The characteristics of raindrop size distributions (DSDs) and vertical structures of rainfall during the Asian summer monsoon season in East China are studied using measurements from a ground‐based two‐dimensional video disdrometer (2DVD) and a vertically pointing Micro Rain Radar (MRR). Based on rainfall intensity and vertical structure of radar reflectivity, the observed rainfall is classified into convective, stratiform, and shallow precipitation types. Among them, shallow precipitation has previously been ignored or treated as outliers due to limitations in traditional surface measurements. Using advanced instruments of 2DVD and MRR, the characteristics of shallow precipitation are quantified. Furthermore, summer rainfall in the study region is found to consist mainly of stratiform rain in terms of frequency of occurrence but is dominated by convective rain in terms of accumulated rainfall amount. Further separation of the summer season into time periods before, during, and after the Meiyu season reveals that intrasummer variation of DSDs is mainly due to changes in percentage occurrence of the three precipitation types, while the characteristics of each type remain largely unchanged throughout the summer. Overall, higher raindrop concentrations and smaller diameters are found compared to monsoon precipitation at other locations in Asia. Higher local aerosol concentration is speculated to be the cause. Finally, rainfall estimation relationships using polarimetric radar measurements are derived and discussed. These new relationships agree well with rain gauge measurements and are more accurate than traditional relations, especially at high and low rain rates.

Convection-permitting numerical experiments using the Weather Research and Forecasting (WRF) Model are performed to examine the diurnal cycles of land and sea breeze and its related precipitation over the south China coastal region during the mei-yu season. The focus of the analyses is a 10-day simulation initialized with the average of the 0000 UTC gridded global analyses during the 2007–09 mei-yu seasons (11 May–24 June) with diurnally varying cyclic lateral boundary conditions. Despite differences in the rainfall intensity and locations, the simulation verified well against averages of 3-yr ground-based radar, surface, and CMORPH observations and successfully simulated the diurnal variation and propagation of rainfall associated with the land and sea breeze over the south China coastal region. The nocturnal offshore rainfall in this region is found to be induced by the convergence line between the prevailing low-level monsoonal wind and the land breeze. Inhomogeneity of rainfall intensity can be found along the coastline, with heavier rainfall occurring in the region with coastal orography. In the night, the mountain–plain solenoid produced by the coastal terrain can combine with the land breeze to enhance offshore convergence. In the daytime, rainfall propagates inland with the inland penetration of the sea breeze, which can be slowed by the coastal mountains. The cold pool dynamics also plays an essential role in the inland penetration of precipitation and the sea breeze. Dynamic lifting produced by the sea-breeze front is strong enough to produce precipitation, while the intensity of precipitation can be dramatically increased with the latent heating effect.

Convection-permitting numerical experiments using the Weather Research and Forecasting (WRF) Model are performed to explore the influence of monsoonal onshore wind speed and moisture content on the intensity and diurnal variations of coastal rainfall over south China during the mei-yu seasons. The focus of the analyses is on a pair of 10-day WRF simulations with diurnally cyclic-in-time lateral boundary conditions averaged over the high versus low onshore wind speed days of the 2007–09 mei-yu seasons. Despite differences in the rainfall intensity, the spatial distributions and diurnal variations of rainfall in both simulations verified qualitatively well against the mean estimates derived from ground-based radar observations, averaged respectively over either the high-wind or low-wind days. Sensitivity experiments show that the pattern of coastal rainfall spatial distribution is mostly controlled by the ambient onshore wind speed. During the high-wind days, strong coastal rainfall is concentrated along the coastline and reaches its maximum in the early morning. The coastal lifting induced by the differential surface friction and small hills is the primary cause for the strong coastal rainfall, while land breeze enhances coastal lifting and precipitation from evening to early morning. In the low-wind days, on the other hand, coastal rainfall is mainly induced by the land–sea-breeze fronts, which has apparent diurnal propagation perpendicular to the coastline. With stronger land–sea temperature contrast, the land–sea breeze is stronger during the low-wind days. Both in the high-wind and low-wind days, the coastal rainfall intensity is sensitive to the incoming moisture in the upstream oceanic airflow, especially to the moisture content in the boundary layer.