Improvement of microphysics schemes for a warm-sector heavy precipitation over South China

Abstract

Numerical prediction of warm-sector heavy precipitation under weak synoptic-scale forcing in South China remains a challenging problem. In this study, the simulation capabilities of four microphysics schemes (WSM6, Thompson, Thompson aerosol-aware, and Morrison) for the heavy rainfall event that occurred during 10–11 May 2022, which featured both a coastal warm-sector rain belt and an inland frontal rain belt, have been evaluated and improved by using polarimetric radar and 2DVDs. The results showed that four schemes effectively reproduced the coastal and inland heavy precipitation amounts but exhibited poor performance in describing raindrop size and number concentration, with noticeable differences among these schemes. Compared to observations, the microphysics schemes tended to produce raindrops with larger size and fewer number concentration. By incorporating the observed relationship between rainwater and generalized intercept parameter into the WSM6 scheme, the simulated raindrop size and number concentration were optimized with real-time diagnosis of raindrop intercept parameter. For the Thompson and Morrison double-moment schemes, modifying the diameter threshold parameter in raindrop self-collection process to enhance raindrop breakup efficiency was the most direct and effective method for improving simulation. Even though the impact of vertical wind shear on raindrop breakup was considered here, there remained a discrepancy between the simulated and observed raindrop sizes and number concentrations. Therefore, the reasons for adjusting this threshold parameter were still unclear. Additionally, using ECMWF-CAMS aerosol reanalysis data as input for Thompson aerosol-aware scheme showed a better representation of aerosol spatial distribution, thereby improving precipitation distribution, especially for the inland rain belt.