Simulations of moist nearly neutral flow over a ridge

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