Systematic Bias in the Prediction of Warm‐Rain Hydrometeors in the WDM6 Microphysics Scheme and Modifications

Abstract

Modifications are made to the calculation of warm‐rain hydrometeor number concentrations of the Weather Research and Forecasting double‐moment six‐class (WDM6) microphysics scheme. These modifications are related with cloud condensation nuclei activation, cloud droplet evaporation, and snow and graupel melting. The effects of the modifications are tested in a North China heavy rain event (Case01) as well as a short‐duration rain event in Hebei Province of China (Case02). The results show that the WDM6 scheme underestimates the cloud mixing ratios (Qc) and number concentrations (Nc), while it overestimates the rain mixing ratios (Qr) and number concentrations (Nr). Meanwhile, the WDM6 scheme overestimates the mass mean diameter of cloud droplets (Dmc) and underestimates that of raindrops (Dmr). Through microphysical budget analysis, it is found that the inappropriate calculations of cloud condensation nuclei activation and cloud evaporation are responsible for the underestimation of Nc and larger Dmc. The overestimation of the autoconversion rate resulting from larger Dmc plays a decisive role in the overestimation of Qr and Nr, as well as in the underestimation of Qc. The overestimation of raindrops produced by the melting of snow and graupel is another reason for higher Qr and Nr. By comparing with the airborne observations in Case02, it is found that the Nc and Dmc simulation are notably improved with the direct modifications of Nc. The Nr simulation is closer to the observation after the melting of snow and graupel are modified.

A Comparison of Spectral Bin Microphysics versus Bulk Parameterization in Forecasting Typhoon In-Fa (2021) before, during, and after Its Landfall

“…In addition, it is also notable in the vertical reflectivity profiles (Figure 8) that BULK scheme shows abnormally lower simulated values than the observed near the melting level, which can partially explain its poorer performances than BIN scheme in simulating warm rain processes. As reported in Lei et al [53], BULK scheme (WDM6 scheme) has systematic bias in the prediction of warm rain hydrometeors, and high concentration of small raindrops tends to appear near the 0 • C layer as validated against airborne observations. To confirm that, we give the number concentration of warm rain hydrometeors in typhoon simulations as shown in Figure 12.…”

“…To confirm that, we give the number concentration of warm rain hydrometeors in typhoon simulations as shown in Figure 12. One can notice that BULK scheme (WDM7 scheme) produces abnormally high raindrop number concentrations at an altitude around the melting level (Figure 12a), possibly due to the fast melting processes of snow, graupel, and hail [53]. Meanwhile, the cloud drop number concentrations of BULK scheme are significantly higher than that of BIN scheme (Figure 12b), probably because of the saturation adjustment strategy in BULK scheme.…”

Improved Weather Forecasting Using Neural Network Emulation for Radiation Parameterization

“…In this study, we considered operational configurations of KLAPS, such as the RRTMG-K radiation (Baek, 2017) and WRF double moment 7-Class (WDM7) microphysics (Bae et al, 2019), Shin and Hong planetary boundary layer (Shin & Hong, 2015), KIAPS Simplified Arakawa-Schubert (SAS) cumulus (Kwon & Hong, 2017), Unified Noah land surface model (Tewari et al, 2004), and revised MM5 Monin-Obukhov surface layer (Jiménez et al, 2012). The systemic biases for the WDM7 microphysics scheme, which were reported by Lei et al (2020), were corrected in this study. Although the KLAPS also includes a local data assimilation system, it was replaced with the use of European Centre for Medium-Range Weather Forecasts Reanalysis v5 (ERA5) data (Hersbach et al, 2020) in the WRF Preprocessing System.…”

Revision of WDM7 Microphysics Scheme and Evaluation for Precipitating Convection over the Korean Peninsula

“…The KF scheme is used only in the outer domain (9 km). The WDM7 cloud microphysics scheme [10] with the bug fix from Lei et al [32] is applied. The cloud microphysics processes in WDM7 are recoded to allow WDM7 to adopt the observed microphysical parameters during the ICE-POP 2018 field campaign.…”