A local thermal non-equilibrium model for rain-on-snow events

Abstract. Liquid-water movement through a snowpack, e.g., during rain-on-snow events or meltwater infiltration, is an essential process to understand runoff generation, flash floods, and snow avalanches. From a physical point of view, water infiltration into snow is a strongly coupled thermo-hydraulic problem with a thermal non-equilibrium between phases because the infiltrating water can be substantially warmer than the snowpack. Contrarily to water infiltration into a frozen soil, the solid volume fraction is highly dynamic due to the melting of snow and (re-)freezing of water. This work presents the first true multi-phase local thermal non-equilibrium model with variable volume fractions of all involved phases, including the snowpack as a solid porous matrix. While the possible value range of hydraulic, geometrical, and thermal parameters within a snowpack can be highly variable, the developed model is subsequently used to systematically study the effects of environmental conditions and parameters on the spatial distribution of melting and freezing within the snowpack. The model can be used to identify the formation of new ice layers due to refreezing, as well as layers of enhanced melting.