Variability of Raindrop Size Distribution during a Regional Freezing Rain Event in the Jianghan Plain of Central China

Abstract

The characteristics of the raindrop size distribution (DSD) during regional freezing rain (FR) events that occur throughout the phase change (from liquid to solid) are poorly understood due to limited observations. We investigate the evolution of microphysical parameters and the key formation mechanisms of regional FR using the DSDs from five disdrometer sites in January 2018 in the Jianghan Plain (JHP) of Central China. FR is identified via the size and velocity distribution measured from a disdrometer, the discrete Fréchet distancemethod, surface temperature, human observations, and sounding data.

With the persistence of precipitation, the emergence of graupel or snowflakes significantly reduces the proportion of FR. The enhancement of this regional FR event is mainly dominated by the increase in the number concentration of raindrops but weakly affected by the diameters. To improve the accuracy of quantitative precipitation estimation for the FR event, a modified second-degree polynomial relation between the shape μ and slope Λ of gamma DSDs is derived, and a new Z-R (radar reflectivity to rain rate) relationship is developed. The mean values of mass-weighted mean diameters (D_m) and generalized intercepts (lgN_w) in FR are close to the stratiform results in the northern region of China. Both the melting of tiny-rimed graupels and large-dry snowflakes are a response to the formation of this regional FR process in the JHP, dominated by the joint influence of the physical mechanism of warm rain, vapor deposition, and aggregation/riming coupled with the effect of weak convective motion in some periods.

摘要

由于观测数据的局限, 我们对于区域性冻雨在相变过程(从液态到固态)中雨滴谱的变化特征知之甚少。本文利用2018年1月中国中部江汉平原地区5个站点的雨滴谱观测资料, 研究了区域冻雨过程微物理参数演变特征及其形成机制。冻雨是通过雨滴谱仪测量的粒子尺寸和速度分布, 并配合离散Fréchet距离法、地表温度、人工观测和探空数据来综合判定的。

该区域冻雨事件的增强主要以雨滴数浓度增加为主, 受雨滴直径影响较弱。随着降水的持续, 霰或雪花的出现显著降低了降水过程中冻雨的占比。为了提高冻雨事件定量降水估算的准确性, 提出了Gamma分布中形状参数与斜率参数之间的修正二次多项式关系, 并建立了新的雷达反射率-降雨率关系。冻雨的质量加权平均直径和归一化截距的平均值与中国北方地区的层状云结果接近。江汉平原区域冻雨的形成过程, 以微淞附的霰和大而干的雪花融化为主, 受到暖雨过程、凝华、聚并/淞附等物理机制以及某些时期弱对流运动的共同影响。