Simulations of nearly moist neutral flow past a two-dimensional ridge

Although a fairly common atmospheric condition in orographic-rain scenarios, there is relatively little known about moist neutral flow over a ridge from theory and modeling. We will present numerical simulations of the orographic-flow modification occurring for a two-dimensional moist 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 < ambient wind velocity/static stability, is always met (since by definition the moist static stability is approximately zero). However, for reasons to be discussed, 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. In this presentation we use the numerical solutions as a guide to the understanding of this outwardly simple problem of moist neutral flow over a two-dimensional ridge.