Relationship between cloud‐to‐ground lightning polarity and the space‐time distribution of solid hydrometeors in isolated summer thunderclouds observed by X‐band polarimetric radar

Abstract

AbstractTo understand the charge distribution in thunderclouds associated with solid hydrometeors, the relationship between cloud‐to‐ground (CG) lightning polarity and the space‐time distribution of solid hydrometeors for isolated summer thunderclouds in Japan is examined using X‐band polarimetric radar. Hydrometeor classification was conducted to examine the space‐time distribution of dry snow, ice crystal, dry graupel (DG), and wet graupel in thunderclouds. Two thunderstorm cases were selected for analysis: 26 July 2010, which generated few positive CG flashes, and 25 August 2010, which generated positive CG flashes in the reflectivity cores. In both cases, negative CG flashes were observed in all reflectivity cores when a large volume of DG was identified above the height of −10°C level. This is consistent with previous studies showing that graupel particles have a negative charge below temperatures of −10°C. Reflectivity cores with positive CG flashes had a large volume of DG up to high altitudes (around or above the −45°C level). Further, reflectivity cores that sustained large DG volumes at high altitudes had a relatively large number of positive CG flashes. The top height of the DG volume reached lower altitudes for reflectivity cores without positive CG flashes compared with those with positive CG flashes. These results suggest that the persistence of graupel particles in reflectivity cores at high altitudes, implying the existence of strong updraft, is a necessary condition for positive CG flashes in summer thunderclouds. This effect would likely be caused by the positively charged graupel particles under high rime accretion rates.