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

Abstract

Idealized numerical simulations of an unsaturated, conditionally unstable flow over a two-dimensional mountain ridge were used to study the effects of the moist Froude number (Fw) and the orographic aspect ratio of mountain height to width (h/a) on the propagation, cloud type, and pattern and amount of rainfall of orographically induced precipitation systems. For low Fw, the flow belonged to an upstream propagating flow regime (i.e., Regime I) and was insensitive to h/a. For large Fw, both Fw and h/a dictated the precipitation pattern and the nature of the convection. When Fw was fixed, the flow shifted toward a downstream propagating regime as h/a increased. This dependence on h/a at higher wind speeds appears to be linked to the advection time and cloud growth time. A slightly larger Fw was required for the regime transition to occur when the fixed aspect ratio was small. We also found that although the local maximum rainfall was not directly controlled by Fw, the total domain rainfall was sensitive to Fw, in particular for Regimes I and II (i.e., flow with a long-lasting orographic convective system over the mountain peak, upslope or lee slope) when the mountain half-width was fixed. On the other hand, for the unstable flow studied here, the total domain accumulated rainfall was not sensitive to h/a when Fw was fixed. It is suggested that flash floods may occur when quasi-stationary convective precipitation systems stay over the mountain area for flow Regime II or when abundant moisture is supplied for a significant period of time due to a low-level jet for flow Regimes III and IV, which are defined as flow with an orographic convective precipitation system over the mountain and a downstream propagating convective system and flow with an orographic stratiform precipitation system over the mountain and possibly a downstream propagating cloud system, respectively. In addition, local orographic rainfall from stratiform precipitation systems (i.e., Regime IV) can be as heavy as that from convective or mixed type precipitation systems belonging to Regime III.