Microphysical mechanisms of stratiform precipitation with different radar reflectivity profiles in South China: Insights from dual-frequency radar observations

Abstract

Investigating radar reflectivity profiles below the freezing level helps improve surface rainfall estimation. In South China, stratiform rain is common, but the microphysical processes and key factors behind its vertical structure variations are still unclear. Based on ground-based dual-frequency radar observations from Longmen in South China, this study categorizes stratiform precipitation into three types: growth, stable, and evaporative, and examines their formation mechanisms. The results indicate that growth-type precipitation is primarily driven by weak updrafts (0 $\sim$ 0.5 m/s), which prolong the residence time of raindrops in the warm cloud layer (temperature above 0 °C), enhancing coalescence growth and leading to increased reflectivity to ground. Stable-type precipitation occurs in a high-humidity environment ($>90\text{\%}$), which suppresses raindrop evaporation, resulting in minimal vertical variation in the reflectivity profile. In contrast, evaporative-type precipitation is closely associated with lower relative humidity ($70\text{\%}\sim 90\text{\%}$), with a pronounced decrease in reflectivity. Although both stable and evaporative precipitation are accompanied by downdraft (0 $\sim$ 0.5 m/s), the difference in relative humidity (maximum difference exceeds 20%) is the key factor driving the distinct reflectivity profile between the two types, while the influence of air motion is relatively secondary.