On the Sensitivity of Downslope Windstorms to the Asymmetry of the Mountain Profile

The influence of terrain asymmetry on the development and strength of downslope windstorms was examined through the numerical simulation of three basic atmospheric configurations: 1) flow beneath a mean-state critical layer, 2) flow in the presence of breaking waves and 3) flow in a two-layer atmosphere without wave breaking or a mean-state critical layer. When a mean-state critical layer was present in the flow and the wind speed and stability beneath that critical layer were essentially constant, the maximum downslope wind speed was nearly independent of mountain asymmetry. Such insensitivity to mountain shape is consistent with hydraulic theory and supports the idea that there is a close mathematical analog between stratified flow beneath a mean-state critical layer and conventional shallow-water hydraulic theory. When downslope winds were generated by breaking waves, and the upstream stability and wind speed were constant with height, the dependence of lee-slope velocities on terrain asymmetry remained weak. When downslope winds were produced in a two-layer atmosphere without wave breaking or a mean-state critical layer, the flow exhibited a noticeable, but not dominating, sensitivity to mountain asymmetry. The preceding results were obtained from simulations without surface friction. When a surface friction parameterization was included in the numerical model, the sensitivity of the downslope wind speed to mountain asymmetry was significantly enhanced. It appears that in the surface friction simulations, the most significant shape parameter is not mountain asymmetry per se, but simply the steepness of the lee slope, with steep lee slopes being most favorable for strong winds.