Sensitivity of flow dynamics and orographic precipitation to changing ambient conditions in idealised model simulations

Abstract

Idealised numerical simulations using the non-hydrostatic weather prediction model of the Consortium for Small-scale Modeling (COSMO) in a three-dimensional (3D) configuration were conducted to investigate the relationship between ambient conditions, flow characteristics, and orographic precipitation patterns. By changing the model input parameters of wind speed, static stability, temperature, and relative humidity, different flow effects from conditionally unstable flow to upstream deceleration are considered. It is shown that latent heat release significantly delays the onset of flow stagnation, which can be understood using the moist stability concept. However, due to the vertical variations of saturated and unsaturated layers, it is not possible to apply this concept for determining gravity waves. Both the drying ratio, expressing the conversion of the upstream moisture flux into precipitation, and the location of the precipitation maxima can be described to a certain extent by the saturated nondimensional mountain height, M m = N m H/U, where H is the mountain height, N m is the saturated stability, and U is the undisturbed wind speed. In the flow around regime, the precipitation maxima are associated with the extended gravity wave and are located downstream of the mountain crest. With increasing direct mountain overflow (decreasing M m ), the precipitation maxima are shifted to a location upstream of the crest. The transition from purely stratiform precipitation to embedded convection occurs abruptly when M m becomes imaginary, indicating conditional instability.