Evaluating the Effects of the Pacific Decadal Oscillation on Winter Precipitation in the Cascades Using a Mixed-Physics WRF Ensemble

In most of Washington and Oregon, mountain snowpack stores water to be available through spring and early summer, when water demand in the region is at its highest. Therefore, understanding the numerous factors that influence winter precipitation variability is a key component in water resource planning. This project examines the effects of the Pacific Decadal Oscillation (PDO) on winter precipitation in the Cascades using the WRF-ARW regional climate model. WRF simulations were for two ten-member ensembles, one for positive PDO Decembers, and one for negative PDO Decembers. WRF output was compared to both station and gridded observational data. Results indicate that elevations greater than 1000 m receive decreased total precipitation under the negative PDO, while elevations below 500 m receive increased total precipitation. A significant element of this work was evaluating the many options that WRF-ARW provides for representing sub-grid scale cloud microphysical processes. Because the “best” choice of microphysics parameterization can vary depending on the application, this project also seeks to determine which option leads to the most accurate simulation of winter precipitation (as compared to observations) in the complex terrain of the Pacific Northwest. As RCMs tend to do, WRF over-predicts mean total precipitation compared to observations, but the double-moment microphysics schemes over-predict to a lesser extent than the single-moment scheme.