Evaluation and Modification of Microphysics Schemes on the Cold Pool Evolution for a Simulated Bow Echo in Southeast China

The microphysical processes were found to be vital in facilitating the system evolution for a merger-formation bow echo (MFBE) in southeast China, where the reinforced precipitation enhanced the cold pool strength via evaporation cooling. However, current numerical model failed to accurately perform such processes, suggesting the large uncertainties for microphysical schemes in simulating MFBE events in southeast China. In this study, three microphysics schemes including Thompson (THOM), Morrison (MORR), and Weather Research and Forecasting Double-Moment 6-Class (WDM6) schemes were evaluated by comparing against polarimetric observations and Variational Doppler Radar Analysis System (VDRAS) analyses. The three schemes captured the basic kinematic structures for this MFBE event after assimilating radar radial velocities, but all underpredicted the cold pool strength by ∼25%. Particularly, THOM produced the best raindrop size distributions (DSDs) and precipitation pattern, and the larger raindrop size bias and the weak cold pool strength were owing to the relatively low rain breakup efficiency and inefficient rain evaporation, respectively. By decreasing the cutoff diameter of rain breakup parameterization from the default 1.6–1.2 mm (i.e., increasing breakup efficiency) and increasing evaporation efficiency by threefold in THOM, the simulated DSDs and precipitation were greatly improved, and the cold pool strength was significantly increased from 77% to 99% compared to that in VDRAS analyses. This study illustrated a plausible approach of combining polarimetric radar retrievals and VDRAS analyses as bases to adjust THOM default settings in simulating a MFBE event in southeast China with physical characteristics more consistent with observations. Since microphysical processes vary from convective organizations and climate regions, it is recognized more cases studies are needed in the future to examine the validity and approach in this study to improve simulations and predictions of MFBEs in southeast China.