Evaluating Cloud Microphysical Schemes in Simulating the Mixed-Phase Microphysics of a Winter Storm Using Field Campaign Measurements

This study evaluates single- and double-moment cloud microphysical schemes within the NASA-Unified Weather Research and Forecasting (NU-WRF) model system in simulating the evolution of a warm frontal band observed during the Global Precipitation Mission Cold-season Precipitation Experiment (GCPEx), which occurred from 17 January to 29 February 2012 in Ontario, Canada. Field measurements revealed that the warm frontal passage on 18 February was rather complex as mixed-phase ice processes led to the development of mesoscale precipitation bands and heavy snowfall. Model forecasts can vary greatly for these mixed-phase precipitation events due to the large uncertainties among the cloud microphysical schemes. Thus, we need to better understand why the various microphysical assumptions within each scheme lead to such large differences in model forecasts for mixed-phase precipitation events.

For this study, three different NU-WRF simulations are conducted using the same model setup and configuration with the exception of the cloud microphysics scheme which varies between the WRF single-moment 6-class scheme (WSM6), Morrison double-moment scheme, and the new NASA single-moment Goddard 4ICE microphysics scheme. A fourth simulation is also conducted where we implement and evaluate the new double-moment Predicted Particle Properties (P3) scheme within the NU-WRF model system. We use a triple nested domain configuration centered over the field study site with a grid spacing of 1 km in the inner most nest. Model output for the inner most nest of each simulation is evaluated against in situ aircraft and ground-based measurements during GCPEx. The field measurements indicated that the warm frontal passage was well simulated by each microphysical scheme when dry snow and minimal cloud water was observed early on in the event. However, the schemes begin to diverge when the mixed-phase precipitation processes become dominant. We closely examine the period when the schemes begin to diverge by comparing the model output to aircraft measurements of ice and water content, number concentration, and particle size information.