Energy balance and melt over a patchy snow cover

A patchy snow cover significantly alters the surface energy exchange. The two opposite effects of horizontal advection of warm air from the bare ground to the snow patch and the development of strong stability close to the ground were assessed in this study. Atmospheric, snow and hydrological models are typically limited to simulating pointwise vertical exchange between the ground and the atmosphere and do not include lateral transport close to the ground. For a patchy snow cover, this limitation leads to an underestimation of modelled melting rates at the upwind edge. We assess the relative contribution of the advective heat flux to the total surface energy balance and therefore snow melt using (i) high-resolution measurements of daily snow depth changes obtained from Terrestrial Laser Scanning, (ii) the distributed and physics-based snow model Alpine3D using standard automatic weather stations as model input, and (iii) Alpine3D, which is forced with air temperature and wind velocity fields calculated from the non-hydrostatic atmospheric model Advanced Regional Prediction model (ARPS). Analysis of measured melt rates have shown a 5 % increase in total snow melting due to the effect of the advective heat flux for a typical spring snow distribution. We numerically investigate the effect of atmospheric flow field dynamics over a patchy snow cover on the total surface energy balance by forcing Alpine3D with fully resolved meteorological fields (air temperature and wind velocity) obtained from ARPS (i) close to the surface in order to represent the small scale variability, and (ii) above the blending height as a reference due to the fact that the model input is typically recorded above the blending height. Quantitative experimental and numerical results show how the snow melt rate increases with decreasing snow cover fraction (SCF) and decreasing mean perimeter of the snow patches.