2015—2017年夏季南京雨滴谱特征

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

利用2015-2017年夏季南京地区的雨滴谱数据,对南京在梅雨开始前、梅雨期及梅雨结束后3个不同时段降水的宏微观特征进行分析发现:梅雨开始前对流活动强度偏弱,但对流降水的雨滴平均质量加权直径、分钟级强降水频率和逐小时累积短时强降水的频率为3个时段中最高;天气尺度强迫提供的有利于降水的持续性条件、弱对流强度下充分的凝结过程及微物理相关过程对云粒子的损耗偏弱,是有利于该时段大雨滴形成和降水效率提高的重要因素。梅雨结束后,高温高湿环境易产生剧烈对流活动,导致对流降水的大尺度雨滴样本比例及分钟级极端降水发生频率位于3个时段的首位。层云降水时,梅雨期降水频率、降水率及雨滴尺度平均值均位于首位,小尺度雨滴样本比例最低;有利天气尺度强迫条件下的充分碰并作用是主要原因之一。不同时段雨滴谱谱形参数(μ)与斜率(A)之间的二项式关系式的差异与μ的取值有关。

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 massweighted 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 largescale synoptic conditions,sufficient condensation and the weakened influence from evaporation,breakingup 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.