Atmospheric flow and associated changes in turbulent sensible heat flux over a patchy mountain snow-cover

In this study we numerically investigated the small-scale boundary layer dynamics over a gradually decreasing snow-cover in spring. We particularly analyzed the effect of heat advection, boundary layer decoupling and changing patterns of secondary flows on the energy balance of a patchy snow-cover. The atmospheric boundary layer flows over a patchy snow-cover were calculated with an atmospheric model (Advanced Regional Prediction System) on a very high resolution of 5 m. The numerical results revealed that the development of local flow patterns and the relative importance of boundary layer processes depend on the snow patch size distribution and the synoptic wind forcing. Energy balance calculations for quiescent wind situations demonstrated that well-developed katabatic winds exerted a major control on the energy balance at the patchy snow cover, leading to a maximum in the mean downward heat flux over snow for high snow-cover fractions. In contrast, stronger synoptic winds increased the effect of heat advection and decreased the impact of boundary layer decoupling on the catchment's melt behavior. For synoptic wind cases, the strong heat advection resulted in a maximum in the mean heat flux over snow for low snow-cover fractions. A sensitivity analysis to grid resolution suggested that the grid size is a critical factor for modeling atmospheric boundary layer processes, that are shown to significantly alter the energy balance of a patchy snow-cover. The comparison of simulation results from coarse (50 m) and fine (5 m) spatial resolutions revealed a difference in the mean turbulent heat flux over snow of 40% - 70% for synoptic cases and 95% for quiescent cases.