Small-scale mechanisms of orographic precipitation enhancement

Dirk Cannon and Dan Kirshbaum

One of the most elusive goals in meteorology is the accurate prediction of precipitation amounts and distributions over complex terrain (orography). Because airflow in such regions is characterized by small physical and temporal scales that often cannot be captured in general circulation models (GCMs) or regional numerical weather prediction (NWP) models, these subgrid-scale processes are often parameterized. The success of these schemes (or lack thereof) is limited by the community's understanding of the physical processes themselves. Errors produced by inadequate parameterizations lead not only to poor local precipitation forecasts, but to atmospheric moisture anomalies that propagate around the globe.

The aim of this project is to improve orographic precipitation forecasts by enhancing the understanding of small-scale mechanisms that are not explicitly accounted for in today's GCMs. Among these mechanisms are shallow embedded convection, cloud billows that arise from shear instabilities, and small-scale variations of the terrain itself. Using a combination of observations and high-resolution numerical simulations, this project will quantify the impact of these processes and work toward developing parameterizations to improve their representation in GCMs.