A laboratory study of the influence of ice crystal growth conditions on subsequent charge transfer in thunderstorm electrification

Laboratory studies of a thunderstorm charging mechanism involving rebounding collisions between ice crystals and riming graupel pellets, have shown the importance of the growth conditions of the interacting ice particles on the sign of the charge transferred. The present study shows a new result: if an ice crystal is not in thermal equilibrium with the environment (immediately following the mixing of two clouds at different saturations) the crystal surface may experience an enhanced growth rate that can influence the sign of the charge transfer and promote negative rimer charging. Furthermore, when an ice crystal in ice saturation conditions is introduced to a cloud at water saturation, leading to transient growth and heating, the period of thermal non-equilibrium is shown to be sufficiently brief that the enhanced negative rimer charging is short lived. These results suggest that the earlier conclusions of Berdeklis and List-that the cloud saturation conditions around a growing ice crystal impart to the crystal surface a property that is carried with it and that influences the sign of subsequent charge transfer-are unfounded. The discrepancy is because in their laboratory simulations of thunderstorm conditions there is adequate time for the growing ice crystal surface to come to equilibrium with its environment. The established concept of the relative diffusional growth rate of the interacting surfaces controlling the sign of charge transfer, such that the faster growing surface charges positively, is consistent with the observations.