Evaluating Cloud Microphysical Schemes in Simulating Orographic Precipitation Using Intensive OLYMPEX field instrumentation

This study evaluates the bulk and bin cloud microphysical schemes within the Weather Research and Forecasting (WRF) model system in simulating the orographic precipitation during the NASA Precipitation Measurement Mission (PMM) Olympic Mountain Experiment (OLYMPEX). An intensive suite of satellite, ground-based, and in situ instruments collected detailed measurements during numerous orographic precipitation events impacting the Pacific Northwest throughout OLYMPEX campaign from November 2015 – February 2016. This robust dataset provides an opportunity to understand the various orographic precipitation structures and associated microphysics for the precipitation events during OLYMPEX, which can be effectively used for validating high resolution model simulations. Previous field experiments over the Pacific Northwest (e.g., IMPROVE) 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, relatively large microphysical uncertainties and errors were associated with the snow and cloud water distributions. OLYMPEX provides an opportunity to diagnose and improve the microphysical issues in the more advanced microphysical schemes within the Weather Research and Forecasting (WRF) model.

This talk will focus on a few heavy precipitation events associated with atmospheric rivers (ARs) during 12-13 November and 8-9 December, along with a moderate precipitation associated with a strong baroclinic system on 3 December 2015. We evaluate the impact of ice microphysical processes (deposition, riming, and melting) on cloud water accretional growth and precipitation characteristics during these events using surface gauges, ground radars (NPOL and DOW), in situ aircraft (Citation), and satellite sensor measurements (GMI and SSMIS). The WRF was nested down to 1-km grid spacing using the Global Forecast System (GFS) analyses for initial and boundary conditions for a relatively short 36-h simulation. Four unique bulk microphysical schemes were evaluated, including the predicted particle properties (P3) scheme, Thompson, YLin-Stony Brook, and Morrison schemes. We also conduct WRF simulations using the Hebrew University spectral bin microphysical (HUJI) scheme to help identify issues in the bulk parameterization schemes. The P3 scheme predicted a layer of light to moderate rimed ice particles with faster fallout speeds that led to the development of a well-defined melting layer, which generally agreed with radar and aircraft measurements. As a result, P3 produced higher precipitation rates and accumulations that were in overall closer agreement than the other schemes. Conversely, the MORR scheme overpredicted snow mass while underpredicting graupel/rime mass, which led to a weak melting layer and a general underprediction in precipitation along the windward slopes. The HUJI scheme encountered similar issues as MORR, which limited its use for improving the bulk microphysical schemes. Our model results highlight the importance of both the cold and warm rain processes on simulating the orographic precipitation for these events.