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

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 (lg N 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.