Microphysical Structure and Lightning Initiation in Hokuriku Winter Clouds

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.