Characteristics of raindrop size distributions in urban, mountainous and island areas over southeastern coastal China in summer

The southeastern coastal region of China exhibits complex and diverse topographic features, which significantly influence local meteorological conditions and precipitation patterns. In-depth research on microphysical characteristics of precipitation in this area enhances the understanding of regional precipitation mechanisms and provides critical theoretical support for weather forecasting and disaster prevention. However, systematic studies on raindrop size distribution (DSD) characteristics under different topographic settings in this region remain notably insufficient. Zhejiang Province, with its diverse terrain that includes mountains, islands, and plains with highly urbanized areas, offers an ideal setting for investigating DSD features in complex topographic environments. Hence, observations collected by Thies CLIMA Laser Precipitation Monitors at representative urban area of Hangzhou, mountainous area of Taishun and island area of Dachen from June to August during 2019 to 2021 are used to analyze characteristics and differences in DSD under different rain rates and rain types. The results are as follows: (1) The mass-weighted mean diameter (D_m) at Hangzhou is lower than that at Taishun and Dachen. This phenomenon may be attributed to high aerosol concentrations in urban areas, which suppress collision-coalescence processes and promote evaporation-breakup effects when combined with a higher cloud base height. These mechanisms collectively lead to reduced large raindrops (diameter≥6 mm) and a narrower width of drop size spectrum for stratiform rain and when rain rate (R) is below 10 mm/h. Conversely, for heavy precipitation (R＞20 mm/h) and convective rain, the combined effects of urban heat island and aerosol activation enhance convective activities, and thereby promote the formation of large raindrops (diameter≥4 mm) and increase their number concentration. (2) At the island station (Dachen), under conditions of rain rate below 20 mm/h and for stratiform rain, higher concentrations of medium-to-large raindrops (diameter≥2 mm) and fewer small raindrops (diameter＜0.5 mm) are observed. This phenomenon may be attributed to enhanced collision-coalescence processes promoted by favorable low-level humidity condition, stronger convective energy, and a marine aerosol-rich environment dominated by sea salt particles. (3) As most of the precipitation in mountainous areas occurs at the cloud base or within the cloud, the evaporation process of small raindrops is relatively weak. Therefore, the number concentration of small raindrops (diameter ≤0.375 mm) in Taishun is higher than that in Hangzhou and Dachen. The classical reflectivity factor (Z)- rain rate (R) relationship Z=300R^1.40 significantly overestimates convective precipitation when the reflectivity factor is high (Z≥10⁵ mm⁶/m³), and the degree of overestimation is different due to differences in raindrop spectrum characteristics in different topographic regions. The classical Z-R relationship overestimates (underestimates) stratiform precipitation when the normalized intercept parameter (lgN_w) is smaller (greater) than 4.5. The stratiform rain with lgN_w greater than 4.5 is characterized by small raindrop size and high concentration. The statistical results of echo top height indicate that it is shallow precipitation.