An Independent Evaluation of Frozen Precipitation from the WRF model and PRISM in the Olympic Mountains for WY 2015 and 2016

Abstract

Estimates of precipitation from the Weather Research and Forecasting Model (WRF) and the Parameter-elevation Relationships on Independent Slopes Model (PRISM) are widely used in regions of complex terrain to obtain spatially consistent precipitation data. We evaluated the ability of both WRF and PRISM to estimate frozen precipitation using a hydrologic model (SUMMA) and a unique set of spatiotemporal snow depth observations collected for the OLYMPEX ground validation campaign during water years (WY) 2015 and 2016. We found that when WRF precipitation was partitioned with the commonly used linear-partitioning scheme based on wet bulb temperature (WRFLP) that its estimation of frozen precipitation was biased low on average. However, we found that when SUMMA was allowed to partition WRF total precipitation based on WRF’s microphysical scheme (WRFMPP), simulations of snow depth were near equal or better than PRISM. WRFMPP and PRISM had unbiased estimates of snow depth in WY 2016, but both simulated errors in snow depth of up to ~ 1 m (~40-50 cm of SWE) on an annual basis at a few locations. In the winter of WY 2015, which was abnormally warm by about 2.2°C (1 November – 31 March), PRISM was unbiased, and WRFMPP over predicted annual snowfall by ~ 20% on average but showed a similar mean absolute difference to PRISM. Lastly, we hypothesize that PRISM’s rain shadow has too sharp of a gradient between the windward and leeward side of the Olympic Mountains and that the coastal proximity and topographic position weights in PRISM may shift estimates of total annual precipitation too far west of the crest.