A physical SNOWPACK model for the Swiss avalanche warning: Part II. Snow microstructure

The snow cover model SNOWPACK includes a detailed model of snow microstructure and metamorphism. In SNOWPACK, the complex texture of snow is described using the four primary microstructure parameters: grain size, bond size, dendricity and sphericity. For each parameter, rate equations are developed that predict the development in time as a function of the environmental conditions. The rate equations are based on theoretical considerations such as mixture theory and on empirical relations. With a classification scheme, the conventional snow grain types are predicted on the basis of those parameters. The approach to link the bulk constitutive properties, viscosity and thermal conductivity to microstructure parameters is novel to the field of snow cover modeling. Expanding on existing knowledge on microstructure-based viscosity and thermal conductivity, a complete description of those quantities applicable to the seasonal snow cover is presented. This includes the strong coupling between physical processes in snow: The bond size, which changes not only through metamorphic processes but also through the process of pressure sintering (included in our viscosity formulation), is at the same time the single most important parameter for snow viscosity and thermal conductivity. Laboratory results are used to illustrate the performance of the formulations presented. The numerical implementation is treated in the companion paper Part I. A more complete evaluation for the entire model is found in the companion paper Part III.