Charges on Graupel and Snow Crystals and the Electrical Structure of Winter Thunderstorms

The shape and electric charge on particles in Hokuriku winter cumulus clouds have been measured using videosondes. The sign of the charge on graupel reversed at about −11°C. Charges on graupel and ice crystals are responsible for the tripole structure. The magnitude of the space charge increased as the particle concentrations increased. Graupel concentrations in excess of 1 L−1 and an average charge on the precipitation particles of a few tenths of pC produced an accumulated space charge sufficient to initiate lightning. These findings support the model results reported by Takahashi in which riming electrification mechanisms were emphasized as the primary charge separation process. It was also observed that the most active particle-charging process occurred at around the −20°C level.