Simulations of Moist Nearly Neutral Flow over a Ridge

Abstract

Although a fairly common atmospheric condition in orographic-rain scenarios, there is relatively little known about moist neutral flows over a ridge from theory and modeling. Presented in this paper are numerical simulations of the orographic-flow modification occurring for a two-dimensional moist nearly neutral flow over a ridge in the regime where the Coriolis force can be neglected. If an initially saturated moist neutral flow were to remain everywhere saturated as it flows over an obstacle, then the expected solution would be the linear solution because the condition for linearity (hill height less than the ambient wind velocity/static stability) is always met. However, for higher mountains, the solutions indicate the development of areas of unsaturated air, with correspondingly larger values of local static stability. This internal switching from small to large values of static stability is an inherent nonlinearity, which has far-reaching consequences for understanding the orographic-flow modification in this regime. The sensitivity of the solution to the mountain height and to the initial cloud water content is analyzed here. The authors find that the solutions fall into three basic categories. If the mountain height is small enough, a saturated flow can be maintained everywhere given sufficient initial cloud water; for tall mountains the atmosphere upwind of the mountain is maintained in a saturated state and transitions to an unsaturated downslope flow on the lee side, which has characteristics associated with downslope windstorms; for mountains of intermediate height, the solutions show the existence of an upwind-propagating disturbance that has the effect of desaturating the atmosphere above the mountain.