Factors Influencing Orographic Precipitation for Uniform Flow over a Two-Dimensional Mountain: Part I. Basic Flow Speed

A series of systematic two-dimensional numerical experiments have been conducted to investigate combined effects of dynamical, thermodynamical and microphysical processes on orographic precipitation (OP) with varying incoming basic flow speed (U) in a conditionally unstable uniform flow. In the first part of this study, the three moist flow regimes that were identified in Chu and Lin have been reproduced using the Cloud Model 1 (CM1). It is found that, the transient regime (Regime II), which is the stationary convective precipitation over the mountain is sensitive to U as well as model settings. In addition, Regime IV is identified with stationary stratiform precipitation over the mountain, without having any significant mid to upper-level downstream wave breaking when U exceeds 35 m/s. 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 (within 6 to 16 m/s), particularly when the density current is balanced or dominated by the orographic and environmental forcings. Beyond this regime, convection is inhibited as advection becomes very strong, thus leading to stratiform type precipitation. In the second part of this study, Chu and Lin's study is extended to very high Froude number flows comparable to category 2-3 hurricane wind speeds (40 to 50 m/s). It is noted that the total precipitation maximum is shifted closer to the mountain peak as the U increases. A weak bi-modal precipitation distribution is obtained on the downwind side of the mountain after 12 hours of the simulation with a very high wind speed. 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 surface precipitation.