Abstract. Detailed studies of snow cover processes require models that offer a fine description of the snow cover properties. The detailed snowpack model Crocus is such a scheme, and has been run operationally for avalanche forecasting over the French mountains for 20 yr. It is also used for climate or hydrological studies. To extend its potential applications, Crocus has been recently integrated within the framework of the externalized surface module SURFEX. SURFEX computes the exchanges of energy and mass between different types of surface and the atmosphere. It includes in particular the land surface scheme ISBA (Interactions between Soil, Biosphere, and Atmosphere). It allows Crocus to be run either in stand-alone mode, using a time series of forcing meteorological data or in fully coupled mode (explicit or fully implicit numerics) with atmospheric models ranging from meso-scale models to general circulation models. This approach also ensures a full coupling between the snow cover and the soil beneath. Several applications of this new simulation platform are presented. They range from a 1-D standalone simulation (Col de Porte, France) to fully-distributed simulations in complex terrain over a whole mountain range (Massif des Grandes Rousses, France), or in coupled mode such as a surface energy balance and boundary layer simulation over the East Antarctic Ice Sheet (Dome C).

Abstract. SURFEX is a new externalized land and ocean surface platform that describes the surface fluxes and the evolution of four types of surfaces: nature, town, inland water and ocean. It is mostly based on pre-existing, well-validated scientific models that are continuously improved. The motivation for the building of SURFEX is to use strictly identical scientific models in a high range of applications in order to mutualise the research and development efforts. SURFEX can be run in offline mode (0-D or 2-D runs) or in coupled mode (from mesoscale models to numerical weather prediction and climate models). An assimilation mode is included for numerical weather prediction and monitoring. In addition to momentum, heat and water fluxes, SURFEX is able to simulate fluxes of carbon dioxide, chemical species, continental aerosols, sea salt and snow particles. The main principles of the organisation of the surface are described first. Then, a survey is made of the scientific module (including the coupling strategy). Finally, the main applications of the code are summarised. The validation work undertaken shows that replacing the pre-existing surface models by SURFEX in these applications is usually associated with improved skill, as the numerous scientific developments contained in this community code are used to good advantage.

) is small, potentially allowing reconstruction of past shifts in tropospheric oxidation pathways from ice cores. Assuming a Rayleigh-type process we find fractionation constants ε of −60±15‰, 8±2‰ and 1±1‰, for δ 15 N, δ 18 O and 17 O, respectively. A photolysis model yields an upper limit for the photolytic fractionation constant 15 ε of δ 15 N, consistent with lab and field measurements, and demonstrates a high sensitivity of 15 ε to the incident actinic flux spectrum. The photolytic 15 ε is process-specific and therefore applies to any snow covered location. Previously published 15 ε values are not representative for conditions at the Earth surface, but apply only to the UV lamp used in the reported experiment (Blunier et al., 2005;Jacobi et al., 2006). Depletion of oxygen stable isotopes is attributed to photolysis followed by isotopic exchange with water and hydroxyl radicals. Conversely, 15 N enrichment of the NO

Abstract. Here we report the measurement of the comprehensive isotopic composition (δ15N, Δ17O and δ18O) of nitrate at the air–snow interface at Dome C, Antarctica (DC, 75°06' S, 123°19' E), and in snow pits along a transect across the East Antarctic Ice Sheet (EAIS) between 66° S and 78° S. In most of the snow pits, nitrate loss (either by physical release or UV photolysis of nitrate) is observed and fractionation constants associated are calculated. Nitrate collected from snow pits on the plateau (snow accumulation rate below 50 kg m−2 a−1) displays average fractionation constants of (−59±10) ‰, (+2.0±1.0) ‰ and (+8.7±2.4)‰ for δ15N, Δ17O and δ18O, respectively. In contrast, snow pits sampled on the coast show distinct isotopic signatures with average fractionation constants of (−16±14) ‰, (−0.2±1.5) ‰ and (+3.1±5.8) ‰, for δ15N, Δ17O and δ18O, respectively. Our observations corroborate that photolysis (associated with a 15N / 14N fractionation constant of the order of –48 ‰ according to Frey et al. (2009) is the dominant nitrate loss process on the East Antarctic Plateau, while on the coast the loss is less pronounced and could involve both physical release and photochemical processes. Year-round isotopic measurements at DC show a~close relationship between the Δ17O of atmospheric nitrate and Δ17O of nitrate in skin layer snow, suggesting a photolytically driven isotopic equilibrium imposed by nitrate recycling at this interface. Atmospheric nitrate deposition may lead to fractionation of the nitrogen isotopes and explain the almost constant shift of the order of 25 ‰ between the δ15N values in the atmospheric and skin layer nitrate at DC. Asymptotic δ15N(NO3−) values calculated for each snow pit are found to be correlated with the inverse of the snow accumulation rate (ln(δ15N as. + 1) = (5.76±0.47) ċ (kg m−2 a−1/ A) + (0.01±0.02)), confirming the strong relationship between the snow accumulation rate and the degree of isotopic fractionation, consistent with previous observations by Freyer et al. (1996). Asymptotic Δ17O(NO3−) values on the plateau are smaller than the values found in the skin layer most likely due to oxygen isotope exchange between the nitrate photoproducts and water molecules from the surrounding ice. However, the apparent fractionation in Δ17O is small, thus allowing the preservation of a portion of the atmospheric signal.

There are many models that attempt to predict physical processes in snow on the ground for a range of applications, and evaluations of these models show that they have a wide range of behaviours. A review of snow models, however, shows that many of them draw on a relatively small number of process parametrizations combined in different configurations and using different parameter values. A single model that combines existing parametrizations of differing complexity in many different configurations to generate large ensembles of simulations is presented here. The model is driven and evaluated with data from four winters at an alpine site in France. Consideration of errors in simulations of snow mass, snow depth, albedo and surface temperature show that there is no "best" model, but there is a group of model configurations that give consistently good results, another group that give consistently poor results, and many configurations that give good results in some cases and poor results in others. There is no clear link between model complexity and performance, but the most consistent results come from configurations that have prognostic representations of snow density and albedo and that take some account of storage and refreezing of liquid water within the snow.

[1] We carried out numerical simulations of the conductivity of snow using microtomographic images. The full tensor of the effective thermal conductivity (k eff ) was computed from 30 three-dimensional images of the snow microstructure, spanning all types of seasonal snow. Only conduction through ice and interstitial air were considered. The obtained values are strongly correlated to snow density. The main cause for the slight scatter around the regression curve to snow density is the anisotropy of k eff : the vertical component of k eff of facetted crystals and depth hoar samples is up to 1.5 times larger than the horizontal component, while rounded grains sampled deeply in the snowpack exhibit the inverse behavior. Results of simulations neglecting the conduction in the interstitial air indicate that this phase plays a vital role in heat conduction through snow. The computed effective thermal conductivity is found to increase with decreasing temperature, mostly following the temperature dependency of the thermal conductivity of ice. The results are compared to experimental data obtained either with the needle-probe technique or using combined measurements of the vertical heat flux and the corresponding temperature gradient. Needle-probe measurements are systematically significantly lower than those from the two other techniques. The observed discrepancies between the three methods are investigated and briefly discussed.

Atmospheric nitrogen oxides (NOx =NO+ NO2) play a pivotal role in the cycling of reactive nitrogen (ultimately deposited as nitrate) and the oxidative capacity of the atmosphere. Combined measurements of nitrogen and oxygen stable isotope ratios of nitrate collected in the Arctic atmosphere were used to infer the origin and fate of NOx and nitrate on a seasonal basis. In spring, photochemically driven emissions of reactive nitrogen from the snowpack into the atmosphere make local oxidation of NOx by bromine oxide the major contributor to the nitrate budget. The comprehensive isotopic composition of nitrate provides strong constraints on the relative importance of the key atmospheric oxidants in the present atmosphere, with the potential for extension into the past using ice cores.

International audienceThe comprehensive isotopic composition of atmospheric nitrate (i.e., the simultaneous measurement of all its stable isotope ratios: 15N/14N, 17O/16O and 18O/16O) has been determined for aerosol samples collected in the marine boundary layer (MBL) over the Atlantic Ocean from 65°S (Weddell Sea) to 79°N (Svalbard), along a ship-borne latitudinal transect. In nonpolar areas, the δ 15N of nitrate mostly deriving from anthropogenically emitted NO x is found to be significantly different (from 0 to 6‰) from nitrate sampled in locations influenced by natural NO x sources (−4 ± 2)‰. The effects on δ 15N(NO3 −) of different NO x sources and nitrate removal processes associated with its atmospheric transport are discussed. Measurements of the oxygen isotope anomaly (Δ17O = δ 17O − 0.52 × δ 18O) of nitrate suggest that nocturnal processes involving the nitrate radical play a major role in terms of NO x sinks. Different Δ17O between aerosol size fractions indicate different proportions between nitrate formation pathways as a function of the size and composition of the particles. Extremely low δ 15N values (down to −40‰) are found in air masses exposed to snow-covered areas, showing that snowpack emissions of NO x from upwind regions can have a significant impact on the local surface budget of reactive nitrogen, in conjunction with interactions with active halogen chemistry. The implications of the results are discussed in light of the potential use of the stable isotopic composition of nitrate to infer atmospherically relevant information from nitrate preserved in ice cores