A fully coupled thermo-hydraulic model for Rain-on-Snow events with relation to avalanches and streamflows

Rain-on-snow (ROS) events are prominent examples of potentially hazardous events controlled by the thermal state of the system, which can occur at almost any rainfall intensity. Severe floods and mud flows can be triggered when warm storm systems release rain on a snow cover, but ROS events can lead to snow avalanches, and slush flows due to the additional load when rainwater freezes during its passage through the snowpack. Due to climate change, the number of ROS events observed, e.g., in the European Alps and North America, is growing and is expected to increase even more in upcoming years.

Understanding ROS events is crucial for hazard mitigation measures. However, the thermal and hydraulic processes within the snow are complex and strongly coupled. The dynamic snowpack, the interaction with the atmosphere, and the dependence on past freeze/thaw cycles contribute to this complexity.

This work introduces a novel mathematical - numerical framework to study the thermo-hydraulic processes during ROS events. The focus point of the model is the complex thermal non-equilibrium condition between snowpack and rain and the hydraulic consequences arising from that. The model results reveal critical combinations of snow state, thermal and atmospheric conditions facilitating avalanches or floods so that the model can subsequently help to improve the hazard assessment.