The winter storm clouds of the Hokuriku region of Japan, though relatively shallow, are famous not only for producing prodigious amounts of snow, but also for producing high frequencies of lightning. An unusually high proportion is positive lightning, that is, lightning that has an electric charge opposite of the more common negative lightning.
Takahashi et al.  present a comprehensive and detailed analysis of balloon-borne and ground-based observations of these storms, attributing microphysical parameters in the thunderclouds to lightning types and origins. The authors found that the space charge increases with the number concentrations of graupel particles and ice crystals, indicating riming electrification. The charge structures of the different stages of the cloud life cycle were also obtained.
In addition, they found enormous numbers of small ice particles which appear to have been ejected from the graupel particle surface in inter-particle collisions. The small particles generated by this new ice multiplication mechanism could contribute substantially to seeding the development of the large numbers of ice crystals which support the high electrification of these clouds.
The results of this manuscript shed substantial light on the relationship between the distribution of hydrometeor particles and charge structure.
Citation: Takahashi, T., Sugimoto, S., Kawano, T., & Suzuki, K. . Microphysical structure and lightning initiation in Hokuriku winter clouds. Journal of Geophysical Research: Atmospheres, 124. https://doi.org/10.1029/2018JD030227
—Minghua Zhang, Editor in Chief, JGR: Atmospheres