Influence of Cold Pools Downstream of Mountain Barriers on Downslope Winds and Flushing

Abstract

The influence of cold pools downstream of mesoscale mountain barriers on downslope winds and flushing is investigated in this study by means of a numerical mesoscale model. The model is compared with existing analytical and numerical solutions. It is found that the numerical model produces phases and amplitudes of nonlinear mountain waves reasonably well. The solutions show that the structure of mountain waves can be modified greatly by the presence of the cold pool. When a cold pool is present downstream of the mountain, the development of a large amplitude mountain wave is inhibited. In the absence of surface heating, downslope winds associated with a mountain wave can be prevented from penetrating the cold pool to reach the surface and flush out the very stable cold air, particularly when the synoptic pressure gradient is oriented so as to continuously replenish the cold air. Results also suggest that shear-induced turbulent mixing at the top of the cold air has little effect on flushing. Based on the observations and the numerical results, in the absence of significant surface heating a favorable large-scale surface-pressure gradient force must be involved to remove the cold pool before the downslope winds can actually reach the surface.