Impact of Extreme Land Surface Heterogeneity on Micrometeorology over Spring Snow Cover

The melting mountain snow cover in spring typically changes from a continuous snow cover to a mosaic of patches of snow and bare ground, inducing an extreme heterogeneity of the land surface. A comprehensive measurement campaign, the Dischma experiment, was conducted during three entire ablation seasons. The aim of this study was to experimentally investigate the small-scale boundary layer dynamics over a melting snow cover with a gradually decreasing snow cover fraction and the associated heat exchange at the snow surface. This study presents a unique dataset combining eddy covariance measurements at different atmospheric levels with maps of snow surface temperatures and snow cover fractions. The experiments evidence diurnal mountain wind systems driving the diurnal cycle of turbulent sensible heat fluxes over snow and the formation of katabatic flows over long-lasting snow patches strongly affecting the temporal evolution of snow surface temperature patterns. The snow cover distribution is also shown to be of vital importance for the frequency of stable internal boundary layer development over snow. For situations with a clear evidence of stable internal boundary layer development over snow, the data reveal a very shallow atmospheric layer adjacent to the snow cover decoupled from the warm-air advection above. Thesemeasurements confirm previous wind tunnel experiments that also evidenced a decoupling of the air adjacent to the snow cover from the warmer air above, especially within topographical depressions and when ambient wind velocities are low. For these situations, in particular, all tested energy balance models strongly overestimated the turbulent sensible heat flux directed toward the snow cover.