Two-Dimensional Simulations of the Factors Influencing Orographic Precipitation: I. Basic Flow

Study of orographic precipitation formation and evolution in terms of orographic forcing and atmospheric instabilities poses an interesting, yet not well understood, problem in mesoscale meteorology. In this study, we are particularly interested in exploring the relative roles played by the convective available potential energy (CAPE) and basic wind speed (U) on orographic precipitation. In order to understand the fundamental dynamical and physical processes and their interactions, a series of two-dimensional numerical experiments are performed using the ARW model. We focus on the parameter space of (U, CAPE) for a conditionally unstable flow over an idealized bell-shaped ridge. Time-distance evolution analysis of precipitation reveals qualitative agreements with the regime diagram conceptualized in Chen and Lin (2005, JAS). Flow with a deep convective system propagating upstream (Regime I) is distinctly observed under high CAPE and low U conditions. Although other regimes are apparent, the classification is not as clear, mainly due to localized precipitation over the ridge. Large precipitation accumulation is observed when both CAPE and U are high. However, precipitation maximum is more sensitive to the variations in U, while CAPE is low. When U is high (>20 m/s), the variations in CAPE have little or no impact in driving the precipitation maxima. The mechanisms behind these control parameters are used to explain extremely heavy precipitation associated with tropical cyclones over mesoscale mountains. In addition to the idealized setup, realistic terrain and environment included simulations will be performed. By combining another key control parameter, ridge aspect-ratio (h/a), results from the sensitivity analyses of orographic precipitation and its associated efficiency will also be presented.