Raindrop Size Distribution Characteristics of Nanjing in Summer of 2015-2017

It's of great significance to study features of raindrop size distribution (DSD) during different stages of the summer monsoon for understanding the precipitation mechanism, which is regarded as credible reference to improve and refine ainfall retrieval algorithms based on satellite and radar observations and the parameterization of microphysics scheme in numerical model. Characteristics of DSD during summer (June to August) of 2015-2017 are investigated using measurements from a ground-based disdrometer in Nanjing. Results show different micro and macro precipitation characteristics among three stages of summer monsoon. Precipitation before Meiyu is characterized by the highest (among the three stages) mean mass-weighted raindrop diameter, average minutely rainfall rate, and intense minutely and strong hourly rainfall occurrences. Despite generally weak convection intensity in this stage, the persistent support from large-scale synoptic conditions, sufficient condensation and the weakened influence from evaporation, breaking-up and entrainment processes are beneficial to produce large raindrops and improve precipitation efficiency. In contrast, precipitation after Meiyu is identified with the greatest frequency of large raindrop and extreme minutely rainfall occurrences. This is mainly caused by severe convective activities under hot and humid atmospheric conditions. Stronger convection is also associated with higher frequency of smaller raindrops. In pace with the northward advancement of the summer monsoon, the convection intensity enhances gradually and breaking-up processes of raindrops heighten as well, which lead to higher ratio of small-raindrop samples with the largest value during the stage after Meiyu. From many aspects of these raindrop and rainfall characteristics, convective precipitation during Meiyu is inferior comparing to that in the other two stages. However, rainfall rates are highest and raindrops are largest during stratiform precipitation due to sufficient coalescence processes under favorable synoptic forcing conditions. The concentration of small raindrops is usually high but the ratio of small raindrops is the lowest in this stage. Among three stages, the binomial relationship between the shape index and slope parameter also differ significantly, depending on the value of the shape index. Compared with Meiyu of 2009-2011, the frequency of intense rainfall occurrence and its contribution to total precipitation decrease while those for weak rainfall increase in terms of both minutely and hourly rainfall. Simultaneously, the binomial relationship of the shape index and slope parameter changes significantly as well.