Microphysical Characteristics of Precipitation during Pre-monsoon, Monsoon, and Post-monsoon Periods over the South China Sea

Abstract

Raindrop size distribution (RSD) characteristics over the South China Sea (SCS) are examined with onboard Parsivel disdrometer measurements collected during marine surveys from 2012 to 2016. The observed rainfall is divided into pre-monsoon, monsoon, and post-monsoon periods based on the different large-scale circumstances. In addition to disdrometer data, sounding observation, FY-2E satellite, SPRINTARS (Spectral Radiation-Transport Model for Aerosol Species), and NCEP reanalysis datasets are used to illustrate the dynamical and microphysical characteristics associated with the rainfall in different periods. Significant variations have been observed in respect of raindrops among the three periods. Intercomparison reveals that small drops (D < 1 mm) are prevalent during pre-monsoon precipitation, whereas medium drops (1–3 mm) are predominant in monsoon precipitation. Overall, the post-monsoon precipitation is characterized by the least concentration of raindrops among the three periods. But, several large raindrops could also occur due to severe convective precipitation events in this period. Classification of the precipitation into stratiform and convective regimes shows that the lg(N_w) value of convective rainfall is the largest (smallest) in the pre-monsoon (post-monsoon) period, whereas the D_m value is the smallest (largest) in the pre-monsoon (post-monsoon) period. An inversion relationship between the coefficient A and the exponential b of the Z—R relationships for precipitation during the three periods is found. Empirical relations between D_m and the radar reflectivity factors at Ku and Ka bands are also derived to improve the rainfall retrieval algorithms over the SCS. Furthermore, the possible causative mechanisms for the significant RSD variability in different periods are also discussed with respect to warm and cold rain processes, raindrop evaporation, convective activities, and other meteorological factors.

摘要

雨滴谱作为降水最基本的微物理特征量之一, 在揭示降水微结构特征, 改进数值模式参数化方案和提高遥感估测降水精度等方面有着广泛的应用. 海洋作为全球降水最为重要的源和汇, 是地球水循环最重要的组成部分. 因此, 对海洋降水的观测研究具有重要价值. 南海夏季风是连接南亚季风和东亚季风的纽带, 对我国夏季降水的分布起决定性作用. 同时, 南海地区的降水对东亚地区的大气环流和能量收支也有着重要的影响, 研究南海夏季风降水结构特征具有重要意义. 本文利用海洋调查过程中收集的降水观测资料, 对比研究了南海海域夏季风前期, 季风期和季风后期降水的微物理特征差异, 结果表明: 南海季风前期降水小个雨滴(D < 1 mm)浓度最高, 季风期降水中含有更多的中端粒子(1–3 mm), 季风后期降水粒子浓度最小; 不同类型降水相比较, 季风前期的对流降水lgN_w(D_m)最大(小), 季风后期的lgN_w(D_m)最小(大); 三个时期降水的Z=ARb关系中, 系数A从季风前期到季风后期逐渐递增, 指数b从季风前期到季风后期逐渐递减; 为提高卫星遥感南海海域降水精度, 对D_m—Z_e 和 N_w—D_m等参数之间的关系进行了拟合. 本文研究可为建立雨滴谱函数关系, 改进数值模式预报效果和提高遥感反演降水精度提供参考.