Density and surface temperature of graupel and the charge separation during ice crystal interactions

Significant amounts of charge are separated when vapor-grown ice crystals interact with riming graupel a mechanism widely believed to be responsible for the generation of electric charge in thunderstorms. This study shows that the density of graupel, by itself, is not an effective parameter in determining the sign of the separated charge. The well-known charge sign reversal temperature is found to be strongly influenced by the surface temperature of the rime at low cloud water content (CWC). At a cloud temperature of -10 degrees C and a CWC of 0.4 g m(-3) the charging current to the graupel reversed from positive to negative as the surface temperature of the graupel exceeded -6 degrees C. The maximum CWC at which the sign could thus be reversed increased as the cloud temperature decreased, ranging from less than 0.7 g m(-3) at -10 degrees C to less than 0.9 g m(-3) at -20 degrees C. At higher cloud water contents the rime surface temperature had no effect on the charging sign. The observations are shown to be broadly explicable in terms of the hypothesis where, during an interaction between two ice particles, the particle that is growing faster from the vapor acquires the positive charge.