Evaluating Simulated Microphysics in the Pacific Northwest: Evidence for a Warm-Rain Problem

This study evaluates moist physics in the Weather Research and Forecasting (WRF) model using observations collected during the Olympic Mountains Experiment (OLYMPEX; 2015-16) field campaign. Even though WRF using Thompson et al. (2008) microphysics was able to realistically simulate water vapor concentrations and synoptic conditions approaching the Olympic Mountains, there was underprediction of cloud water content and rain rates offshore and over western slopes of terrain. Simulated drop sizes are larger and rain drop concentrations much smaller than observed from surface disdrometers. Evaluations of WRF using the GPM satellite show that WRF produced too little cloud and rain water content below 2.5 km, with excessive snow above this altitude. Furthermore, analyzing synoptic midlatitude storm sectors suggests that warm sectors had the largest errors in cloud and rain water contents. Finally, a case study is examined from summer 2019 with very high melting levels and observed warm-rain conditions. Results from this case highlight a substantial lack simulated clouds in the low and middle levels of the atmosphere, with WRF producing too much ice cloud. These evaluations indicate a problem with warm-rain processes in the WRF model over the Pacific Northwest.