The purpose of this study is to investigate the performance of 3DVAR radar data assimilation in terms of the retrievals of convective fields and their impact on subsequent quantitative precipitation forecasts (QPFs). An assimilation methodology based on the Weather Research and Forecasting (WRF) model threedimensional variational data assimilation (3DVAR) and a cloud analysis scheme is described. Simulated data from 25 Weather Surveillance Radar-1988 Doppler (WSR-88D) radars are assimilated, and the potential benefits and limitations of the assimilation are quantitatively evaluated through observing system simulation experiments of a dryline that occurred over the southern Great Plains. Results indicate that the 3DVAR system is able to analyze certain mesoscale and convective-scale features through the incorporation of radar observations. The assimilation of all possible data (radial velocity and reflectivity factor data) results in the best performance on short-range precipitation forecasting. The wind retrieval by assimilating radial velocities is of primary importance in the 3DVAR framework and the storm case applied, and the use of multipleDoppler observations improves the retrieval of the tangential wind component. The reflectivity factor assimilation is also beneficial especially for strong precipitation. It is demonstrated that the improved initial conditions through the 3DVAR analysis lead to improved skills on QPF.

Riming electrification is the main charge separation mechanism of thunderstorms, occurring mainly during graupel particle–ice crystal collisions. Laboratory experiments have found that charge separation polarity and magnitude depend critically on cloud water content and temperature. Several groups have mapped this dependence, but there are substantial differences between their results. These conflicting laboratory-derived riming electrification topographies can be tested by comparing them to field observations. Here, direct and simultaneous sonde-based measurement of both precipitation particle type and charge (videosonde) and cloud water content [hydrometeor videosonde (HYVIS)] in lightning-active Hokuriku winter clouds at Kashiwazaki, Niigata Prefecture, Japan, are reported. With decreasing height, summed graupel charge transitioned from negative to positive at a mean temperature of −11°C, and the mean peak cloud water content in the positive graupel domain was 0.4 g m−3. Thus, in cloud regions of relatively high temperature (≥−11°C) and low cloud water content (CWC; ≤0.4 g m−3), graupel particles were mainly positively charged. This result can be compared with those of laboratory riming experiments; for example, in this temperature/cloud water content domain, graupel electrification has been reported to be positive by Takahashi, largely negative in early reports using the Manchester cloud chamber, positive in later reports using the Cordoba and Manchester modified cloud chambers, and partially positive in a more recent report using the Cordoba cloud chamber.

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Hokuriku winter clouds produce frequent, positive lightning from relatively shallow clouds. To understand this phenomenon, data from Videosondes and Videosonde‐HYVIS conjoined sondes, launched from Kashiwazaki, Japan, were analyzed with radar and Lightning Location System network data. The main charge carriers were graupel particles and ice crystals, and space charge increased with their number concentrations consistent with riming electrification. Cloud structure evolved greatly over the course of cloud life. In the mature stage, space charge was positive in the cloud upper level (carried by positive ice crystals), negative in the middle level (carried by negative graupel and negative ice crystals), and positive in the lower level (carried by positive graupel). Lightning was only seen in clouds that had all of the following characteristics: cloud top temperature < −14°C, −10°C isotherm >1.2 km, space charge >2–3 pC/L, ice crystal number concentration >500 m−3, and graupel particle number concentration >20 m−3. Predominance of positive cloud‐to‐ground (+CG) lightning was associated with graupel number concentration <200 m−3 and graupel peak diameter ≤4 mm. The +CG strike zone tended to be downshear from the −CG strike zone. The data suggest that −CG is initiated mainly by middle‐level negative graupel in the convective cell, while +CG is initiated by positive ice crystals in the upper domain, displaced by wind shear and descending. A major contributor to ice crystal number density is a novel ice multiplication process in which graupel‐surface ice branches are broken‐off in large numbers and subsequently grow into ice crystals.

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