Wind-Driven Processes at the Snow-Atmosphere Interface: Challenges and Approaches in Snow-Hydrological Modeling

The strong heterogeneity of the mountain snow-cover across different scales is mainly induced by complex snow -atmosphere interactions. The main focus of this study is to improve the representation of wind-driven coupling processes between snow and atmosphere in snow hydrological models We present findings from recent experimental studies on snow-atmosphere interactions which motivate new modelling strategies within the framework of the Swiss operational snow-hydrological model system. We will show selected examples of experimental studies conducted during the snow accumulation and the ablation season. For the snow accumulation season, we can show that the local flow field at the ridge scale determines precipitation patterns by advecting hydrometeors downwind of mountain ridges and peaks. This process is known as preferential deposition of snow and plays a decisive role in the final distribution of snow and snow loading on leeward slopes on the same magnitude as redistribution of fallen snow. For the melting season our measurements provide detailed insight into mechanisms of local heat advection processes over melting snow. We further identified that the increase in mean air temperature due to a snow-albedo temperature-feedback significantly affects turbulent heat exchange towards the snow cover and thus snow melt. Our experiments also highlight high temporal and spatial dynamics of local turbulence profiles of momentum and heat, indicating strong changes of the local thermodynamics at a glacier when larger-scale flows disturb the persistent katabatic flow.

These wind-driven coupling processes strongly affect the mass and energy balance of mountain snow covers in mountain catchments. The key to improved operational snow cover predictions for large mountainous catchments is the coupling of physically based state-of-the art models for both atmospheric and snow cover processes to better resolve processes that act at the snow – atmosphere interface. We therefore fully couple a snow process model (FSM) to an inter-complexity atmospheric model (ICAR). The model framework is using Consortium for Small-Scale Modeling (COSMO1) reanalysis as initial and boundary conditions (COMSO-ICAR). The model coupling will not only allow to solve the energy and mass exchange at the snow-atmosphere interface in the vertical but also in the horizontal direction and therefore allows to account for the lateral interaction between grid cells. This allows us by the first time to fully account for complex snow-atmosphere interactions and feedback mechanisms. Furthermore, the intermediate complexity of the model allows to run simulations for very large mountain catchments on a very high resolution of 250 m. The purpose of the new model chain is to: i) develop a model for preferential deposition of snowfall, ii) assess snow albedo-temperature feedbacks in spring, iii) assess local heat advection effects on snow melt dynamics and iv) directly account for changes of local flow systems on the energy balance of the local snow cover. The development of a model chain that fully describes all relevant atmospheric and snow-cover processes at very high resolutions helps us to assess the relevance of snow deposition processes at the ridge scale for seasonal snow-cover dynamics and the mass balance of glaciers as well as the change of local meteorology with decreasing snow-cover fraction. In this presentation we will demonstrate how this model approach is able to reproduce wind-driven coupling processes at the snow-atmosphere interface that have been observed during field experiments.