The Influence of Diffusional Growth Rates On the Charge Transfer Accompanying Rebounding Collisions Between Ice Crystals and Soft Hailstones

Laboratory experiments designed to investigate the charge transfer accompanying rebounding collisions between ice crystals and soft hailstones were performed inside a cold room. They constitute an extension of those conducted with the same apparatus, reported in 1983 by Jayaratne et al. In particular, the range of temperature was extended (to cover −1.5°C to −35°C) and specific tests were performed in an effort to establish relationships between the sign and magnitude of the charging and the growth characteristics of both types of hydrometeor. Significant charging was obtained only when the interacting surfaces were growing by vapour diffusion. the sign of the current, I, flowing to the simulated soft hailstone target in these circumstances was a sensitive function of time, liquid water content, and temperature. the complete set of results is qualitatively consistent with the hypothesis that I is positive if the target surface is growing more rapidly from the vapour than the ice crystals and is negative for the opposite case. In formulating a qualitative model of this hypothesis, account is taken of the contribution made to the diffusional growth of the target by the flux of vapour from droplets freezing onto its surface. Crude calculations designed to assess the implications of these studies with respect to thunderstorm electrification, yield the conclusion that negative soft hailstone charging is likely to predominate over positive in most circumstances. Copyright © 1987 Royal Meteorological Society