Effects of Moist Froude Number and Orographic Aspect Ratio on a Conditionally Unstable Flow over a Mesoscale Mountain

Idealized simulations for an unsaturated, conditionally unstable flow over a two-dimensional mountain ridge were performed to investigate how the unsaturated moist Froude number (Fw) and the orographic aspect ratio, which is defined as the ratio of mountain height to half-width (h/a), affect the propagation, cloud type and rainfall amount of orographically induced precipitation systems. This was tested through a series of idealized simulations using the Weather Research and Forecast (WRF) model wherein the basic uniform wind (i.e. Fw) and the mountain half-width (i.e. h/a) were varied. The simulation results indicate that for low Fw, the flow is insensitive to h/a and the flow is in an upstream propagating flow regime. For large Fw, both Fw and h/a dictate the precipitation pattern and the nature of the convection, with flow shifting more toward a downstream propagating regime with increasing h/a if Fw is fixed. This dependence on h/a for larger wind speeds appears to be linked to nonhydrostatic effects. With a fixed h/a, the flow shifts more toward downstream propagating regime with increasing Fw.