2000-2004

Abstract: A simplified model representing the small reservoir-ebb tide system by a set of different interacting water storage reservoirs has been proposed. The first one is the small reservoir, which represents the main water storage, accumulated during the rainy season. The second reservoir is the ebb tide zone. The link between the two reservoirs, the saturated zone, represents a buffer reservoir for water transfer. The water transfer in the system is described by a set of two balance equations, with two unknowns: water level in the reservoir and piezometric levels in the ebb tide area, both time-dependent. The solution for the set of equations depends on the equivalent saturated hydraulic conductivity of the medium and on atmospheric conditions. In order to evaluate the preferential flow throughout the network of fissures located in the deepest part of the reservoir two approaches have been considered, denominated the equivalent medium and the new model formulation. Four parameters were calibrated by inverse method using data of 70 days during a the monitoring period (23rd July – 1st October 1998) and the model was validated for a 243 days period (17th April – 16th December 1998) with different measured initial conditions and evaporation flux variation in time. Both the approaches were capable of reproducing the water levels in the reservoir and in the aquifer. In spite of differences in the estimated daily values, the two approaches predicted well-cumulative water flow and preferential flow. This study allowed a better understanding. of the water transfer process in the reservoir-ebb tide system during the evaporation period. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: The Antarctic Vostok ice core provided compelling evidence of the nature of climate, and of climate feedbacks, over the past 420,000 years. Marine records suggest that the amplitude of climate variability was smaller before that time, but such records are often poorly resolved. Moreover, it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records. Here we report the recovery of a deep ice core from Dome C, Antarctica, that provides a climate record for the past 740,000 years. For the four most recent glacial cycles, the data agree well with the record from Vostok. The earlier period, between 740,000 and 430,000 years ago, was characterized by less pronounced warmth in interglacial periods in Antarctica, but a higher proportion of each cycle was spent in the warm mode. The transition from glacial to interglacial conditions about 430,000 years ago ( Termination V) resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases, but there are significant differences in the patterns of change. The interglacial stage following Termination V was exceptionally long – 28,000 years compared to, for example, the 12,000 years recorded so far in the present interglacial period. Given the similarities between this earlier warm period and today, our results may imply that without human intervention, a climate similar to the present one would extend well into the future.

Abstract: Cr, Cu, Zn, Co, Ni, Mo, Rh, Pd, Ag, Cd, Sb, Pt, Au, and U have been determined in clean room conditions by inductively coupled plasma sector field mass spectrometry and other analytical techniques, in various sections of two dated snow/ice cores from the high-altitude (4450 m asl) glacier saddle Colle Gnifetti, Monte Rosa massif, located in the Swiss-Italian Alps. These cores cover a 350-year time period, from 1650 to 1994. The results show highly enhanced concentrations for most metals in snow/ice dated from the second half of the 20th century, compared with concentrations in ancient ice dated from the 17th and 18th centuries. The highest increase factors from the pre-1700 period to the post-1970 period are observed for Cd (36), Zn (19), Bi (15), Cu (11), and Ni (9), confirming the importance of atmospheric pollution by heavy metals in Europe. Metal concentrations observed in Colle Gnifetti snow around 1980 appear to be quantitatively related to metal emissions from Italy, Switzerland, Germany, France, Belgium, and Austria at that time, making it possible to reconstruct past changes in metal emission in these countries during the last centuries.

Abstract: The objective of the paper is to provide recommendations on the temporal and spatial resolution of rainfall measurements required for urban hydrological applications, based on quantitative investigations of the space-time scales of urban catchments and rainfall. First the temporal rainfall-runoff dynamics is studied using lag time values derived from rainfall and discharge time series for a set of urban catchments. Then the temporal and spatial structure of rainfall is analysed with high resolution rain gauge and radar measurements from the HYDROMET Integrated Radar Experiment '98 experiment for three typical intense Mediterranean rain events. In particular the evolution of the structure of the rainfall field at different time steps is investigated using geostatistics. Finally the required space-time resolution of rainfall for Mediterranean regions is estimated as a function of the surface of the catchments. According to the results, hydrological applications for urban catchments of the order of 1000 ha require a temporal resolution of about 5 min and a spatial resolution of about 3 km. For urban catchments of the order of 100 ha, it becomes a resolution of about 3 min and 2 km, that common operational networks or radars cannot provide. These results complement the scarce recommendations on resolution of rainfall for urban hydrology reported in the literature. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: The rapid wastage of mountain glaciers and their contribution to sea level rise require worldwide monitoring of their mass balance. In this paper, we show that changes in glacier thickness can be accurately measured from satellite images. We use SPOT image pairs to build Digital Elevation Models (DEMs) of the Mont Blanc area (French Alps) for different years. To register the DEMs, we adjust their longitude, latitude and altitude over motionless areas. The uncertainty of the thickness change measurement is greatly reduced by averaging over areas covering altitude intervals of 50 m. Comparisons with topographic profiles and a differential DEM from aerial photographs obtained on the Mer de Glace indicate an overall accuracy of 1 m for the thickness change measurement. Below 2100 m, satellite DEMs show an evolution of the thinning rate from 1 +/- 0.4 m.a(-1) (years 1979-1994) to 4.1 +/- 1.7 m.a(-1) (2000-2003).

Abstract: For a regional assessment of water needs and consumption in semiarid agricultural zones, one needs robust and simple tools that provide space-time estimates of evaporation losses. Most operational evaporation estimates rely on semiempirical relationships that are not generally applicable. Several authors have proposed physically based simple expressions to model the “energy-limited'' (stage-one) and the ”supply-limited'' (stage-two) evaporation rates during a dry down. They use the time compression approximation (TCA) to relate stage-one and stage-two drying. Two asymptotic approximations of the TCA corresponding to the diffusion-dominated and the drainage-dominated flows have been proposed in the literature. In this study a full analytical solution of the TCA is presented. The derived "supply-limited'' evaporation rate is evaluated for a wide range of soil conditions and vegetation cover against a physically based complex soil-vegetation-atmosphere transfer model, the Simple Soil-Plant-Atmosphere Transfer scheme (SiSPAT). SiSPAT solves the differential equations of water flow in a vertical soil column and computes estimates of soil evaporation and transpiration. SiSPAT is used to evaluate the performance of the full solution as well as the two existing asymptotic approximations for the case of sparse to dense vegetation in a semiarid environment. The full analytical solution gives accurate predictions of first- to second-stage evaporation time series for the bare soil and vegetated cover conditions with a leaf area index of 3 or higher. The results of the full solution are closer to the evaporation rate time series simulated by SISPAT than the asymptotic approximations.

Abstract: We have developed a spectrogonio radiometer to measure in the laboratory (-35 degreesC to +30 degreesC) the bidirectional reflectance and polarization distribution functions of various types of planetary material from the UV to the near-IR (310-4800 nm). The major, to our knowledge, novel feature of this instrument is that it is capable of measuring dark to translucent materials with a high degree of radiometric accuracy under most viewing geometries. The sample surface is illuminated with a large monochromatic and polarized parallel beam (incidence: 0degrees-90degrees), and the total intensity and the two polarized components of the reflected light are measured (observation, 0degrees-80degrees; azimuth, 0degrees-180degrees). The scientific and technical constraints, the design, and the performances and limitations of the system are presented in this first paper. (C) 2004 Optical Society of America.

Abstract: As a part of the ZAIANGO IFREMER/TOTAL-FINA-ELF program, gas hydrates were collected from a gravity sediment core on the Congo-Angola margin during the ZAI-ROV cruise (December 2000). Gas hydrates, associated with a deep giant (800 in in diameter) active depression called “pockmark,” occurred from the sediment surface down to 12 in depth, at 3160 in water depth. Elevated concentrations of particles, manganese, iron, and methane detected in the water column close to the sediment surface reveal intense fluid circulation transporting methane-rich turbid fluids into the overlying bottom waters. Free gas is liberated via diffusion and/or advection and from destabilizing gas hydrates. The gas hydrates occur as small fragments and massive crystal aggregates, mostly disseminated irregularly in the highly disturbed sediment and escaping in the overlying deep seawater, creating methane-rich plumes. The dissociation of solid CH4 hydrate particles rising in a turbulent flow may explain the high heterogeneity of methane concentration and CH4 peaks measured in vertical profiles. Raman spectroscopy revealed that the gas hydrates recovered from the Congo-Angola are mainly 100% methane gas hydrate of structure I cubic with a lattice constant of a = 12 Adegrees, but H2S and CO2 are co-clathrated with CH4 in cages. Analyses of hydrate water show depletions of Cl, SO4, Na, Mg, Ca, and Sr, and enrichments of Si, Cs, and Ba, compared to the ambient deep seawater. Gas analysis shows that methane is the major component (99.1%), but CO2 (0.83%) and heavier gases such as C2H6 (0.043%) and H2S (0.02%) are also present as traces. Helium concentrations were in the range of 0.04-1.3 ppm, with slightly radiogenic He-3/He-4 ratios between 6.6 x 10(-7) and 7.6 x 10(-7). The hydrate methane has delta(13)C of – 69.3% (PDB) and deltaD of -199% (V-SMOW), and the hydrate CO2 has delta(13)C of – 17.5 % (PDB). These values indicate a primarily microbial origin for the CH4, which is generated through bacterial CO2 reduction, as previously observed on many continental margins where solid gas hydrates were sampled. (C) 2004 Elsevier B.V All rights reserved.

Abstract: We present here the first comprehensive study of the chemical composition of accretion ice from Lake Vostok. Ion chromatographic analyses were performed on samples obtained along the deeper part of the Vostok ice core. Samples were taken from 3350 down to 3611 in depth, both in glacier ice and subglacial lake ice. The total ionic contents of two accretion ice layers-a few meters thick and centered around 3540 and 3590 in depth-are several times lower than those of glacier ice. Very low concentrations were also observed in the deeper part of accretion ice, below 3609 in depth. Elsewhere, the total ionic content is variable but remains 5 to 50 times higher than in glacier ice. Whatever its total ionic content, the ionic composition of accretion ice is significantly different from what is observed in glacier ice. It is dominated by sodium chloride, homogeneously distributed throughout the ice lattice, as well as calcium and magnesium sulfate, likely located in solid inclusions, or to a lesser extent at grain boundaries. Chemical considerations combined with additional studies of sulfur and oxygen isotopes in sulfate, and iron measurements strongly suggest that glacier water recycling and bedrock hydrolysis do not play a prominent role in providing impurities to accretion ice. It is more likely that NaCl rich water carrying fine sulfate salt particles is sporadically incorporated in the ice accreting in a shallow bay upstream from Vostok. The origin of such salty water, which should also contribute to Lake salinity, is discussed. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: A new aeolian dust record from the first 2200 m of the EPICA-Dome C ice core (75degrees06' S, 123degrees21' E) covering about 220,000 years of climatic history is compared to the Vostok (78degrees28' S, 106degrees48' E) ice core [Nature 399 (1999) 429]. The two dust profiles are very similar and several common dust events allow to establish stratigraphical links. The late Quaternary period is characterized at both sites, and likely overall East Antarctic plateau, by high dust input during glacial periods. In the EPICA-Dome C ice core, the dust flux rises by a factor of similar to25, similar to20 and similar to12 in glacial stages 2, 4 and 6 with respect to interglacial periods (Holocene and stage 5.5). The magnitude and pattern of changes are comparable in the Vostok, ice core. In this study, the geographical origin of ice core dust (ICD) in cold periods has been investigated at both sites through Sr-87/Sr-86 versus Nd-143/Nd-144 isotopic tracers, following the previous studies of Grousset et al. [Earth Planet. Sci. Lett. 111 (1992) 175] and Basile et al. [Earth Planet. Sci. Lett. 146 (1997) 573]. The new data and the existing ones allow to define the isotopic fields for dust at the two Antarctic sites that are almost identical and restricted into the 0.708<Sr-17/Sr-16<0.711 and -5<&epsilon;(Nd)(0)<+5 ranges. This suggests a common geographical provenance for dust at Vostok and Dome C and for all the glacial periods of the late Pleistocene. To decipher the ICD provenance, more than 50 samples of loess and aeolian deposits, sands and fluvioglacial sediments from the Potential Source Areas (PSAs) of the Southern Hemisphere have been collected. However, the methodology has been refined with respect to former studies. First, the isotopic fractionation that can occur in function of grain size has been taken into account, and the PSA's signature has been defined in the <5 &mu;m size range, within which fine-grained dust reaching Antarctica is found. Moreover, a possible contribution from carbonates on the samples from PSAs has also been also considered. South Africa and Australia can be excluded as dominant sources, but a partial overlap arises among southern South America, New Zealand and the Antarctic Dry Valleys isotopic fields, these latter two documented for the first time. A possible contribution from all these three sources cannot be excluded, but complementary arguments suggest the dominant contribution to East Antarctic dust in glacial times deriving from the southern South American region of Patagonia and the Pampas. This study shows a first-order uniformity in the dust flux and geographical provenance to the East Antarctica plateau during glacial periods. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: The Vostok (East Antarctica, 78 degrees S, 106 degrees E) ice core preserves, below the meteoric ice keeping the climatic memory of the last 420 000 years, ice formed by freezing of subglacial Lake Vostok water. This latter contains some bedrock fragments representing unique samples for the geological investigation of the East Antarctic Plateau, covered by similar to 2-4 km of ice. The first geochemical (Sr-87/Sr-86 versus Nd-143/Nd-144) and mineralogical characterization of these inclusions as well as the dating of one of them (Nd model age on whole-rock sample) has given evidence for a Mid-Proterozoic age of the basement lying below the ice sheet, consistent with recent geophysical data. The geochemical characteristics of bedrock inclusions within the accreted ice zone are markedly different from those of the mineral dust of aeolian origin archived in the uppermost part of the Vostok ice core and originating from deflation of the Southern Hemisphere continents, and easily discriminates between the two contributions.

Abstract: Nowadays, it is necessary to understand and identify the reactions governing the fate of heavy metals introduced into the environment with low complexing organic compounds, particularly when they are transferred through soils in urban areas. In this work the concomitant influence of pH and acetate on the fate of zinc on siliceous sand was studied in batch and non-saturated column experiments. Total zinc concentrations varied between 2 and 20 mg/l, and total acetate concentrations were fixed at 22, 72, 132, and 223 mM to obtain solution pHs of 4, 5, 6 and 7, respectively. Natural sand (diameter, 0.3-2 mm), mainly constituted of silica, was used. In batch adsorption experiments, zinc adsorption is insignificant at pH 4, low and linear at pH 5, and increasingly nonlinear, of the Langmuir type, at pH 6 and 7 indicating near-saturation conditions of surface sites at these high pH values. In column experiments, Zn retardation increases and the maximum outlet concentration of Zn decreases with rising pH and acetate concentrations. Previous column tracer experiments revealed the occurrence of regionalized water transport in the column. Modeling these data was based on a non-electrostatic approach. Batch and column data modeling was based on the PHREEQC code that allows concomitant resolution of chemical speciation and regionalized water transport. The speciation calculation indicates that the ZnAcetate(+) species is the dominant Zn species in the solutions used. Batch experimental curves are correctly modeled assuming the formation of the three surface species equivalent toSiOZn(+), equivalent toSiOH-Zn Acetate(+) and equivalent toSiO-Zn(Acetate)(2)(-). The column data could be adequately modeled assuming a two-region water transport and the formation of the same three species with the same thermodynamic constants determined in the batch experiments. The hypothesis of the modeling leads to a slight overestimation of the quantities of zinc eluted (10%) at pH 6 and 7, mostly in the desorption phase. These results show that the methodology used facilitates the correct modeling of both batch and transport experiments and formulation of the hypothesis on the interactions between the low reactive sand and a complex solution. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: The reliability of model predictions used in meteorology, agronomy or hydrology is partly linked to an adequate representation of the water and energy balances which are described in so-called SVAT (Soil Vegetation Atmosphere Transfer) models. These models require the specification of many surface properties which can generally be obtained from laboratory or field experiments, using time consuming techniques, or can be derived from textural information. The required accuracy of the surface properties depends on the model complexity and their misspecification can affect model performance. At various time and spatial resolutions, remote sensing provides information related to surface parameters in SVAT models or state variables simulated by SVAT models. In this context, the Simple Soil-Plant-Atmosphere Transfer-Remote Sensing (SiSPAT-RS) model was developed for remote sensing data assimilation objectives. This new version of the physically based SiSPAT model simulates the main surface processes (energy fluxes, soil water content profiles, temperatures) and remote sensing data in the visible, infrared and thermal infrared spectral domains. As a preliminary step before data assimilation in the model, the objectives of this study were (1) to apply a multiobjective approach for retrieving quantitative information about the surface properties from different surface measurements and (2) to determine the potential of the SiSPAT-RS model to be applied with 'little' a priori information about input parameters. To reach these goals, the ability of the Multiobjective Generalized Sensitivity Analysis (MOGSA) algorithm to determine and quantify the most influential input parameters of the SiSPAT-RS model on several simulated output variables, was investigated. The results revealed the main influential input parameters according to different contrasted environmental conditions, and contributed to the reduction of their a priori uncertainty range. A procedure for specifying surface properties from MOGSA results was tested on the thermal and hydraulic soil parameters, and evaluated through the SiSPAT-RS model performance. Although slightly lower than a reference simulation, the performance were satisfactory and suggested that complex-SVAT models can be driven with little a priori information

Abstract: Ice crystals were grown in the laboratory at -15degreesC, at different growth rates and in the presence of a partial pressure of HCl of 1.63 x 10(-3) Pa, to test whether the ice growth rate influences the amount of HCl taken up, X-HCl, as predicted by the ice growth mechanism of Domine and Thibert (1996). The plot of HCl concentration in ice as a function of growth rate has the aspect predicted by that mechanism: X-HCl decreases with increasing growth rate, from a value that depends on thermodynamic equilibrium to a value that depends only on kinetic factors. The height of the growth steps of the ice crystals is determined to be about 150 nm from these experiments. We discuss that the application of these laboratory experiments to cloud ice crystals and to snow metamorphism is not quantitatively possible at this stage, because the physical variables that determine crystal growth in nature, and in particular the step height, are not known. Qualitative applications are attempted for HCl and HNO3 incorporation in cloud ice and snowpack crystals.

Abstract: Image analyses, carried out on thin sections made in consolidated and sheared kaolinite test pieces, allow the identification of three 'microstructural domains': (1) the initial isotropic matrix; (2) a partly anisotropic matrix resulting from simple particle arrangement; and (3) an anisotropic matrix resulting from rearrangement plus flattening and delamination of particles. In order to explain the micromechanisms of the clay matrix behaviour, this paper proposes to link the 'microstructural domains' represented in the e vs. log p Cam-clay diagram and domains of hydraulic conductivity in the k vs. e diagram. The hydraulic conductivities are calculated following the Kozeny-Carman relations, which take into account the micro-arrangement of particles via a tortuosity calculation. The generation of 2D images shows that the preservation of the isotropic arrangement of particles is limited by a minimum porosity value. A decrease of the porosity value below this limit can be explained only by a progressive anisotropic rearrangement of the particles. The microtexture behaviour, induced by the superimposition of the compaction, orientation and particle flattening and delamination stages, causes an anisotropy of the hydraulic conductivity which affects (1) the interstitial water flow direction, (2) the rotation of particles itself, and (3) the damage mechanism of the clay.

Abstract: Meteorological variables were recorded (14 March 2002 to 14 March 2003) at 4890 m above sea level (asl) on the Antizana Glacier 15 (0.71 km(2); 0degrees28'S, 78degrees09'W) in the tropical Andes of Ecuador (inner tropics). These variables were used to compute the annual cycle of the local surface energy balance (SEB). The four radiative fluxes were directly measured, and the turbulent fluxes were calculated using the bulk aerodynamic approach, calibrating the roughness length by direct sublimation measurements. The meteorological conditions are relatively homogeneous throughout the year (air temperature and air humidity). There is a slight seasonality in precipitation with a more humid period between February and June. During June-September, wind velocity shows high values and is responsible for intense turbulent fluxes that cause reduction of melting. Considering the SEB over the whole year, it is dominated by net radiation, and albedo variations govern melting. During the period under consideration the net short-wave radiation S (123 W m(-2)) and the sensible turbulent heat flux H (21 W m(-2)) were energy sources at the glacier surface, whereas the net long-wave radiation L (-39 W m(-2)) and the latent turbulent heat flux LE ( -27 W m(-2)) represented heat sinks. Since the OdegreesC isotherm-glacier intersection always oscillates through the ablation zone and considering that the phase of precipitation depends on temperature, temperature indirectly controls the albedo values and thus the melting rates. This control is of major interest in understanding glacier response to climate change in the Ecuadorian Andes, which is related to global warming and ENSO variability.

Abstract: Mercury (Hg) species have been measured in the snow pack at Station Nord, Greenland both in the snow and in the air of snow from February 25 to March 15, 2002, during twilight and low solar irradiation periods. More than 99% of Hg is in the snow itself ( similar to 94 – 97% as Hg2+ and similar to 5% as MeHg+) while less than 1% is in the interstitial air of snow as Hgdegrees. Gaseous Elemental Mercury (Hgdegrees) concentrations, decreased exponentially with depth from similar to 1.5 ng/m(3) outside to similar to 0.1 ng/m(3) at 120 cm depth in the snow air. Hgdegrees incorporation flux to the snow pack has been evaluated to similar to 5.8 – 7.0 pg/m(2)/h which is weak, indicating that this process does not change significantly the Hg content in the snow. We believe that this decrease in the air of snow is the result of fast oxidation processes of Hgdegrees rather than adsorption of Hgdegrees onto snow surfaces.

Abstract: Greenhouse gas measurements along polar ice cores provide important insight into the former composition of the atmosphere, its natural variations, and the responses to fast climatic changes in the past. We present high-resolution nitrous oxide (N2O) and methane (CH4) records measured along two ice cores from central Greenland covering part of Marine Isotope Stages 3 and 4 in the last glacial epoch. The N2O data confirm the hypothesis that N2O shows variations in phase to fast climatic changes observed in the Northern Hemisphere, the so-called Dansgaard-Oeschger (D-O) events. Variations exist not only for events with a long duration (1500 years and more) but also for the shorter ones. The comparison with CH4 unveils interesting differences between the response of CH4 and N2O to D-O events. While the average amplitudes of CH4 oscillations associated with D-O events are similar to those of the Northern Hemisphere summer insolation, the magnitude of the N2O concentration change instead correlates with the duration of the D-O events. The records give further insight into the timing of concentration changes at the beginning of D-O events. They show that for long-lasting events the N2O concentration starts to increase before both the sharp increase in the CH4 concentration and the temperature reconstructed for Greenland.

Abstract: Surface mass balance (SMB) distribution and its temporal and spatial variability is an essential input parameter in mass balance studies. Different methods were used, compared and integrated (stake farms, ice cores, snow radar, surface morphology, remote sensing) at eight sites along a transect from Terra Nova Bay (TNB) to Dome C (DC) (East Antarctica), to provide detailed information on the SMB. Spatial variability measurements show that the measured maximum snow accumulation (SA) in a 15 km area is well correlated to firn temperature. Wind-driven sublimation processes, controlled by the surface slope in the wind direction, have a huge impact (up to 85% of snow precipitation) on SMB and are significant in terms of past, present and future SMB evaluations. The snow redistribution process is local and has a strong impact on the annual variability of accumulation. The spatial variability of SMB at the kilometre scale is one order of magnitude higher than its temporal variability (20-30%) at the centennial time scale. This high spatial variability is due to wind-driven sublimation. Compared with our SMB calculations, previous compilations generally over-estimate SMB, up to 65% in some areas.

Abstract: An iridium anomaly at the Cretaceous/Tertiary boundary layer has been attributed to an extraterrestrial body that struck the Earth some 65 million years ago(1). It has been suggested that, during this event, the carrier of iridium was probably a micrometre-sized silicate-enclosed aggregate(2) or the nanophase material of the vaporized impactor(3). But the fate of platinum-group elements ( such as iridium) that regularly enter the atmosphere via ablating meteoroids remains largely unknown. Here we report a record of iridium and platinum fluxes on a climatic-cycle timescale, back to 128,000 years ago, from a Greenland ice core(4). We find that unexpectedly constant fallout of extraterrestrial matter to Greenland occurred during the Holocene, whereas a greatly enhanced input of terrestrial iridium and platinum masked the cosmic flux in the dust-laden atmosphere of the last glacial age. We suggest that nanometresized meteoric smoke particles(5,6), formed from the recondensation of ablated meteoroids in the atmosphere at altitudes > 70 kilometres, are transported into the winter polar vortices by the mesospheric meridional circulation(7) and are preferentially deposited in the polar ice caps. This implies an average global fallout of 14 +/- 5 kilotons per year of meteoric smoke during the Holocene.

Abstract: By using recently developed automatic instruments for fabric and texture measurements on ice, both the c-axis orientation and area of the individual crystals can be determined. Each grain can then be associated with its volume fraction, defined as a function of its measured cross-sectional area, to describe the microstructure of a polycrystal. The relevance of this approach is studied using a three-dimensional microstructure obtained from the Potts model. In particular, the area weighting is compared to the classical implicit equal weighting used by glaciologists, which assumes that all the grains have the same volume fraction (discrete uniform distribution). Then, using the measurements of c-axis orientation and crystal size performed on the North Greenland Icecore Project (NorthGRIP) ice core, we compare area-weighted and equal-weighted fabrics. All these comparisons are made with respect to the orientation tensor. According to the ability of the Potts model to reproduce the ice microstructure, it is shown that using the grain cross-sectional area to infer its volume fraction improves the description of the actual polycrystal fabric.

Abstract: An analysis of the effects of nonuniform beam filling on the measurement of the propagation phase shift for X-band radar is presented. An analytical expression is derived for the measured differential phase shift. It is shown that the phase measurement is weighted by the reflectivities and the attenuation within the sampling volume and that this may lead to negative values in the retrieved specific differential phase shift K-DP. These effects are compared with those due to the influence of the backscattering phase shift delta. Both effects are quantified using numerical simulations. A very simple rain model with one cell is used to analyze and understand the problem. Comparison of X and S bands shows that the effects of azimuthal gradients are stronger at X band. The problem is also illustrated with radar measurements carried out with an X-band coherent polarimetric radar. For practical purposes, the problem appears to be small for beamwidths of less than 1degrees and the cell sizes considered here, or if the differential phase shift is evaluated over a long path. This study illustrates some of the practical problems that may be encountered using rain measurement algorithms based on K-DP at X band, even though the high values of K-DP at X band compared to longer wavelengths make its use attractive for rain estimation, especially for lower rain rates.

Abstract: Hard X-ray diffraction experiments were performed on ice single crystals deformed in pure torsion. The torsion strain appears to be totally accommodated by geometrically necessary basal screw dislocations. Dislocation slip is concentrated on macroscopic basal slip bands, which are clearly seen on the prismatic diffraction lines. The slip band spacing is decreasing with the increase of the finite torsion strain. The deformation appears to result from the increase of the number of slip bands and not from the accumulation of dislocations within these bands. The elastic interaction between dislocations located in neighbouring slip bands is weak since they all have the same sign and move in the same direction.

Abstract: V, Co, Cu, Zn, As, Rb, Sr, Ag, Cd, Ba, Pb, Bi and U have been analysed by inductively coupled plasma sector field mass spectrometry in various sections of a dated snow/ice core drilled at an altitude of 6542 m on the Sajama ice cap in Bolivia. The analysed sections were dated from the Last Glacial Stage (similar to22 000 years ago), the Mid-Holocene and the last centuries. The observed variations of crustal enrichment factors (EFc) for the various metals show contrasting situations. For V, Co, Rb, Sr and U, EFc values close to unity are observed for all sections, then showing that these elements are mainly derived from rock and soil dust. For the other metals, clear time trends are observed, with a pronounced increase of EFc values during the 19th and 20th centuries. This increase shows evidence of metal pollution associated with human activity in South America. For Pb an important contribution was from gasoline additives. For metals such as Cu, Zn, Ag and Cd an important contribution was from metal production activities, with a continuous increase of production during the 20th century in countries such as Peru, Chile and Bolivia.

Abstract: Large proglacial lakes cool regional summer climate because of their large heat capacity, and have been shown to modify precipitation through mesoscale atmospheric feedbacks, as in the case of Lake Agassiz(1). Several large ice-dammed lakes, with a combined area twice that of the Caspian Sea, were formed in northern Eurasia about 90,000 years ago, during the last glacial period when an ice sheet centred over the Barents and Kara seas(2) blocked the large northbound Russian rivers(3). Here we present high-resolution simulations with an atmospheric general circulation model that explicitly simulates the surface mass balance of the ice sheet. We show that the main influence of the Eurasian proglacial lakes was a significant reduction of ice sheet melting at the southern margin of the Barents – Kara ice sheet through strong regional summer cooling over large parts of Russia. In our simulations, the summer melt reduction clearly outweighs lake-induced decreases in moisture and hence snowfall, such as has been reported earlier for Lake Agassiz1. We conclude that the summer cooling mechanism from proglacial lakes accelerated ice sheet growth and delayed ice sheet decay in Eurasia and probably also in North America.

Abstract: Our knowledge of the temperature evolution over Greenland during Dansgaard-Oeschger events (DO) is currently qualitatively described through the water isotopic profile. Using two independent paleothermometry methods, one based on air isotopic measurements and the other on the combined measurements of water isotopes (deltaD and delta(18)O), we show a complete and quantitative reconstruction of temperature at the NorthGRIP site over the period 76 to 62 kyr BP ( DO 18, 19 and 20). We confirm that the associated warmings are larger than those conventionally depicted by the water isotopes (11degreesC, 16degreesC and 11degreesC for DO 18, 19 and 20). Secondly, we demonstrate that the relationship between temperature and delta(18)O varies rapidly during the last glacial period, even over a DO. Finally, our temperature reconstruction over DO 19 agrees well with that predicted from simple climate models linking the DO to iceberg discharges.

Abstract: Isotopic measurements of air trapped in ice to quantify temperature changes. Isotopic measurements in polar ice core have shown a succession of rapid warming periods during the last glacial period over Greenland. However, this method underestimates the surface temperature variations. A new method based on gas thermal diffusion in the firn manages to quantify surface temperature variations through associated isotopic fractionations. We developed a method to extract air from the ice and to perform isotopic measurements to reduce analytical uncertainties to 0.006 and 0.020parts per thousand for delta(15)N and delta(40)Ar. It led to a 16 +/- 1.5degreesC surface temperature variation during a rapid warming (-70000 yr). (C) 2004 Academie des sciences. Publie par Elsevier SAS. Tous droits reserves.

Abstract: Quantifying the specific surface area (SSA) of snow and its variation during metamorphism is essential to understand and model the exchange of reactive gases between the snowpack and the atmosphere. Isothermal experiments were conducted in a cold room to measure the decay rate of the SSA of four snow samples kept in closed systems at -15 degreesC. In all cases, a logarithmic law of the form SSA=B-A ln(t+Deltat) fits the SSA decrease very well, where A, B and Deltat are adjustable parameters. B is closely related to the initial SSA of the snow and A describes the SSA decay rate. These and previous data suggest the existence of a linear relationship between A and B so that it may be possible to predict the decay rate of snow SSA from its initial value. The possibility that grain coarsening theories could explain these observations was investigated. The logarithmic equation was shown to be an approximation of a more general equation, that describes the time evolution of the mean grain radius R in most grain coarsening theories, such as Ostwald ripening: (R) over bar (n)-(R) over bar (n)(0)=Kt. (R-0) over bar is the initial mean grain radius, (R) over bar is the mean grain radius, n and K are the growth exponent and the growth rate, respectively. Values of n between 2.8 and 5.0 are found. It is concluded that snow metamorphism and Ostwald ripening processes are governed by similar rules. Ostwald ripening theories predict that a steady-state regime is reached after a transient stage, but our results suggest that the steady-state regime is not reached after a few months of isothermal snow metamorphism. This last feature makes is difficult to predict the rate of decrease of snow SSA using the theory of Ostwald ripening. (C) 2004 American Institute of Physics.

Abstract: In this paper the mathematical macroscopic modeling of unsaturated water flow in a porous medium (soil) with highly permeable porous inclusions is presented. It is supposed that water flow in each sub domain can be described by the strongly non-linear Richards' equation. Gravity effects are considered. The upscaling process of this stiff problem is performed using the homogenization method of periodic structures with asymptotic expansions. The resulting non-linear macroscopic description is a one equation model, revealing the local equilibrium of the capillary pressure head. The effective water retention capacity was found to be the volume average of the water retention capacities of the two porous sub-domains. The effective conductivity tensor is obtained from a linear and non-stiff boundary value problem at the heterogeneity scale. (C) 2003 Academie des sciences. Published by Elsevier SAS. All rights reserved.

Abstract: Stress and temperature dependences of the strain rate for ice single crystals are derived from evolution equations of mobile dislocation densities. This model is based on the mechanisms generally agreed for multiplication and annihilation of mobile dislocations in monocrystalline ice. It gives the correct stress exponent value n = 2 for steady state creep or constant strain rate deformation. The activation energy predicted by the model is that for dislocation climb, in agreement with experimental activation energies that are closer to the self-diffusion energy than to the activation energy associated with dislocation glide velocities.

Abstract: The relationships between the initial area expansion rate of tropical convective systems and their total life duration are analyzed during the period of the Wet Season Amazon Mesoscale Campaign/Large-Scale Biosphere Atmosphere (WETAMC/LBA) experiment over tropical South America, using an objective tracking of convective systems during their life cycle from infrared Geostationary Operational Environmental Satellite (GOES) images. The results show that it is possible to estimate the probable lifetime of a convective system, within certain error bars, considering only its initial area expansion. This result shows that the initial area expansion could be used as a predictor of the life cycle of convective systems. The area expansion is also a good indicator of convective activity such as the diurnal cycle of convection. Over the southwest Amazon, the maximum area expansion occurs close to the time of maximum precipitation and about 4 h before the maximum cold cloud fraction at the same threshold (235 K). Also, the hypothesis that the area expansion, and hence the convective activity, impacts the high-level wind divergence has been investigated using satellite wind observations. It is found that the wind divergence fields derived are able to describe the large-scale patterns but are not able to capture the small-scale features. The diurnal cycle of the high-level wind divergence generally shows a flat response over tropical South America, although a coherent but not significant signal is observed over the WETAMC/LBA area. It is shown that the area of the cloud shield of convective systems varies not only in association with the upper-level wind divergence but also with the condensation-evaporation process. The increase of area in this initial stage is mainly due to the condensation process. During the ensuing mature stage, the upper-air wind divergence also contributes to the expansion.

Abstract: During the Quaternary period, ice sheets centred over the Barents and Kara seas expanded several times onto mainland Russia and blocked northflowing rivers, such as the Yenissei, Ob, Pechora and Mezen. Large ice-dammed lakes with reversed outlets, e.g. toward the Caspian Sea, formed south of these ice sheets. Some lakes are reconstructed from shorelines and lacustrine sediments, others mainly from ice-sheet configuration. Ice-dammed lakes, considerably larger than any lake on Earth today, are reconstructed for the periods 90-80 and 60-50 ka. The ages are based on numerous optically stimulated luminescence (OSL) dates. During the global Last Glacial Maximum (LGM, about 20 ka) the Barents-Kara Ice Sheet was too small to block these eastern rivers, although in contrast to the 90-80 and 60-50 ka maxima, the Scandinavian Ice Sheet grew large enough to divert rivers and meltwater across the drainage divide from the Baltic Basin to the River Volga, and that way to the Caspian Sea. Climate modelling shows that the lakes caused lower summer temperatures on the continent and on the lower parts of the ice sheet. The final drainage of the best mapped lake is modelled, and it is concluded that it probably emptied within few months. We predict that this catastrophic outburst had considerable impact on sea-ice formation in the Arctic Ocean and on the climate of a much larger area. (C) 2003 Elsevier Ltd. All rights reserved.

Abstract: Alpine valleys represent some of the most important crossroads for international heavy-duty traffic in Europe, but the full impact of this traffic on air quality is not known due to a lack of data concerning these complex systems. As part of the program “Pollution des Vallees Alpines” (POVA), we performed two sampling surveys of polycyclic aromatic hydrocarbons (PAHs) in two sensitive valleys: the Chamonix and Maurienne Valleys, between France and Italy. Sampling campaigns were performed during the summer of 2000 and the winter of 2001, with both periods taking place during the closure of the “Tunnel du Mont-Blanc”. The first objective of this paper is to describe the relations between PAH concentrations, external parameters ( sampling site localization, meteorological parameters, sources), and aerosol characteristics, including its carbonaceous fraction (OC and EC). The second objective is to study the capacity of PAH profiles to accurately distinguish the different emission sources. Temporal evolution of the relative concentration of an individual PAH (CHR) and the PAH groups BghiP+COR and BbF+BkF is studied in order to differentiate wood combustion, gasoline, and diesel emissions, respectively. The results show that the total particulate PAH concentrations were higher in the Chamonix valley during both seasons, despite the cessation of international traffic. Seasonal cycles, with higher concentrations in winter, are also stronger in this valley. During winter, particulate PAH concentration can reach very high levels (up to 155 ng.m(-3)) in this valley during cold anticyclonic periods. The examination of sources shows the impact during summer of heavy-duty traffic in the Maurienne valley and of gasoline vehicles in the Chamonix valley. During winter, Chamonix is characterized by the strong influence of wood combustion in residential fireplaces, even if the temporal evolution of specific PAH ratios are difficult to interpret. Information on sources given by PAH profiles can only be considered in qualitative terms.

Abstract: A crystallographic analysis of the size-dependent geometrically necessary dislocations (GND) stored in a cylindrical hexagonal single crystal deformed by torsion with its c-axis aligned with the torsion axis has been made. The only activation of basal slip and strain rate dependence of critical resolved shear stresses have been assumed. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Abstract: The influence on precipitation of regional sea surface temperature (SST) during a drought period of the West African monsoon is determined, using a regional climate model (RCM). The results from three simulations of two realistic dry years are compared. The first two experiments are initialised and nested respectively in 1983 and 1984 reanalysis data sets. The third experiment is a hybrid simulation of 1983 which is the same as the first experiment except that the SST field is the 1984 SST. Precipitation from the RCM is compared with several precipitation data sets and, as in observations, the RCM reasonably simulates the West African monsoon (seasonal cycle and monsoon sub-period) for the two different years. In particular, the model reproduces stage by stage the motion of the monsoon band well: installation phase, high rain period with abrupt northward shift of the rain band, and the retreat southward phase. Interannual variability and wet or dry tendencies are also represented. The most significant effect of SST is shown by the hybrid simulation, when the regional SST appears as a major factor in the seasonal and interannual monsoon precipitation regime over the African continent (up to 12degreesN) although this influence is modulated both by the surface conditions (soil and vegetation) and by the reanalysis flow introduced at the lateral boundaries. Dynamically, a warmer SST leads to a decrease in the magnitude of the African Easterly Jet and an increase in northward equivalent water content transport (from equator to 12degreesN).

Abstract: High resolution records of atmospheric CO2 concentration during the Holocene are obtained from the Dome Concordia and Dronning Maud Land (Antarctica) ice cores. These records confirm that the CO2 concentration varied between 260 and 280 ppmv in the Holocene as measured in the Taylor Dome ice core. However, there are differences in the CO2 records most likely caused by mismatches in timescales. Matching the Taylor Dome timescale to the Dome C timescale by synchronization of CO2 indicates that the accumulation rate at Taylor Dome increased through the Holocene by a factor two and bears little resemblance to the stable isotope record used as a proxy for temperature. This result shows that different locations experienced substantially different accumulation changes, and casts doubt on the often-used assumption that accumulation rate scales with the saturation vapor pressure as a function of temperature, at least for coastal locations. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: In many industrial or civil engineering applications, design of structural elements may require analysing the mechanical interaction between a flowing material and an open structure made of a metallic mesh. This paper presents some theoretical results, first in a two-dimensional case and then extended to a three-dimensional context. In particular, we establish that the distribution of forces applied to a wire structure by a flowing material whose behaviour is viscous does not depend upon the viscosity of the material. Further numerical computations provide valuable elements to confirm these results. An application to the case of a snowpack interacting with an avalanche net structure is presented, focusing on the fundamental numerical consequences resulting from the mechanical analysis. (C) 2004 Elsevier Ltd. All rights reserved.

Abstract: Gibbs sampling is used to simulate Sahelian rain fields conditional to an areal estimate provided either as the output of an atmospheric model or by a satellite rainfall algorithm. Whereas various methods are widely used to generate simulated rain fields conditioned on point observations, there are many fewer simulation algorithms able to produce a spatially disaggregated rain field of known averaged value. The theoretical and practical aspects of Gibbs sampling for the purpose of conditional rain field simulation are explored in the first part of the paper. It is proposed to used a so-called acceptation-rejection algorithm to ensure convergence of the conditional simulation. On a Sahelian case study, it is then showed that Gibbs sampling performs similarly to the well-known turning band method in an unconditional mode. A preliminary validation of the method in conditional mode is presented. Several rain fields are simulated conditionally on an observed rainfield, whose only the spatial average over a 100 x 100 km(2) area is supposed to be known. These conditional simulations are compared with the observed rain field and to other rain fields of similar magnitude. For a given class of events, the conditional rain fields have a distribution of point values similar to the distribution of observed point values. At the same time, the model is producing a wide range of spatial patterns corresponding to a single area average, giving an idea of the variety of possible fields of equal areal value.

Abstract: [1] We have used new spaceborne ( elevation) and airborne ( ice thickness) data to constrain a 2D1/2 model of snow accumulation and ice flow along the Ridge B-Vostok station ice flow line ( East Antarctica). We show that new evaluations of the ice flow line geometry ( from the surface elevation), ice thickness ( from low-frequency radar data), and basal melting and sliding change significantly the chronology of the Vostok ice core. This new Vostok dating model reconciles orbital and glaciological timescales and is in good agreement with the Dome Fuji glaciological timescale. At the same time, the new model shows significantly older ages than the previous GT4 timescale for the last glacial part, being thus in better agreement with the GRIP and GISP2 chronologies.

Abstract: We introduced a simple scheme of soil freezing in the LMDz3.3 atmospheric general circulation model (AGCM) to examine the potential effects of this parameterization on simulated future boreal climate change. In this multi-layer soil scheme, soil heat capacity and conductivity are dependent on soil water content, and a parameterization of the thermal and hydrological effects of water phase changes is included. The impact of these new features is evaluated against observations. By comparing present-day and 2 x CO2 AGCM simulations both with and without the parameterization of soil freezing the role of soil freezing in climate change is analysed. Soil freezing does not have significant global impacts, but regional effects on simulated climate and climate change are important. In present-day conditions, hydrological effects due to freezing lead to dryer summers. In 2 x CO2 climate, thermal effects due to freeze/ thaw cycles are more pronounced and contribute to enhance the expected future overall winter warming. Impact of soil freezing on climate sensitivity is not uniform: the annual mean warming is amplified in North America (+ 15%) and Central Siberia (+ 36%) whereas it is reduced in Eastern Siberia ( – 23%). Nevertheless, all boreal lands undergo a strong attenuation of the warming during summertime. In agreement with some previous studies, these results indicate once more that soil freezing effects are significant on regional boreal climate. But this study also demonstrates its importance on regional boreal climate change and thus the necessity to include soil freezing in regional climate change predictions.

Abstract: A thermomechanical ice sheet model ( ISM) is used to investigate the sensitivity of the Laurentide and Fennoscandian ice sheets to tropical sea surface temperature (SST) perturbations during deglaciation. The ISM is driven by surface temperature and precipitation fields from three different atmospheric general circulation models (AGCMs). For each AGCM, the responses in temperature and precipitation over the ice sheets nearly compensate, such that ice sheet mass balance is not strongly sensitive to tropical SST boundary conditions. It was also found that there is significant variation in the response of the ISM to the different AGCM output fields.

Abstract: The increasing need for distributed hydrological modelling leads to an intense use of spatially distributed predictions of physically based models, such as TOPMODEL as addressed here. The ability of these models to reproduce the internal behaviour of catchments physically is increasingly tested through field experiments (geochemical investigation, distributed measurements network, etc.). This paper will show that, in the case of TOPMODEL, an implicit approximation remains in the classic derivation of the equations that consists in neglecting the surface of saturated areas with respect to the total surface of the catchment. This simplifying, though unnecessary, approximation leads to a systematic underestimation of the catchment water storage deficit and to divergence in the water budget accounting. This may also significantly change the predicted ratio between Subsurface and surface water fluxes in the total discharge. An analytical solution is Suggested that leads to water balance accounting which is better defined, and more consistent in comparison with field water storage recording. It is expected that this work will ensure more accurate TOPMODEL predictions, consistent with the assumptions of the model. This will then improve the interpretation of comparisons between results of simulation and field experiments. Copyright (C) 2004 John Wiley Sons, Ltd.

Abstract: A good calibration of satellite sensors is necessary to derive reliable quantitative measurements of the surface parameters or to compare data obtained from different sensors. In this study, the snow surface of the high plateau of the East Antarctic ice sheet, particularly the Dome C area (75degreesS, 123degreesE), is used first to test the quality of this site as a ground calibration target and then to determine the inter-annual drift in the sensitivity of the VEGETATION sensor, onboard the SPOT4 satellite. Dome C area has many good calibration site characteristics: The site is very flat and extremely homogeneous (only snow), there is little wind and a very small snow accumulation rate and therefore a small temporal variability, the elevation is 3200 m and the atmosphere is very clear most of the time. Finally, due to its location, it is frequently within view of many satellites. VEGETATION visible blue channel data (0.43 – 0.47 mum) of a 716 X 716 km(2) area centred on the French-Italian Dome Concordia station, during the 1998-1999, 1999-2000, 2001-2001, and 2001-2002 austral summers were cloud masked and atmospherically corrected. The snow surface Bidirectional Reflectance Distribution Function is very high with little spatial and seasonal variability, which is a major advantage for sensor calibration. The inter-annual variation is found to be very small, proving that the stability of the site is very good. (C) 2003 Elsevier Inc. All rights reserved.

Abstract: The paper concerns the solution of non-linear system of equations arising while solving a steady gradually varied flow in an open channel network. To reduce the dimensions of final system the depths at each cross-section and the discharge at each branch are considered as unknowns. Newton and Picard iteration can suffer from non-convergent saw-tooth errors. An approach to improve and consequently to ensure the convergence of the Picard method is proposed. Using the average from two successive iterations suppresses the saw-teeth and gives a robust convergent iteration method to solve a steady gradually varied flow in any type of open channel network. The improvement in performance is confirmed by numerical tests. Copyright (C) 2004 John Wiley Sons, Ltd.

Abstract: Over fifty years of cumulative annual mass balance data for several glaciers in the Alps shows similar fluctuations which seem to provide evidence of a common climatic signal. Separate winter and summer mass balance measurements from the Claridenfirn (glacier in Switzerland) since 1914 and the Sarennes glacier (France) since 1949 show that (1) the annual mass balance is primarily driven by the summer mass balance term and (2) melting rate variations with time are very similar for these two glaciers located 290 km apart. The increase in the ablation rate of 0.5 cm w.e. day(-1) between the two periods 1954-1981 and 1982-2002 over these two glaciers corresponds to a 20 Wm(-2) rise in the energy flux at the glacier surface. These results suggest that a common summer melting rate change may have affected the Alps as a whole. Detailed observations on the Sarennes glacier show that the origin of this strong increase in summer ablation since 1982 is not only a rise in the summer melting rate, but also an increase in the ablation period during the months of September and October.

Abstract: Recent investigations of crevassing on alpine glaciers and ice shelves have been based on linear elastic fracture mechanics (LEFM). However, LEFM is unable to explain some aspects of crevasse formation such as the initiation of crevasse propagation from crystal-scale (mm) microcracks, the slow propagation of large fractures in ice shelves, and the acceleration of crevasse opening before breaking of the ice terminus. Here another mechanism to account for these observations is proposed: subcritical crevassing. Subcritical crack growth, documented in many materials though not yet explored in ice, is characterized by a crack velocity that scales as a power of the tensile stress intensity factor, but is much less than that associated with critical crack propagation. This mechanism allows crevasse propagation from mm-scale microcracks at velocities much lower than body wave speeds, and explains crevasse-opening accelerations in a natural way. Subcritical crevassing is theoretically explored for several simplified situations but is limited by a lack of available data on crevasse evolution.

Abstract: Recently, mechanical tests on ice as well as dislocation dynamics simulations have revealed that plastic flow in materials exhibiting a largely dominating slip system displays a scale-free intermittent dynamics characterized by dislocation avalanches with a power law distribution of amplitudes. To further explore the complexity of dislocation dynamics during plastic flow, we present a statistical analysis of dislocation avalanche correlations and avalanche triggering. It is shown that the rate of avalanche triggering immediately after any avalanche is larger than the background activity due to uncorrelated events. This self-induced triggering increases in intensity, and remains over the background rate for longer times, as the amplitude of the mainshock increases. This analysis suggests that stress redistributions and the associated collective dislocation rearrangements may be responsible for aftershock triggering in the complex process of plastic deformation. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: [1] Both sulfur and oxygen isotopes of sulfate preserved in ice cores from Greenland and Antarctica have provided information on the relative sources of sulfate in the ice and their chemical transformation pathways in the atmosphere over various time periods. The mass-independent fractionation in the oxygen isotopes of sulfate from the Vostok ice core from east Antarctica suggests that gas-phase oxidation by the hydroxyl radical (OH) was relatively greater than aqueous-phase oxidation by O-3 and H2O2 during the last glacial period than during the Eemian and preindustrial Holocene. The complete sulfur isotopic composition (delta(33) S, delta(34)S, delta(36)S) from the same Vostok ice core samples along with delta(34)S measurements from the Dome C, east Antarctic ice core from this study lend support to these conclusions and reveal significant isotopic fractionation of delta(34)S during chemical transformation and transport to east Antarctica. These findings reveal that conservation of sulfur isotopic signatures upon transport cannot be assumed for the East Antarctic plateau over the time periods considered.

Abstract: [1] This study, based on a statistical analysis of simulated warm rain event and radar data, aims at highlighting the main physical mechanisms that lead to organize shallow convection on the relief. The region of investigation, the Cevennes-Vivarais, is located in the southeast part of France. Radar images from the Cevennes experiment ( fall 1986 1988) reveal a characteristic and repetitive structure of the rain distribution organized in narrow bands or plumes, oriented south-north in the case of stationary southerly Mediterranean flow. The event of 14 November 1986 has been selected and constitutes the data set of this numerical study. This work is closely associated with the previous work by Miniscloux et al. [2001] which presents in detail the results of a geostatistical analysis of the radar data set extracted from the Cevennes experiment data base. The main results highlight the physical characteristics and the dynamics of the rain patterns. Following the recent work of Cosma et al. [2002], high-resolution (Delta = 1 km) simulations have been continued with the nonhydrostatic three-dimensional ( 3D) atmospheric model MesoNH, in order to reproduce the observed rainbands over the Cevennes region. The numerical model correctly reproduces the structure and the dynamics of the rainbands. The geostatistical analysis has been applied for the simulated rain fields. The model slightly overestimates the northward advection velocity of the rain cells within the bands (75 km h(-1) against 60 km h(-1) for the observation), and the simulated rainbands are narrower and more organized around the N180degrees direction than the observed rain field. The comparison allows the qualification and validation of the choice of the numerical methodology and realism of the physical parameterizations. The analysis of the 3D simulated fields confirms the physical mechanisms responsible for the rain organization demonstrated by Cosma et al. [ 2002] through idealized simulations. The statistical analysis highlights the presence of mean topographic features under low-level convergence zones composed of a succession of ridges and penetrating valleys orientated east-west. The rainbands are generated upstream of these topographic features and enhanced on the leeside due to the convergence created by the flow deflection around the obstacle and its penetration into the valleys. The simulated triggering takes place further to the south than the observed one, and the triggering is active as soon as the relief is suitably described in the model.

Abstract: Measurements of atmospheric and snow mixing ratios of nitrates and nitrites and their fluxes above the snow surface were made during two intensive campaigns during spring time 2001 at Ny-Alesund, Svalbard as part of the EU project “The NItrogen Cycle and Effects on the oxidation of atmospheric trace species at high latitudes” (NICE). At this coastal site close to the unseasonably unfrozen fjord, of the measured nitrogen species, only HNO3 showed a significant flux on to the snow surface; a mean deposition of -8.7 nmol h(-1) m(-2) was observed in late April /early May 2001. These fluxes may be due to the reaction of HNO3 with sea salt, and especially NaCl, or may be simply uptake of HNO3 by ice, which is alkaline because of the sea salt in our marine environment. During snowfall periods dry deposition of HNO3 may contribute up to 10% of the N budget in the snow; however, the main source for N is wet deposition in falling snow. The surface snow at Ny-Alesund showed very complex stratigraphy; the NO3- mixing ratio in snow varied between 65 and 520 ng g(-1), the total NO3- content of the snowpack was on the order of 2700 ng cm(-2). In comparison the atmospheric boundary layer column showed a NO3- content of only 8 ng cm(-2). The limited exchange, however, between the snow and the atmosphere was attributed to low mobility of NO3- in the observed snow. Contrary to other Arctic sites (i.e. Alert, Nunavut or Summit, Greenland) deposition of sea salt and crustal aerosols in this marine environment made the surface snow alkaline; snow NO3- was associated with heavier cations and was not readily available for physical exchange or photochemical reactions.

Abstract: The global sulphur cycle has been simulated using a general circulation model with a focus on the source and oxidation of atmospheric dimethylsulphide (DMS). The sensitivity of atmospheric DMS to the oceanic DMS climatology, the parameterisation of the sea-air transfer and to the oxidant fields have been studied. The importance of additional oxidation pathways (by O3 in the gas- and aqueous-phases and by BrO in the gas phase) not incorporated in global models has also been evaluated. While three different climatologies of the oceanic DMS concentration produce rather similar global DMS fluxes to the atmosphere at 24-27 Tg S yr(-)1(,) there are large differences in the spatial and seasonal distribution. The relative contributions of OH and NO3 radicals to DMS oxidation depends critically on which oxidant fields are prescribed in the model. Oxidation by O3 appears to be significant at high latitudes in both hemispheres. Oxidation by BrO could be significant even for BrO concentrations at sub-pptv levels in the marine boundary layer. The impact of such refinements on the DMS chemistry onto the indirect radiative forcing by anthropogenic sulphate aerosols is also discussed.

Abstract: During the second phase of the European International Project on Climatic and Hydrological Interactions between Vegetation, Atmosphere, and Land Surface (ECHIDA) Field Experiment in a Desertification Threatened Area (EFEDA) the spatial variability of the soil water retention and hydraulic conductivity characteristics of layers at 2-12- and 17-27- cm depth was characterized. A simplified method, based on particle size distribution and simple infiltration tests, was used. It provided these characteristics at the nodes of a 1-km grid over 10 x 10 km(2) around the town of Tomelloso (Castilla-La Mancha, Spain). A total number of 78 sample points were used to address the problem of soil surface properties variability and its consequences on the monthly and annual water balance. The Simple Soil Plant Atmosphere Transfer model (SiSPAT) 1D Soil-Vegetation-Atmosphere Transfer (SVAT) model was run with a 1-yr climatic forcing for the 78 soil profiles until equilibrium was reached. As no runoff was generated, the spatial variability of the water budget components only concerned soil evaporation, transpiration, and deep drainage. It was found that (i) the choice of the type of boundary condition at the bottom of the soil profile was greatly influencing the final variability, (ii) the variability of transpiration was the largest in situations of water stress for the vegetation, and (iii) soil evaporation was the most sensitive component when plants were well supplied with water. Various aggregation methods of soil surface parameters (use of the arithmetic mean, median of the parameters, or parameters associated to the average soil texture of the Clapp and Hornberger classification) were assessed. The use of median parameters in a single 1D simulation was found to provide the best agreement with the average of the 78 simulations performed for each grid cell using locally measured soil properties. The use of average soil texture parameters led to a significant bias, especially in the case of water stress.

Abstract: The snowpack can impact atmospheric chemistry by exchanging adsorbed or dissolved gases with the atmosphere. Modeling this impact requires the knowledge of the specific surface area (SSA) of snow and its variations with time. We have therefore measured the evolution of the SSA of eight recent surface snow layers in the Arctic and the French Alps, using CH4 adsorption at liquid nitrogen temperature (77 K). The SSA of fresh snow layers was found to decrease with time, from initial values in the range 613-1540 cm(2)/g to values as low as 257 cm(2)/g after 6 days. This is explained by snow metamorphism, which causes modifications in crystal shapes, here essentially crystal rounding and the disappearance of microstructures. A parametrization of the rate of SSA decrease is proposed. We fit the SSA decrease to an exponential law and find that the time constant alpha(exp) (day(-1)) depends on temperature according to alpha(exp) = 76.6 exp (-1708/7), with Tin kelvin. Our parametrization predicts that the SSA of a snow layer evolving at – 40 degreesC will decrease by a factor of 2 after 14 days, while a similar decrease at 1 degreesC will only require 5 days. Wind was found to increase the rate of SSA decrease, but insufficient data did not allow a parametrization of this effect.

Abstract: The analysis of air bubbles from ice cores has yielded a precise record of atmospheric greenhouse gas concentrations, but the timing of changes in these gases with respect to temperature is not accurately known because of uncertainty in the gas age-ice age difference. We have measured the isotopic composition of argon in air bubbles in the Vostok core during Termination III (similar to240,000 years before the present). This record most likely reflects the temperature and accumulation change, although the mechanism remains unclear. The sequence of events during Termination III suggests that the CO2 increase tagged Antarctic deglacial warming by 800 +/- 200 years and preceded the Northern Hemisphere deglaciation.

Abstract: An image-based approach for discharge measurements is evaluated for river gauging in an experiment on the Iowa River at Iowa City, Iowa. Over a twenty-day period, ten discharge measurements were made using the image-based approach. A ten-minute video recording was made of the river flow for each measurement. Particle image velocimetry (PIV) was used to estimate surface velocities for the imaged area using naturally occurring foam as a flow tracer. The surface velocities were then estimated along a surveyed, river section, and river discharge was computed using standard velocity-area methods over a selected cross-section. Several unique aspects of this experiment were the use of PIV for unseeded flow conditions, and the evaluation of discharge estimates over a range of flow conditions. A comparison of the PIV discharge measurements with traditional current meter measurements, which have been made at the site since 1984, showed that the PIV measurements were consistent with the observed stage-discharge relationship. Discharge for the experiment ranged from 50 to 300 m(3)s(-1), which covers a large portion of the existing rating curve. The experimental results suggest that image-based approach may be a reliable way of establishing a stage-discharge relationship at a site, perhaps even remotely, by making repeated measurement with a camera mounted at the site. Still, there are inherent limitations with the approach: These limitations include the need for recognizable tracer particles or flow patterns to detect motion, problems associated with shadows and reflections on the water surface, as well as the common the problem for all discharge measurement of the need for survey information on the channel cross-section. (C) 2003 Elsevier Science B.V. All rights reserved.

Abstract: We have examined several MSA (methanesulfonic acid) records from the upper 200 m of the Antarctic ice street and in particular the new Dome F profile. At all the four sites studied, concentration profiles exhibit similar patterns as a function of depth. They suggest that snow metamorphism and solid phase migration are responsible for a marked release of gaseous MSA to interstitial firm air as kell as probably to the free atmosphere, in particular at extremely low accumulation sites. Snow acidity can also modify MSA concentration. It is proposed that, belong the upper few metres where the communication with the free atmosphere is possible, gaseous MSA may remain in the firn layers and be entrapped later in air bubbles at pole close-off. i.e. when firn is transformed into ice. Chemical measurements on the firn core do not take into account the MSA released to the gaseous phase. but this fraction is measurable in ice samples. In spite of these alterations occurring in the firn layers, relative charges of the atmospheric MSA concentration in the past are probably still there deep within the Antarctic ice street. However. for glacial periods, different processes have to he considered in relation to modified aerosol properties.

Abstract: The Cevennes-Vivarais Mediterranean Hydro-meteorological Observatory aims at improving the understanding and modelling of the intense Mediterranean precipitation events that frequently result in devastating flash-floods in southern France. A primary objective is to bring together the skills of meteorologists and hydrologists, modellers and instrumentalists, researchers and practitioners, to cope with these rather unpredicatable events. The three complementary observation strategies implemented within the observatory are reviewed in the present communication.

Abstract: Northern Mexico is suffering a severe drought since the beginning of 1990's. Rainfall amount was at least 25% lower than average during 8 over 11 years an the 1992-2002 period. This drought caused an “hydropolitic” problem with USA and led to drastic consequences both on agriculture and breeding. The current water scarcity is due to a plurality of processes occurred during the last years, in the historical and geographical context of this northern region. New strategies of management appeared as an adaptation to maintain the activities in spite of the drought; new activities that generate more added value with the same quantity of water were developed. Water scarcity must lead to consider drought and flood hazards. A new “'patrimonial” management must take over from the “mining-type” of natural resources in order to preserve the environment. That should avoid that land degradation continues to enhance scarcity by a reduction in water reserves of soils.

Abstract: The GAMAS (gaseous mercury in interstitial air in snow) instrument developed in our laboratory is a new device devoted to sampling and determination of gaseous mercury concentration in interstitial air in snow. Sampling probes inserted in the snowpack, coupled with a Gardis mercury vapour analyser, provide reliable and original data of vertical profiles of both snow temperature and gaseous mercury concentration at several depths in a snow mantle. This instrument has been tested successfully in Station Nord in Greenland in February-March 2002. A description of this instrument, of the sampling area and its setting up is presented with precise details. Illustrations of the first investigations are given showing a rapid decrease of gaseous mercury concentration simultaneously with depth. A concentration of 0.10 ng/m(3) is reached at 120 cm depth. It may be the result of fast oxidation processes occurring within the snowpack. Gaseous mercury behaviour in the snowpack is a central parameter to elucidate the fate of deposited mercury after mercury depletion events in polar regions. With our new device, we have now the opportunity to determine this key parameter.

Abstract: The snowpack is a medium that greatly interacts with a variety of atmospheric gases. Its role in the mercury depletion events in arctic and sub-arctic regions seems to be crucial though it is poorly understood. We studied an environmental component of mercury that is interstitial gaseous mercury (IGM) present in the air of the snowpack at Kuujjuarapik (55degreesN, 77degreesW), Quebec in April 2002. Our data demonstrate that the snow-pack plays a major role in the global mercury cycle. Indeed, IGM concentrations exhibit a well-marked diurnal cycle with uninterrupted events of Hgdegrees depletion and production within the snowpack. During snowmelt episode, high IGM concentrations (similar to20 ng/m(3) at a depth of 37 cm) have been recorded. This study gives us the confirmation that Hgdegrees is involved in oxidation processes occurring in the snowpack. Additionally, we assume that the impressive production of Hgdegrees during the daytime may be the results of photoreduction and photo-initiated reduction of Hg(II) complexes.

Abstract: The behavior of the terrestrial glacial regime during the Neoproterozoic glaciations is still a matter of debate. Some papers claim that the glacial sequences cannot be explained with the snowball Earth scenario. Indeed, the near shutdown of the hydrological cycle simulated by climatic models, once the Earth is entirely glaciated, has been put in contrast with the need for active, wet-based continental ice sheets to produce the observed thick glacial deposits. A climate ice-sheet model is applied to the older extreme Neoproterozoic glaciation (around 750 Ma) with a realistic paleogeographic reconstruction of Rodinia. Our climate model shows that a small quantity of precipitation remains once the ocean is completely ice-covered, thanks to sublimation processes over the sea-ice at low latitudes acting as a water vapor source. After 10 ka of the ice-sheet model, the ice volume in the tropics is small and confined as separate ice caps on coastal areas where water vapor condenses. However, after 180 ka, large ice sheets can extend over most of the supercontinent Rodinia. Several areas of basal melting appear while ice sheets reach their ice-volume equilibrium state, at 400 ka, they are located either under the two single-domed ice sheets covering the Antarctica and the Laurentia cratons, or near the ice-sheet margins where fast flow occurs. Only the isolated and high-latitude cratons stay cold-based. Finally, among the simulated ice sheets, most have a dynamic behavior, in good agreement with the needs inferred by the preserved thick formations of diamictite, and share the features of the Antarctica present-day ice sheet. Therefore, our conclusion is that a global glaciation would not have hindered the formation of the typical glacial structures seen everywhere in the rock record of Neoproterozoic times. (C) 2003 Elsevier Science B.V. All rights reserved.

Abstract: Anion retention is important in highly weathered soils that contain large amounts of iron and aluminium oxides with surfaces of variable charge. Sorption mechanisms retard anionic solute transfer through these soils. We determined the retardation factor for nitrate in highly weathered Ferralsols from New Caledonia from dynamic experiments using a transient-flow method, and we evaluated the effect of soil solution concentration and organic matter content. A simple method with sectionable tubes was used to determine the nitrate isotherm during non-steady-state water flow under unsaturated conditions. The topsoil retarded the movement of nitrate, and the sorption followed a linear isotherm. In subsoils, retardation factors were larger and increased from 1.15 to 2.05 at soil pH as the NO3--N concentration of the input solution decreased from 71.43 to 0.35 mM, indicative of a non-linear isotherm. Positive surface charge sites were considered to be of two types: one with strong affinity for nitrate at small concentrations and one with weak affinity for adsorption of nitrate at larger concentrations. This type of isotherm with high- and low-energy sites is similar to those found for oxyanions and heavy metals. The related anion exchange capacity was larger than that usually observed in soils of variable charge. Not all exchange sites were detected with our method, and some sites were obviously not available for nitrate retention.

Abstract: Runoff production conditions in a small gully catchment are studied at four different scales: the point scale (0.001 m(2)), the local scale (1 m(2)), the field scale (of the order of 100 m(2)) and the catchment scale (0.2 km(2)). At the point scale, infiltration measurements were conducted using a tension infiltrometer. At the local and the field scale, runoff plots were setup on typical soil surface conditions of the catchment (plateau bare soil, hillslope bare soil and fallow grassland). At the catchment scale, stream discharges were measured at two gauging stations. The overland flow yield is significantly nonuniform in space, due to the high spatial variability of infiltration capacities and the depressional. storage of the soil surface. The runoff and the infiltration data collected confirmed the major role played by soil crusting on runoff generation in that part of Sahel. At the point scale, hydraulic conductivity measurements have shown that infiltration and runoff were driven by the hydraulic properties of the crust. At the field scale, microtopography and heterogeneity in the soil surface crusting decreased discharge volumes. The influence of vegetation growth on runoff yield was evident in the case of the fallow sites. Analysis of discharge data at the catchment scale highlights that infiltration through the bottom of the gully between two gauging stations leads to considerable runoff water transmission losses. (C) 2003 Elsevier Science B.V. All rights reserved.

Abstract: A strong linear relationship is found between Special Sensor Microwave/Imager (SSM/I) microwave (19 and 37 GHz) surface emissivities at horizontal and vertical polarizations over snow- and ice-free land surfaces. This allows retrieving the land surface emissivity and temperature from satellite microwave brightness temperatures after atmospheric corrections. Over the Canadian sub-arctic continental area, we show that the main factor modifying the emissivity is the fraction of water surface (FWS) within a pixel. Accordingly, a map of the fraction of water surface across the Canadian landmass is derived, given a correspondence within 6% as compared to the I km 2 Canadian National Topographic Database of water-covered areas. The microwave-derived surface temperatures are compared to synchronous in situ air and ground surface temperatures and also with independent satellite IR measurements over areas without snow or ice. Root mean square differences range between 2degrees and 3.5degrees, with mean bias error of the order of 1-3degrees. Better results are always obtained with the 37 GHz channel rather than with the 19 GHz channel. Over dense vegetation, the microwave-derived surface temperature is closer to the air temperature (at surface level) than to the ground temperature. The proposed simple retrieval algorithm, not sensitive to cloud cover, appears very useful for monitoring summer interannual or seasonal trends of the fraction of surface water, as well as the daily land surface temperature variation, which are very important parameters in environmental change analysis. (C) 2003 Elsevier Science Inc. All rights reserved.

Abstract: During the last decades, ice cores drilled in Antarctica and in Greenland have provided time series of data that have allowed the characterisation of variations of natural and anthropogenic heavy metals in the past atmosphere. Nevertheless, the interactions of heavy metals with climate change and their transport patterns remain largely unknown during the last climatic cycles. Hereafter we present our project dealing with the assessment of past changes of various heavy metals in Antarctic and Greenland ice during the past approximate to 500 kyr anticipating some preliminary Zn measurements in the EPICA Dome C ice core back to about 200000 years. In our project special emphasis is given to Pt group elements (PGE) that are tracers of interplanetary dust particles (IDPs). Tracers of crustal material, volcanic activity and ocean paleoproductivity are also investigated. At the moment we are focusing especially on the ongoing EPICA Dome C Antarctic ice core, decontaminating mechanically some section and performing preliminary measurements of Zn and At using Graphite Furnace Atomic Absorption Spectrometry and adopting ultraclean procedures. These data confirm the prominent continental origin of Zn in the East Antarctic ice during the last and penultimate glacial period.

Abstract: In order to better understand MDE chemistry, which are thought to happen in the presence of halogenated (chloride and bromide) species, we have led several kinetic experiments with gaseous mercury, ozone and chlorine in Teflon bags under polar atmospheric conditions. The chlorine was expected to produce, with natural solar radiation, the reactive chloride species assumed to play a role in the MDE mechanism. The aim of the first experiment was to study the reactivity between ozone and gaseous mercury: we obtained a rate constant of (13 +/- 6)*10(-20) cm(3)/molec/s, which is roughly comparable to constants given in the literature. Concerning the second experiment, for which chlorine has been introduced, a rate constant of (6.2 +/- 2.2))*10(-17) cm(3)/molec/s for the reactivity of gaseous mercury was obtained. This constant is too weak to recover the atmospheric lifetime of gaseous mercury during MDE. This difference is in discussion in this paper.

Abstract: Precipitation data from the new ERA40 reanalyses and from a 200-year simulation confirm a robust main mode of precipitation variability in west Antarctica. An intermittently strong ENSO signature is found in this mode. However, high correlation with ENSO indices appears infrequent. Thus, the high correlation found in ERA40, and previously in other chronologically realistic data, in the late 1980s and the 1990s may not be expected to last. Unlike previously suggested by others, the sign of the correlation between ENSO indices and west Antarctic precipitation, when significant, does not appear to change in time: Precipitation variability at the ENSO pace in the Bellingshausen-Weddell (Ross-Amunsden) region is consistently in phase (phase opposition, respectively) with the Southern Oscillation Index. This is consistent with a tropospheric wave train connecting the tropical Pacific and west Antarctic regions, which modulates in phase opposition the advection of air and moisture in the 2 regions.

Abstract: Wilkes Land annual snow accumulation variability is explored together with ice core chemistry data as proxies for circumpolar atmospheric circulation in the 110degreesE-135degreesE sector of the circumpolar trough. Four accumulation time series spanning 1930-1985 show a pattern of accumulation related to maritime air mass incursions over the East Antarctic ice sheet slope, which is regionally consistent on interannual and interdecadal timescales. The temporal accumulation pattern in Wilkes Land shows decadal fluctuations of +/-10%. Both these fluctuations and the overall trend of increasing accumulation since the middle 1960s are consistent with a poleward shift and intensification of the circumpolar trough. This is consistent with a shift toward the high index state of the southern annular mode. Interannual variability of snow accumulation rate throughout Wilkes Land was found to be dependent upon shifts in the preferred tracks of cyclones along or across the coast and in the location of cyclolysis regions. Significant differences in this temporal accumulation pattern over Wilkes Land can occur as a result of the time-varying location of atmospheric ridging and the circumpolar long-wave structure south of New Zealand and Australia. Snow accumulation at an additional site (GD09) displays a well-defined interdecadal pattern, and the mean annual nitrate (NO3-) concentration of snow is correlated to the winter sea level pressure gradient between East Antarctica and the sub-Antarctic and the upper geopotential height and zonal wind data over East Antarctica. Strong, surface wind drainage during tropospheric ridging in Wilkes Land produces anomalous snow accumulation and annual mean NO3- concentration in firn at GD09.

Abstract: The implementation of a physically based parameterization scheme for computation of wind gusts in a numerical regional climate model (RCM) is described in this paper. The method is based on an innovative approach proposed by Brasseur [2001] that assumes that gusts occurring at the surface result from the deflection of air parcels flowing higher in the boundary layer. Our parameterization scheme is developed so as to use quantities available at each model time step: consequently, the gusts are also computed for each of these time steps. To illustrate the performances of this novel method, gusts simulated for two severe midlatitude windstorms with the Canadian RCM at various resolutions are compared with observed gust speeds. The study is carried out concurrently for the complex terrain of Switzerland and for the smoother topography of Belgium. A preliminary analysis indicates that this parameterization performs equally well over flat and over mountainous regions; it also responds properly in the strengthening as well as the weakening phases of wind storms. The storm-dependent results rely on the model configuration associated with the downscaling procedure, as well as on the accuracy of the simulated flow fields. The model response is dependent on the resolved topography distribution and height and on the types of lower boundary conditions that affect the stability of the boundary layer. The simulated gusts are generally more realistic at higher resolution over the complex topography of Switzerland but are less sensitive to resolution over the flat terrain as in Belgium. On the basis of these two storms, this study also shows that simple scaling coefficients relating gust speeds and resolution are not an appropriate method for addressing such issues.

Abstract: [1] Isotope records from Andean ice cores provide detailed and high-resolution climate information on various time scales. However, the relationship between these valuable isotope records and local or regional climate remains poorly understood. Here we present results from two new drillings in Bolivia, from the Illimani and the Sajama ice caps. All four high altitude isotope signals in the Andes now available (Huascaran, Quelccaya, Illimani and Sajama) show near identical decadal variability in the 20th century. Comparison with general circulation model results and meteorological data suggest that the Andean high altitude records are primarily controlled by precipitation variability over the Amazon basin.

Abstract: Nine sections from the Sajama ice core recovered from the summit of an extinct Andean volcano in Bolivia were analysed for various heavy metals to assess anthropogenic changes in the tropical upper troposphere over South America. These samples include two sections dating from 14,100 to 19,200 years BP, corresponding to the Last Glacial Stage (LGS), one section dating from 2800 years BP (the late Holocene), and six sections dating from AD 790 to 1988. Our preliminary data, combined with data reported elsewhere, demonstrate that the heavy metal concentrations since the late Holocene are higher than in the period from the LGS to the early Holocene. The enrichment factors calculated by normalizing for crustal abundance using the metal/Al ratio in the mean upper continental crust indicate that a significant input of heavy metals attributed to anthropogenic contributions has occurred since the beginning of the 19th century for Pb and from the last century for Cd, Cu, and Zn.

Abstract: Mercury speciation analysis has been performed in an ice/snow core drilled at an altitude of 4250 m in the Col du D (o) over cap me glacier on the Mont Blanc, France. The study provides the first time series for mercury speciation analysis in the Alpine snow/ice for the last century. Measured concentrations range from 0.25 to 3.96 pg g(-1) for MeHg+ and 0.20 to 1.80 pg g(-1) for Hg2+. Since the beginning of 20(th) century, a clear increase in total mercury concentration is observed with a maximum for 1965 with an approximately 5-fold increase compared to 1885. A drop in both methylmercury and total mercury concentrations was observed after 1965 and might be linked with the reduction in the mercury emissions from anthropogenic activities.

Abstract: [1] In order to establish a chronology of two nearby ice cores from a glacier at Illimani ( 6438 m), Bolivia, a broad dating approach is presented here, which in particular makes use of the fast, simple, and nearly nondestructive electrical conductivity method (ECM) that provides a highly resolved record. Thus, ECM is suited for counting annual layers in the ice, especially for ice cores extracted from high-mountain glaciers with a fast layer thinning. Furthermore, ECM can be used for detecting volcanic signals. Annual signals in the ECM record of the Illimani ice core were identified using the 1964 A. D. tritium reference horizon and were counted along 125 m or 90% of the core, representing the time period from 1200 +/- 240 A. D. ( estimated accumulated error) to 1999 A. D. The resulting age-depth relationship was supported by counting annual peaks in the microparticle record as well as by nuclear dating using the decay of Pb-210. The identification of volcanic signals originating from eruptions such as Pinatubo (1991 A.D.), El Chichon (1982 A.D.), Agung (1963 A.D.), Krakatoa (1883 A.D.), Tambora (1815 A.D.), and the Unknown 1258 A.D. significantly reduced the uncertainty of annual layer counting (ALC) to +/- 2 years in the vicinity of these events.

Abstract: We present a method to analyse tracer transit time climatologies based on the concept of tracer age, The method consists of introducing idealized, short-lived radioactively decaying tracers in a general circulation model of the atmospheric. Tracer age since emission is calculated at any given place in the atmosphere from the ratio of the concentrations of tracers with different lifetimes emitted over the same source area. An obvious use of this method is the analysis of transport of real tracers with similar lifetime, (such as dust particles) during different climatic periods. Here, this method is applied to transport from southern hemisphere continental source areas towards Antarctica at the present, the last glacial maximum (21 kyr BP) and the last glacial inception (115 kyr BP). It is found that the variation over time of atmospheric transport efficiency towards Antarctica depends on the tracer source region: changes for Patagonian tracers differ from those for tracers originating over Australia and southern Africa. Transport towards Antarctica during the last glacial maximum (LGM) is faster for Patagonian. but not for Australian and Southern African tracers. It is shown that for the time of the last glacial inception, tracer transit time towards Antarctica is not different from the present, although signs of a more vigorous atmospheric circulation can be seen in the simulation.

Abstract: [1] The disturbed stratigraphy of the ice in the lowest 10% of the Greenland GRIP ice core prevents direct access to climatic information older than 110 kyr. This is especially regretful since this period covers the previous interglacial corresponding to marine isotopic stage 5e (MIS 5e, 130 – 120 kyr B. P.). Here we present a tentative reconstruction of the disturbed GRIP chronology based on the succession of globally well mixed gas parameters. The GRIP delta(18)O(ice) chronological sequence is obtained by comparing a new set of delta(18)O of atmospheric O-2 and CH4 measurements from the bottom section of the GRIP core with their counterpart in the Vostok Antarctic profiles. This comparison clearly identifies ice from the penultimate glacial maximum ( MIS 6, 190 – 130 kyr B. P.) in the GRIP core. Further it allows rough reconstruction of the last interglacial period and of the last glacial inception in Greenland which appears to lay its Antarctic counterpart. Our data suggest that while Antarctica is already entering into a glaciation, Greenland is still experiencing a warm maximum during MIS 5e.

Abstract: [1] Sahelian rainfall is characterized both by a strong interannual variability and by periods of long-lasting droughts, such as the years 1970 – 1997. The controlling factors of this variability have been the subject of a significant amount of research, but most of this research is carried out using low-resolution averages, typically, monthly to seasonal in time and over 5degrees x 5degrees grid boxes (or larger) in space. This paper is an attempt at characterizing the Sahelian rainfall regime at finer scales, with the objective of establishing links between the seasonal cycle and the interannual variability. To that end, high space-time resolution data sets are analyzed. One is composed of around three hundred daily rain gauges covering a 1,700,000 km(2) area for the period 1951 – 1990. The second is a set of full resolution Meteosat images covering the years 1989 – 1999, allowing for a systematic tracking of the mesoscale convective systems (MCSs). The third data set was produced from an experimental network of recording rain gauges covering 16,000 km 2 in the region of Niamey, Niger, during the years 1990-2000. The analysis of the regional daily rainfall data set tends to revisit the common vision of the seasonal cycle of the Sahelian rainfall. It is shown that the average regime is in fact composed of two subregimes. One is an oceanic regime characterized by a progressive increase of the moist air flow from the ocean into the continent, associated with the seasonal migration of the ITCZ from its southern position in the boreal winter to its northern position in the boreal summer. The second regime is a continental regime in which rain is mostly produced by large convective systems embedded in the easterly circulation. This continental regime sets in abruptly during the second half of June, and 90% of the Sahelian rainfall is then produced by a small number (12% of the total number) of large and organized mesoscale convective systems. The mean event rainfall associated with these systems is larger than the mean event rainfall observed in the oceanic regime. The average proportion of the Sahelian rainfall occurring during the continental regime represents between 75% and 90% of the total annual rainfall. It is thus necessary to study this regime in order to understand the interannual rainfall variability of the region better. It is shown, for instance, that the main factor of interannual variability is the variability of the number of the large convective systems from year to year. It is also shown, using NC
EP/NCAR reanalysis, that the easterly waves, which are a major synoptic feature of the region, are not systematically associated with rain-efficient convective systems and that further studies are needed to understand the differences between wet and dry waves.

Abstract: The quantification of the specific surface area (SSA) of snow crystals and of its variation during metamorphism are essential to understand and model the exchange of reactive gases between the snowpack and the atmosphere. Therefore, the decay rate of SSA of five fresh snow samples was studied in the laboratory at -4, -10, and -15degreesC under isothermal conditions in closed systems. The time-evolution of the snow SSA can in all cases be very well described by an empirical law of the form, SSA = – A log(t + Deltat) + B, where A, B, and Deltat are adjustable parameters. B seems to be closely related to the initial SSA of the snow, and A describes the SSA decay rate. Our preliminary findings at -15degreesC suggest that a linear relationship exists between A and B, so that it may be possible to predict the decay rate of snow SSA from its initial value. For the first time, images obtained from scanning electron microscopy show that crystal rounding of snow is the main process taking place during isothermal metamorphism. New grain boundaries also form. More surprising, however, was the formation of new basal, prismatic, and pyramidal crystal faces, sometimes with very sharp angles, especially at -15degreesC. The growth of facets with sharp angles is not fully explained by current theories of snow metamorphism and has not been observed before.

Abstract: Greenhouse effect: gaps of knowledge and perception. The anthropogenic emissions of greenhouse gases should result in a marked warming of the Planet. Although quantitatively uncertain, this message is well apprehended by the society. Development of more investigations upon future climate and sources of energy should help to limit the extensiveness of this risk. (C) 2003 Academie des sciences. Publie par Editions scientifiques et medicales Elsevier SAS.

Abstract: In gridpoint regional climate models (RCMs), the lateral boundary conditions are usually provided by a procedure called relaxation. The technique was originally studied in the context of numerical weather forecasting. This paper complements the preceding theoretical studies in order to assess the practical choices of model relaxation coefficients. Several profiles of coefficients used in RCMs are then evaluated. The complexity of actual model numerics makes any definite choice of the coefficients out of reach of simple theoretical considerations, but these provide practical guidelines. The latter are confirmed by pragmatic considerations such as minimizing discontinuities and keeping relaxation rates in the range of the represented physical processes. The last part of the paper presents a sensitivity study with the Modele Atmospherique Regional (MAR).

Abstract: We present results obtained from a pristine site of French Guiana (South America), at the Pic Matecho, a region far from gold mining activity. This area contains a small permanent lake, a rare situation in French Guiana. Tree canopy, soils and Lake Matecho sediments were analysed in total Hg and Pb-210. The annual deposit of litterfall is estimated at 40 mug (Hg)/m(2)/y. The top sediment content of 0.2 mug (Hg)/g indicates a large contribution of the atmospheric deposit. In sediment, the decrease of total Hg with depth is neither due to local human influence, nor to increase of atmospheric background during this century. It is attributed to remineralization of organic matter in the upper part of the sediment : the majority of the total Hg deposited at the water/sediment interface is not permanently trapped in the sediment but remobilized in water. Interpretation of the historical sediment profile suggests a constant deposit rate over the last millennium.

Abstract: Gold-mining is a major source of Hg pollution, especially in the tropical rain forest of South America. This pollution has been shown to strongly increase the local natural background. Does this pollution extend far beyond the local areas? Here we present results concerning sediments cored in the large dam take of Petit-Saut in French Guiana. This dam lake, created in 1994, collects waters from a large catchment area where gold-mining activity has been developing since the last century, and still is. Sediment depth profiles in Pb-210 and in total-Hg revealed a high Hg content in the last 4 years. This can only result from anthropogenic activities, which could be due either to propagation of sediments from bold mining sites, or chemical processes acting in recently flooded soils (both processes being related to the dam creation).

Abstract: Hard X-ray diffraction experiments were performed on ice single crystals deformed in torsion. This work shows the relationship between the density of geometrically necessary dislocations and strain gradients. The torsion strain appears to be totally accommodated by geometrically necessary basal screw dislocations. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Abstract: A new method of constraining divergent attenuation corrections for weather radar systems is presented. This was motivated by the need for reliable attenuation corrections when making quantitative precipitation estimates using a small, mobile X-band radar at short range. The approach is suitable for systems requiring attenuation correction in real-time and requires no auxiliary data. An outline of the literature on attenuation and its correction for single-polarisation weather radar is presented. The traditional form of correction is known to be problematic due to the divergence, which may occur in its estimation. The form of constraint presented is based on the representation of attenuation correction originally derived for space-borne radar configurations. This method determines when divergent estimates will occur and allows a more realistic application of the constrained correction. Examples of attenuation correction applied to X-band radar observations in comparison with ground clutter returns are presented, showing good agreement and hence good absolute calibration. Selected profiles are then used to determine the influence of measurement errors on attenuation estimation in the context of this representation. The paper concludes with a discussion of the practical limitations and considerations in applying an attenuation correction to quantitative weather radar rainfall estimates.

Abstract: The astronomical theory of climate, in which the orbital variations of the Earth are taken to drive the climate changes, explains many features of the paleoclimatic records. Nevertheless, the precise link between insolation variations and climatic changes during the Quaternary remains mysterious in several aspects. In particular, the largest sea level changes of the past million years occurred when insolation variations were minimal, like during stage 11, and vice versa like during stage 7. Moreover, recent data from terminations 11 and III show surprising phase lead and lag between insolation and sea level variations. To explain these paradoxical amplitude and phase modulations, we suggest here that deglaciations started when a combination of insolation and ice volume was large enough. To illustrate this new idea, we present a simple conceptual model that simulates the sea level curve of the past million years with very realistic amplitude modulations, and with good phase modulations. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: The diffusion and solubility of formaldehyde (HCHO) in ice were measured at -15degrees by exposing large single crystals of ice to low vapor pressures of HCHO (P-HCHO) for several days. The solubility was found to be ( 30 +/- 3) x 10(-6) mol L-1 at -15degrees under P-HCHO = 1.7 x 10(-2) Pa. The diffusion coefficient is of the order of 8 x 10(-11) cm(2) s(-1) at -15degrees. Extrapolation of these preliminary results to the lower P-HCHO values observed in the polar atmospheric boundary layer is consistent with the hypothesis that the concentration of HCHO in aged snow crystals is governed by thermodynamic equilibrium.

Abstract: A record of the concentrations of Pb and Ba and the isotopic composition of Pb has been established for a remote, low accumulation site in the Atlantic sector of Antarctica (Coats Land) by means of thermal ionization mass spectrometry. The snow samples cover the period similar to 1840 to 1990. They were taken from the walls of a pit to a depth of 7.8 nu and as a core to 16 m; ultraclean procedures were used. Detailed laboratory subsampling provided both long-term (secular scale) and short-term (intra-annual) Pb, Ba, and Pb isotope variations. The results show that there have been significant variations in Ph concentrations (range, 0.1 to 9.3 pg/g) and isotopic composition (range, 1.096 to 1,208 for Pb-206/Pb-207 ratio) since the 1840s. The data show evidence of pollution for this metal in Antarctica as early as the 1880s. Several Pb maxima were observed: the first at the beginning of the 20th century and the last in the 1970s to 1980s, with a clear decrease during recent years. Although the last maximum is clearly linked to the rise and fall in the use of leaded gasoline in the Southern Hemisphere, especially in South America, the reason for the first remains uncertain. The pattern of changing isotopic composition of Ph reveals the changing origin and character of the anthropogenic inputs to Antarctica. An interesting feature in this pattern is the relatively large contribution of unradiogenic Pb in the similar to1890s, possibly originating from Australia. Another interesting feature is the pronounced intra-annual variation in the isotopic composition of Pb, which illustrates the complexity of the changing inputs of Pb to Antarctica. Copyright (C) 2003 Elsevier Science Ltd.

Abstract: [1] Continuous high- resolution records from Col du Dome ( CDD, 4250 m above sea level, French Alps) ice cores were used to investigate the impact of growing NO emissions on the midtroposphere nitrate levels over Europe from 1925 to 1997. The large snow accumulation rate at the CDD site largely minimized diffusional and depositional perturbations of the nitrate record as commonly encountered in ice cores. That permits for the first time examination of the winter and summer trends separately with a high degree of confidence. Being close to 100 ng g(-1) in 1925, summer levels increased at a mean rate of 2 ng g(-1) per year from 1925 to 1960 and 8.5 ng g(-1) yr(-1) from 1960 to 1980. These summer nitrate changes follow rather well the course of growing NO emissions from source regions ( France, Italy, Spain, and Switzerland, denoted WE4) located 1000 km around the Alps as estimated by NMI and EDGAR-HYDE inventories. After 1980, the summer nitrate levels continue to increase at a slightly weaker rate than during the 1960 – 1980 time period. Such a lasting increase of nitrate levels is inconsistent with NO emission estimates, which indicate a decrease after 1993. The nitrate record shows a weaker increase ( 1 ng g(-1) yr(-1) from 1930 to 1990) in winter than in summer, corresponding to a lower contamination of the European wide midtroposphere as preferentially recorded in CDD winter samples. Using ice core and emission data, we calculate a preindustrial NO emission ( including agricultural and natural emissions) of 0.8 +/- 0.2 Mt NO2 per year for WE4 countries, which represents 15 – 20% of the recent NO emissions. These data would help to reduce existing uncertainties in NO inventories which represent key information to assess past and future ozone changes over Europe.

Abstract: Watercress (Lepidium sativum) is consumed as a vegetable, especially by the indigenous community in New Zealand. An investigation was carried out on the accumulation of arsenic, by watercress, following earlier reports of inordinate arsenic concentrations in some aquatic macrophytes collected from the Waikato River, North Island, New Zealand. The Waikato River and some other aquatic systems in Taupo Volcanic Zone, New Zealand have elevated arsenic concentrations due to geothermal activity. Watercress, river water and sediment samples were collected from 27 sites along the Waikato river and analysed for arsenic. Greenhouse trials with watercress grown in beakers containing added arsenic were conducted to confirm the ability of this species to accumulate arsenic. At a number of sites, the concentration of arsenic in both the water and the. watercress samples exceeded the World Health Organisation (WHO) limit for drinking water (0.01 mg l(-1)) and foodstuffs (2 mg kg(-1) on a fresh weight basis). The average leaf and stem arsenic concentrations were, respectively, 29.0 and 15.9 mg kg(-1) on a fresh weight basis. Plants grown in solutions of > 0.4 mg l(-1) arsenic concentration had fresh weight arsenic concentrations above the WHO limit. Despite these higher concentrations, arsenic levels in plants grown under greenhouse conditions were approximately fivefold lower than in plants growing in the Waikato River, possibly because under natural conditions, the watercress is rooted in sediment containing on average approximately 35 mg kg(-1) arsenic. It is recommended that watercress from the Waikato River, or other areas with elevated water arsenic concentrations, should not be. consumed. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: Measurements of Pb and Sr isotopes and Pb, Sr, Ba and Rb concentrations in ice-core samples from the Sajama icecap in Bolivia were made to investigate the changing environmental conditions in the region over the past 22 ky.. The Pb isotopic measurements indicate there has been a significant change in the Pb-206/(207) Pb ratio in the ice by 1700 AD which can be attributed to human activity. This change was also accompanied by increasing Pb/Ba ratios that reach similar to10 times natural values by the beginning of the 20(th) century, confirming an increase of anthropogenic Pb levels. Sr and Rb concentrations are highly enhanced in 3 ky and 0.5 ky BP ice relative to the upper-crustal rock indicating a different origin for a significant proportion of the dust in these samples. Likely sources of this dust are dry lake beds that occur on the Altiplano as a result of changing climatic conditions. This dust adds to the background level of dust accumulating on the Sajama icecap.

Abstract: This study demonstrates the sensitivity of reflectivity-rainfall rate (Z-R) relationships, which were derived from disdrometer-based drop size distribution measurements, to the fall velocity of the drops. The dataset used comes from the simultaneous observation of a series of five moderate rainfall events with a Joss and Waldvogel disdrometer and an optical spectropluviometer. The signal-processing software of the latter was able to measure the residence time of the drops in its sampling volume, enabling computation of the volumetric drop concentration without any assumption on the fall velocity of the drops. The Z-R relationships derived from the two instruments are significantly different. This difference is shown to essentially arise from the drop fall velocities in a twofold manner. First, it comes from the use of a theoretical terminal fall velocity in still air to compute the drop concentration from the drop flux. Second, it comes from the principle of drop size measurement by the impact-type disdrometer that relies on the "energy'' of the drop, and thus on its size and fall velocity.

Abstract: [1] Sulfuric acid aerosols produced in the stratosphere following massive volcanic eruptions possess a mass-independent sulfur isotopic signature, acquired when volcanic SO2 experiences UV photooxidation. The volcanic data are consistent with laboratory SO2 photooxidation experiments using UV light at 248 nm ( maximum absorption of ozone), whereas sulfur isotopic anomalies previously observed in Archean samples are consistent with photodissociation at 190 – 220 nm. A mechanism of SO2 photooxidation, occurring in the early stage of a stratospheric volcanic plume, in the range of 220 – 320 nm ( weak band absorption of SO2), is also proposed. Since mass- independent sulfur isotope anomalies in stratospheric volcanic sulfate appear to depend on the exposure of SO2 to UV radiation, their measurements might therefore offer the possibility to determine the degree of UV penetration in the ozone- absorption window for the present and past atmospheres. They can also be used to determine the stratospheric or tropospheric nature of volcanic eruptions preserved in glaciological records, offering the possibility to reassess the climatic impact of past volcanic eruptions.

Abstract: Sublimation and melt disturb the environmental information obtained from ice core records in the Andes. In two case studies we demonstrate to what extent these post-depositional processes may remove major parts of the accumulated snow cover. Dark ash layers from the Tungurahua eruption changed the albedo of surface snow on Chimborazo glacier (6268 m, 1degrees30'S, 78degrees36' W, Ecuador) between two ice core drilling campaigns and forced substantial melt. Redistribution and washout of the chemical constituents shifted the concentration profiles obtained in December1999 as compared to an equivalent core drilled in December 2000. The stable isotope records showed that approximately the water equivalent (weq) of an annual layer had melted, and that the percolating melt water penetrated within the firn layer to a depth of at least 16.5 m without refreezing. In the second example, from a site on the dry axis between the tropical and extra-tropical precipitation belts, significant loss of accumulated snow layers occurred by sublimation. A surface experiment at Cerro Tapado glacier (5536 m, 30degrees08'S, 69degrees55' W, Chile) revealed that losses of approximate to2 mm weq (approximate to5 mm snow) per day occurred during the dry period following the 1997/98 El Nino. This loss generally included the entire surface layer enriched in stable isotopes, and thus caused minimal disturbance of the isotopic signature ( and hence climatic information) of the net accumulation, yet chemical constituents again experienced considerable changes in concentration. From annual layer counting and direct dating it is obvious that the major part of the accumulated ice on both glaciers is younger than 100 years; however, isotopic and chemical variations at least in the basal ice from Cerro Tapado clearly point to climate conditions different from the recent centuries. This evidence is supported by mass balance considerations derived from a glacier-climate model. The possibility of a third type of disturbance aside from sublimation and melting – in this case a significant hiatus in the environmental chronology – also deserves consideration for other ice core records from this region. Potential disruptions or discontinuities need to be carefully evaluated given the profound changes in climatic and glaciological conditions since the Last Glacial Maximum throughout Holocene times.

Abstract: Chemical reactions of dissolved gases in the liquid phase play a key role in atmospheric processes both in the formation of secondary atmospheric compounds and their wet removal rate but also in the regulation of the oxidizing capacity of the troposphere. The behavior of gaseous species and their chemical transformation in clouds are difficult to observe experimentally given the complex nature of clouds. During a winter field campaign at the summit of the Puy de Dome (central France, 1465ma. s. l), we have deployed an experimental set-up to provide a quantification of phase partitioning of both organic ( CH3COOH, HCOOH, H2C2O4) and inorganic (NH3, HNO3, SO2, HCl) species in clouds. We found that nitric and hydrochloric acids can be considered close to Henry's law equilibrium, within analytical uncertainty and instrumental errors. On another hand, for NH3 and carboxylic acids, dissolution of material from the gas phase is kinetically limited and never reaches the equilibrium predicted by thermodynamics, resulting in significant sub-saturation of the liquid phase. On the contrary, S-IV is supersaturated in the liquid phase, in addition to the presence of significant aerosol-derived S-VI transferred through nucleation scavenging. Upon droplet evaporation, a significant part of most species, including SIV, tends to efficiently return back into the gas phase. Overall, gas contribution to the droplet solute concentration ranges from at least 48.5 to 98% depending on the chemical species. This is particularly important considering that aerosol scavenging efficiencies are often calculated assuming a negligible gas-phase contribution to the solute concentration. Our study emphasizes the need to account for the in-cloud interaction between particles and gases to provide an adequate modeling of multiphase chemistry systems and its impact on the atmospheric aerosol and gas phases.

Abstract: In the northern part of Michoacan State in Mexico, between the Patzcuaro and Cuitzeo lakes, we used aerial photographs and Spot satellite data, to define the spatial and temporal extensions of soil erosion over indurated volcanic material called tepetates. Segmentation techniques combined with graphic mask were applied over the aerial photographs in order to obtain the extension of eroded areas for 1975, 1995 and 2000. An unsupervised classification was performed over various band combinations of Spot. The eroded areas were checked and improved with ground control points. The results show that there are no more eroded areas today than 25 years ago. There is, however, an evolution of the eroded areas; some of them have disappeared while others have become larger. A change in land use intensity explains the regression of eroded area in the north and the augmentation of eroded surfaces in the south. The result obtained with Spot imagery classification allows discrimination of four different erosion degrees, where only one could be distinguish with aerial photographs. However, it is not possible to distinguish outcropping tepetates and outcropping andesit with Spot. As well, additional socioeconomic studies and studies over spectral signatures of eroded materials in relation with their mineral contents are in process. (C) 2003 Elsevier Science B.V. All rights reserved.

Abstract: A laboratory experiment was set up in to determine the role of plant litter in decreasing bare soil evaporation. Two undisturbed samples, collected in the field instrumented during the MUREX (Monitoring the Usable Reservoir Experimentally) experiment were used, The first one was unchanged as compared to in situ conditions. The litter and vegetation layers were cut on the second one. These samples were inserted into a design where the climatic evaporation demand could be modified and maintained constant. The bottom of the soil samples was maintained at constant soil water pressure, so that evaporation rates were not limited by water availability. Total evaporation rates were measured under steady states conditions. The results showed that a linear relationship between litter + vegetation evaporation and bare soil evaporation could be fitted, with a slope around 0.4. Testing conditions were extended using the version of the Simple Soil Plant Atmosphere Transfer (SiSPAT) to model heat and water transfer within the litter It allowed a better quantification of climatic variables (wind speed, air temperature and humidity, incoming solar radiation) and vegetation cover influence on this relationship. Model results showed that this dependence was more complex than experimentally found, the apparent linear relationship being the consequence of mixing various climatic conditions. However, results showed potential for deriving simple parameterisations of litter influence which could be used within atmospheric models.

Abstract: [1] To reconstruct fast atmospheric trace gas variations from polar ice cores it has to be considered that their amplitudes are attenuated during the enclosure process in the ice. Relevant processes for the attenuation are the molecular diffusion in the open pores of the firn column and the gradual bubble close off in the depth of the transition from firn to ice. These processes depend mainly on temperature and accumulation rate and lead e. g. to a strong attenuation for cold sites with low accumulation rates. With a diffusion and enclosure model it is possible to calculate the attenuation for a single event and to compare ice core records from different sites. We investigate the atmospheric methane (CH4) variation during the cold event 8200 years ago and calculate that its amplitude as recorded in the EPICA Dome C ice core is attenuated to a magnitude between 34% and 59%.

Abstract: Contaminant distribution coefficient determined under saturated conditions are often used to model solute transport in the vadose zone. However, the water content of porous media may affect the transport behaviour of conservative and sorbing solutes. We performed column experiments using sand from Chernobyl area and we investigated the transport of tritiated water as conservative tracer, Sr-85 and Cs-134 as reactive solutes at different water saturation levels. The results indicate that when the Kd model is applicable, a decrease of the water content leads to a decrease of the Kd value.

Abstract: Attenuation in rainfall is recognized as one of the most significant limitations in rain-rate estimation from weather radar returns at X- or C-band wavelengths. This paper introduces a radar measurement correction as an inverse problem that accounts for attenuation effects in rainfall. First, a direct theoretical model, relating radar returns at attenuating wavelengths to the rainfall rates between the radar and the point of measurement, is presented. Second, the inverse algorithm used to identify rain-rate estimates from radar returns is described and its application to the attenuation correction is discussed, with the well-known characteristics of the attenuation model (i.e., instability, underdetermination, and nonlinearity) receiving particular attention. Third, a sensitivity analysis is then performed to test the influence of the raindrop size distribution, radar measurement features, and statistical parameters involved in the inverse method. The sensitivity analysis allows for establishing the application conditions of the method. Last, a preliminary evaluation of the method is provided, through simulated radar rainfall measurements and through a limited case study. Various attenuation correction methods are compared with the inverse algorithm. These methods include the standard radar reflectivity – rainfall rate algorithm and two versions of the Hitschfeld – Bordan algorithm. In the simulation exercise, various examples of rainfall field, with different characteristics, are tested. The case study confirms the utility of the proposed method and its ability to provide a robust and stable solution. The method consistently provides better results than the well-known Hitschfeld – Bordan algorithm.

Abstract: [1] The objective of this study is to evaluate the surface energy balance (SEB) of a cold, high-altitude tropical glacier, Illimani (16degrees39'S; 67degrees47'W, 6340 m above sea level (asl)), where a 137 m ice core was drilled down to the bedrock in June 1999. During the dry austral winter, tropical glaciers are known to experience strong sublimation, which may be responsible for snow composition changes through postdepositional processes. In order to help toward the interpretation of this climatic archive, SEB experiments were carried out in 1999, 2001, and 2002, during the dry season (mostly clear and cold atmosphere, strong westerly winds). The daily net all-wave radiation is usually negative during this austral dry winter because of the highly reflective snow surface and because of reduced incoming long-wave radiation due to a low cloudiness compared to outgoing long-wave radiation. The turbulent heat fluxes were evaluated using the bulk aerodynamic approach, including stability correction. The roughness parameters are deduced from direct sublimation measurements and serve as calibration parameters. The sensible heat flux strongly heats the surface at night but changes to negative values during daytime unstable conditions (between 1000 and 1600 LT). The latent heat flux is always negative, which means that the surface loses mass through sublimation, particularly in the daytime (sublimation rates are -1.2 mm w.e. d(-1), -0.7 mm w.e. d(-1), and -0.8 mm w.e. d(-1) during the 2001, 2002, and 1999 measuring periods, respectively, where w.e. is water equivalent). The winter SEB of this high-altitude cold tropical glacier is comparable to the summer SEB over snow surfaces of the intermediate slopes of Antarctica.

Abstract: The scaling properties of fracture and faulting of ice on Earth are reviewed. Numerous evidences for the scaling of fracture and faulting of ice are given, including self-affine fracture surfaces, fractal fracture networks at small (laboratory) and large (geophysical) scales, power law distributions of fracture lengths or of fragment sizes within fault gouges. These scaling laws are discussed in terms of the underlying mechanics. Scaling of the observables associated with fracture and faulting argues for the scale invariance of the fracture and faulting processes and indicates that small scales cannot be arbitrarily disconnected from large scales. Consequently, quantitative links between scales cannot be performed through classical homogenization procedures. Scaling can also induce scale effects on different mechanical parameters such as fracture energy, strength or stiffness. Although scaling is ubiquitous for the fracture of ice on Earth, important exceptions exist such as the nucleation of microcracks or the crevassing of glaciers. These exceptions are stressed and discussed.

Abstract: Type II polar stratospheric cloud particles are made up of ice that forms by water vapor condensation in the presence of numerous trace gases, including HCl. These gaseous species can co-condense with water molecules and perturb ice structure and reactivity. In order to investigate the effect of co-condensing dopants on the structure of ice, we have designed an experimental system where ice films can be stabilized at 190 K, a temperature relevant to the polar stratosphere. We have co-condensed different HCl : H2O gaseous mixtures, with ratios 5: 1, 1: 10, 1: 50 and 1: 200 and studied the solids formed by infrared spectroscopy. The IR spectra obtained show that: ( 1) HCl is likely undergoing ionic dissociation when it is incorporated by co-condensation into the ice at 190 K; (2) this dissociation is done by several water molecules per HCl molecule; and (3) significant differences between our spectra and those of crystalline solids were always detected, and indicated that in all cases the structure of our solids retained some disorganized character. Considering the major impact of HCl on ice structure observed here, and the well known impact of the structure of solids on their reactivity, we conclude that the actual reactivity of stratospheric ice particles, that catalyze reactions involved in ozone depletion, may be different from what has been measured in laboratory experiments that used pure ice.

Abstract: [1] Ice cores have provided a wealth of information about past atmospheric composition and climate variability. However, relatively little is known about how the chemistry of the atmosphere has responded to natural climate change and anthropogenic influences. The oxygen isotopes (delta(17)O and delta(18)O) of sulfate serve as a recorder of the relative amounts of gas and aqueous-phase oxidation pathways in the atmosphere. This quality, along with its stability, renders sulfate an ideal proxy to investigate changes in oxidation pathways of S(IV) species in present and ancient atmospheres. The oxygen isotopic composition of sulfate in eight samples from the Vostok, Antarctica ice core, covering one full climate cycle, is presented. Assuming tropospheric-derived sulfate only, isotope data reveal that the ratio of gas-phase over aqueous-phase oxidation of S(IV) species was greater during the last glacial than the surrounding interglacial periods.

Abstract: Dating recent lake sediment records yielding disturbed (210) Pb profiles has been a problem of wide interest in palaeoclimatic and palaeoseismic studies over the last few centuries. When applied to an alpine lake sedimentary record, a high-resolution sedimentological study reveals that the Pb-210 profile is disturbed by the occurrence of single-event deposits triggered by two different mechanisms: flood events deposits and gravity reworking. Removing disturbed layers from the (210) Pb profile yields a logarithmic depth-activity relationship. Using a simple (210) Pb decay model (CFCS) provides an assessment of mean accumulation rate of `continuous sedimentation', as opposed to `event-linked sedimentation'. The correlation of the thickest four gravity-reworked deposits with historically known earthquakes permits both validation and refinement of the age-depth relationship. This refinement highlights variations in accumulation rate consistent with post-Little Ice Age climatic variations.

Abstract: Snow chamber and snow-pile experiments performed during the 'Alert 2000' campaign show significant release of NO, NO2, and HONO in steady ratios under the influence of irradiation. Both light and a minimal degree of heating are required to produce this effect. We suggest diffusion and re-distribution of NO3 in the form of HNO3 as an important step in the mechanism of active nitrogen release from the snowpack. (C) 2002 Elsevier Science Ltd. All rights reserved.

Abstract: It seemed to us interesting to recall, (- as impassioned amateurs but not being able to claim neither with the rigour nor with exhaustiveness ? -) history of the development of Hydrology in France from Perrault to Parde. The facilitate the reading one will cut out the period in various times.

Abstract: Oxygenated hydrocarbons, including for the first time alcohols, in the atmosphere and snow-pack interstitial air were measured at Alert, Nunavut, Canada from 15 February to 5 May 2000. Unexpectedly high concentrations of oxygenated hydrocarbons were observed. Acetone, acetaldehyde and methanol represent about 90% of all oxygenated hydrocarbons measured in this work, and together with formaldehyde their total concentration was higher than the sum of measured NMHCs. During sunlit hours, concentrations in the snow-pack interstitial air were higher than those measured in the gas-phase, implying a positive flux from the snow-pack to the Arctic boundary layer. Fluxes of acetaldehyde, acetone and methanol at that time were estimated to be 26, 7.5 and 3.2 x 10(8) molecules cm(-2) s(-1), respectively. These rates would deplete the local snow of acetaldehyde and acetone in about 2 days if degassing was driving the flux. Additional evidence suggests that photochemical production in the snow-pack could explain these fluxes, especially for acetaldehyde. Diel variations were observed at Alert after polar sunrise in the snow-pack interstitial air and in ambient air. During decreasing 03 conditions, positive correlation with acetaldehyde was observed which is interpreted as implying local Br driven chemistry, but acetone mixing ratios showed a strong negative correlation. Crown Copyright (C) 2002 Published by Elsevier Science Ltd. All rights reserved.

Abstract: Techniques have been developed to measure the isotopic composition and concentration of Sr at sub-nanogram per gram levels in polar snow and ice samples. A Sr-84 spike was used to determine Sr concentrations on a single sample aliquot of a few millilitre. Microlitre scale columns of Sr-Spec resin were used to purify Sr samples. Thermal ionisation mass spectrometry (TIMS) was used to measure the Sr isotopic ratios. A single TIMS measurement of the Sr sample yielded both isotopic composition and concentration after deconvolution of spike and sample spectra. This allows isotopic variations in Sr to be used to identify source regions of crustal dust. Early Holocene and last glacial maximum ice core samples from both Antarctica and Greenland were analysed to demonstrate the applicability of the technique. Sr isotopes were analysed in similar to20 g sized samples of Antarctic early Holocene ice where the concentration was only similar to31 pg g(-1). This represents an improvement of two orders of magnitude in sample consumption over previous studies allowing for a much higher time resolution in the analysis of polar ice cores. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: A 3-dimensional thermomechanical ice-sheet model is used to simulate the evolution of the geometry of Northern Hemisphere ice sheets through the Last Deglaciation. The ice-sheet model is forced by a time-evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the LMD5.3 atmospheric general circulation model (AGCM) at different periods of the Last Deglaciation (21, 15, 9, 6 and 0 kyr BP). The AGCM is driven by insolation, atmospheric CO2 content, ice-sheet configuration and sea surface temperatures. Although our approach is able to produce the complete continental ice retreat, our simulated deglaciation presents a phase-lag with reconstructions based on observational evidences. This suggests that physical mechanisms related to climate forcing and/or ice-sheet internal dynamics are not properly represented. The influence of millennial-scale forcing, feedback mechanisms between ice-sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments. The rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature-mass balance relationships. Fennoscandia appears to be strongly sensitive to the small-scale ice-sheet instability. Both ice sheets are to some extent sensitive to an increased basal sliding. (C) 2001 Elsevier Science Ltd. All rights reserved.

Abstract: As part of a regional project of the study of pollution in two alpine valleys (POVA), an intensive campaign of measurements for VOCs, carbonyl compounds, PM10, particles and major ionic species was performed in Maurienne Valley (French Alps) during one week in August 2000. In summer, under anticyclonic conditions, the dispersion of air pollutants in deep valleys is mainly dependent on an usual characteristic cycle: up-valley wind for the day, down-valley wind for the night. During the sampling wind, campaign, the diumal variation west was changed by a South wind for two days. A saharan dust episode occurred, characterized by large increases of PM10, large particles, and Ca2+. This dust episode was followed by an increase of fine particles (0.3 mum) and of SO42-, indicating the intrusion of an anthropogenic air mass. The evolution of VOCs have been influenced by these events and the concentration levels were higher during this period. The ratio benzene/ toluene was low (0.7) during the episode, which indicates an urban pollution.

Abstract: [1] We used a suite of two models: a global climate model, and a hydrological routing scheme, to estimate the changes in the surface water budget and extent of natural wetlands, at the last interglacial (126000 years ago) and at the last glacial maximum (21000 years ago). At both time periods, in northern tropical Africa as well as in northern South America, our simulations exhibit, in many places, an indirect relationship between the surface water budget and the extent of natural wetlands. In relatively moist regions, decreasing (increasing) rainfall and runoff at the last glacial maximum (last interglacial) result in increased (decreased) wetland area due to the reduction (increase) in lake depth. This counter-intuitive result has never been hypothesized before and may shed a new light on the interpretation of past changes in atmospheric methane, as derived from ice core analyses. It also points to the importance of using a bottom-up modelling approach in this field of study.

Abstract: Insoluble dust concentrations and volume-size distributions have been measured for the new 581 m deep Dome C-EPICA ice core (Antarctica). Over the 27000 years spanned by the record, microparticle measurements from 169 levels, to date, confirm evidence of the drastic decrease in bulk concentration from the Last Glacial Maximum (LGM) to the Holocene (interglacial) by a factor of more than 50 in absolute value and of about 26 ill flux. Unique new features revealed by the EPICA profile include a higher dust concentration during the Antarctic Cold Reversal phase (ACR) by a factor of 2 with respect to the Holocene average. This event is followed by a well-marked minimum that appears to be concomitant with the methane peak that marks the end of the Younger Dryas in the Northern Hemisphere. Particle volume-size distributions show a mode close to 2 mum in diameter, with a slight increase from the LGM to the Holocene; the LGNI/Holocene concentration ratio appears to be dependent on particle size and for diameters from 2 to 5 mum it changes from 50 to 6. Glacial samples are characterised by well-sorted particles and very uniform distributions, while the interglacial samples display a high degree of variability and dispersion. This suggests that different modes of transport prevailed during the two climatic periods with easier penetration of air masses into Antarctica in the Holocene than during Glacial times. Assuming tat southern South America remained the main dust source for East Antarctica over the time period studied, the higher dust content recorded during the ACR which preceded the Younger Dryas period, represents evidence of a change in South America environmental conditions at this time. A wet period and likely mild climate in South America is suggested at circa 11.5-11.7 kyr BP corresponding to the end of the Younger Dryas. The Holocene part of the profile also shows a slight general decrease in concentration, but with increasingly large particles that may reflect gradual changes at the source.

Abstract: The aim of this paper is to assess the influence of the aerodynamic resistance modelling in a two-layer Soil-Vegetation-Atmosphere-Transfer (SVAT) model, on the simulation of both surface fluxes and temperatures. For this purpose, a large review of the existing schemes was conducted and seven parameterisations were retained. They are all based on the first approximation of the turbulence theory, called the K-theory. All these parameterisations were implemented in the SiSPAT-RS (Simple Soil Plant Atmosphere Transfer and Remote Sensing) SVAT model, already calibrated on the experimental fields of the Alpilles-ReSeDA (Remote Sensing Data Assimilation) experiment. The impacts on the simulation of the surface transfers were analysed. It is shown that the different parameterisations lead to similar simulations of total surface fluxes, but to different soil and vegetation contributions, especially in high moisture conditions and low LAI values.

Abstract: Jurassic black marls constitute one of the most extensive outcrops in the Alpine massif. They include the most eroded terrains in the French Alps. Black marls are subject to strong weathering and their erosion rate can reach values close to 1 cm year(-1) (i.e. 100 m(3) ha(-1) year(-1)). Many studies have focused on the processes and forms of erosion of these black marls and on the development of an original metrology to improve our understanding of the seasonal processes in these badlands areas. This study shows that alternating freeze-thaw and wet-dry cycles are the main causes of disaggregation in the marts. The exposure of hillslopes and the dip-slope angle are the main site factors explaining the variability in the action of these processes and thus, in the soil thickness lost as a result of erosion. The kinetic energy of rainfall is similar everywhere, but the volumes of regolith transported by runoff depend on the effects of previous weathering.

Abstract: In a subtropical mountain of northern Mexico. soils and slopes seem to have a hortonian functioning, as it is commonly observed in other tropical or subtropical areas. So it is important to know previous soil moisture before any rainfall event in order to properly estimate the runoff coefficients and to get a better understanding of the hydrologic behaviour of the catchments. A very simple deterministic model, named NAZASM, is developed and tested for 6 microplots (1 and 10 m(2)), 13 plots (50 m(2)) and 5 catchments (1-50 km(2)). It is based on the 'antecedent precipitation index' with an exponential decay parameter (a) into the rainfall-runoff relationship, Measurements of rainfall and runoff were made during four rainy seasons. Between 22 and 157 rain events were considered in the analysis. depending on the sizes of the plots and catchments. 'NAZASM' gives reasonable estimations of runoff if the calibration data set includes events occurring during wet and dry years. due to the difference of hydrological processes involved in these two situations. All the results have shown that the use of such a model significantly improves the explained variances between measured runoff and rainfall as compared to classical regression analysis whatever the size of plots and catchments are. The alpha soil water content decay parameter which in theory is a soil characteristic. appeared in fact to be scale dependent. (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: The data about 15 years of mass balance of sforzellina Glacier (Lombardy Alps, Italy) were analysed; the mass balance data were constantly negatives from 1987 to 2000 and positive only in 2001. The mass balance data of Sforzellina Glacier represents a good statistical sample that was compared with data of other Alpine glaciers (Careser (I), Hintereisferner (A), Vernagh (A), Sarennes (F), Saint Sorlin (F) Gries (S), Kesselvand (S), Aletsch (S), Wurten (A)). The study about the relations between glacier mass balance data allowed to analyze the temporal variability of alpine mass balances, evidencing past and present glacial trend, moreover evidenced the sensibility of this glaciological parameter that encloses not only a temporal index of climate variability but also a spatial determination of the climate variability. In this way it was possible to characterize a regionalism of glacier mass balances that allows the mapping of the climate. and of its spatial and temporal variations. The study of spatial and dimensional relations of Alpine glaciers in the period 1967-1997 and 1987-1998 evidenced that the correlation between the mass balance series decreases if the distance between the studied glacier increases (according to exponential function if we consider emispheric mass balance data, according to linear function if we use only italian data) and contemporaly the standard deviation of the predicted data increases if the distance from glacier used to predict the data increases. From the experimental data it is possible to assert that the critical distance beyond which the correlation between the Italian glacier mass balances and the others is not meaningful (r < 0.5) is about 200 km. Within such spatial range, instead, the correlations with tire series of mass balance data were meaningful. This distance can be interpreted as the limit of the region introduced previously. In this way was possible to quantify and to map the comparability zone of Italian glaciers. If we consider the other Alpine glaciers, the comparability zones are in some cases more extended (350 km for Hintereisferner) even if the mean value is about 200 km. Moreover the dimensional influences on the single mass balance and on the relations between the different glaciers turned out negligible. It is possible to conclude that in order to reconstruct mass balance data using a modelistic method it is necessary to employ data of close glaciers (with a maximum spatial range of 200 km) while the dimensional factor does not seem important; that is to say that on the Alps also the small glaciers (as Sforzellina Glacier) can be compared with large glacier having good results if they are in a spatial range of 200 km. This result is important to use the limited series of mass balance data currently available in order to reconstruct past and present mass balance of other alpine glaciers that are not directly monitored. The study supplies moreover suggestions on spatial distribution of glaciers to measure directly.