Understanding wind-driven melt of patchy snow cover

Abstract. The simplified representation of snow processes in most large-scale hydrological and climate models is known to introduce considerable uncertainty in the predictions and projections of water availability. During the critical snowmelt period, the main challenge in snow modeling is that net radiation is spatially highly variable for a patchy snow cover, resulting in large horizontal differences in temperatures and heat fluxes. These differences can drive advection of turbulent heat from the snow free areas to the snow patches, potentially enhancing the melt rates at the leading edge and increasing the variability of subgrid melt rates. To get more insight in these processes, we examine the melt along the upwind and downwind edges of a 50 meter long snow patch in the Finseelvi catchment, Norway, and try to explain the observed behaviour with highly idealized simulations of heat fluxes and air movement over patchy snow. The melt of the snow patch was monitored from 11 June until 15 June 2019 by making use of height maps obtained through the photogrammetric Structure-from-Motion principle. A vertical melt of 23 ± 2.0 cm was observed at the upwind edge over the course of the field campaign, whereas the downwind edge melted only 3 ±  0.4 cm. When comparing this with meteorological measurements, we estimate the turbulent heat fluxes to be responsible for 60 to 80 % of the upwind melt of which a significant part is caused by the latent heat flux. The melt at the downwind edge approximately matches the melt occurring due to net radiation. To better understand the dominant processes, we represented this behaviour in idealized direct numerical simulations, which are based on the measurements on a single snow patch by Harder et al. (2017) and resemble a flat patchy snow cover with typical snow patch sizes of 15, 30 and 60 m. Using these simulations, we found that the reduction of the vertical temperature gradient over the snow patch was the main cause for the reductions in sensible heat over distance from the leading edge, independent of typical snow patch size. Moreover, we observed that the sensible heat fluxes at the leading edge and the decay over distance were independent of snow patch size as well, which resulted in a 15 % and 25 % reduction in average snowmelt for respectively a doubling and quadrupling of typical snow patch size. These findings lay out pathways to include the effect of local-scale heat advection based on the typical snow patch size in large-scale hydrological and climate models to improve snowmelt modelling.