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

Laboratory experiments, in which vapour grown ice crystals interact with ri ming graupel targets, simulate charging processes in thunderstorms. The int roduction of cooled, moist, laboratory air into a supercooled droplet and i ce crystal cloud enhances charge transfer and, when the air-stream is direc ted at the riming target, can reverse its charge sign. The suggestion is th at the extra water vapour introduced increases the supersaturation and infl uences particle diffusional growth. The results have been considered in ter ms of the Relative Growth Rate Hypothesis, which states that the interactin g ice surface growing fastest by vapour diffusion charges positively. A cor ollary to this was noted, when dry air is introduced into a cloud of ice cr ystals so that both the crystals and target surface sublimate, the ice surf ace that sublimates fastest charges negatively. The experiments are relevant to considerations of the reasons why earlier s ets of charge transfer results give different liquid water and temperature boundaries between positive and negative graupel charge sign. The differenc es appear to be connected to the techniques used, in particular, to the mix ing of separate droplet and ice crystal clouds before riming, which can lea d to positive rimer charging in conditions of low-rime accretion rate, as o bserved in the present study. Further work is needed to resolve questions c oncerning the most naturally representative manner of performing these labo ratory simulations. (C) 2001 Elsevier Science BN. All rights reserved.