Microphysical Characteristics Of The Raindrop Size Distribution Observed In East China During The Asian Summer Monsoon Season

Raindrop size distribution (DSD) is a fundamental microphysical property of precipitation. Fully recognizing the variability of DSDs is important for understanding precipitation microphysics and its parameterization, improving radar quantative precipitation estimation, and forecasting. In this study, the data from the four collocated instruments, i.e. a two-dimensional video disdrometer (2DVD), a vertically pointing micro rain radar (MRR), and two laser-optical OTT Particle Size Velocity (PARSIVEL) disdrometers (first-generation OTT-1 and second-generation OTT-2), were collected during the OPACC 2014-2015 field campaign. This unique dataset are used for evaluating the performance of different kinds of disdrometers. Then, the statistical characteristics of DSDs in three different types of precipitation are revealed using the advanced instruments of 2DVD and MRR.Firstly, a quantitative analysis of DSD, rain rate, and drop fall velocity measurements are revealed using 2-yr DSD measurements from the four instruments. Overall, the 2DVD have the most accurate DSDs and drop velocity measurements among the four instruments. The DSD measurements from the different instruments show high consistency with each other for midsized (1-4 mm) raindrops. The OTT-1 and OTT-2 both underestimate the small drops (<1 mm), which can be largely attributed to measurement error rather than the nature of the rain. Compared to the OTT-1, the OTT-2 shows improvement for fall velocity measurements. Unlike the 2DVD and OTT, which can only observe the DSD at surface, the MRR can measure the vertical structures of DSDs. The MRR observation at the lowest level (-200 m) underestimates the concentration of drops larger than 5 mm and overestimates the concentration of drops smaller than 1 mm. The impacts of small drops that are not detected by the instruments will lead to distinctive differences in the computed physical quantities, derived relations and model microphysical processes.Secondly, using advanced instruments of 2DVD and MRR, three types of precipitation, i.e., convective, stratiform and shallow precipitation, are classified and analyzed. The results shows that summer rainfall in the study region is found to consist mainly of stratiform rain in terms of frequency of occurrence while shallow rain shows the lowest, but is dominated by convective rain in terms of accumulated rainfall amount. The shallow precipitation forms directly in liquid form and no melting is present, which result in a relatively small mean (and maximum) diameter and high concentration of raindrops with small diameters. Overall, higher raindrop concentrations and smaller diameters are found when compared to monsoon precipitation at other locations in Asia (e.g., Japan), and the convective precipitation characterized as maritime convection. Higher local moisture and aerosol concentration is speculated to be the cause. Further separation of the summer season into time periods before, during, and after the Meiyu season reveals that intra-summer 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. 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.Finally, DSDs from three different rainfall systems, i.e. Meiyu frontal rainband, landing typhoon rainband and squall line are quantitatively compared. The results suggest that precipitation of typhoon and Meiyu rainband are mainly composed of small raindrops with high concentration, while the spectrum of squall line DSD is wider with its convective center is characterized as continental convection and a few large raindrops produced by melting ice particles at its stratiform region. The distinctive raindrop shape differences exist among different rainfall systems. Typhoon precipitation in East China have more spherical raindrops with a larger axis ratio than those in similar climatic region (e.g., Taiwan), which result in a larger Dm but lower Nw retrieved from a given polarimetric radar observation, as well as a larger differential reflectivity derived from a given DSD observation.