Winter Drying Ratio across the Sierra Nevada from Regional Climate Model Simulations

Seasonal snowfall in the Sierra Nevada of California is critical to the water supply of the region. Orographically-induced precipitation is the major contributor to the annual accumulation of snow each year. The physics of orographic precipitation is well understood – mountains pull moisture out of the atmosphere when air lifts, cools, and precipitates. Here we consider the ability for the Weather Research and Forecasting (WRF) regional climate model to simulate orographic precipitation at a range of resolutions across the Sierra Nevada. We run WRF over water year 2009, an average snow accumulation year, with 27 km, 9 km, and 3 km grids. At each resolution, we consider winter atmospheric moisture fluxes on the windward and the lee side of the mountain range to calculate the drying ratio of the Sierra Nevada, i.e. what percentage of atmospheric moisture is precipitated over the mountain due to orographic life. Previous studies report drying ratios of 32-50% over mountain ranges worldwide, though most studies consider rainfall events. We focus on the drying ratio averaged over the entire winter, instead of only one precipitation event as previous work has done, though we do consider the main precipitation events during the water year, too. The drying ratio can additionally be used to describe how mountain ranges upstream may affect those downstream (e.g. moisture movement over the Sierra Nevada in California affecting that of the Rocky Mountains in Colorado). By comparing the three WRF resolutions, we demonstrate the importance of resolution for reliable simulations in regions with complex topography and we suggest that future mountain range-scale studies utilize fine resolution (< 9 km) model estimates.