Summer characteristics of raindrop size distributions in urban, mountainous and island areas over southeastern coastal China

The southeastern coastal region of China exhibits complex and diverse topographic features that significantly influence local meteorological conditions and precipitation patterns. In-depth research on the microphysical characteristics of precipitation in this area contributes to understanding 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 encompassing mountains, islands, and plains (including highly urbanized areas), offers an ideal setting for investigating DSD features in complex topographic environments. Hence, observations collected by Thies CLIMA Laser Precipitation Monitor at representative urban area Hangzhou, mountainous area Taishun and island area Dachen stations from June to August in 2019 to 2021 are used to analyze characteristics and differences of raindrop size distribution under different rain rates and rain types. The results show that: (1)The mass-weighted mean diameter (Dm) at the Hangzhou is lower than that at Taishun and Dachen. This phenomenon may be attributed to high aerosol concentrations in urban areas suppressing collision-coalescence processes, combined with higher cloud base heights promoting evaporation-breakup effects. These mechanisms collectively led to reduced large raindrops (diameter ≥6 mm) and narrower drop size distributions during stratiform rains and rain rate (R) below 10 mm/h. Conversely, under heavy precipitation (R>20 mm/h) and convective rains, the combined effects of urban heat island and aerosol activation enhance convective activity, thereby promoting the formation of large raindrops (diameter ≥4 mm) and increasing their number concentration. (2) At the island station (Dachen), under conditions of rain rates below 20 mm/h and stratiform rains, 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 conditions, stronger convective energy, and a marine aerosol-rich environment dominated by sea salt particles. (3) As the majority of 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 = 300R1.40 significantly overestimated the precipitation of convective rain with high reflectivity factor ( Z > 105mm6/m3 ), and the degree of overestimation is different due to the difference of raindrop spectrum characteristics in different topographic regions. The classical Z-R relationship overestimates (underestimates) the precipitation of stratiform rain with the normalized intercept parameter (lgNw) less than (greater than) 4.5. The stratiform rains with lgNw 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.