Evaluating Radar-Derived Microphysical Processes Near Terrain during OLYMPEX

There is mounting evidence of major WRF model microphysical biases along coastal regions and windward slopes of the Pacific Northwest, particularly with regard to warm rain microphysics. Model biases include rain drop size distributions (DSDs) characterized by larger but fewer drops. Precipitation along this coastal/windward zone has also been underpredicted for at least the last decade. This study utilizes observations collected during the OLYMPEX field campaign over Washington State during the Winter of 2015-16. From surface observations of DSDs, several radar retrievals were developed to produce estimations of median volume diameter (D0), liquid water content (LWC), and rain rate (RR). These quantities were then used to produce estimates of warm rain microphysical processes using Kessler-style equations. Results demonstrate significant differences between radar-derived and WRF-simulated microphysics, especially with regard to processes of rain-cloud collection and evaporation. These radar-derived processes are also used in WRF as an attempt to improve model biases in these coastal and windward regions.