A numerical model has been developed to simulate energy and mass evolution of snow cover at a given location , as a function of meteorological conditions: precipitation, air temperature, humidity, wind velocity, and incoming short-wave and long-wave radiation.This model, named CROCUS, was first tested on a well-instrumented field during a whole winter, showing its ability to simulate the important phenomena affecting the evolution of the snow layers: high temperature gradients, wetting, compaction, and melting-freezing cycles. A second test was conducted at two locations in the French network used for operational avalanche forecasting . Though the weather observations are made there only twice daily, the snow profiles calculated by the model were very close to those obtained once a week by a pit observation . CROCUS proved itself sufficient to be considered now as a useful objective tool for operational avalanche forecasting .

ABSTRACT. A numerical model has been developed to simulate energy and mass evolution of snow cover at a given location , as a function of meteorological conditions: precipitation, air temperature, humidity, wind velocity, and incoming short-wave and long-wave radiation.This model, named CROCUS, was first tested on a well-instrumented field during a whole winter, showing its ability to simulate the important phenomena affecting the evolution of the snow layers: high temperature gradients, wetting, compaction, and melting-freezing cycles. A second test was conducted at two locations in the French network used for operational avalanche forecasting . Though the weather observations are made there only twice daily, the snow profiles calculated by the model were very close to those obtained once a week by a pit observation . CROCUS proved itself sufficient to be considered now as a useful objective tool for operational avalanche forecasting .

Snow, from its fall until its full melting, undergoes a structural metamorphism that is governed by temperature and humidity fields. Among the many possible mechanisms that contribute to snow metamorphism, those that depend only on curvature are the most accessible to modelling. In this paper, techniques of volume data analysis adapted to the complex geometry of snow are introduced and then applied to experimental tomographic data coming from the isothermal metamorphism of snow near 0°C. In particular, an adaptive algorithm of curvature computation is described. Present results on the evolution of specific surface area and anisotropy already show that such image-analysis methods are relevant tools for the characterization of real snow microstructures. Moreover, the evolution of the curvature distribution with time provides valuable information for the development of sintering models, in the same way as a possible quantitative calibration of snow-grain coarsening laws.

Snow, from its fall until its full melting, undergoes a structure metamorphism governed by temperature and humidity fields. Among the many possible mechanisms that contribute to snow metamorphism, those that depend only on curvature are the most accessible to modelling. The isothermal metamorphism of a dry snow sample near 0˚C is addressed in this paper. Near 0˚C, the vapour pressure of water is high: the metamorphism can be considered, in first approximation, as fully curvature-driven. This corresponds to neglect crystallographic orientation and diffusion-limited effects. Based on Kelvin's and Langmuir-Knudsen equations, a growth law of the ice phase can be analytically obtained. In this law, the variation of the local volume fraction is proportional to the difference between integral and local curvatures. A simple numerical model was implemented in three dimensions and applied on real tomographic images.

The porosity of wet snow is often about 50%; however, liquid water generally fills less than 10% of this pore volume. In order to relate the irreducible water content trapped in snow to its characteristics, we have conducted experiments in a cold laboratory. The results show that irreducible water content, expressed as per cent of mass, depends only on porosity. Experimental studies were restricted to homogeneous wet snow samples. Therefore, we can only achieve a valid result in natural snowpacks when applying to an homogeneous layer of wet snow. Nevertheless, the results may be incorporated into snow-cover energy-balance models to improve the retention and percolation predictions. The thickness of the water-saturated layer observed at the base of the sample in our experiments, was related to the ratio of the mean convex radius of curvature to dry density.

For the first time, three-dimensional (3-D) high-resolution images of snow were obtained using X-ray absorption tomography. Images with a spatial resolution of 10 mm were taken on four different cylindrical snow samples (9 mm high, 9 mm diameter). About 1000 two-dimensional X-ray absorption images were recorded at angular positions of the object around an axis spanning 180³. An appropriate algorithm was then used for these data to reconstruct a 3-D image. In the case of snow, experimental problems have been solved to prepare the samples and prevent both melting and metamorphism of snow during the experiments. This tomographic method provided 3-D data files from which images of 600 3 voxels were extracted. Several physical parameters of snow microstructure can be processed from these data. Porosity P and discrete local (3-D) curvature C of the grain/pore interface were computed for the four snow samples. Representative elementary volume (REV, in the sense of porous media) is a relevant index to the significance of the sample size with respect to a given parameter. From each image, the values of P and C are compared for subsamples of different size, as an attempt to assess the REVs for porosity and curvature. Results show that the observed volume of snow is statistically significant to achieve the porosity and the curvature distribution. Fig. 9. Comparison between the histogram of 3-D convex curvatures and the 2-D one, for each snow sample.

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The porosity of wet snow is often about 50%; however, liquid water generally fills less than 10% of this pore volume. In order to relate the irreducible water content trapped in snow to its characteristics, we have conducted experiments in a cold laboratory. The results show that irreducible water content, expressed as per cent of mass, depends only on porosity. Experimental studies were restricted to homogeneous wet snow samples. Therefore, we can only achieve a valid result in natural snowpacks when applying to an homogeneous layer of wet snow. Nevertheless, the results may be incorporated into snow-cover energy-balance models to improve the retention and percolation predictions. The thickness of the water-saturated layer observed at the base of the sample in our experiments, was related to the ratio of the mean convex radius of curvature to dry density.