LOCAL ADVECTION OF MOMENTUM, HEAT, AND MOISTURE DURING THE MELT OF PATCHY SNOW COVERS

A numerical atmospheric boundary layer model, based on higher-order tu rbulence closure assumptions, is developed and used to simulate the lo cal advection of momentum, heat, and moisture during the melt of patch y snow covers over a 10-km horizontal domain. The coupled model includ es solution of the mass continuity equation, the horizontal and vertic al momentum equations, an E-epsilon turbulence model, an energy equati on, and a water vapor conservation equation. Atmospheric buoyancy is a ccounted for, and a land surface energy balance model is implemented a t the lower boundary. Model integrations indicate that advective proce sses occurring at local scales produce nonlinear horizontal variations in surface fluxes. Under conditions of the numerical experiments, the energy available to melt snow-covered regions has been found to incre ase by as much as 30% as the area of exposed vegetation increases upwi nd of the snow cover. The melt increase is found to vary in a largely linear fashion with decreasing snow-covered area for snow-covered area s greater than 25% and in a strongly nonlinear fashion below that valu e. Decreasing the ratio of patch size to total area, or increasing the patchiness, of the snow cover also leads to nonlinear increases in th e energy available to melt the snow. In the limit of a snow cover comp osed of Small patches, melt energy is found to increase linearly as th e fractional snow-covered area decreases. In addition, for the purpose of computing grid-average surface fluxes during snowmelt in regional atmospheric models, the results of this study indicate that separate e nergy balance computations can be performed over the snow-covered and vegetation-covered regions, and the resulting fluxes can be weighted i n proportion to the fractional snow cover to allocate the total energy flux partitioning within each surface grid cell.