Moist dynamics and orographic precipitation

Abstract

Uniformly stratified moist flow over a Gaussian-shaped circular mountain is investigated using anonhydrostatic mesoscale model. The focus is the interaction between flow stagnation and orographicprecipitation. Two closely related issues are addressed: the effect of condensation and precipitation onmountain flow stagnation, and the influence of flow blocking and latent heat on upslope precipitation. Itis demonstrated that latent heat release and precipitation can significantly delay the onset of mountainflow stagnation. The dynamical and thermodynamical nature of this modification can be qualitativelyunderstood 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 nonlinearityof moist orographic flow. The effect of flow blocking and splitting on the intensity and distribution oforographic precipitation is found to be significant. For lowmountains, the upslope ascent dominates andthe precipitation intensity is roughly proportional to the mountain height and windspeed as predicted byboth a slab model and the mesoscale model. For high mountains, this relationship breaks down becausethe mountain lift effect is reduced as the low level moist flow passes around the peak. An arc-shapedprecipitation band forms further upstream of the peak where the terrain slope is gentle, associatedwith the secondary circulation forced by the upstream flow blocking/reversal. The removal of latentheat processes leads to reduced upslope lift and enhanced windward blocking, thereby reducing themaximum precipitation rates and increasing the precipitation area.