Moist dynamics and orographic precipitation

Abstract

Uniformly stratified moist flow over a Gaussian-shaped circular mountain is investigated using a nonhydrostatic mesoscale model. The focus is the interaction between flow stagnation and orographic precipitation. Two closely related issues are addressed: the effect of condensation and precipitation on mountain flow stagnation, and the influence of flow blocking and latent heat on upslope precipitation. It is demonstrated that latent heat release and precipitation can significantly delay the onset of mountain flow stagnation. The dynamical and thermodynamical nature of this modification can be qualitatively understood using the moist stability concept. However, due to the vertical variation of the moist stability, it is not possible to define a single nondimensional mountain height to describe the general nonlinearity of moist orographic flow. The effect of flow blocking and splitting on the intensity and distribution of orographic precipitation is found to be significant. For low mountains, the upslope ascent dominates and the precipitation intensity is roughly proportional to the mountain height and windspeed as predicted by both a slab model and the mesoscale model. For high mountains, this relationship breaks down because the mountain lift effect is reduced as the low level moist flow passes around the peak. An arc-shaped precipitation band forms further upstream of the peak where the terrain slope is gentle, associated with the secondary circulation forced by the upstream flow blocking/reversal. The removal of latent heat processes leads to reduced upslope lift and enhanced windward blocking, thereby reducing the maximum precipitation rates and increasing the precipitation area.