (central Antarctica): palaeoclimatic implications and prospects for refining ice core chronologies. Tellus Ser. B-Chem. Phys. Meteorol., 61(5), 768–790.
Abstract: A semi-empirical model has been developed to reproduce glacial-interglacial changes of continental dust and marine sodium concentrations (factor of similar to 50 and similar to 5, respectively) observed in inland Antarctic ice cores. The model uses conceptual pathways of aerosols within the high troposphere; assumes the dry deposition of impurities on the Antarctic surface; uses estimates of aerosol transit times taken independent of climate; assumes a temperature-dependent removal process during aerosol pathways from the mid-latitudes. The model is fitted to the data over the last four climate cycles from Vostok and EPICA Dome C Antarctic sites. As temperature is cooling, the aerosol response suggests different modes of climate couplings between latitudes, which can be continuous or below temperature thresholds for sodium and dust, respectively. The model estimates a southern South America dust source activity two to three times higher for glacial periods than for the Holocene and a glacial temperature over the Southern Ocean 3-5 degrees C cooler. Both estimates appear consistent with independent observations. After removal of temperature effects, dust and sodium residuals for both sites show orbital frequencies in opposite phase at the precession timescale. Such long-term insolation-related modulation of terrestrial and marine aerosol input, could provide a chemical pacemaker useful for refining ice core chronologies.
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Picard, G., Arnaud, L., Domine, F., & Fily, M. (2009). Determining snow specific surface area from near-infrared reflectance measurements: Numerical study of the influence of grain shape. Cold Reg. Sci. Tech., 56(1), 10–17.
Abstract: Determining the specific surface area of snow from reflectance measurements in the near infrared domain represents a promising technique to rapidly and quantitatively acquire snow stratigraphic profiles in the field. In this paper, we develop a ray tracing model that simulates the albedo, of snowpacks composed of geometric crystals (spheres, cubes. cylinders, etc) and model simulations are exploited to study the influence of the grain shape on the SSA-albedo relationship. The results clearly show that the relationship depends on the grain shape at 1310 nm: Cubic (resp. cylindrical) grains reflect about 40% (resp. 20%) more than spherical grains at equal SSA. Depth-hoar modeled as a collection of hollow cubes is found to reflect exactly as much as cubes. None of the tested shapes (including concave and hollow shapes) reflects more than cubes. These results suggest that determining SSA from albedo measurement is uncertain when the snow grain shape is unknown. This uncertainty reaches +/- 20% considering that spherical and cubic grains are the two extreme cases in terms of reflexion. This large value is probably over-pessimistic for practical applications as only perfect crystals are considered in this theoretical study and natural snow is always a mixture of curved and plane faces. Therefore, further experimental studies should focus on jointly measuring SSA and albedo in order to assess the influence of the grain shape (or snow type) on the SSA-albedo relationship in natural snows. (C) 2008 Elsevier B.V. All rights reserved.
Keywords: Snow; Specific surface area; Albedo; Ray tracing
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Picard, G., Brucker, L., Fily, M., Gallee, H., & Krinner, G. (2009). Modeling time series of microwave brightness temperature in Antarctica. J. Glaciol., 55(191), 537–551.
Abstract: This paper aims to interpret the temporal variations of microwave brightness temperature (at 19 and 37GHz and at vertical and horizontal polarizations) in Antarctica using a physically based snow dynamic and emission model (SDEM). SDEM predicts time series of top-of-atmosphere brightness temperature from widely available surface meteorological data (ERA-40 re-analysis). To do so, it successively computes the heat flux incoming the snowpack, the snow temperature profile, the microwaves emitted by the snow and, finally, the propagation of the microwaves through the atmosphere up to the satellite. Since the model contains several parameters whose value is variable and uncertain across the continent, the parameter values are optimized for every 50 km x 50 km pixel. Simulation results show that the model is inadequate in the melt zone (where surface melting occurs on at least a few days a year) because the snowpack structure and its temporal variations are too complex. In contrast, the accuracy is reasonably good in the dry zone and varies between 2 and 4 K depending on the frequency and polarization of observations and on the location. At the Antarctic scale, the error is larger where wind is usually stronger, suggesting either that meteorological data are less accurate in windy regions or that some neglected processes (e.g. windpumping, surface scouring) are important. At Dome C, in calm conditions, a detailed analysis shows that most of the error is due to inaccuracy of the ERA-40 air temperature (similar to 2 K). Finally, the paper discusses the values of the optimized parameters and their spatial variations across the Antarctic.
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Pitanga, H. N., Gourc, J. P., & Vilar, O. M. (2009). Interface shear strength of geosynthetics: Evaluation and analysis of inclined plane tests. Geotextiles And Geomembranes, 27(6), 435–446.
Abstract: The inclined plane test (IPT) is commonly performed to measure the interface shear strength between different materials as those used in cover systems of landfills. The test, when interpreted according to European test Standards provides the static interface friction angle, usually assumed for 50 mm displacement and denoted as phi(stat)(50). However, if interpreted considering the several phases of the sliding process, the test is capable of yielding more realistic information about the interface shear strength such as differentiating interfaces which exhibit the same value of phi(stat)(50) but different behavior for displacement less than 50 mm. In this paper, the IPT is used to evaluate the interface shear strength of some materials usually present in cover liner systems of landfill. The results of the tests were analyzed for both, the static and the dynamic phases of the sliding and were interpreted based on the static initial friction angle, phi(0), and the limit friction angle, phi(lim). It is shown that depending on the sliding behavior of the interfaces, phi(stat)(50), which is usually adopted as the designing parameter in stability analysis, can be larger than phi(0) and phi(lim). (C) 2009 Elsevier Ltd. All rights reserved.
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Pollacco, J. A. P., & Angulo-Jaramilo, R. (2009). A Linking Test that investigates the feasibility of inverse modelling: application to a simple rainfall interception model for Mt Gambier, southeast South Australia. Hydrological Processes, 23(14), 2023–2032.
Abstract: Interception loss has an important influence oil the water yield of forested areas. Nevertheless, in Most Studies stemflow is not measured. therefore the question of how to determine the feasibility of optimizing interception and stemflow parameters simultaneously by matching daily simulated throughfall to fortnightly measurements of cumulative throughfall is an important one. By applying a daily empirical interception model, a goodness fit of 2.2 mm/day is obtained between observed and Simulated cumulative throughfall. However, by applying the simple but robust Linking Test, it was shown that the parameters are non-unique and falsely linked, i.e. inter-relationships between different vegetation parameter sets give similar throughfall but non-unique net precipitation. The Linking Test investigates the causes of obtaining falsely linked parameters and shows that objective equifinality is not the Source of the problem. Objective equifinality occurs when all inappropriate objective function is used. The Linking Test also shows that falsely linked parameters are not caused by measuring throughfall on a non-daily basis (termed frequency, sampling equifinality). By expanding the interception model to the second degree, it was found that the non-uniqueness is due to the inherent nature of interception and stemflow functions that behave similarly and therefore can easily compensate each other (termed similarity equifinality). It is also shown that a simple daily empirical exponential interception model developed for conifers in the uplands of the United Kingdom is suitable to model interception in Pinus radiata plantations in the Mediterranean climate of Southern Australia by using only daily gross precipitation data and two parameters. Copyright (C) 2009 John Wiley & Sons, Ltd.
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Prado, B., Duwig, C., Marquez, J., Delmas, P., Morales, P., James, J., et al. (2009). Image processing-based study of soil porosity and its effect on water movement through Andosol intact columns. Agricultural Water Management, 96(10), 1377–1386.
Abstract: The soil pore network and marcoporosity are important factors affecting water and solute transport. The transfer of contaminants to water resources is of particular importance in the Valle de Bravo watershed as it provides 10% of the drinking water for the 20 million inhabitants of Mexico City. This watershed is composed mainly of Andosols with unique mineralogical and physical characteristics. Soil porosity is usually examined on thin sections, using various image analysis techniques. We propose a novel methodology combining image analysis and a displacement experiment to study relationships between soil structure and water tracer transport parameters. H(2)(18)O displacement experiments were conducted through intact soil columns sampled at three depths from a representative cultivated Andosol profile. The soil structure and pore characteristics were obtained by image analysis on thin sections obtained from each column at the end of the displacement experiment. The total 2D porosity (for pores larger than 50 μm) varied from 80% of the total section area in the topsoil to around 60% in the subsoil. Tubular pores were the most abundant in the soil profile, but ploughing of the topsoil had destroyed sections of these pores and replaced them with packing pores. Water transport in the intact subsoil columns was always in physical non-equilibrium, showing the existence of preferential flow pathways. In the topsoil, one column out of three showed no preferential flow, demonstrating that soil ploughing also homogenised pore connections. Pore connectivity was larger in the ploughed topsoil than in their deeper soil horizon counterparts. Our methodology offers a 2D quantitative characterisation of the macroporous network at 50 μm resolution and the determination of water transport parameters on the same intact soil samples. 3D characterisation of soil porosity using X-ray computed tomography (CT) gives a better picture of pore connection but usually has lower spatial resolution and a larger cost. (C) 2009 Elsevier B.V. All rights reserved.
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Proisy, C., Gratiot, N., Anthony, E. J., Gardel, A., Fromard, F., & Heuret, P. (2009). Mud bank colonization by opportunistic mangroves: A case study from French Guiana using lidar data. Continental Shelf Research, 29(3), 632–641.
Abstract: Mud bank colonization by mangroves on the Amazon-influenced coast of French Guiana was studied using light detection and ranging (lidar) data which provide unique information on canopy geometry an sub-canopy topography. The role of topography was assessed through analysis of vegetation characteristics derived from these data. Measurements and analyses of mangrove expansion rates over space and time led to the identification of two distinct colonization processes. The first involves regular step-by-step mangrove expansion to the northwest of the experimental site. The second is qualified as 'opportunistic' since it involves a clear relationship between specific ecological characteristics of pioneer Avicennia and mud cracks affecting the mud bank surface and for which probabilities of occurrence were computed from terrain elevations. It is argued from an original analysis of the latter relationship that mud cracks cannot be solely viewed as water stress features that reflect desiccation potentially harmful to plant growth. Indeed, our results tend to demonstrate that they significantly enhance the propensity for mangroves to anchor and take root, thus leading to the colonization of tens of hectares in a few days. The limits and potential of lidar data are discussed with reference to the study of muddy coasts. Finally, the findings of the study are reconsidered within the context of a better understanding of both topography and vegetation characteristics on mangrove-fringed muddy coasts. (C) 2008 Elsevier Ltd. All rights reserved.
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Ramier, D., Boulain, N., Cappelaere, B., Timouk, F., Rabanit, M., Lloyd, C. R., et al. (2009). Towards an understanding of coupled physical and biological processes in the cultivated Sahel-1. Energy and water. Journal Of Hydrology, 375(1-2), 204–216.
Abstract: This paper presents an analysis of the coupled cycling of energy and water by semi-arid Sahelian surfaces, based on two years of continuous vertical flux measurements from two homogeneous recording stations in the Wankama catchment, in the West Niger meso-site of the AMMA project. The two stations, sited in a millet field and in a semi-natural fallow savanna plot, sample the two dominant land cover types in this area typical of the cultivated Sahel. The 2-year study period enables an analysis of seasonal variations over two full wet-dry seasons cycles, characterized by two contrasted rain seasons that allow capturing a part of the interannual variability. All components of the surface energy budget (four-component radiation budget, soil heat flux and temperature, eddy fluxes) are measured independently, allowing for a quality check through analysis of the energy balance closure. Water cycle monitoring includes rainfall, evapotranspiration (from vapour eddy flux), and soil moisture at six depths. The main modes of observed variability are described, for the various energy and hydrological variables investigated. Results point to the dominant role of water in the energy cycle variability, be it seasonal, interannual, or between land cover types. Rainfall is responsible for nearly as much seasonal variations of most energy-related variables as solar forcing. Depending on water availability and plant requirements, evapotranspiration pre-empts the energy available from surface forcing radiation, over the other dependent processes (sensible and ground heat, outgoing long wave radiation). In the water budget, pre-emption by evapotranspiration leads to very large variability in soil moisture and in deep percolation, seasonally, interannually, and between vegetation types. The wetter 2006 season produced more evapotranspiration than 2005 from the fallow but not from the millet site, reflecting differences in plant development. Rain-season evapotranspiration is nearly always lower at the millet site. Higher soil moisture at this site suggests that this difference arises from lower vegetation requirements rather than from lower infiltration/higher runoff. This difference is partly compensated for during the next dry season. Effects of water and vegetation on the energy budget appear to occur more through latent heat than through albedo. A large part of albedo variability comes from soil wetting and drying. Prior to the onset of monsoon rain, the change in air mass temperature and wind produces, through modulation of sensible heat, a marked chilling effect on the components of the surface energy budget. (C) 2008 Elsevier B.V. All rights reserved.
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Rampal, P., Weiss, J., & Marsan, D. (2009). Positive trend in the mean speed and deformation rate of Arctic sea ice, 1979-2007. J. Geophys. Res.-Oceans, 114, 14 pp.
Abstract: Using buoy data from the International Arctic Buoy Program, we found that the sea ice mean speed has substantially increased over the last 29 years (+17% per decade for winter and +8.5% for summer). A strong seasonal dependence of the mean speed is also revealed, with a maximum in October and a minimum in April. The sea ice mean strain rate also increased significantly over the period (+51% per decade for winter and +52% for summer). We check that these increases in both sea ice mean speed and deformation rate are unlikely to be consequences of a stronger atmospheric forcing. Instead, they suggest that sea ice kinematics play a fundamental role in the albedo feedback loop and sea ice decline: increasing deformation means stronger fracturing, hence more lead opening, and therefore a decreasing albedo. This accelerates sea ice thinning in summer and delays refreezing in early winter, therefore decreasing the mechanical strength of the cover and allowing even more fracturing, larger drifting speed and deformation, and possibly a faster export of sea ice through the Fram Strait. The September minimum sea ice extent of 2007 might be a good illustration of this interplay between sea ice deformation and sea ice shrinking, as we found that for both winter 2007 and summer 2007 exceptionally large deformation rates affected the Arctic sea ice cover.
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Rampal, P., Weiss, J., Marsan, D., & Bourgoin, M. (2009). Arctic sea ice velocity field: General circulation and turbulent-like fluctuations. J. Geophys. Res.-Oceans, 114, 17 pp.
Abstract: Using buoy trajectories of the IABP data set, we analyze the Arctic sea ice velocity field as the superposition of a mean field and fluctuations. We study how the mean field can be objectively defined, using appropriate spatial and temporal averaging scales depending on the season considered: 400 km and 5 1/2 months for winter (i.e., approximately all the polar winter duration), and 200 km and 2 1/2 months for summer (i.e., approximately all the polar summer duration). The mean velocity field shows a strong intra-annual (between winter and the following summer) as well as interannual variability. The fluctuations, i.e., the remaining part of the velocity field after subtracting the mean field, are analyzed in terms of diffusion properties. Although the Arctic sea ice cover is a solid, we show that the fluctuations follow the same diffusion regimes as the ones predicted for turbulent flows, as observed in geophysical fluids like the ocean or the atmosphere. We found that the integral time and the diffusivity of sea ice are in the same ranges as those estimated for the ocean, i.e., 1.5 days in winter and 1.3 days in summer and 0.44 x 10(3) m(2)/s for winter and 0.45 x 10(3) m(2)/s in summer, respectively. However, the statistics of the sea ice fluctuating velocity deviate from classical turbulence theory, as they show exponential instead of Gaussian distributions. Sea ice velocity and acceleration are intermittent, and both are characterized by a multifractal scaling. The oceanic and atmospheric dynamic forcing cannot explain solely the statistical properties of sea ice kinematics and dynamics. We argue that sea ice dynamic is significantly influenced by the interplay of multiple fractures that are activated intermittently within the ice pack.
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Recking, A., Frey, P., Paquier, A., & Belleudy, P. (2009). An experimental investigation of mechanisms involved in bed load sheet production and migration. Journal Of Geophysical Research-Earth Surface, 114.
Abstract: Field measurements indicate that, in gravel bed rivers, bed load may not be a one-to-one response to shear stress but may instead fluctuate a great deal over time for a given flow condition. Both in flume and field experiments, these fluctuations were often associated with migration of low-relief bed forms called bed load sheets. Whereas several studies have described bed load sheets as a consequence of grain sorting, little is known about the mechanisms responsible for their production and migration. These were investigated in flume experiments. A set of 20 experiments was conducted under constant feeding rate conditions, with mixtures of different uniform sediments and for slopes varying from 0.8 to 9%. Except for runs performed in high flow conditions, we observed periodic bed load sheet production and migration associated with fluctuations of bed slope, bed state (bed fining and paving), and bed load. Observations allowed us to conclude that bed load sheets resulted from very efficient vertical and longitudinal grain sorting that produced periodic local bed aggradation and erosion clearly observed in the upstream section of the flume. Fractional transport rates were measured in one run. Combined with the results of experiments previously conducted by authors with uniform sediments, this experiment showed that the highest (peak) solid discharges were essentially caused by the much greater mobility of the coarser gravels when transported within bed load sheets. A scenario is proposed for the mechanisms involved in bed load sheet production and migration.
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Rubio, A., Barnier, B., Jorda, G., Espino, M., & Marsaleix, P. (2009). Origin and dynamics of mesoscale eddies in the Catalan Sea (NW Mediterranean): Insight from a numerical model study. Journal Of Geophysical Research-Oceans, 114.
Abstract: Past observations and satellite sea surface temperature imagery indicate the presence of mesoscale anticyclonic eddies drifting along the Catalan coast. In September 2001 one of these anticyclonic eddies was surveyed over the shelf break during an oceanographic cruise which permitted the 3-D description of its structure. In this work we investigate the origin and dynamics of such "Catalan eddies'' using a numerical circulation model of the northwest Mediterranean at 3 km resolution driven by high-resolution atmospheric analyses and compare model eddies with the observations in the Catalan Sea. We identify two zones of eddy formation in the Gulf of Lions, in front of the city of Marseille and at the southeast of coast of Roussillon, from which anticyclonic eddies are observed to drift toward the Catalan Sea. The hydrology and dynamics of the structures observed in the simulations are characterized. Sensitivity experiments and energy analysis are performed which allow us to identify the mechanisms associated with their generation. Properties of the eddy observed during the 2001 cruise at the Catalan shelf break are found to compare well with model eddies generated at the southeast of the Roussillon coast. The model relates the origin of these eddies to the separation of the coastal current downstream from Creus Cape: flow separation is linked to intense downwelling taking place in front of the Roussillon coast when strong northwesterly winds events occur.
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Saux-Picart, S., Ottle, C., Decharme, B., Andre, C., Zribi, M., Perrier, A., et al. (2009). Water and energy budgets simulation over the AMMA-Niger super-site spatially constrained with remote sensing data. Journal Of Hydrology, 375(1-2), 287–295.
Abstract: The SEtHySSavannah model [Saux-Picart et al., submitted for publication. SEtHySSavannah: a multiple source land surface model applied to sahelian landscapes. Agricultural and Forest Meteorology] was developed as an extension of the SEtHyS land surface model to simulate the water and energy fluxes over dry savannah landscapes. The vegetation cover is represented by a two layer model and a mulch approach is used for the soil description. The SEtHyS_Savannah model was regionalized over the AMMA-Niger super-site (about 50 km by 40 km), with the help of remote sensing data. The model uses a regular 1 km grid and each cell is divided in sub-grid patches in order to represent land cover and soil heterogeneities (the approach). The vegetation cover parameters were prescribed according to the land cover map and the seasonal evolution of the Leaf Area Index (LAI), both derived from SPOT-HRV (Satellite Pour l'Observation de la Terre – High Resolution Visible) data imagery. The atmospheric forcing was assumed homogeneous over the area and provided by a meteorological station installed at the Fakara experimental site. The surface water and energy budgets were simulated over a one-year period (2005) at a 5-min time step and validated against MSG-SEVIRI (Meteosat Second Generation – Spinning Enhanced Visible and Infra-red Imager) land surface temperature and ENVISAT-ASAR (ENVIronnement SATellite – Advanced Synthetic Aperture Radar) soil humidity products. The results show realistic surface fluxes and good agreement with the MSG-SEVIRI temperature observations. The soil moisture comparison presents significant correlation but large root mean square errors. These discrepancies are the consequence of both the use of a non-spatialized atmospheric forcing and to residual vegetation effects on the radar signal. Despite these uncertainties. the results increase confidence in the model representation of Sahelian soil-vegetation processes and open new perspectives to quantify the effects of vegetation changes on evapotranspiration and runoff over the region. (C) 2009 Elsevier B.V. All rights reserved.
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Schupbach, S., Federer, U., Kaufmann, P. R., Hutterli, M. A., Buiron, D., Blunier, T., et al. (2009). A New Method for High-Resolution Methane Measurements on Polar Ice Cores Using Continuous Flow Analysis. Environ. Sci. Technol., 43(14), 5371–5376.
Abstract: Methane (CH4) is the second most important anthropogenic greenhouse gas in the atmosphere. Rapid variations of the CH4 concentration, as frequently registered, for example during the last ice age, have been used as reliable time markers for the definition of a common time scale of polar ice cores. In addition, these variations indicate changes in the sources of methane primarily associated with the presence of wetlands. In order to determine the exact time evolution of such fast concentration changes, CH4 measurements of the highest resoution in the ice core archive are required. Here,we present a new, semicontinuous and field-deployable CH4 detection method, which was incorporated in a continuous flow analysis (CFA) system. In CFA, samples cut along the axis of an ice core are melted at a melt speed of typically 3.5 cm/min. The air from bubbles in the ice core is extracted continuously from the meltwater and forwarded to a gas chromatograph (GC) for high-resolution CH4 Measurements, The GC performs a measurement every 3.5 min, hence, a depth resolution of 15 cm is achieved at the chosen melt rate. An even higher resolution is not necessary due to the low pass filtering of air in ice cores caused by the slow bubble enclosure process and the diffusion of air in firn. Reproducibility of the new method is 3%, thus, for a typical CH4 concentration of 500 ppb during an ice age, this corresponds to an absolute precision of 15 ppb, comparable to traditional analyses on discrete samples. Results of CFA-CH4 measurements on the ice core from Talos Dome (Antarctica) illustrate the much higher temporal resolution of our method compared with established melt-refreeze CH4 measurements and demonstrate the feasibility of the new method.
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Seghieri, J., Vescovo, A., Padel, K., Soubie, R., Arjounin, M., Boulain, N., et al. (2009). Relationships between climate, soil moisture and phenology of the woody cover in two sites located along the West African latitudinal gradient. Journal Of Hydrology, 375(1-2), 78–89.
Abstract: The study quantifies the relationships at local scale between phenology and determinants of climate and soil water resources at two sites located along the latitudinal gradient of West Africa, one in the central Sahel (Mali), the other in the Sudanian bioclimatic zone (Benin). The aim is to improve our knowledge on possible vegetation response to possible climate change. Within the Sudanian site, average annual rainfall is 1200 mm, extending from April to October, while, in the Sahelian site, it is 370 mm, occurring from June to September. Physical data were collected from the African Monsoon Multidisciplinary Analysis research programme. The phenology of the dominant species was monitored in four types of vegetation cover at the wetter site, and in three types of vegetation cover at the drier site. For each sampled plant, leafing, flowering and fruiting were recorded as binary variables in terms of the presence/absence of phenophases. A small proportion of the variability of each phenophase occurrence is explained by the logit models. However, rainfall rise is significantly linked to leafing probability increase in the Sahelian site but not in the Sudanian site. Day length extension and temperature decrease are significantly correlated with an increase in leafing in the Sudanian site, but not in the Sahelian. On both sites, the increase in cumulative rainfall is not found to be linked to an increased probability of reproductive phenophases (negative or non-significant relationships). Air temperature is positively correlated with flowering rate in the Sudanian site, but, all other factors being constant, no climate factors are found to be highly significant of flowering occurrence in the Sahel. Fruiting probability is positively correlated mainly with temperature within the Sahelian site. Leafing occurrence is positively correlated with soil moisture in the 0-1 m layer for the Sudanian site, but not for the Sahelian site. Significant relationships between fruiting occurrence and soil moisture may reflect a prior selection of plants on fruiting period that maximizes seed dispersion and germination differently at the two sites. While vegetative and reproduction schedules may be determined by specific genetic factors, the physical environment controls the possibility of their expression. Reduction of the rainfall amount and intensity may increase reproduction rates in wet areas. Although this factor should decrease leafing rate, it does not influence reproduction at dry sites, except through the decrease in air hu
midity. In wetter areas, increasing temperature may reduce leafing, but may increase reproduction rates. Cover reduction may have an impact on local physical factors and, consequently, probably also affects vegetation phenology. (C) 2009 Elsevier B.V. All rights reserved. |
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Sigl, M., Jenk, T. M., Kellerhals, T., Szidat, S., Gaggeler, H. W., Wacker, L., et al. (2009). Towards radiocarbon dating of ice cores. J. Glaciol., 55(194), 985–996.
Abstract: A recently developed dating method for glacier ice, based on the analysis of radiocarbon in carbonaceous aerosol particles, is thoroughly investigated. We discuss the potential of this method to achieve a reliable dating using examples from a mid- and a low-latitude ice core. Two series of samples from Colle Gnifetti (4450 m a.s.l., Swiss Alps) and Nevado Illimani (6300 m a.s.l., Bolivian Andes) demonstrate that the C-14 ages deduced from the water-insoluble organic carbon fraction represent the age of the ice. Sample sizes ranged between 7 and 100 μg carbon. For validation we compare our results with those from independent dating. This new method is thought to have major implications for dating non-polar ice cores in the future, as it provides complementary age information for time periods not accessible with common dating techniques.
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Sirguey, P., Mathieu, R., & Arnaud, Y. (2009). Subpixel monitoring of the seasonal snow cover with MODIS at 250 m spatial resolution in the Southern Alps of New Zealand: Methodology and accuracy assessment. Remote Sens. Environ., 113(1), 160–181.
Abstract: This study describes a comprehensive method to produce routinely regional maps of seasonal snow cover in the Southern Alps of New Zealand (upper Waitaki basin) on a subpixel basis, and with the MODerate Resolution Imaging Spectroradiometer (MODIS). The method uses an image fusion algorithm to produce snow maps at an improved 250 m spatial resolution in addition to the 500 m resolution snow maps. An iterative approach is used to correct imagery for both atmospheric and topographic effects using daily observations of atmospheric parameters. The computation of ground spectral reflectance enabled the use of image-independent end-members in a constrained linear unmixing technique to achieve a robust estimation of subpixel snow fractions. The accuracy of the snow maps and performance of the algorithm were assessed carefully using eight pairs of synchronic MODIS/ASTER images. 'Pixel-based' metrics showed that subpixel snow fractions were retrieved with a Mean Absolute Error of 6.8% at 250 m spatial resolution and 5.1% after aggregation at 500 m spatial resolution. In addition, a 'feature-based' metric showed that 90% of the snowlines were depicted generally within 300 m and 200 m of their correct position for the 500-m and 250-m spatial resolution snow maps, respectively. A dataset of 679 maps of subpixel snow fraction was produced for the period from February 2000 to May 2007. These repeated observations of the seasonal snow cover will benefit the ongoing effort to model snowmelt runoff in the region and to improve the estimation and management of water resources. (C) 2008 Elsevier Inc. All rights reserved.
Keywords: MODIS; Snow; Image fusion; Subpixel snow fraction; Mountainous terrain; Spectral unmixing
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Six, D., Wagnon, P., Sicart, J. E., & Vincent, C. (2009). Meteorological controls on snow and ice ablation for two contrasting months on Glacier de Saint-Sorlin, France. Ann. Glaciol., 50(50), 66–72.
Abstract: The influence of meteorological variables on snow/ice melting has been analyzed for two very contrasting months, in summer 2006, on Glacier de Saint-Sorlin, French Alps. July 2006 was the warmest July since 1950, and August 2006 was the coldest August since 1979. The total energy available for melting was just over half as much in August as in July, due to a sharp decrease in net shortwave radiation and in turbulent flux. This decrease of net shortwave radiation was mainly controlled by a strong increase in albedo responsible for an increase of reflected shortwave radiation, as well as by a reduction in incident shortwave radiation. During the two months, net longwave radiation remained almost unchanged. The mass balance computed from energy-balance modelling or with a degree-day approach was in good agreement with measured mass balance. Differences were attributed to space and time surface aspect variations which mainly controlled the observed mass balance.
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