Modelled sensitivity of the snow regime to topography, shrub fraction and shrub height

Abstract. Recent studies show that shrubs are colonizing higher latitudes and altitudes in the Arctic. Shrubs affect the wind transport, accumulation and melt of snow, but there have been few sensitivity studies of how shrub expansion might affect snowmelt rates and timing. Here, a blowing snow transport and sublimation model is used to simulate premelt snow distributions and a 3-source energy balance model, which calculates vertical and horizontal energy fluxes between the atmosphere, snow, snow-free ground and vegetation, is used to simulate melt. Vegetation is parametrized as shrub cover and the parametrization includes shrub bending and burial in winter and emergence in spring. The models are used to investigate the sensitivity of the snow regime in an upland tundra valley to varying shrub cover and topography. Results show that topography dominates the spatial variability of snow accumulation, which in turn dominates the pre and early melt energy budget. With topography removed from the simulations, modelled snow cover is uniform when there is no vegetation but increasing vegetation introduces spatial variability in snow accumulation which is then decreased as further increases in shrub cover suppress wind-induced redistribution of snow. The domain-averaged simulations of premelt snow accumulation also increases with increasing shrub cover because suppression of blowing snow by shrubs decreases sublimation. In simulations with topography, the increase in snow accumulation and its spatial variability with increasing vegetation is less marked because snow is also held in topography-driven drifts. With topography, the existence of wind-scoured snow-free patches at the onset of snowmelt causes exposed ground to contribute to the energy balance such that sensible, advective and radiative heat fluxes are higher than in the flat domain during this period. However, as snowmelt evolves, differences in the energy budget between runs with and without topography dramatically diminish. These results suggest that, to avoid overestimating the effect of shrub expansion on the energy budget of the Arctic, future large scale investigations should consider wind redistribution of snow, shrub bending and emergence, and sub-grid topography as they affect the variability of snowcover.