A laboratory study of the influence of water vapour and mixing on the charge transfer process during collisions between ice crystals and graupel

Abstract

Laboratory experiments, in which vapour grown ice crystals interact with riming graupel targets, simulate charging processes in thunderstorms. The introduction of cooled, moist, laboratory air into a supercooled droplet and ice crystal cloud enhances charge transfer and, when the air-stream is directed at the riming target, can reverse its charge sign. The suggestion is that the extra water vapour introduced increases the supersaturation and influences particle diffusional growth. The results have been considered in terms of the Relative Growth Rate Hypothesis, which states that the interacting ice surface growing fastest by vapour diffusion charges positively. A corollary to this was noted, when dry air is introduced into a cloud of ice crystals so that both the crystals and target surface sublimate, the ice surface that sublimates fastest charges negatively. The experiments are relevant to considerations of the reasons why earlier sets of charge transfer results give different liquid water and temperature boundaries between positive and negative graupel charge sign. The differences appear to be connected to the techniques used, in particular, to the mixing of separate droplet and ice crystal clouds before riming, which can lead to positive rimer charging in conditions of low-rime accretion rate, as observed in the present study. Further work is needed to resolve questions concerning the most naturally representative manner of performing these laboratory simulations.