Comparative analysis of raindrop size distribution characteristics before and after monsoon onset in southern coast of Fujian province in 2018—2019

Observations collected by precipitation phenomenometers at Xiamen and Tong’an stations from April to June in 2018 and 2019 are used to analyze differences in drop size distribution (DSD) and microphysical parameters before and after the monsoon onset in the pre-flood season in southern coast of Fujian. The results show that the frequency of stratiform precipitation is significantly higher than that of convective precipitation. The spectrum width of convective precipitation is much larger than that of stratiform precipitation in every interval above the peak value. Variations of the mass-weighted mean diameter (D_m) and the generalized intercept parameter (N_w) have opposite trends in 2018 and 2019. D_m decreased while N_w increased for both stratiform and convective precipitation after the monsoon onset in 2018. However, D_m increased and N_w of stratiform precipitation decreased while that of convective precipitation increased after the monsoon onset in 2019. Rain rate (R) and liquid water content (W) are consistent with N_w for stratiform precipitation, while R and W are consistent with D_m for convective precipitation. Microphysical parameters in southern coast of Fujian show consistent features with those in southern coast of China, while the particle number concentration is higher but the particle size is smaller compared with those in eastern China. The coefficient of Z-R relationship varies significantly for stratiform and convective precipitation with better fitting effect for convective precipitation. QPE based on Z=300R^1.4 relationship probably would underestimate stratiform or weak convective precipitation, but most likely overestimates strong convective precipitation. Circulation influence on DSD is discussed. Results indicate that specific humidity can affect particle concentration. Dynamic condition such as southwesterly winds and thermal condition such as CAPE can affect DSD through convection height and microphysical processes of precipitation.