Numerical simulations of sheared conditionally unstable flows over a mountain ridge

In two recent papers, the authors performed numerical simulations with a three-dimensional, explicitly cloud-resolving model for a uniform wind owing past a bell-shaped ridge and using an idealized unstable (Weisman-Klemp) sounding with prescribed values of the relevant parameters. More recently, some ob-served cases of orographically forced wind proles were analyzed, showing that, in order to reproduce larger rainfall rates, it was necessary to initialize the sounding with low-level ow toward the mountain with weak ow aloft (as observed). Additional experiments using the Weisman-Klemp sounding, but with nonuniform wind proles, are performed here to identify the conditions in which the presence of a low-level cross-mountain ow together with calm ow aloft may increase the rain rates in conditionally unstable ows over the orography. The sensitivity of the solutions to the wind speed at the bottomand the top of a shear layer and the effect of different mountain widths and heights are systematically analyzed herein. Large rainfall rates are obtained when the cold pool, caused by the evaporative cooling of rain from pre-cipitating convective clouds, remains quasi stationary upstream of the mountain peak. This condition occurs when the cold-pool propagation is approximately countered by the environmental wind. The large pre-cipitation amounts can be attributed to weak upper-level ow, which favors stronger updrafts and upright convective cells, and to the ground-relative stationarity of the cells. This solution feature is produced with ambient wind shear within a narrow region of the parameter space explored here and does not occur in the numerical solutions obtained in the authors' previous studies with uniform wind proles.