Evaluating Warm and Cold Rain Processes in Cloud Microphysical Schemes Using OLYMPEX Field Measurements

Abstract

Field observations from the Olympic Mountain Experiment (OLYMPEX) around western Washington State during two atmospheric river (AR) events in November 2015 were used to evaluate several bulk microphysical parameterizations (BMPs) within the Weather Research and Forecasting (WRF) Model. These AR events were characterized by a prefrontal period of stable, terrain-blocked flow with an abundance of cold rain over the lowland region followed by less stable, unblocked flow with more warm rain, and a shift in the largest precipitation amounts to over the windward Olympic slopes. Our WRF simulations underpredicted the precipitation by 19%–36% in the Morrison (MORR) and Thompson (THOM) BMPs and 10%–23% in the predicted particle properties (P3) BMP, with the largest underpredictions over the windward slopes during the more convective, unblocked flow conditions. Several important processes related to the BMPs led to the differences in simulated precipitation. First, the prognostic single ice category parameterization in the P3 scheme promoted a more realistic evolution of rimed particles and larger cold rain production, which led to the lowest underpredictions in precipitation among the schemes. Second, efficient melting processes associated with the production of nonspherical ice and snow in the P3 and THOM BMPs, respectively, promoted a more realistic transition to rain fall speeds within the warm layer compared to the spherical snow assumption in MORR. Last, all BMPs underpredict the contribution of warm rain processes to the surface precipitation, particularly during the unblocked flow period, which may be partly explained by too weak condensational and collisional growth processes due to the neglect of turbulence parameterizations within the schemes.

Evaluating Simulated Raindrop Size Distributions and Ice Microphysical Processes With Polarimetric Radar Observations in a Meiyu Front Event Over Eastern China

“…Nowadays, polarimetric radars also play an important role in providing information of ice‐phase processes in convective systems. Based on the polarimetric radar data sets, uncertainties of simulating ice particles in different precipitation systems have been investigated (Barnes & Houze, 2016; Matsui et al., 2020; Naeger et al., 2020; Schrom & Kumjian, 2018). Barnes and Houze (2016) have evaluated four types of ice‐phase processes of three microphysics schemes utilized for tropical oceanic MCSs and found that none of the parameterizations predicted the correct microphysical spatial patterns.…”

A Double‐Moment SBU‐YLIN Cloud Microphysics Scheme and Its Impact on a Squall Line Simulation

“…Naeger et al. (2020) found that the P3 scheme improved the underestimated precipitation by giving a realistic representation of rimed ice and cold rain process based on the Olympic Mountain Experiment (OLYMPEX) field observations. Milbrandt et al.…”

“…Han et al (2019) indicated that the P3 scheme enhanced stratiform precipitation and gave a better agreement with observations. Naeger et al (2020) found that the P3 scheme improved the underestimated precipitation by giving a realistic representation of rimed ice and cold rain process based on the Olympic Mountain Experiment (OLYMPEX) field observations. Milbrandt et al (2021) indicated that the triple-moment P3 scheme had a better representation of the reflectivity and size of rimed ice using both a 1D kinematic model and 3D simulations of a supercell.…”

Modification of raindrop size distribution due to seeder–feeder interactions between stratiform precipitation and shallow convection observed by X‐band polarimetric radar and optical disdrometer

“…Zwiebel et al (2016) reported that the mass-weighted mean diameter of raindrops was smaller in the mountains than in plains. In the Olympic Mountains Experiment (OLYMPEX), extensive studies on precipitation over the mountains were conducted using disdrometers (Zagrodnik et al, 2019), ground-based and airborne radars (McMurdie et al, 2018;Zagrodnik et al, 2019), and numerical simulations (Morales et al, 2018;Naeger et al, 2020). In these studies, precipitation was strongly enhanced when the warm sector of extratropical cyclones passed over the mountains, and small-to medium-sized raindrops increased while the concentrations of large raindrops were varied.…”