Comparison of Simulated Orographic Precipitation Structures Using Different Microphysical Schemes With OLYMPEX Field Program Observations

The primary goal of the Olympic Mountains Experiment (OLYMPEX) from November 2015 to February 2016 in Washington State was to verify and validate satellite measurements of precipitation from the Global Precipitation Measurement (GPM) satellites. However, this dataset also provides an opportunity to observe various orographic precipitation structures and associated microphysics and compare them with high resolution models. Previous field experiments over the Pacific Northwest (e.g., IMPROVE in early 2000s) illustrated the importance of mountain gravity waves in modifying the precipitation distribution from cloud water generation and riming above the narrow windward ridges to enhanced snow generation aloft over the broader windward slope. During IMPROVE there were relatively large microphysical uncertainties and errors associated with the snow and cloud water distributions. OLYMPEX offers an opportunity to revisit some of these issues with some more advanced microphysical schemes in the Weather Research and Forecasting (WRF) model.

For this presentation three cases will be highlighted (13 November 2015, 17 November 2015, and 8-9 December 2015). All three events were heavy precipitation events (100-300 mm over lower windward/southwest slope) associated with an atmospheric river extending from the eastern Pacific to the Pacific Northwest. Freezing levels were relatively high (~2 km ASL), so these events were used to evaluate the riming and cloud water accretional growth in the various WRF microphysical schemes. The WRF was nested down to 1-km grid spacing using the Rapid Refresh (RR) analyses for initial and boundary conditions for a relatively short 36-h simulation. Four different microphysical schemes were evaluated using surface gauge, ground radars (DOW, NPOL, and WSR-88D), and aircraft (Citation): the new predicted particle properties (P3) scheme, WSM6, Thompson, and YLin-Stony Brook schemes. This presentation will highlight some of windward flow and precipitation structures between the observations and WRF. The importance of properly simulating the partition between rimed and unrimed snow will be discussed as well as potential challenges in simulating the upstream moisture flux as demonstrated by using a few other analyses for initial/boundary conditions.