An Evaluation of Precipitation and Microphysics Forecasts during OLYMPEX

Although substantial progress has been made, the accurate prediction of precipitation in complex terrain continues to be a challenge for numerical models. In the Olympic Mountains of Washington State, the variety of wintertime frontal systems that impact the complex topography of the region provides an ideal testbed for current microphysical parameterization schemes. We evaluate WRF precipitation and microphysics forecasts during the OLYMPEX field campaign of 2015-16, with emphasis on the atmospheric river events of Nov. 12-15 and Nov. 16-19. A variety of microphysics schemes are tested, including Goddard, Milbrandt 2-moment, P3, SBU-YLin, Thompson, WSM5, WSM6, and WDM5. Attention is also given to the influence of boundary-layer (PBL) scheme on these forecasts, with ACM2, MRF, MYJ, MYNN3, and YSU tested. 

Results indicate that higher-resolution (1.33 km vs. 36, 12, or 4 km) WRF simulations are more skillful, but do tend to underpredict precipitation along the windward slopes of coastal terrain. Resolution matters for resolving topographical complexities, but forecast performance does not substantially differ based on elevation. Finally, we analyze model performance along the heavily instrumented Quinault River. Orographic enhancement is noted, with pre-frontal regimes experiencing underprediction, while postfrontal periods are overpredicted. Forecast rain rates and cross-sections of microphysical species vary greatly among the microphysics and PBL schemes, especially snow, cloud ice, and graupel.