Orographic Precipitation Response to Microphysical and Environmental Perturbations for Idealized Moist Nearly Neutral Flow

Orographic precipitation plays a vital role in providing water resources to mountain communities. The precipitation type, amount, and location are determined through complex interactions between cloud microphysics and environmental conditions. Previous studies have explored the sensitivities to these factors through "one-at-a-time" perturbations, where one parameter is perturbed while keeping the rest constant. Although this method provides information on parameter sensitivity, it is limited, and perhaps misleading, as the experiments are performed with respect to a constant default value for other input factors other than the parameter being varied. This study explores the sensitivity of orographic precipitation to changes in microphysical (MP) and environmental (ENV) parameters using the Morris screening method, which is a robust statistical tool allowing for simultaneous perturbation of numerous parameters. This method is applied for the first time to the study of orographic precipitation. Idealized simulations of quasi-2D moist nearly neutral flow representative of an atmospheric river over a bell-shaped mountain are performed and the controls on surface precipitation are analyzed. Results show MP parameter perturbations can produce changes in precipitation similar to those caused by ENV parameter perturbations. The most influential parameters can be either ENV or both ENV and MP, depending on the location over the mountain. Experiments testing the impact of different upstream environments (i.e., temperature profiles with high versus low freezing level and slow versus fast wind speed profiles) on MP parameter sensitivities and process-based analysis of influential parameters will also be presented. Results from this study have implications for ensemble forecasting, data assimilation, and orographic precipitation predictability.