Factors Influencing Orographic Precipitation for Uniform Flow over a Two-Dimensional Mountain: Variations in Basic Flow Speed and CAPE

A series of systematic two-dimensional numerical experiments have been conducted to investigate dynamical and physical processes on orographic precipitation (OP). Variations in both basic flow speed (U) and convective available potential energy (CAPE) are considered in a conditionally unstable uniform flow. The three moist flow regimes that were identified in Chu and Lin have been reproduced using the Cloud Model 1 (CM1). In addition, Regime IV is identified with stationary convective/stratiform precipitation over the mountain, without having any significant mid to upper-level downstream wave breaking when U exceeds 35 m s-1. When U is pushed further, a stationary convective precipitation regime appears, however this might be due to growing numerical instabilities associated with high wind stress in the simulations. Larger precipitation accumulations are produced in Regime II (at 8 m s-1), particularly when the density current is balanced or dominated by the orographic and environmental forcings. Beyond this regime, 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. A weak bi-modal precipitation distribution is obtained on the downwind side of the mountain after 12 hours of the simulation. Time and spatial accumulated distributions of hydrometeors indicate that the most dominant microphysical processes are melting of graupel, auto-conversion of cloud water and accreation in producing precipitation. Preliminary sensitivity studies on CAPE, similar to the study of Chen and Lin, show differences in regime classification. As the updrafts are mainly driven by CAPE and convection is modulated by advection speed, larger precipitation accumulations tend to yield for larger CAPE and smaller U.