1990-1999

Abstract: This study present an experimental non destructive method which is performed in laboratory, for the characterization of an infiltration achieved under axisymmetrical condition, in an unsaturated porous medium. We show that this method based on the visualization of phenomena using fluorescence, allows to distinguish clearly the infiltrated zone and to observe with a great accuracy the evolution of the wet front during the infiltration phase, Simultaneously, a technique of images treatment based on morphological binary analysis is proposed for determining flow parameters.

Abstract: Flood propagation and sediment transport differ by their characteristic time scale. As a consequence, flood damages are more familiar than morphological evolution, which remains more often a domain for the specialist. The objective of this paper is to show, through some examples, how much these two domains are interrelated, and to convince that flood hazard protection must take into account sediment transport and morphological consequences.

Abstract: In this paper, Opus, an agricultural contaminant transport model, was used to simulate a year of soil water flow and nitrate transport as well as crop growth for an arable cultivated soil, near Grenoble, France. This study is explicitly related to the cultivation of irrigated maize, a major crop in the area. The results have shown that, in terms of water balance, the simulations of water flow match the general measured trends. Drainage and evapotranspiration were estimated with errors of 4 and 20%, respectively. Runoff estimation was exactly accurate. Nitrogen movement and nitrogen concentration in the root zone were simulated well, and the cumulative leach beyond the root zone was estimated with an error less than 11%. The results for crop growth were good. Some reasons for these results are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: Numerical experiments have been carried out with a soil-vegetation-atmosphere transfer (SVAT) model to study the impact of spatial variability in soil and land surface parameters on regional-scale water balance components. A statistical-dynamical approach has been used to account for the spatial variability of the selected parameters and to determine the seasonal evolution of the impact on the water budget. Point data are used to derive probability-density functions for the thickness of the permeable top soil layer, coefficients for the water retention and hydraulic conductivity curves, the leaf area index and the minimum stomatal resistance. These distributions are incorporated in the model's mathematical framework in order to generate univariate distributions (sensitivity patterns) for evaporation, transpiration, total evaporation and runoff. The means of these univariate distributions of outputs yield catchment-scale averages. Next, the study obtains catchment-scale evaporation estimates by running simulations with aggregated parameters obtained as statistical descriptors of parameter distributions. The difference between the catchment-scale averages and values obtained with aggregated parameters describes the non-linear response of the model to spatial variability of the particular parameter. Finally, the study also investigates several effective parameters based on recently described hydrometeorological aggregation rules. Results show significant differences in sensitivity patterns between individual parameters and between seasons. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: The Precordillera area of the Andes Mountains (Mendoza, Argentina) is affected by severe flash floods, caused by heavy rainfall events of short duration and high intensities. A, telemetric network, installed on a pilot zone since 1983, provided a set of about 150 rainfall events. In addition, about 50 hood events were recorded at the outlet of a 5.47 km(2) catchment. The distributed model Areal Non-point Source Watershed Environment Response Simulation was adapted to this catchment and applied continuously over the period 1983-1994. The year 1985 was used for calibration. The model was able to reproduce runoff volume with an efficiency of 0.6, and peak discharge with an efficiency of 0.46. The largest events were however underestimated, although the model was able to reproduce the sharp increases in streamflow registered by the sensor. Based on the assumption that the model had captured the catchment behaviour, it was used to determine the main processes involved in runoff generation. The combination of rainfall and soil variability, mainly associated with a quasi-impervious area in the middle of the catchment, was found to explain the rapid increases in streamflow. Vegetation, surface storage capacity, and initial soil moisture were also influential but with a much smaller magnitude than the combination of rainfall and soil variability. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: Several terrain investigations, with specialized organizations such as services R.T.M., the P.G.H.M., allow us to count nearly 100 events which have occurred in the French Alps during 200 last years. The whole of this inventory has been centralized and synthesized in the form of data sheets; each glacier having presented at least one event has a data sheet, containing a description of the glacier (localization and a photograph), and the description of the various listed events. The data sheets were gathered by massif (mont Blanc, Vanoise, Ecrins and the South Alps massif). After having indexed the events of glacial origin, it appeared necessary to classify them; 3 principal categories thus are distinguished: drainings of glacial lakes, falls of ice (distinguishing the falls from seracs, repetitive, from the glacial tongues ruptures or the falls of suspended glaciers, much rarer), ruptures of water pockets.

Abstract: Experiments have been conducted to investigate the effect of melt on the creep behavior of polycrystalline ice deformed in the dislocation creep regime. The transition between a mode with a stress exponent n = 3 and a made with n < 2 is observed for both melt-free and melt-added ice samples. The large influence of the melt phase is not related to the wetting characteristics of the liquid. Owing to the large plastic anisotropy of the ice crystal, the liquid phase would attenuate the internal stress field which develops during the primary creep. The contribution to the deformation of the basal slip (the weaker slip system) would increase with the melt content.

Abstract: The variability of soil water content over short distances was studied at seven experimental fields, located in southern Sweden, eastern Germany, southern France, and in the Netherlands. The soils all in use as arable land, are a sandy soil, a stony sandy soil, a loamy sand, a sandy loam, and three clay soils. Samples with a volume of 100 cm(3) were taken at close intervals in trenches at several depths to determine potential water repellency and soil water content. When dry, the topsoils of the Mellby site in Sweden (loamy sand) and the Vredepeel site in the Netherlands (sandy soil) are water repellent, whereas the soils at the other five sites are wettable, according to the water drop penetration time (WDPT) test. The variation in water content within short distances was high at most depths at all sites, with differences of 7 to 18 vol% within a horizontal distance of only centimeters to decimeters. Drier as well as wetter soil areas were visualized by contour plots of the soil water content distributions in the transects. Large differences in wetting capacity between samples taken in the topsoil and in the subsoil at several sites were assessed by measurements of the wetting rate. In many cases, subsoil samples wetted faster than topsoil samples. The severity of water repellency of sandy samples from the Vredepeel site increased remarkably at oven temperatures above 65 degrees C, The wetting rates of dried samples from this soil were also evidently influenced by the oven temperature; samples dried at 25 degrees C wetted immediately and reached soil water contents of 23 to 32 vol% within one hour, whereas samples dried above 85 degrees C hardly wetted during 72 hours, as a result of the increased water repellency at higher drying temperatures. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: This paper is devoted to a sensitivity study of the equation describing attenuation effects in rain for groundbased weather radar systems operating at X- or C-band wavelengths. First, the so-called attenuation equation, also termed the HE solution or HE algorithm in reference to the well-known paper by Hitschfeld and Bordan, is recalled. A procedure aimed at obtaining consistent relations between average values of the equivalent reflectivity factor Z(e), the attenuation coefficient k, and the rain rate R as function of two parameters of the drop size distribution (DSD) is also presented. Then. a numerical simulation framework based on a simple description of rainfall characteristics and accounting for some of the radar measurement features is proposed to test the ability of the HE algorithm to perform attenuation correction of hypothetical rain-rate profiles. In a first step, the well-known instability of the solution is illustrated. For instance, it is shown that, even in the absence of radar calibration error and with perfect knowledge of the DSD. the algorithm is nor able to correct profiles with path-integrated attenuation (PIA) greater than about 20 dB when typical values are considered for the radar parameters. Owing to this inherent instability, the sensitivity study with respect to the DSD parameters is therefore limited to profiles with PIAs less than 15 dB. The two following results are obtained: 1) a PIA of about 10 dB should be considered as the upper limit that the algorithm is able to correct and 2) given the choice of the (Z(e), k. R) relations, optimization of one parameter is necessary and sufficient to obtain improvement over the standard ZR method for this range of PIAs. This parameter plays the role of a correction term for the radar calibration error, the uncertainty in the knowledge of the DSD, and other sources of bias. These results are confirmed by an X-band radar-rain gauge comparison with a dataset collected during the Marseilles Hydrometeorological Experiment.

Abstract: Comet C/1996 B2 Hyakutake was extensively observed at the Pic du Midi observatory during late March of 1996, Bright condensations were observed in the near-nucleus coma. We have performed a detailed data analysis in order to derive the position of these features with respect to the nucleus. We make the hypothesis that they are induced by fragments of the nucleus. Despite the frequency of fragmentation of cometary nuclei, the dynamics of the fragments is not yet well understood. We propose a general approach in order to study the motion of the fragments in the orbital plane of the comet. An estimate of the non-gravitational forces is used to describe the motion of the fragment and of the nucleus with respect to their centre of mass. Then the equations of the theory of perturbed Keplerian motion are solved in order to study the motion of the centre of mass, This approach is applied to Comet C/1996 B2 Hyakutake, The results are in good agreement with the observations, An excellent fit is obtained for a fragment size of 20 m, assuming a density of 300 kg m(-3).

Abstract: To document the long-term energy and water budget of a fellow land, a complete dataset was collected in the south-west of France. it included climatic data, surface fluxes, vegetation evolution and soil moisture monitoring. The soil hydrodynamic and thermal properties were also determined in situ and through laboratory measurements. The experiment was conducted during 3 years (1995-1997) on a 20 ha held. A natural plant-residue mulch layer was constantly covering the field. Its thickness artificially increased in June 1995 when the farmer unexpectedly cut the grass and left the residue on the ground. Two versions of the simple soil plant atmosphere transfer (SiSPAT) model were compared using the 1995 dataset: the original version which did not take into account the mulch layer; and a modified version where explicit modelling of the mulch was included (SiSPAT-mulch). The prediction of soil temperature, surface fluxes and surface soil moisture was improved when considering a mulch layer. Annual surface evaporation was divided by a factor two to four and transpiration increased by 30-50% when considering the mulch layer. Annual total evaporation decreases by 4-10% according to mulch characteristics, leading to a better simulation of the annual and daily water balance of the sitz. A sensitivity analysis investigated the robustness of these results and possible causes of inaccuracies on half hour values. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: Two atmospheric aldehyde sampling techniques, the impinger method in which aldehydes are trapped in a 2,4-dinitrophenylhydrazine acidified acetonitrile solution, and the method in which aldehydes are captured in 2,4-dinitrophenylhydrazine acidified coated cartridges commercialised by the WATERS(R), were compared for blank values, sample preservation, ozone action, and water influence. The two methods were tested in a Paris atmosphere and the influence of a KI ozone scrubber has been evaluated. Except for acetaldehyde and benzaldehyde, the two techniques do not give coherent results. When no ozone scrubber is used, the cartridge technique gives systematically lower values. Bad correlation has been obtained between data with scrubber and data without scrubber for the impinger technique except for acetaldehyde. For the cartridge technique, the correlation is better for all the aldehydes but higher values are found when a scrubber is used, except for formaldehyde. Ozone action leads to 2,4-dinitrophenylhydrazone destruction, but also to formaldehyde 2,4-dinitrophenylhydrazone formation by reaction between airborne volatile organic compounds and ozone on the cartridge surfaces.

Abstract: Nitrous oxide (N2O) is an important greenhouse gas that is presently increasing at a rate of 0.25 percent per year. Records measured along two ice cores from Summit in Central Greenland provide information about variations in atmospheric N2O concentration in the past. The record covering the past millennium reduces the uncertainty regarding the preindustrial concentration. Records covering the last glacial-interglacial transition and a fast climatic change during the last ice age show that the N2O concentration changed in parallel with fast temperature variations in the Northern Hemisphere. This provides important information about the response of the environment to global climatic changes.

Abstract: The tiger bush is a patterned woodland with alternating bare area and vegetated stripes. In Niger, it covers one third of the Sahelian zone. These natural forests are of considerable economical interest since they are the main source of livestock forage and domestic energy. Its sustainable exploitation needs improved understanding of its dynamics. The redistribution of water between thicket and intervening bare areas is decisive for the water supply of the vegetation. Tiger bush patterning replicates an elementary unit composed of a bare area, the upslope border, the core and the downslope margin of the thicker. (Each zone of tiger bush is characterised by specific soil crusting associated with vegetation). Both water storage and runoff have been monitored after each rain, over a period of 4 yr, including contrasting rainy seasons, on the different zones composing the tiger bush. On the three crusted zones, runoff has a piecewise linear relationship with rain: on closed plots, runoff yield vs. annual rainfall ratio reaches 54% on bare soil, 2% on upslope border and 18% on downslope border. The measured infiltration confirms these rates on independent plots. In the core of the thicket, measured infiltration corresponds with the sum of the contributions of upslope zones, weighted by their relative lengths. This model predicts that bare area contributes up to 62% of the thicket supply, while direct rain is 27%, the senescence zone is 10% and the upslope border contribution is negligible (1%). The average water infiltration in the thicket is equal to 4 X the incident rainfall, but water redistribution is not homogeneous within the core of the thicket. By the most favourable location, infiltration depth is measured to be about 8 X the rainfall. The important runoff, mainly generated on the impervious bare area crosses the upslope border of the thicket without infiltrating, and entirely benefit to the core. Nothing is left to the downslope border, only rainfed. The upslope border, often described as favourable location for young plants is only rainfed most part of the year. By the end of the season, its increasing porosity, due to vegetation and termite activity let it benefit of the last rains. The simple water balance model based on runoff measurement is satisfactorily validated by independent observed infiltration. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: A digital image-processing approach is proposed which allows the extraction of two-dimensional polycrystalline ice microstructure (grain boundaries) from thin sections observed between cross-polarisers. It is based on image segmentation of colour images. The method is applied to the preliminary analysis of the shallow (Holocene) ice of the European Project for Ice Caring in Antarctica (EPICA) ice core at Dome Concordia. Structural parameters, such as the mean cross-sectional area, shape anisotropy and grain morphology, are obtained. The interest and limitations of this automatic procedure are discussed.

Abstract: For the purpose of numerically studying sahelian storm rainfields, a family of random functions is described with a characterization of its finite dimensional law. Some problems appearing when fitting its functional parameters are put forward and two solutions to bypass those problems are provided, according to the regularity properties of the marginal cumulative distribution function. An illustration of this method is implemented on a set of sahelian rainfields of event accumulation displaying a strong spatial intermittency.

Abstract: The discharge-duration-frequency approach, called QdF, makes it possible to describe the flood hydrological regime of a catchment from a statistical point of view. Equivalent to intensity-duration-frequency curves for rain, QdF curves represent discharge distributions for different d durations. The multiduration continuous model, presented in this paper, aims to describe these distributions continuously, as a function of d. When the statistical law is the exponential law, this model contains only four parameters, due to observed scale invariance properties. Consequently, these four parameters characterize the flood hydrological regime of the studied catchment. ((C) Academie des sciences / Elsevier, Paris.).

Abstract: Atmospheric records of sulfate and MSA in aerosols were obtained by year-round samplings in the boundary layer at two coastal Antarctic sites, Neumayer (1983-1995) and Dumont d'Urville (1991-1997), At Dumont D'Urville and Neumayer the non-sea-salt sulfate (nssSO(4)(2-)) level observed from October 1991 to October 1993 exceeded by 44 and 48 ng m(-3), respectively, the mean level (similar to 150 ng m(-3)) observed during nonvolcanic years. The Neumayer and Dumont d'Urville records first exhibit spring (October to December) 1991 nssSO(4)(2-) concentrations exceeding mean spring concentrations by some 100 to 150 ng m(-3) caused by a significant poleward transport of the Cerro Hudson volcanic aerosol in the lower stratosphere beneath the polar vortex and possibly in the middle/upper troposphere. Limited to 10 ng m(-3) during winter 1992, the nssSO(4)(2-) perturbation reached 78 ng m(-3) in fall 1993 at Neumayer. The latter enhancement corresponds to the input of the Pinatubo volcanic aerosol advected through stratosphere/troposphere air mass exchanges. An enhancement of nssSO(4)(2-) concentrations is also recorded in snow layers deposited between 1991 and 1993 at various sites located over the high Antarctic plateau. On the basis of these firn data the relative impact of the three latest volcanic eruptions of global concern, namely, Mount Agung (1963), El Chichon (1982), and Pinatubo (together with Cerro Hudson) (1991), was evaluated in terms of sulfate input at high southern latitudes. The fallout of sulfate in central Antarctic snow following the Cerro Hudson/Pinatubo eruptions was slightly lower than fallout after the Mount Agung eruption. Sulfate fallout after El Chichon was 2 to 3 times lower than that from either Mount Agung or Pinatubo. The enhancement of sulfate levels detected in central Antarctic snow deposits during the 1991-1993 time period allows us to estimate a mean atmospheric sulfate perturbation of around 60 ng m(-3) in the boundary layer of these regions (i.e., at least 35% higher than that seen at coastal sites). This observation suggests that coastal Antarctic regions are less sensitive than the high plateau in detecting volcanic sulfate fallouts following eruptions of global concern. Atmospheric near-surface, year-round sampling achieved at coastal Antarctic sites reveals no significant enhancement of the nitrate level over 1992 and 1993, suggesting that volcanic aerosols have had a limited effect on the transfer of nitric acid from the lower Antarctic stratosphere to the boundary layer in these regions.

Abstract: Detailed measurements of delta(18)O of atmospheric oxygen performed on air trapped in the Vostok ice cores (Antarctica) are used to extend the record of the Dole effect over two climatic cycles (back to 240 kyr B.P.). Except for glacial terminations I and ii and for an unexpected minimum occurring around 175 kyr, the Dole effect shows small variations (Delta Dole within +/- 0.5 parts per thousand). These small variations, however, show a well-marked 23 kyr precessional periodicity, thus confirming the results obtained by Bender et al. [1994a] for the first climatic cycle. To explain the minimum value reached around 175 kyr, we invoke the possibility of a peak in the oceanic productivity linked to climatic events induced at low latitudes under glacial conditions.

Abstract: A rectangular conduit with a closed end has water flowing in/out at the other end. The water level at the open end has an imposed sinusoidal movement. When this level is higher than the ceiling of the conduit, a certain mass of air is trapped under the ceiling. In a previous article (T.D. Nguyen, La Houille Blanche, No. 2, 1990), it was supposed that this air is flowing out freely through the ceiling, so the relative pressure at the water surface is zero, and the water hammer at the dead end of the conduit was calculated when the conduit was thoroughly filled. In this article, it is supposed that the trapped air is compressed isothermally or adiabatically. The set of equations is resolved (water continuity and movement equations, air state equation) by supposing a regime of flow at each section (section submerged or not), a certain value for the air pressure and by using the sweep method to determine the water flow characteristics. The air volume calculated by iteration must converge, and the calculated regimes at each section (submerged or free) must agree with the supposed regimes. The simulation is performed first with a horizontal conduit then with an inclined conduit. As expected, adiabatic compression gives higher pressure than isothermal compression. The simulation shows also that when there is an air cushion, compared with the case when air is flowing out freely, the shock of the water hammer at the closed end of the conduit is significantly reduced. This method is aimed at calculating the now with entrapped air in the inlet/outlet tunnel of a hydroelectric plant, or in sewer system pipe when a sudden discharge surge (due to turbin opening/closing or to urban storm) changes a previously free-surface flow in a mostly full-pipe flow, but with some air entrapped under the ceiling. Copyright (C) 1999 John Wiley & Sons, Ltd.

Abstract: The structural features accompanying the lithium extraction/insertion process in a fluorine-substituted spinel oxide with formula LiMn2(O3.74F0.26) were studied in situ using Bellcore-type plastic batteries directly placed in a synchrotron X-ray beam. The initial material contains two phases, a cubic spinel and a slightly orthorhombically distorted one (major phase), It becomes entirely cubic on extraction of 0.17 Li atoms per formula unit, The lithium extraction reaction around 4 V vs Li/Li+ can be divided into three regions as a function of lithium content x: (1) a single-phase range for 0.59 less than or equal to x less than or equal to 0.83, (2) a two-phase range for 0.23 less than or equal to x less than or equal to 0.59 with equilibrium potential 4.122 V, (3) a narrow single-phase range III for 0.18 less than or equal to x less than or equal to 0.23. These three domains are fully reversible. With the exception of the presence of the initial orthorhombic distortion, the main features of the so-called electrochemical 4 V plateau are very similar to those of stoichiometric LixMn2O4. (C) 1999 Academic Press.

Abstract: This paper presents the analysis of near-infrared observations of the icy surface of Triton, recorded on 1995 September 7, with the cooled grating spectrometer CGS4 at the United Kingdom Infrared Telescope (Mauna Kea, HI). This analysis was performed in two steps. The first step consisted of identifying the molecules composing Triton's surface by comparing the observations with laboratory transmission spectra (direct spectral analysis); this also gives information on the physical state of the components. Most of the bands in Triton's spectrum were assigned to specific vibration bands of the CH4,N-2,CO, and CO2 molecules previously discovered. A detailed comparison of the frequencies of the CH4 bands confidently indicated that this molecule exists in a diluted state in solid beta-N-2 Three new bands peaking at 5717, 5943, and 6480 cm(-1) (1.749, 1.683, and 1.543 μm, respectively) were also observed. Laboratory experiments have shown that C2H6 isolated in solid N-2 fits well the second band, but this would imply the appearance of unobserved bands and thus rules out this assignment. However, C2H6 may exist in another physical state, and more experiments are necessary. No plausible candidate was found for these three bands when comparing with the spectra of nine molecules (C2H2, C2H4, C3H8, NH3, SO2, HC3N, CH3OH, NO, NO2). In view of the results of D. P. Cruikshank et al. (1993, Science 261, 742; in preparation), the work presented here leads to two possible representations of the surface of Triton. First, a two-region surface composed of a N-2 : CH4 : CO terrain, N-2 : CH4 : CO consisting of a solid solution in which N-2 is the dominant molecule, and of a H2O + C-2 terrain, composed of a mixture of pure crystalline H2O and CO2 grains. The second representation is a three-region surface composed of a N-2 : CH4 : CO terrain and two geographically separated H2O and CO2 terrains. The second step of the analysis consisted of using a bidirectionnal reflectance model (S. Doute and B. Schmitt 1998, J. Geophys. Res, Planets 103, 31367). The modeling first confirms the direct spectral analysis in that CH4 is diluted in solid beta-N-2, giving a high degree of confidence to the conclusion that the N-2 : CH4 :CO terrain is in fact a solid solution. It also provides numerical information on this terrain, namely the size of the grains, the geographical abundance, and the CH4 and CO concentrations. The large grain size(around 10 cm) would mean that the texture of this terrain is a compact crystalline solid rather than granular, which is in agreement with calculations from J. Eluszkiewicz (1991, J. Geophys. Res. 96, 19,217). In addition, an accurate modeling of the N-2 band could suggest that the temperature is greater or equal to 35.6 K. Although undistinguishable in the spectra, a maximum of 10% surface area of pure CH4 ice can be present at the surface of Triton, thus explaining the high atmospheric CH4 abundance observed by Voyager 2. Finally, the modeling showed that none of the two- or three-region representations was able to fit simultaneously the K and H regions of the spectrum of Triton. The origin of this misfit is not yet elucidated, but an instrumental effect is suspected. Some questions about the physical state of the H2O and CO2 molecules are thus raised, but unfortunately observational constraints are missing. New near-infrared observations could partly provide these missing constraints, and would be important for detecting new molecules on Triton's surface. Such new data would be especially useful to identify the three bands at 5717, 5943, and 6480 cm(-1) (1.749, 1.683, and 1.543 μm). (C) 1999 Academic Press.

Abstract: A mechanistic simulation model was developed for predicting, under field conditions, the unsaturated water flow and nitrogen transfer in an area located in Mnasra. The model parameters can be calculated from easily measured physical and chemical soil properties or obtained from the literature. Comparison between observed and calculated groundwater nitrate concentrations revealed that the application rate of 120 kg N . ha(-1).yr(-1) should be reduced on agricultural soils, especially during winter, which coincides with heavy rainfall and the absence of vegetation cover. (C) 1999 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.).

Abstract: Transport models have often been tested in laboratory studies using soil columns, usually of the order of 1 dm(3) in size. Even if the columns are undisturbed, their small size does not allow water flow and solute transport to occur as they would in the field. We therefore used a 1.7 m(3) column and applied steady-state flow rates of the order of 1 mm h(-1), and then applied pluses of tracers Br-, Cl- and (H2O)-H-2 and of atrazine (2-chloro-4-ethylamino-6-isopropylamino-S-triazine) to the surface. Classical models for tracers with these boundary conditions are the Convection-Dispersion model (CD), the two-region (mobile-immobile water) model with first-order exchange of solutes (MIM), and the transfer function models, among which the most widely used is the Convective Lognormal Transfer Function model (CLT). Thanks to simple boundary and initial conditions, analytical solutions are available for all these models. The CD model (1 parameter) was not able to fit the tracer elution curves. but use of the MIM model was satisfactory. The CLT model and the CD model (2 parameters) also gave satisfactory fits. To choose the best model we used the parameters fitted to the elution curves to predict vertical concentration profiles in the lysimeter. These predictions are compared to the profile obtained after thorough sampling of the soil when tracers reached about half way down the lysimeter. The MIM model yielded a better prediction. However, accurate predictions would require taking into account the highly stratified characteristics of this soil. Atrazine simulation was done with the CD-based one-site kinetic sorption and first-order decay equation. Again analytical solutions are provided for our experimental conditions. Values of decay and absorption parameters are in agreement with previous studies. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: As polar ice descends from the free surface to depth in a large ice sheet, it undergoes deformations which give rise to the formation and subsequent evolution of a fabric and associated anisotropy. In this paper two orthotropic models of such strain-induced anisotropy are considered. Model A is based on analysis of the microscopic behaviour of an individual ice crystal with transversely isotropic response and assumed uniform stress in a polycrystal. The macroscopic response of the ice aggregate is then derived by applying the concept of an orientation distribution function, and the resulting viscous law relates the strain rate to the stress and three structure tensors. In model B it is assumed that the macroscopic response of ice is determined by the fabric induced entirely by macroscopic deformations, and all microprocesses taking place at the grain level are ignored. A constitutive relation is derived from a general frame-indifferent law for orthotropic materials, and expresses the stress in terms of the strain rate, strain and three structure tensors. The two models are applied to determine the viscous response of ice to continued uniaxial compression and simple shearing in order to compare the predictions of both theories.

Abstract: In a series of three articles Calder, Hall and colleagues have recently presented an extension of Calder's original stochastic model of rainfall interception. In their model, the dependence of rainfall interception on raindrop size is explicitly taken into account. All raindrops are assumed to have the same characteristic size, the so-called median-volume raindrop diameter D(0), which is taken to be a power law function of the rain rate R. However, the D(0)-R relationship used in their model results from an erroneous interpretation of the general theory of the raindrop size distribution recently proposed by Sempere Torres and colleagues. As a result, this relationship is not consistent with the postulated underlying raindrop size distribution. Moreover, it seriously underestimates D(0) for a given R as compared to the consistently derived D(0)-R relationships and to the D(0)-R relationship reported in a classical article on the subject of raindrop size distributions. This may seriously affect the model results presented by Calder, Hall and colleagues. (C) 1999 Elsevier Science B.V. All rights reserved.

Abstract: Temperature, density and accumulation data were obtained from shallow firn cores, drilled during an overland traverse through a previously unknown part of Dronning Maud Land, East Antarctica. The traverse area is characterised by high mountains that obstruct the ice flow, resulting in a sudden transition from the polar plateau to the coastal region. The spatial variations of potential temperature, near-surface firn density and accumulation suggest that katabatic winds are active in this region. Proxy wind data derived from firn-density profiles confirm that annual mean wind speed is strongly related to the magnitude of the surface slope. The high elevation of the ice sheet south of the mountains makes for a dry, cold climate, in which mass loss owing to sublimation is small and erosion of snow by the wind has a potentially large impact on the surface mass balance. A simple katabatic-wind model is used to explain the variations of accumulation along the traverse line in terms of divergence/convergence of the local transport of drifting snow The resulting wind- and snowdrift patterns are closely connected to the topography of the ice sheet: ridges are especially sensitive to erosion, while ice streams and other depressions act as collectors of drifting snow.

Abstract: An 18-month meteorological data set recorded at 5150 m above sea level (asl) on Zongo Glacier, in the tropical Andes of Bolivia, is used to derive the annual cycle of the local energy balance and to compare it to the local mass balance. The roughness parameters needed to calculate the turbulent fluxes over the surface are deduced from direct sublimation measurements performed regularly on the field site and serve as calibration parameters. For the hydrological year September 1996 to August 1997, net all-wave radiation (16.5 W m(-2)) is the main source of energy at the glacier surface and shows strong fluctuations in relation to the highly variable albedo. An important peculiarity of tropical glaciers is the negative latent heat flux (-17.7 W m(-2)) indicating strong sublimation, particularly during the dry season. The latent heat flux is reduced during the wet season because of a lower vertical gradient of humidity. The sensible heat flux (6.0 W m(-2)), continuously positive throughout the year, and the conductive heat flux in the snow/ice (2.8 W m(-2)) also bring energy to the surface. There is a good agreement between the monthly ablation calculated by the energy balance and the ablation evaluated from stake measurements. The seasonality of the proglacial stream runoff is controlled by the specific humidity, responsible for the sharing of the energy between sublimation and melting.

Abstract: Speciation of the elements emitted into the atmosphere plays an important role in their long range transport over the globe and their eventual pollution of remote environments. This paper describes recent results of our laboratory in: (1) organolead determinations in archives of snow and ice in Greenland and the Mont Blanc region in Western Europe. Speciation analysis together with the determination of total inorganic lead gives a clear indication on the extent of global pollution when compounds are transported over long distances in the atmosphere; and (2) determinations of volatile halocarbons in macroalgae whose transport in the stratosphere could interfere with the ozone destruction process. It is shown that the analytical methodology for the determination of a number of species is now well enough developed to allow ultra-sensitive and reliable measurements in samples collected in the remote environment and to derive interesting conclusions on long range transport processes.

Abstract: The uplift, transport and deposition of dust have been implemented in the LMDz AGCM. Simulations of atmospheric dust transport have been performed under present day (PD) and Last Glacial Maximum (LGM) conditions. For the PD climate the large-scale atmospheric dust transport as inferred from atmospheric measurements is reproduced. Increased dust amounts are simulated almost everywhere for the LGM climate, and the concentration of dust in Antarctic ice is increased as inferred from ice cores. The model simulates substantially increased dust concentrations in Greenland ice, however they are still lower than values reported from ice cores.

Abstract: A simple Rayleigh-type isotope model, typical of those used to develop algorithms for extracting climatic information from stable water isotope paleodata, is evaluated against the more complex and presumably more reliable calculations of a general circulation model (GCM) fitted with isotope tracer diagnostics. The evaluation centers on an analysis of how the temperature T-e of an oceanic moisture source affects the deuterium excess d of Greenland precipitation. The annual T-e-d relationship derived from the GCM diagnostics is largely reproduced by the simple isotope model when the latter is properly initialized. This, coupled with the fact that the GCM itself reproduces observed isotope behavior, suggests that the simpler model's atmospheric calculations are indeed adequate for isotope studies. Furthermore, the GCM results support the idea, originally developed with the simpler models, that polar deuterium excess values contain information on meteorological conditions at distant evaporative sources.

Abstract: For glaciological and meteorological reasons Mount Elbrus, Caucasus, was chosen as a site for physical and chemical pilot studies of ice cores. This study was the first step towards systematic studies of impurities in glacier ice on Mount Elbrus. In 1990 two ice cores, each 17 m deep and spaced 10 m apart, were taken at an elevation of 4100 m on the Bolshoy Azau glacier on the western slopes of Mount Elbrus. The cores were used for different physical and chemical analyses. Structure, texture and ice microstructure Mere studied. Chemical analyses of major ions (SO(4)(2-), NO(3-), K(+) and Na(+)) and measurements of insoluble light-absorbing material and radioactivity ((137)Cs and total beta activity) were made. With the results of the physical and chemical analyses of these two ice cores? the possibilities of utilising the ice for the study of trace substances deposited after long-range transport from Europe were explored. Ice-stratigraphic methods made it possible to establish the annual accumulation rate. A reference horizon was established From the depth variation of (137)Cs and total beta activities that showed a well-defined peak of their activities coinciding in time with the 1986 Chernobyl accident. The results of this study show that Mount Elbrus is a useful archive for atmospheric composition downwind of Europe.

Abstract: The influence of moisture content on the thermal effusivity of wood cement-based composite was investigated. Measurements of thermal effusivity were performed inside a closely controlled climatic cell at ambient temperature (20 +/- 0.5 degrees C) in conditions ranging from fully saturated to oven-dry. Shrotriya et al's model based on an Ohm's law approach was used for predicting the effective thermal effusivity of studied materials. The topological parameters of the model, such as sphericity of particles and resistivity formation factor, have been estimated as a first approximation from both thermal conductivity measurements and formulae proposed in the literature. Test results confirm that moisture content tends to increase the thermal effusivity significantly. It is also found that the Shrotriya et ars model yields predictions which agree quite closely with experimental data for wood aggregates-clay-cement composites for different amounts of wood aggregates and degrees of saturation.

Abstract: Hg has been measured using ultraclean procedures in snow deposited in central Greenland from 1949 to 1989. Concentrations range from less than or equal to 0.05 to 2.0 pg/g (mean : 0.43 pg/g), i.e. values which are orders of magnitude lower than those obtained in earlier studies. It indicates that earlier data were plagued by major contamination problems. Combined estimated contributions from natural Hg sources cannot explain Hg concentrations observed in the snow. It suggests that Hg deposition to the Greenland ice sheet is now significantly influenced by anthropogenic inputs from North America, Asia and Europe linked especially to coal burning and solid waste incineration. Although our data suggest that Hg concentrations were higher in snow dated from the late 1940s to the mid 1960s than in more recent snow, further studies are needed to clearly assess Hg temporal trends.

Abstract: Mechanical tests have been performed on strongly textured ice samples coming from a wide range of depths (from 1328 down to 2868 m) of the GReenland Ice core Project (GRIP) for different sample orientations with respect to the prescribed stress. In this way, two directional viscosities, corresponding to the “easy glide” and to the “hard glide” orientations, were determined along the core. The viscoplastic anisotropy gradually increases down to a depth of similar to 2600 m and slightly decreases below, revealing a clear correlation between rheology and texture. The experimental mechanical response compares well with that predicted by the ViscoPlastic Self-Consistent (VPSC) model. The VPSC model is also applied to ice samples that exhibit an axisymmetric texture to show in more detail the sensitivity of the rheology to specific texture parameters, This leads to a number of recommendations for future mechanical tests on anisotropic samples, A large-scale ice flow model is finally used to estimate the influence of ice anisotropy on the flow along the GRIP-GISP2 flow line, The particular mechanical behavior of deep GRIP ices in the stress regime corresponding to an ice divide leads to deformation rates that are highly sensitive to bedrock topography and texture pattern, This feature is likely to favour the formation of stratigraphic disturbances in deep ice layers, as observed in the last 300 m of both GRIP and GISP2 cores. (C) 1998 Elsevier Science B.V.

Abstract: The lack of appropriate data has long been a major obstacle to the study of the Sahelian rainfall at the event scale. In this paper, use is made of the EPSAT-Niger recording rain-gauge data to characterize the convective rain events of the central Sahel. Although some considerations lead to the identification of two main types of mesoscale convective systems, it is shown here that the most relevant stratification in terms of statistical analysis of the event rainfall distribution is between the events of the margins and those of the core of the rainy season. In fact, the average storm rain-depth appears to be non-stationary in time, with storm rain-depths slightly higher in the core of the rainy season than on the margins. Separation between the core and the margins thus allows the fitting of an exponential model to the observed storm rain-depth distributions of each period (core and margins), although a better fit would certainly be obtained if a proper modelling of the time non-stationarity was carried out. It is then shown that there is little, if any, correlation between the mean storm rain-depth of a given year and the overall abundance of the corresponding rainy season. This is a validation of previous works, which reached the same conclusion using daily rainfall data only. One major result of this work is that the statistics characterizing the rain events in the Sahel display little fluctuations, either in space or from year-to-year, as compared with those observed for the total seasonal rainfall. Each year and at each station the average storm rainfall remains close to 12 mm during the margins of the rainy season and close to 15 mm in the core. During the same period, the average seasonal rainfall over the study area ranged from 400 to 660 mm and for any given year the ratio between the maximum and the minimum point seasonal rainfall was of the order of 2. It is therefore concluded that the main source of rainfall variability in the Sahel is linked to the variability in the number of events rather than in the magnitude of these events. (C) 1998 Royal Meteorological Society.

Abstract: Sommerfeld et al. (1998) (hereafter: SKL) attempted to measure HNO3 diffusion in ice. Although they used HNO3 partial pressures much higher than atmospheric, they encountered contamination problems, and conclude that they could not obtain meaningful values of the diffusion coefficient of HNO3 in ice. In earlier measurements, we used a method having some similarities with theirs (Domine and Thibert, 1995). SKL suggest that our data may have been affected by contamination. Here we examine their methods, find many differences with ours, and demonstrate that our data was not affected by contamination. We point out questionable choices in their experimental procedure, and in the earlier work of Diehl et al. (1995) and discuss the difference between their solubility data and ours. We propose changes to the procedure of SKL, so that an independent set of reliable data can be obtained on the diffusion and solubility of HNO3 in ice.

Abstract: We present a practical, timely, and effective radiative transfer algorithm, suitable for qualitative and quantitative analyses of high-resolution hyperspectral images of planetary surfaces in the visible and near-infrared domains. The bidirectional reflectance of a plane parallel, absorbing, scattering, and slightly stratified medium is generated. The local mean properties of scattering and absorption of such media are obtained apart, using semiempirical approaches. The functions which express the diffuse reflection and transmission behaviors of each homogeneous layer are then derived. For the multiple scattering term, we numerically resolve the equations appearing in the H, X and Y function method of radiative transfer, reducing the real phase function to a simplified one which can nevertheless be anisotropic. A better approach to the physical realism is obtained for the single and double scattering contributions, using their real analytical expressions. This contrasts with the Hapke model dedicated to homogeneous and semi-infinite media, where only an isotropic reduced phase function is adopted and the single scattering correction is applied. The bidirectional reflectance and the derived;quantities (albedos) of an optically semi-infinite homogeneous medium are then easily derived from these quantities. For a stratified medium, a simple adding algorithm based on principles of invariance is presented. Compared to earlier and more complete theoretical developments, this model in most cases reproduces the dependence of the bidirectional reflectance according to the different geometrical and radiative parameters with a maximum of 10% relative error. It leads to important gains of computation time and significantly extends the validity of Hapke's or similar practical approaches.

Abstract: Ambient levels and night and day profiles of aromatic hydrocarbons (benzene, toluene, (m+p)xylene, o-xylene and ethyl benzene) and aldehydes (formaldehyde, acetaldehyde, n-butanal, n-pentanal, n-hexanal and benzaldehyde) were measured in Grenoble, France, during a selected week from May, 4, 1995 to May, 11, 1995. Most abundant aromatic hydrocarbons observed were toluene (up to 3.5 ppbv), benzene and (m+p)-xylene (0-3 ppbv). Formaldehyde was the most abundant aldehyde with concentrations ranging from 2 to 18 ppbv followed by acetaldehyde with levels from 2 to 10 ppbv. Gas phase concentrations of four other aldehydes were in the range 0-2 ppbv. Results are discussed with respect to direct emissions and photochemical production. Formaldehyde appears to come mainly from exhaust car emissions but acetaldehyde from other primary sources. (C) 1998 Elsevier Science Ltd. All rights reserved.

Abstract: Modelling of water and solute movement in soils generally requires a great number of parameters which are imprecise. In this paper, we focus on various types of imprecisions introduced either by subjective or state-of-the-art estimates of coefficients, or through measurement error, and on their evaluation by fuzzy set and fuzzy logic theory. After summarising the theoretical framework, the paper provides an example of an application dealing with estimates of imprecisions associated with the advective flux of atrazine in the unsaturated calcareous soil of the Grand Morin watershed, France. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract: Understanding soil swelling is hampered by the difficulty of simultaneously measuring water content and bulk density. A number of studies have used dual-energy gamma rays to investigate soil swelling. The long counting time of this technique makes it impracticable for studying the rapid changes in moisture content and soil swelling shortly after infiltration is initiated. In this paper, we use the dual-energy synchrotron X ray to measure, for the first time, the water content and bulk density changes during the fast, initial phase of the swelling process. Ponded infiltration experiments were performed with two soils: a bentonite-sand mixture and a vertisol. Swelling curves and hydraulic diffusivity were determined. Deformation was very rapid immediately after water application and then became progressively slower. The hydraulic diffusivity decreased with time, which can partially explain the very rapid decrease in infiltration rates observed in the field.

Abstract: The intra-annual variability of Antarctic precipitation from the European Centre for Medium-range Weather Forecasts short-term meteorological forecasts and from climate simulations by the ARPEGE and LMD-Zoom general circulation models is presented and discussed. The spatial resolution of forecasts and simulations is high over the Antarctic region, about 100 km, so that the impact of topography and small-scale atmospheric dynamics are better resolved than with mc,re conventional model grids (about 300 km). All the models and forecasts show that the seasonality of precipitation is spatially very variable. Meridional coast-to-interior contrasts are marked, but equally strong variations are unexpectedly found where more homogeneity might be expected because of the homogeneity of the environment, e.g. on the high Antarctic plateau. Neither the forecasts nor the simulations confirm that precipitation is mostly maximum in winter over much of East Antarctica as suggested by scarce and potentially unreliable observations (Bromwich, 1988). Spring and fall maxima are quite frequent too, though summer maxima are rare. Daily precipitation statistics show more spatial pattern, with increasingly infrequent precipitation as distance from the coast toward the interior of the ice sheet increases. Several aspects of the intra-annual variability of precipitation can be interpreted in terms of atmospheric dynamics, but at both daily and seasonal time-scales the different forecasts and climate simulations often locally and regionally disagree with each other. Discrimination between models and their ability to reproduce the dynamics of Antarctic hydrology and progress on simulating such aspects of the Antarctic climate, is limited by the lack of reliable observation of precipitation variability.

Abstract: Observations with tensiometers and piezometers near the Maurets River in the Real Collobrier basin supply interesting information about hydrological mechanisms during floods. Series of weekly time-step measurements show a good correlation between river flow and piezometric levels near the river. These levels can rise only when the soil capillary pressure before the storm event is approximately zero. More detailed observations of the suction values were recorded with a half-hour time step. They show that only high intensity rainfall can provoke a quick increase of positive pressures in the soil profiles (saturation), as long as these pressures are very close to zero before the rain. Associated with these positive pressures a quasi-null vertical hydraulic head gradient and a strong flow in the river are observed. In such a case, the soil saturation can be of a very short duration. Therefore, in Mediterranean conditions, saturated contributing areas can appear and develop causing big floods, but they seem to be very limited in time.

Abstract: For quantitative precipitation forecasting, model output statistics (MOS) are very useful. We have tried to improve a MOS based on a selection of analogue and past situations. Our study concerns the analogue criterion: its form, what variables to introduce (predictors), among those available (700 and 1000 hPa geopotential fields) and their format (principal components or PCs, gridded data, etc.). Initially, an Euclidean distance using predictors as PCs was used. Its replacement by the Teweles-Wobus score, with predictors known in grid format, gave significantly improved results. ((C) Academie des sciences / Elsevier, Paris.)

Abstract: Pb, Cd, Cu, and Zn have been measured using ultraclean procedures in eight sections taken from two well-dated ice cores from Law Dome, an independent small size ice cap with high accumulation rate situated in the coastal area of East Antarctica. Seven sections were dated from the 1830s to 1940s and one was dated from three millennia ago. The data show that there are strong seasonal variations in the concentrations of Pb and Cd, with values similar to two- to fourfold higher in winter than in spring-summer. Evaluation of the contributions from the different sources suggests that contribution from sea salt spray is relatively important, especially for Cd. Contribution from marine biogenic emissions could also be very significant. The importance of marine contributions is consistent with strong intrusions of marine air masses at this coastal site, especially during wintertime. (C) 1998 Academic Press.

Abstract: Daily atmospheric concentrations of particulate oxalate measured at the Summit of the Greenland Ice Sheet are presented for the summers 1992 – 1995. We believe that four episodes of elevated concentrations are due to biomass burning plumes passing over the site. In at least two cases the source regions of the fires are located in northern Canada. Further characteristics of the aerosol are examined during one of these events. A large increase of particle number concentrations in the accumulation mode can be observed, while the increase is much more limited for total particle number, The suite of chemical species enriched in the aerosol includes typical biomass burning tracers like fine K, large concentrations of ammonium, particulate formate and acetate, as well as other organic species like glycolate. The size distributions of K, oxalate, and glycolate are skewed toward the accumulation mode and exhibit the very same shape as sulfate, suggesting internal mixing of these species in the same particles. Molar ratios S/K indicate incorporation of S during transport, most probably by production of sulfate, Concentrations of these species were measured in fog samples for radiative events that occurred during the plume passage. There is a good agreement in the relative variation of concentrations between the aerosol and fog for oxalate and glycolate, while the gas phase probably dominates incorporation in the fog droplets for acetate, formate, chloride, nitrate, and sulfate (incorporated as SO2, which is further oxidized), The complexity of the transfer of the organic acids from the atmosphere to fog is underlined.

Abstract: Relationships between size-resolved aerosol chemistry and various atmospheric processes were studied at Summit over the Greenland ice sheet. The Summit summer aerosol displayed an Aitken mode below 0.1 μm aerodynamic particle diameter, one or two modes in the accumulation size range (0.1-1 μm), and a supermicron mode. The Aitken mode contained little particulate mass (< 5%) but accounted for most of the particle number concentration. The accumulation size range frequently had two overlapping modes with a minimum around 0.4 μm. A potential reason for this bimodality is fog processing which occurs frequently over the ice sheet during summer nights. Most of the particulate sulfate, ammonium, methane sulfonic acid (MSA), and dicarboxylic were found in the accumulation size range, where they displayed quite a similar modal structure, suggestive of an internal mixture of these compounds in submicron particles. The ratio of MSA to sulfate varied with particle size over the accumulation size range, so that different deposition pathways are likely to cause different MSA to sulfate ratios in the surface snow. Less than 20% of particulate sulfate and >95% of particulate nitrate were usually found in supermicrometer particles. It is likely that the supermicron sulfate and nitrate are produced when SO2 and HNO3 react with particles of mostly crustal origin. Supermicron sulfate centered at a somewhat smaller size (< 2 μm) than nitrate (2-3 μm) The particulate phase contained only minor amounts of semivolatile acidic compounds, except when the ice sheet was impacted by apparent biomass burning plumes. These plumes are characterized by elevated levels of many low-volatility compounds, and they may contain submicron ammonium nitrate and ammonium formate.

Abstract: Multiple-year time series of the weight ratio R of methane sulfonic acid (MSA) to non-sea-salt sulfate (nss SO4) in aerosols collected at three coastal Antarctic stations Neumayer (70 degrees 39'S, 8 degrees 15'W), Halley (75 degrees 35'S, 26 degrees 19'W), and Dumont D'Urville (66 degrees 40'S, 140 degrees 1'E) are presented here. Discarding the November 1991 to October 1992 time period, during which the Antarctic atmosphere is thought to have been contaminated by volcanic plumes (Pinatubo and Cerro Hudson), the multiple-year data sets indicate mean annual R values of 17.9 +/- 10.7% at Neumayer (1983-1995) and 11.7 +/- 6.7% at Dumont D'Urville (1991-1996). On the basis of a more limited time series (February 1991/February 1992), which partly overlaps the volcanic time period, the Halley data had R values of 25.6 +/- 11.9% from October to April. The R seasonal variations are broadly consistent at the three sites, with maxima in mid to late summer (from December to April) ranging from 18.2 +/- 4.6% at Dumont d'Urville to 29.2 +/- 6.5% at Neumayer and 32.2 +/- 6.9% at Halley. Minima are lower than 10% during winter (from May to September) at Neumayer (8.5 +/- 2.5%) and Dumont d'Urville (6.0 +/- 3.0%). The interpretation of these R values in terms of relative abundance of MSA and nss SO4 produced by the oxidation of dimethylsulfide (DMS) at high southern latitudes is not straighforward. In winter, nss sulfate present at coastal Antarctic sites is likely originating in long-range transported by-products from marine DMS emissions taking place at 50 degrees S and non-DMS sulfate from the continental free troposphere. Midwinter R values show a mean ratio of up to 20% for the pure marine input. From September to November, both MSA and nss sulfate concentrations increase in relation to enhanced marine DMS emissions from 50 degrees to 60 degrees S. R values are only weakly enhanced. In summer, the further increase of MSA and nss sulfate concentrations is related to increased biogenic activity south of 60 degrees S and is characterized by R values in the range of 25 to 35%. The mean ratio for DMS marine emissions south of 60 degrees S is close to 62% at Neumayer and 32 to 38% at Dumont d'Urville. These baseline data obtained at coastal Antarctic sites allow us to investigate the role of various parameters that modulate seasonal variations of R at high southern latitudes including the oxidative capacity of the atmosphere, the temperature of the atmospheric oxidation of DMS, and the aging of marine air masses during their transport from source regions toward coastal Antarctic regions.

Abstract: Ground-based measurements were carried out in the French Alps in March 1995 to study the bidirectional and polarized bidirectional reflectances of various snow covers. Atmospheric (optical thickness) and snow characteristics (grain size and type) were measured simultaneously. The ground reflectance data are compared with snow reflectance modeling results. A radiative transfer model based on the adding-doubling method is used to compute the snow reflectance. The single-scattering parameters are determined by a Mie or a ray-tracing code depending on the particle shape introduced in the model (spherical or hexagonal ice particles). Special attention is paid to the comparisons in the near-infrared spectral range (1.65 μm) because of the sensitivity of snow reflectance to the grain size and shape. In the visible and near-infrared domains of the solar spectrum, the amplitude of the polarized signal is very low and independent of the snow grain size. The agreement between measurements and model results is good if hexagonal particles are introduced in the model instead of the spherical snow grains commonly used in reflectance studies.

Abstract: A general approximation for the solution of the one-dimensional nonlinear diffusion equation is presented. It applies to arbitrary soil properties and boundary conditions. The approximation becomes more accurate when the soil-water diffusivity approaches a delta function, yet the result is still very accurate for constant diffusivity suggesting that the present formulation is a reliable one. Three examples are given where the method is applied, for a constant water content at the surface, when a saturated zone exists and for a time-dependent surface flux.

Abstract: Vintages of French wine from 1950 to 1991 were analyzed for lead isotopes and concentration to investigate whether they might be used to archive the isotopic composition of the anthropogenic lead in aerosols to which the vineyard was exposed. Early vintages (1950-1980) contained 78-227 ng/g of lead with Pb-206/Pb-207 ratios between 1.152 and 1.173, while the later vintages displayed significantly lower concentrations and a smaller range of isotopic ratios. The concentration of trimethyl lead, which is associated with automobile emissions, was found to be poorly correlated with total lead in the wines, suggesting that automobile aerosols were not a significant source of the lead. This result was supported by lead isotope data which showed a poor correlation with the available petrol and aerosol data. To identify its origin lead isotopes were measured in vineyard aerosols, soil particles, bottle caps, corks, and brass components used to dispense the wine. Although a dominant source could not be identified there was some evidence to suggest that brass which had a high lead concentration was a significant contributor. Because the lead contribution from the processing of wine was probably relatively high in the past it is unlikely that old vintages of wine will be a suitable archive for lead isotopes in aerosols. (C) 1998 Academic Press.

Abstract: An interpretation of the deuterium profile measured along the Vostok (East Antarctica) ice core down to 2755 m has been attempted on the basis of the borehole temperature analysis. An inverse problem is solved to infer a local “geophysical metronome,” the orbital signal in the surface temperature oscillations expressed as a sum of harmonics of Milankovich periods. By correlating the smoothed isotopic temperature record to the metronome, a chronostratigraphy of the Vostok ice core is derived with an accuracy of +/-3.0-4.5 kyr. The developed timescale predicts an age of 241 kyr at a depth of 2760 m, The ratio delta D/delta T-i between deuterium content and cloud temperature fluctuations (at the top of the inversion layer) is examined by fitting simulated and measured borehole temperature profiles. The conventional estimate of the deuterium-temperature slope corresponding to the present-day spatial ratio (9 per mil/degrees C) is confirmed in general. However, the mismatch between modeled and measured borehole temperatures decreases noticeably if we allow surface temperature, responsible for the thermal state of the ice sheet, to undergo more intensive precession oscillations than those of the inversion temperature traced by isotope record. With this assumption, we obtain the long-term temporal deuterium-temperature slope to be 5.8-6.5 per mil/degrees C which implies that the glacial-interglacial temperature increase over central Antarctica was about 15 degrees C in the surface temperature and 10 degrees C in the inversion temperature. Past variations of the accumulation rate and the corresponding changes in the ice-sheet surface elevation are simultaneously simulated.

Abstract: We have analyzed the soluble portion of impurities trapped in solid precipitation that accumulated at Summit (central Greenland) from 1193 A.D. to the present. Seventy-three ice layers show elevated concentrations of ammonium and formate, caused by high-latitude biomass burning debris reaching Greenland. While a mixture of ammonium and formate close to the molar ratio is generally observed in these ice layers, a large depletion of formate relative to ammonium is found in a few cases. The chemical composition of such layers indicates the presence of a mixture of ammonium, formate, and nitrate with a NH(4)(+)/(HCOO(.)+NO(3)(.)) molar ratio close to 1. These differences may be related to the fire type (flaming versus smoldering) or to meteorological conditions encountered by plumes during their transport toward Greenland. The high-resolution ammonium and formate profiles are used to reconstruct the frequency and the intensity of high-latitude biomass burning input having reached central Greenland since 1193 A.D. Three periods of enhanced biomass burning input over central Greenland are identified: 1200-1350 A.D., 1830-1930 A.D., and to a lesser extent 1500-1600 A.D. The 1200-1350 A.D. time period coincides with warm and dry conditions which characterized the Medieval Warm Period. After a period of infrequent biomass burning input during the coldest period of the Little Ice Age (1600-1850 A.D.), the frequency was enhanced at the turn of the last century and then decreased throughout this last century. Aside from high-latitude biomass burning, the background levels of formate show a slight and persistent decreasing trend over the last 800 years probably reflecting the deterioration of the boreal vegetation from North America.

Abstract: An inverse procedure was used to estimate the soil hydraulic characteristics of a two-layered soil system-soil surface crust and subsoil-from data obtained during a tension-disc infiltration experiment. The inverse procedure combined the Levenberg-Marquardt nonlinear parameter optimization method with a numerical solution of the axisymmetric variably-saturated flow equation. The objective function was defined in terms of the cumulative infiltration curve and the final water content measured directly below the tension-disc infiltrometer at the end of the experiment; this final water content was assumed to correspond to the final supply pressure head. We analyzed two infiltration experiments carried out with a 25-cm diameter tension-disc infiltrometer. One experiment was carried out on a two-layered system, and a second after removal of the surface crust covering the sandy subsoil. Both experiments were performed with six consecutive supply tensions. We first analyzed the infiltration experiment for the subsoil only, thus yielding its hydraulic characteristics. Subsequent analysis of the infiltration experiment for the two-layered system with known hydraulic properties of the subsoil provided estimates of the hydraulic properties of the surface crust. We further compared the estimated hydraulic parameters of the subsoil with those obtained using Wooding's analytical method [Wooding, R.A., 1968. Steady infiltration from a shallow circular pond. Water Resour. Res. 4, 1259-1273] and predictions based on a neural network model requiring textural input information. All three methods generated roughly the same results. The numerical inversion technique proved to be a convenient tool for estimating the soil hydraulic properties of both the surface crust and the subsoil. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract: During the 1992 rainy season, in the sahelian region (Niger), some important convective systems were sampled at a scale smaller than the event, to measure the variability of stable isotopes of water (oxygen-18 and deuterium). The evolution of the the isotopic content of rainfall shows a good agreement with the processes described by the physical meteorological model of the sahelian convective system. With the knowledge of the main mechanisms governing the min system, isotopes point out the contribution of the the different sources of vapour during the storm. (C) Academie des sciences/Elsevier, Paris.

Abstract: A detailed dielectric profiling (DEP) conductivity profile (delta(infinity)) measured in the 6 m of the basal silty ice sequence from the Greenland Ice Core Project (GRIP) ice core (Summit, Central Greenland) is presented and compared to previous multiparametric studies. DEP conductivities span the whole glacial-interglacial range observed higher up in the GRIP core (9-25 μS m(-1)). Values in the bottom meter of the sequence reach the level of some of the highest peaks from Holocene volcanic layers in the core (33 μS m(-1)). On a steady increase of the sigma(infinity) values down the sequence are superimposed large fluctuations “inphase” with other variables measured in the core such as delta(18)O, debris content, or gas compositions in CO2 and CH4. Analysis of the type and strength of intercorrelations shows that the controlling variable for the DEP signal must be closely related to the gas content and composition of the ice. Plausible candidates for this causality link are investigated. Enhancing of the sigma conductivity by CO2 and CH4 encaged in the ice lattice as gas hydrates is ruled out since these are nonpolar clathrates of structure I, known as having negligible impact on the orientational stability of the water molecules under ac currents. NH4+ is proposed as the best candidate since it has been shown to enhance DEP conductivities by introducing Bjerrum defects in the ice lattice and since it could have been initially present partly as gaseous NH3 in the ice. This proposition is supported by the NH4+ profile in the basal ice sequence. Using calibration curves from higher up in the core, it is shown that sigma is in fact fully explained by intracrystalline conductivity of pure ice solely disrupted by ammonium impurities in the ice lattice, The origin of the NH4+ signal is discussed in the light of organic acid profiles (formate, acetate, and oxalate). It appears that the most likely source is local degradation of biological residues, which supports the hypothesis that part of the basal ice was formed locally, in the absence of the present-day ice sheet.

Abstract: We present here the first reliable data on the occurrence of Pb, Zn, Cu, Cd, Bi, Mn and Al in high altitude cold Alpine ice. They were obtained by analyzing with three different analytical techniques various sections of a 140 meter long ice core drilled near the summit of Mont Blanc at the French-Italian border. Special emphasis was given to the study of seasonal variations in ice dated from 1960/61 and 1967/68, i.e, ice deposited just before and after the opening of the Mont Blanc road tunnel. For all the metals, concentrations vary by two orders of magnitude from summer to winter, with the highest concentrations being observed in summer. For ph, Zn, Cu, Cd and Bi, the lowest concentrations range from 0.1 pg/g (for Bi) to 50 pg/g (for Pb), i.e, values which are comparable to concentrations observed in central Greenland. Anthropogenic inputs are dominant for Pb, Zn, Cd, and Bi while inputs from natural sources are important for Cu, Mn and Al. Both source and meteorological parameters are responsible for the heavy metal content of high altitude alpine ice. These metals mainly originate from Western European sources as indicated by back air trajectories and emissions data for the different countries. The observed seasonal variations are however largely linked with the changing vertical structure of the regional troposphere, which prevented the transfer of pollutants to higher altitudes in winter but not in summer. Our data do not indicate a significant change linked with the opening of the Mont Blanc tunnel. (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract: Records of atmospheric nitrate were obtained by year-round aerosol sampling at Neumayer and Dumont D'Urville stations, located in the Atlantic and Pacific sector of coastal Antarctica, respectively. Where possible, evaluation of the nitrate records is mainly based on concurrently measured radioisotopes (Be-10, Be-7, Pb-210) as well as delta(15)N in nitrate nitrogen. Observations made at these land two other coastal Antarctic sites [Savoie et al., 1993]) reveal a uniform nitrate background near 10 ng m(-3) persisting throughout coastal Antarctica between approximately April and June. The dominant seasonal nitrate maximum, which occurred between spring and midsummer and ranged from 20 to 70 ng m(-3), tended to increase with latitude. An estimate based on Neumayer mineral dust concentrations suggests that an average of less than 5% of the observed atmospheric nitrate load may be associated with continental tropospheric sources, while a separate estimate based on Pb-210 records implies a much higher proportion of up to 60%. Stratospheric nitrate influx rates seen at coastal sites, deduced from Neumayer Be-10/Be-7 records for stratospheric air mass intrusions and from tritium for the sedimentation of polar stratospheric clouds (PSC), exceed the theoretical stratospheric odd nitrogen production rate from N2O oxidation by almost a factor of 5 and are found to be in close agreement with the observed surface nitrate flux, implying again that the continental source contribution is relatively unimportant. Consideration of nitrate reemission from near-surface snow layers reveals a minor effect of this flux on the global Antarctic troposphere but possibly a substantial influence on the nitrate load of a persistent surface inversion layer. Evaluation of the mean seasonal nitrate pattern, based on concurrent Be-10, Pb-210 and delta(15)N records at Neumayer and on tritium in precipitation at Halley, suggests that the period of significant enhancement above the background mainly reflects inputs of stratospheric nitrate with secondary peaks in winter and late summer most likely dominated by PSC sedimentation and stratospheric air mass intrusions, respectively.

Abstract: Fracturing of freshwater granular ice up to failure under uniaxial compression creep was investigated from series of interrupted creep tests and from a multifractal analysis of the corresponding fracture patterns. At the early stages of damage corresponding to primary and secondary creep, the fracturing process is dominated by the nucleation of microcracks from stress concentrations within the material (unlike rocks, artificial freshwater ice does not contains starter flaws). Because of the clack nucleation mechanisms, the microstructure of the material (e.g., the nonfractal grain size distribution) strongly influences the organization of fracturing which is therefore nonfractal. As fracturing proceeds during tertiary creep, a hierarchical (fractal) organization of the fracturing emerges progressively over a wider scale range. At failure, this fractal organization is fully developed without detectable lower or upper bound, and the role of the initial microstructure has completely disappeared. Similarly, cracks are preferentially oriented along the compression axis at the early stages of damage, but this anisotropy vanishes as failure is approached. The simultaneity between the onset of tertiary creep and the emergence of fractal organization suggests that the acceleration of the deformation during tertiary creep is due to the cataclasis of a material which becomes granular. An important consequence of the fractal organization of fracturing is that homogenization procedures, as well as damage mechanics, developed to study the behavior of damaged materials, cannot be used to describe tertiary creep and failure.

Abstract: Aerosol samples have been collected at three stations on the coast of Antarctica. At Dumont d'Urville Station, samples have been collected for 4 years, at Halley Station for 2 years, and at Neumayer Station for 12 years. Fresh snow samples have also been collected at the three sites. At Halley, daily surface snow samples were taken, as well as firn cores covering the sampling period. Meteorological (including upper air) data are available at the three sites. In the subsequent papers of this special section, the data from the three stations have been combined in order to make general statements about the chemistry of aerosol in coastal Antarctica, about the differences between sectors of Antarctica, and about the relationship between air concentrations and the chemistry that is eventually seen in ice cores. This paper summarizes the samples collected and the sampling methods used at the three sites. It also highlights the main role of each paper in this special section.

Abstract: In order to calculate the electrical permittivity property of a multiphase mixture, a beta function distribution of the geometrical shapes of inclusions is considered. By using the effective medium theory together with the assumption of self-consistency, it is shown that the beta function distribution leads to Lichtenecker's formulae. This gives a theoretical model for this mixing formula, which had hitherto been considered empirical. This result is important because the first Lichtenecker's is already used for time domain reflectometry (TDR) calibration purposes, namely conversion from permittivity measurement to water content of heterogeneous materials.

Abstract: This paper addresses rainfall radar measurement in mountainous areas where numerous catchments subject to flash flooding are located. The main sources of error affecting the quantitative use of weather radar data are examined with special attention paid to the influence of topography and altitude. A radar in a mountainous region is affected by specific measurement errors such as beam blockage; other sources of error like bright band interception can be enhanced by the effect of altitude, This work is based on data collected during the 86-88 Cevennes Experiment conducted in a mountainous Mediterranean region in the south of France prone to Bash floods, Analysis of the main sources of error leads to the following observations. Beam blockage can be corrected using digital terrain models with the correction efficiency depending on the accuracy of the radar antenna pointing direction. The influence of vertical variations in radar reflectivity is shown to be taken into account at an hourly time step in order to provide satisfactory range-dependent corrections. Additionally, the control of the stability of the radar signal using strong ground clutter is described. Because of attenuation effects, the use of a 10-cm wavelength is recommended for surveying intense rain events which regularly affect Mediterranean regions, The error due to an unadapted Z-R relationship is shown to depend on the statistical distribution of reflectivity over the catchment where the rainfall measurement is utilized. (C) 1997 Elsevier Science B.V.

Abstract: CO2 ice-core records show an increase in the atmospheric concentration of 80-100 parts per million by volume (ppmv) from the last glacial maximum (LGM) to the early Holocene. We present CO2 measurements performed on an ice core from central Greenland, drilled during the Greenland Ice Core Project (GRIP). This CO2 profile from GRIP confirms the most prominent CO2 increase from the LGM, with a mean concentration of 200 ppmv, to the early Holocene with concentrations between 290 and 310 ppmv. Some structures of the new CO2 record are similar to those previously obtained from the Dye 3 ice core (Greenland), which indicated a dilemma between Greenland and Antarctic CO2 records [Oeschger et al., 1988]. Both Greenland cores show high CO2 values for rather mild climatic periods during the last glaciation, whereas CO2 records from Antarctica do not show such high CO2 variations during the glaciation and, furthermore, the CO2 values in the early Holocene are about 20-30 ppmv higher in the GRIP record than in Antarctic records. There is some evidence that the difference could be due to chemical reactions between impurities in the ice leading to an increase of the CO2 concentration under certain conditions. If in situ processes can change the CO2 concentration in the air bubbles, the question arises about how reliably do CO2 records from ice cores reflect the atmospheric composition at the time of ice formation. The discrepancies between the CO2 profiles from Greenland and Antarctica can be explained by in situ production of excess CO2 due to interactions between carbonate and acidic species. Since the carbonate concentration in Antarctic ice is much lower than in Greenland ice, CO2 records from Antarctica are much less affected by such in situ-produced CO2.

Abstract: The potential of a double focusing ICP-MS instrument in terms of high sensitivity, sample throughput and low volume of sample consumed was investigated for the direct, simultaneous determination of Co, Cu, Zn, Mo, Pd, Ag, Cd, Sb, Pt, Ph, Ri and U at the low and sub-pg g(-1) level in polar snow, The entire analytical procedure, including cleaning of material, field sampling, sample handling, determination of the blanks and instrumental analysis, is described. The mean concentrations detected in snow samples collected in Central Greenland (2.7 m deep pit) are (in pg g(-1)): Co 5.8, Cu 4.6, Zn 47, Mo 1.6, Pd 1.1, Ag 0.60, Sb 0.86, Pt 0.61, Bi 2.5 and U 1.8, The Cd, Pb and U concentrations in a snow core section collected in East Antarctica are: Cd 0.39, Pb 5.0, U 0.04 pg g(-1). Repeatability of measurements ranges between 8 and 25% depending on the element considered. For some of the elements investigated these results constitute the first available for polar snow, The results of direct analysis by double focusing ICP-MS on Cd and Pb in the Antarctic snow samples and on Zn and Cu in Greenland samples are consistent with those obtained by differential pulse anodic stripping voltammetry (DPASV) and graphite furnace atomic absorption spectrometry (GFAAS), respectively.

Abstract: The transition from the Last Glacial to the Holocene is a key period for understanding the mechanisms of global climate change. Ice cores from the large polar ice sheets provide a wealth of information with good time resolution for this period. However, interactions between the two hemispheres can only be investigated if ice core records from Greenland and Antarctica can be synchronised accurately and reliably. The atmospheric methane concentration shows large and very fast changes during this period. These variations are well suited for a synchronisation of the age scales of ice cores from Greenland and Antarctica. Here we confirm the proposed lead of the Antarctic Cold Reversal on the Younger Dryas cold event. The Antarctic cooling precedes the Younger Dryas by at least 1.8 kyr. This suggests that northern and southern hemispheres were in anti-phase during the Younger Dryas cold event. A further result of the synchronisation is that the long-term glacial-interglacial increase of atmospheric CO2 was not interrupted during the Younger Dryas event and that atmospheric CO2 changes are not necessarily dominated by changes in the North Atlantic circulation.

Abstract: In predicting the overall thermal conductivity of composite porous structures such as building materials or soils, the thermal conductivities of their solid components must be known in order to apply some theoretical models. Horai proposed a method to determine the thermal conductivity of solid particles. The material under study is first ground to a fine powder. This powder is then carefully saturated with a fluid and the thermal conductivity of this mixture is measured using the 'needle probe' method. Finally, the conductivity of the solid phase is evaluated using a mixing-law model, namely the mean of the so-called 'Hashin and Shtrikman bounds'. We have introduced a slightly different version of this method that uses the 'transient plane source' (TPS) technique recently developed by Gustafsson for simultaneously measuring both the thermal conductivity and the thermal diffusivity of solids or fluids. An adapted experimental device has been designed and our approach has been validated through measuring, at room temperature, the thermal conductivities of the well-known minerals quartz, calcite and kaolinite. Afterwards, while considering that our method had become fully operational, we studied the thermal conductivity of the solid aggregates of a light-weight, energy-saving concrete whose solid phase is a mixture of natural clay, hardened cement paste and wood aggregates in various proportions.

Abstract: As part of the framework of the EFEDA (Echieval field experiment in a desertification threatened areal experiment, an 11-day data set including atmospheric Forcing, turbulent fluxes and soil water monitoring was gathered on a bare soil site, characterized by a large diurnal cycle of temperature, and with very dry conditions near the surface, A version of the SiSPAT (Simple Soil Plant Atmospheric Transfer) model, restricted to bare soil, was applied to this data set. The first objective was to validate model results against observations. This was restricted somewhat by the narrow range of soil moisture variations during the 11 days of the simulation. In addition some discrepancies were observed between total evaporation as calculated from neutron probe measurements and micrometeorological ones. However, because the SiSPAT results were more in agreement with evaporation calculated from soil water balance, it was assumed that the model could be used as a tool to investigate the mechanisms of the water movement and evaporation within the soil under such dry conditions. In the first 25 cm of soil, vapour flux is the most important transfer term, and the evaporation front (change of phase from liquid to vapour) can be localized at this depth. The contribution of temperature gradients to mass Bur is also found to be important, and tends to lower the total evaporation at the surface by creating a downward vapour flux component which partly cancels the upward one induced by large matric potential gradients at the soil surface. (C) 1997 Elsevier Science B.V.

Abstract: In the framework of the HAPEX-Sahel experiment, a data set was gathered on a fallow savannah site of the Central East Supersite. This includes 54 days of atmospheric forcing (air temperature and humidity, wind speed, solar and long-wave radiation and rainfall), net radiation, sensible, latent and soil heat fluxes and soil temperature series at a time step of 20 min. Furthermore, 17 soil moisture profiles, the evolution of the leaf area indices and some soil characteristics were available. The data set was used, at the field scale, to calibrate and validate the SiSPAT (simple soil plant atmosphere transfer) model, a 1D model of coupled heat and mass transfer in the soil-plant-atmosphere continuum. The objectives of the study were (i) to assess the performances of the model in the prediction of the diurnal cycle of net radiation, turbulent fluxes, soil temperatures and the evolution of soil water content over a period of 54 days (day of the year 239-292, 1992), characterized by early stage intense rainfall events and fast drying afterwards, (ii) to analyse the influence of soil surface crust on the water balance and (iii) to identify the 1D modelling limits when the surface area consists of two strates: a ground sparse herb laver, characterized by a large spatial variability of surface properties and water content with scattered bushes. The model was calibrated over a 2-week period and then nln over the whole 53-day period. We were able to reproduce the main characteristics of the observed net radiation, turbulent fluxes, soil temperature and soil moisture for the intense rainfall events and for an elongated dry period. Nevertheless, when the crust was not taken into account, the rainfall-runoff-infiltration process and the evapotranspiration after rain were poorly predicted (overestimation of evapotranspiration and infiltration). When a crust was considered to model the water balance at the field scale, its influence was found to be substantial on the runoff generation and the infiltration, and consequently on the bare soil evaporation. However, runoff predictions were much larger than the observations, indeed, at the field scale, no runoff was generally observed. Lateral redistribution of water between crusted and non-crusted zones was observed in the plot. However, this cannot be taken into account with the presented 1D deterministic modelling. Hence further model development is needed to yield a better representation of soil water fluxes at the field scale.

Abstract: N-2 and O-2 hydrates (clathrates) from a polar ice core (Vostok) were analysed by high-sensitivity Raman microspectrometry. Three new lines in the region of the stretching modes of N-2 and O-2 guest molecules were observed and were interpreted by considering both small (5(12)) and large (5(12)6(4)) cages in type II clathrates, isotopic effects and the possibility of double occupancy of the large cages. The isotopic effect for N-2 and O-2 guest molecules in clathrates is demonstrated. (C) 1997 John Wiley & Sons, Ltd.

Abstract: The suggestion of climatic instability during the last interglacial period (Eem), based on the bottom 10% of the Greenland Ice core Project (GRIP) isotopic profile, has been questioned because the bottom record from the neighboring Greenland Ice Sheet Project 2 (GISP2) core (28 km away) is strikingly different over the same interval and because records of the delta(18)O of atmospheric O-2 from both cores showed unexpected rapid fluctuations. Here we present detailed methane records from the Vostok (Antarctica), GRIP, and GISP2 cores over the relevant intervals. The GRIP and GISP2 data show rapid and large changes in methane concentration, which are correlative with variations of the delta(18)O of the ice, while the Vostok record shows no such variations. This discrepancy reinforces the suggestion that the bottom sections of the Greenland records are disturbed. By combining the methane data with measurements of delta(18)O of O-2 in the same samples, we attempt to constrain the nature of the stratigraphic disturbance and the age of the analyzed ice samples. Our results suggest that ice layers from part of the last interglacial period exist in the lower section of both ice cores and that some of the apparent climate instabilities in the GRIP core would be the result of a mixture of ice from the last interglacial with ice from the beginning of the last glaciation or from the penultimate glaciation.

Abstract: Since 1980 the electrical conductivity method (ECM) has been used to infer volcanic acid signals in Greenland ice cores. The method reveals the great majority of major volcanic eruptions, including several known from historic records. Subsequent ion chromatographic analyses of the acid volcanic layers show the chemical composition, i.e., the concentration of the volcanic acids H2SO4, HCI, and HF plus, e.g., the nonvolcanically derived HNO3. While ECM data are available from a large number of shallow depth Greenland ice cores, covering the past 500-1500 years, only the Greenland Ice Core Project (GRIP), Greenland Ice Sheet Project 2 (GISP2), and Dye 3 deep ice cores exist for a detailed comparative study of volcanic signals in Greenland ice cores representing several thousand years. Comparison of the volcanic signals registered in the GRIP and GISP2 cores will be presented elsewhere. The latter cores were augered 30 km apart and essentially represent the same atmospheric conditions such as temperature, snow accumulation, and chemical composition of the air. Here we present a comparison between the major volcanic signals over the past 4000 years in the GRIP core from central Greenland and the Dye 3 core from SE Greenland in order to investigate the depositional differences. Many of the major signals are detected in both cores, but some of the differences in the records can be used to infer the latitudinal band of some eruption sites. Furthermore, the influence of the amount of annual precipitation and glaciological postdepositional processes on the volcanic signals is discussed.

Abstract: This paper investigates the validation of radar rainfall measurement in a mountainous region by comparison with raingage data, Initially, the comparison serves as a basis for defining the assessment factor introduced to correct the systematic bias often affecting radar data. Several methods are available for determining the assessment factor and all are shown to provide a similar result. Corrections for both beam blockage and vertical profile of reflectivity are also evaluated. Both corrections provide an improvement in radar measurement accuracy. The validation of radar rainfall measurement is performed at the catchment scale of a few hundred square kilometres and at an hourly time period, these conditions being consistent with hydrological needs. The reference rainfall is determined by interpolating data from a dense raingage network. The accuracy of the areal reference value is defined by its standard deviation. A validation criterion taking into account the accuracy of the reference value is introduced. Validation results show a good level of agreement between reference measurements and radar data at the catchment scale. The confidence associated with the reference value suggests a combined use of radar and raingages based on the control of radar estimates by raingage measurements. (C) 1997 Elsevier Science B.V.

Abstract: The Marseilles Hydrometeorological Experiment has been designed to improve rainfall measurement techniques over the space scales and timescales useful for urban hydrology applications. Among other sensors, an X-band light configuration weather radar system was set up near the city. The main objective of the present paper is to test the feasibility of using mountain return to correct rain attenuation effects, an application of the well-known ''surface reference technique'' developed for spaceborne radar configurations. The radar siting and scan procedures were defined so as to obtain both (i) rain reflectivity measurements free of ground detection over a large domain and (ii) strong mountain return for the path-integrated attenuation (PIA) estimation in a reduced azimuthal sector. The so-called PIA constraint equation is proposed to relate the measured rain reflectivity profile in one direction to the corresponding mountain PIA. A study of this equation for the 23 September 1993 rain event provides valuable information concerning (i) the parameterization of the radar data processing and (ii) the comparison of various models proposed for the estimation of the mountain and blind-range PIAs. Three attenuation correction algorithms, already proposed in the literature, are then reviewed. Two of them make direct use of the mountain PIA, while the other is a modified version of the well-known Hitschfeld and Bordan algorithm. A consistency test between the various sources of rainfall data available for the considered rain event is presented. First, disdrometer drop size distribution measurements are modeled and used to derive the (Z k, R) relations required for radar data processing. Then, a validation procedure is used to test the effectiveness of the attenuation correction algorithms by comparison with rain gauge measurements. The performance characteristics of the three algorithms are basically equivalent-a result to be considered with respect to the range of PIAs observed during the rain event (0-20 dB)-and the overall consistency of the various sources of rainfall data used is found to be satisfactory. Further improvements could be obtained from (i) a better understanding of the blind-range attenuation effects and (ii) a higher radar data sampling rate.

Abstract: The Greenland Ice Sheet Project 2 (GISP2) and Greenland Ice Core Project (GRIP) deep drilling programs at Summit, Greenland included support (both logistical and scientific) of extensive investigation of atmospheric transport and air-snow exchange processes of gases and particles relevant to the interpretation of the ice-core records. Much of the sampling for the air-snow exchange investigations was conducted at a unique solar-powered camp 30 km southwest of the GISP2 drill camp (even further from the GRIP camp) and was characterized by a high degree of international collaboration and cooperation. The wide range of expertise and analytical capabilities of the 20-plus investigators participating in these studies has provided important insight into the meteorological, physical, and chemical processes which interact to determine the composition of snow and firn at Summit. Evolving understanding of this system will allow improved reconstruction of the composition of the atmosphere over Greenland in the past from the detailed Summit ice-core records. This Paper provides an overview of air-snow exchange investigations at Summit, including their development through the course of the drilling programs (1989-1993), significant findings related to both air-snow exchange issues and the present state of the Arctic free troposphere, as well as the major outstanding questions which are being addressed in ongoing experiments at Summit.

Abstract: The sum of winter accumulation and summer losses of mass from glaciers and ice sheets (net surface mass balance) varies with changing climate, In the Arctic, glaciers and ice caps, excluding the Greenland Ice Sheet, cover about 275,000 km(2) of both the widely glacierized archipelagos of the Canadian, Norwegian, and Russian High Arctic and the area north of about 60 degrees N in Alaska, Iceland, and Scandinavia, Since the 1940s, surface mass balance time-series of varying length have been acquired from more than 40 Arctic ice caps and glaciers, Most Arctic glaciers have experienced predominantly negative net surface mass balance over the past few decades. There is no uniform recent trend in mass balance for the entire Arctic, although some regional trends occur, Examples are the increasingly negative mass balances for northern Alaska, due to higher summer temperatures, and increasingly positive mass balances for maritime Scandinavia and Iceland, due to increased winter precipitation. The negative mass balance of most Arctic glaciers may be a response to a step-like warming in the early twentieth century at the termination of the cold Little Ice Age. Arctic ice masses outside Greenland are at present contributing about 0.13 mm yr(-1) to global sea-level rise. (C) 1997 University of Washington.

Abstract: Ammonium, nitrate, and organic acid records from the Greenland Ice Core Project deep ice core are discussed. All species have a continental biogenic source that is situated predominantly on the North American continent for species deposited in Summit, central Greenland. The record therefore can be used to trace back the biomass history of the North American continent. Difficulty in the interpretation of these records arises from their unknown transfer behavior in a more alkaline atmosphere, which characterizes glacial time periods compared to interglacial stadials. This may have implications not only for weak acids such as formate and acetate, but also possibly for the transport and incorporation of HNO3 into aerosols, whereas ammonium is probably not affected by the alkalinity change of the atmosphere. Our approach is to compare samples with similar H+ concentrations throughout the record. From the records we infer several significant fluctuations in the extent of the Laurentide Ice Sheet during the last glaciation. We find evidence against the occurrence of an extensive Younger Dryas event in North America.

Abstract: Current models of water flow in deforming soils generally involve a transformation from spatial to material coordinates, Existing forms of this coordinate transformation either assume that soil deformation is one-dimensional, or that it is isotropic, In the present article, we propose a new expression of the transformation gradient tensor, that allows different extents of deformation in the vertical and horizontal directions, The resulting generalized water flow equation is calibrated with experimental data obtained for one-dimensional vertical infiltration in a bentonite sample, The hydraulic characteristics obtained from this calibration are then used to analyze, via simulations, the sensitivity of water flow to anisotropy in soil deformation, The results indicate that the extent of the lateral deformation strongly influences not only the height of the soil surface, as expected, but also the distribution of water and the total volume of water in a swelling/shrinking soil undergoing infiltration or drainage, Consequently, this lateral deformation should be taken into account explicitly in modeling efforts or in the determination of the hydraulic characteristics of soils that deform anisotropically.

Abstract: The in situ determination of the field-saturated hydraulic conductivity of low-permeability porous materials is a major concern for both geotechnics and soil physics with regards to environmental protection or water resources management. Recent early-time single-ring infiltration experiments, involving sequential constant head and falling head conditions, allow its efficient estimation. Nevertheless, the theory on which the interpretation was based was still strictly valid to nondeformable soils and implicity relied on a particular form of the hydraulic conductivity-soil water pressure head relationship. This theory is now extended to deformable materials, without any restrictive hypothesis. A new concept, bulk sorptivity, which characterizes the solid phase movement, is introduced. Field experiments, conducted on two liners of swelling and slowly permeable materials, revealed that neglecting the soil deformation induces an underestimation of the actual coefficient of permeability of the soil.

Abstract: The boundary layer development over the Central sites of HAPEX-Sahel has been simulated for 4 individual days. The study is divided into two parts. In the first part a surface scheme is calibrated for the dominant land cover of the area, the fallow savanna. This calibration is performed for a period of 54 days encompassing the transition from wet to dry. A comparison is then made with another more detailed surface scheme. The model, in spite of its drastic simplification of reality, is able to reproduce the overall water balance for the period. In terms of relative contribution of soil evaporation and vegetation transpiration it does not depart significantly from the more complex scheme. In the second part, the surface scheme is coupled to a one-dimensional version of the ARPEGE model (operational forecast model at Meteo-France). The importance of the direct heating of the boundary layer by aerosols is stressed. With minor modifications of calibration parameters from the first phase, boundary layer profiles of temperature and also mixing ratio are reproduced with a good accuracy.

Abstract: Simulations of Rn-222 and other short-lived tracers are used to evaluate and intercompare the representations of convective and synoptic processes in 20 global atmospheric transport models. Results show that most established three-dimensional models simulate vertical mixing in the troposphere to within the constraints offered by the observed mean Rn-222 concentrations and that subgrid parameterization of convection is essential for this purpose. However, none of the models captures the observed variability of Rn-222 concentrations in the upper troposphere, and none reproduces the high Rn-222 concentrations measured at 200 hPa over Hawaii. The established three-dimensional models reproduce the frequency and magnitude of high- Rn-222 episodes observed at Crozet Island in the Indian Ocean, demonstrating that they can resolve the synoptic-scale transport of continental plumes with no significant numerical diffusion. Large differences between models are found in the rates of meridional transport in the upper troposphere (interhemispheric exchange, exchange between tropics and high latitudes). The four two-dimensional models which participated in the intercomparison tend to underestimate the rate of vertical transport from the lower to the upper troposphere but show concentrations of Rn-222 in the lower troposphere that are comparable to the zonal mean values in the three-dimensional models.

Abstract: The seasonal variation in atmospheric transport patterns to Summit, Greenland, is examined using a 44-year record of daily, 10-day, isobaric back trajectories at the 500-hPa level. Over 24,000 modeled trajectories are aggregated into distinct patterns using cluster analysis. Ten-day trajectories reaching Summit are longest during winter, with 67% extending upwind (westward) as far back as Asia or Europe. Trajectories are shortest during summer, with 46% having 10-day origins over North America. During all seasons a small percentage (3-7%) of trajectories originate in west Asia/Europe and follow a meridional path over the Arctic Ocean before approaching: Summit from the northwest. Trajectories at the 700-hPa level tend to be shorter than at 500 hPa, with many of the 700-hPa trajectories from North America tracking over the North Atlantic and approaching Summit from the south. The long-range transport climatology for Summit is similar to a year-round climatology prepared for Dye 3, located 900 lan to the south [Davidson et al., 1993b]. An analysis of several aerosol species measured at Summit during summer 1994 reveals examples of the usefulness and also the limitations of using long-range air trajectories to interpret chemical data.

Abstract: A high resolution GCM is used to examine the effect of changes in local surface climate parameters on the ice sheets that can influence the interpretation of the isotopic signal of the ice from deep cores. The model suggests that the 10 degrees C difference between the LGM surface temperature deduced from borehole thermometry and that deduced from the water isotope analysis to a great extent may be due to a modification of the precipitation seasonality in central Greenland. For central East Antarctica, the model tends to suggest a weak opposite bias.

Abstract: Snow accumulation records from central Greenland are explored to improve the understanding of the accumulation signal in Greenland ice core records. Results from a “forest” of 100 bamboo poles and automated accumulation monitors in the vicinity df Summit as well as shallow cores collected in the Summit and Crete areas are presented. Based on these accumulation data, a regression has been calculated to quantify the signal-to-noise variance ratio of ice core accumulation signals on a variety of temporal (1 week to 2 years) and spatial (20 m to 200 km) scales. Results are consistent with data obtained from year-round automated accumulation measurements deployed at Summit which suggest that it is impossible to obtain regional snow accumulation data with seasonal resolution using four accumulation monitors positioned over a length scale of similar to 30 km. Given this understanding of the temporal and spatial dependence of noise in the ice core accumulation signal, the accumulation records from 17 shallow cores are revisited. Each core spans the time period from 1964 to 1983. By combining the accumulation records, the regional snow accumulation record has been obtained for this period. The results show that 9 of the 20 years can be identified as having an accumulation different from the 20 year mean with 99% confidence. The signal-to-noise variance ratio for the average accumulation signal sampled at annual intervals is 5.8 +/- 0.5. The averaged accumulation time series may be useful to climate modelers attempting to validate their models with accurate regional hydrologic data sets.

Abstract: In the Sahel, rainfall is the single most important factor conditioning the hydrology and the climate, but comprehensive statistical analyses of the rainfall climatology in the region are rare. Yet, even though in the Sahel rainfall data are scarce by the standards of the temperate countries, it is shown here that it is possible to obtain a reasonably good idea of what the rainfall has been over Sahelian Niger for the past 40 years, both in terms of interannual variability and spatial distribution. To that aim a statistical model is used, which decomposes the space-time fluctuations of long-term rainfall averages into the fluctuations of the mean event rainfall on the one hand, and of the mean number of rainfall events over any period of accumulation, on the other hand. This model is first applied to the analysis of monthly rainfall data over the whole of Niger. It is shown that the lasting drought which has affected Niger for more than 20 years is associated with a decrease in the number of rainy events, rather than to a decrease of the mean event rainfall, and that this decrease is more pronounced for the core of the rainy season. Because these fluctuations are not homogeneous over Niger, a 5 degrees x 4 degrees zone centred on the HAPEX-Sahel 1 degrees x 1 degrees square is selected in order to characterise more accurately the rainfall climatology of the HAPEX-Sahel area between 1950 and 1990. In comparison with what it was between 1950 and 1970, the average length of the rainy season has not changed significantly during the dry period 1970-1990. Rather, it is the decrease of rainfall in July and August that explains most of the diminution of the total annual rainfall over this part of the Sahel since 1970. The average number of rainy events in August was reduced by about 30%, while the mean event rainfall remained roughly constant. Finally, the analysis of the daily rainfall series for Niamey (which constitutes the longest record available in Niger, starting in 1905) enables the comparison of four periods of 20 years between 1910 and 1990. The period 1970-1989 appears to be by far the longest and most severe dry spell of the past century. Almost 90% of the annual rainfall decrease over this period is explained by the decrease of the mean number of rainfall events during July and August, while both the length of the rainy season and the mean event rainfall remained stable.

Abstract: The HAPEX-Sahel experiment took place in the midst of the most severe drought that has ever plagued the region since rainfall records have been available in the Sahel, The aim of this paper is to describe the rainfall conditions that were observed during HAPEX-Sahel by a network of 100 recording raingauges, and to analyse them in the perspective of the long-term statistics at the Niamey station. Globally it is found that the average rainfall over the HAPEX-Sahel study area, as well as over Niger as a whole, during the experiment has been moderately (1991 and 1992) to markedly dry (1990 and 1993), indicating that the drought of the past 20 years has not ended, More detailed statistics point to the high intermittency of the Sahelian rainfall, both in space (each year the ratio between the maximum and minimum recorded seasonal rainfall was of the order of 2) and in time (half the annual rain falls in 5 h). Particular attention is paid to the climatology of the rainy events. The point event rainfall is nearly exponentially distributed with a mean of about 14 mm, while the average areal event rainfall over the HAPEX-Sahel 1 degrees x 1 degrees square is of the order of 10.5 mm. The probability of zero rainfall is thus close to 1/4. Rain rates an often heavy, with half the annual rain falling at rain rates higher than 35 mm h(-1) and one third of it falling at rain rates higher than 50 mm h(-1). The year to year statistics of both the event rainfall and the rain rates vary little, indicating that most of the inter-annual rainfall variability is due to the variation of the number of rainfall events, rather than to variations in rainfall intensities or in the mean event rainfall.

Abstract: Human activities have already modified the chemical composition of the natural atmosphere even in very remote regions of the world. The study of chemical parameters stored in solid precipitation and accumulated on polar ice sheets over the last several hundred thousand years provides a unique tool for obtaining information on the composition of the preindustrial atmosphere and its natural variability over the past. This paper deals with the chemistry of polar ice focused on the soluble mineral (Na+, NH4+, K+ Ca++, Mg++, H+, F-, Cl-, NO3-, SO4--, and H2O2) and organic (methanesulfonate (CH3SO3-), formate (HCOO-), acetate (CH3COO-), and formaldehyde (HCHO)) species and their interpretation in terms of past atmospheric composition (aerosols and water soluble gaseous species). We discuss ice core dating, the difficulties connected with trace measurements, and the significance of the ionic composition of snow. We examine temporal (from the last decades back to the last climatic cycle) and spatial (including examples from coastal as well as central areas of Greenland and Antarctica) variations in the ionic budget of the precipitation and evaluate ice core studies in terms of the chemical composition of our past atmosphere. We review (1) how Greenland and Antarctic ice cores that span the last few centuries have provided information on the impact of human activities and (2) how the chemistry of deep ice cores provides information on various past natural phenomena such as climatic variations (glacial-interglacial changes, El Nino), volcanic eruptions, and large boreal forest fires.

Abstract: A mathematical model for simulating the densification of bubbly glacier ice is used to interpret the following experimental data from the Vostok (central Antarctica) ice core: two ice-porosity profiles obtained by independent methods and a bubble-pressure profile obtained by direct measurements of air pressure within individual bubbles. The rheological properties of pure polycrystalline ice are deduced from the solution of the inverse problem. The model and the inferred ice-now law are then validated, using porosity profiles from seven other ice cores drilled in Antarctica and Greenland, in the temperature range from -55 degrees to -20 degrees C. The following expression is adopted for the constitutive law: 2<(e)over dot> = (tau/mu(1) + tau(alpha)/mu(2))exp[Q(1/T-s – 1/T)/R-s] where <(e)over dot> and tau are the effective strain rate and stress, respectively, ct is the creep exponent taken as 3.5, R-s is the gas constant and T(T-s) is the temperature (standard temperature). The numerical values obtained for the “linear” and “non-linear” viscosities are: mu(1) = 2.9 +/- 1.3 MPa year and mu(2) = 0.051 +/- 0.019 MPaalpha year, and the apparent activation energy Q is confirmed to be 60 kJ mole(-1). The corresponding flow law is in good agreement with results of both mechanical tests and independent estimations based on the analysis of different natural phenomena associated with glacier-ice deformation. When the model is constrained by the porosity and bubble-pressure profiles from Vostok, the mean air content in Holocene ice is inferred to be about 0.088 cm(3) g(-1). The corresponding mean air pressure in bubbles at the end of pore closure is about 0.083 MPa, whereas the atmospheric pressure at this depth level would be 0.063 MPa. The influence of the climatic change on the ice-porosity profile is discussed. It resulted in an increased air content in ice at Vostok during the Last Glacial Maximum: 0.096 cm(3) g(-1).

Abstract: Few ice sheet flow models have been developed that solve the complete set of mechanical equations. Until now, these models were limited to isotropic conditions. We present here a two-dimensional, finite difference method capable of solving the equations for the steady flow of a viscous, incompressible, anisotropic fluid with a free surface under isothermal conditions. It is not a standard method, especially with respect to the time discretization of the numerical scheme, and converges for very low Reynolds numbers. This method is applied here to the planar flow of anisotropic ice over flat or irregular bedrock, with no-slip boundary conditions at the ice-bedrock interface. The results are presented here for Newtonian behavior in the vicinity of an ice divide. The ice is assumed to be isotropic at the ice sheet surface, with continuous and prescribed development of anisotropy with increasing depth. Going from isotropic to anisotropic situations, our results indicate that the free surface becomes flatter and the shear strain rates larger and more concentrated near the bedrock. The flow is less sensitive to variations of the bedrock topography in the anisotropic case than in the isotropic case. Furthermore, a new phenomenon appears in the anisotropic case: the partial stagnation of ice in the holes of the bedrock. These effects have significant consequences when dating the; ice. The isochrones obtained in the anisotropic case are flatter and the anisotropic ice is more than 10% younger above the bumps and more than 100% older within the holes than for the isotropic ice.

Abstract: The HAPEX-Sahel experiment was organized to investigate the impact of water, energy and CO2 fluxes at the soil-vegetation-atmosphere interface on climate processes in the Sahelian region, Measurements of the energy balance components, CO2 flux and soil moisture were conducted over a savanna area at the East Central Supersite of the one degree square during a 3 month period in 1992. The aim of this particular investigation was to understand the role of surface conditions (i.e. vegetation and moisture) in the partitioning of available energy at the surface into sensible and latent heat flux. It also aimed to improve the understanding of how water and carbon cycles are affected by vegetation functioning, soil water availability and atmospheric demand. The analysis presented in this paper showed that the relative contribution of the soil and the vegetation to latent heat flux varies intimately with the temporal rainfall distribution and the growth of the savanna grass species, which is more sensitive to the distribution of precipitation than to its amount. Finally, semi-empirical parameterizations were developed to formulate (1) the daily evapotranspiration rate of the savanna in terms of available energy at the surface and soil water content, and (2) the instantaneous carbon uptake in terms of photosynthetically active radiation received at the surface and soil water availability.