Factors Influencing Orographic Precipitation for Uniform Flow over an Idealized Mountain: Variations in Basic Wind Speed and CAPE

A series of systematic two and three-dimensional (2D and 3D) numerical experiments with Cloud Model 1 (CM1) have been conducted to investigate dynamical and physical processes and their interactions on orographic precipitation (OP). Both basic wind speed (U) and convective available potential energy (CAPE) in a conditionally unstable uniform flow are varied. With a fixed high CAPE, in addition to the three moist flow regimes identified in Chu and Lin (2000), a new flow regime (Regime IV) is found. Regime IV is characterized with stationary convective/stratiform precipitation over the mountain, without having any significant mid to upper-level downstream wave breaking when U exceeds a certain threshold. For higher wind regimes, convection is weakened as advection becomes very strong, thus leading to more stratiform precipitation. At higher wind speeds, the total precipitation maximum is shifted closer to the mountain peak. Time and spatial accumulated distributions of hydrometeors indicate that the most dominant microphysical processes are melting of graupel, auto-conversion of cloud water and accretion in producing precipitation. With a low CAPE, the OP increases as U increases due to strong orographic forcing. An unusual OP regime transition occurs at high wind speeds. Specifically, at a fixed high wind speed, OP reduces when CAPE increases. This is explained partially by the increase of evaporative cooling below cloud base, but, more importantly, by the lack of three-dimensionality and turbulence in the 2D simulations with planetary boundary layer (PBL) parameterization. This is verified by performing three-dimensional, high-resolution large-eddy simulations (LES's), which show a consistent increase in OP with increasing wind speed for both low and high CAPE environments. Much larger precipitation accumulations are resulted in low CAPE high wind simulations compared to high CAPE, as the convective system transitions to stratiform, similar to that reported in Chen and Lin.