Modelling fluxes of momentum, sensible heat and latent heat over heterogeneous snow cover

A boundary-layer model is used to simulate turbulent fluxes of momentum, sensible heat and latent heat over heterogeneous surfaces. Results for heterogeneous snow cover are compared with results for heterogeneous snow-free vegetation, and effective parameters characterizing the surfaces are discussed. A tile model, which calculates area-average fluxes from given area-average windspeeds, temperatures and humidities at a reference height in the atmosphere, is assessed in comparison with the boundary-layer model. The performance of the tile model is found to be degraded for surfaces with regions of both upward and downward sensible-heat flux, as can occur over heterogeneous snow cover. Better results can be obtained by using different reference heights for heat flux and momentum flux calculations.