Analysis of Orographically Enhanced Precipitation during an Atmospheric River Event for the OLYMPEX Field Campaign

Heavy, cold-season precipitation events in the Pacific Northwest are frequently driven by concentrated bands of moisture known as atmospheric rivers. The propagation of atmospheric rivers into areas of complex terrain has the potential to result in severe flooding, as strong flow perpendicular to the windward edge of a mountain range coupled with an influx of warm, moist air can significantly increase precipitation rates due to orographic lifting. These events pose both significant observational and modeling challenges. Surface and upper-air observations are hindered by the difficulties in obtaining valid data in mountainous regions, while models must rely on the use of bulk microphysical schemes (BMPs) to simulate sub-mesoscale ice particle processes. A significant atmospheric river event brought over 200 mm of rainfall to some parts of the Olympic Peninsula of Washington on November 16-17, 2015, during the OLYMPEX campaign, a collaborative field project which involved the deployment of a widespread observation network to examine precipitation processes in mountainous areas. As a result, the passage of a surface warm front, the landfall of the atmospheric river, the development of a narrow cold frontal rain band (NCFR), and the orographic modification occurring throughout the event were all observed in great detail. The event was also simulated with the Weather Research and Forecasting Model (WRF) to examine the strengths and weaknesses of different BMPs in cases of orographically enhanced precipitation. Results show that the Predicted Particle Properties (P3) scheme, which uniquely uses a single ice category, was able to resolve rimed ice particles significantly better than other BMPs. This resulted in a more accurate representation of orographic precipitation processes and thus closer precipitation totals to what was observed by the OLYMPEX network during the event.