Theory, Observations, and Predictions of Orographic Precipitation

There have been rapid advances in the understanding of orographic precipitation during the past two decades given the advent of high resolution mesoscale modeling and numerous field studies around the world. This chapter begins with introducing the fundamental ingredients, processes, and scaling parameters for orographic precipitation. If the low level air is stable, terrain flow blocking can occur, which shifts the precipitation enhancement upwind of the barrier. Low-level diabatic cooling from evaporation and melting can enhance the near-surface stability and flow blocking. In contrast, latent heating within the cloud reduces the effective stability and the potential for flow blocking, which in turn results in stronger vertical motions over the ridges and increases the riming and precipitation fallout. For less stable flows, there are mountain gravity waves over the mountain that can change the depth and upstream extent of the orographic cloud, and lead to precipitation enhancements over individual windward ridges. Barrier dimension is also important, with wide barriers favoring precipitation enhancement upwind of the crest and more efficient water vapor removal from the ambient flow. If the ridges are very narrow, the mountain waves decay with height and have less impact on the precipitation. For unstable flow, the upslope flow can trigger convective systems or quasi-stationary banded precipitation. The various orographic processes are illustrated using several field study and modeling results. The prediction of orographic precipitation is discussed using a 1.33-km grid spacing simulation and sensitivity results from a linear orographic precipitation model for a flooding event over southwest Washington in early November 2006. The chapter concludes with a summary and areas for future work.