Validation of Simulated Orographic Precipitation Structures in OLYMPEX Using Different Microphysical Schemes

The Olympic Mountains Experiment (OLYMPEX) from November 2015 to February 2016 in Washington State 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 flow blocking in shifting the precipitation upstream of the barrier and mountain gravity waves in modifying the precipitation distribution from cloud water generation and riming above the narrow windward ridges. 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, Morrison, Thompson, and YLin-Stony Brook schemes. For the 12-13 November 2015 event, relatively large low-level stability early in the event resulted in flow splitting and maximum precipitation immediately west of the lower windward slope. As the stability decreased the maximum precipitation shifted over the Olympics higher terrain. For all three cases the WRF BMPs underpredicted the precipitation over the lower windward slope by 10-30%. Some of this was the result of the precipitation ending too soon, but other microphysical reasons will be explored. For all three cases, the new WRF P3 scheme more realistically predicted the amounts and precipitation rate.  The Morrison scheme has more (too much) snow aloft, and less riming and precipitation fallout over windward slope.