A laboratory study of the effects of rime ice accretion and heating on charge transfer during ice crystal graupel collisions

In a laboratory study of thunderstorm electrification involving charge tran sfer between ice crystals and a riming graupel pellet, the effect on charge transfer of rimer heating by droplet accretion has been separated from the associated influence of the vapour flux to the rimer surface. This was acc omplished by heating internally a riming target rod whose surface condition s represent those of a falling graupel pellet in thunderstorms while keepin g the rate of rime accretion constant. The results show that the positive c harging of a rimer may be reversed to negative by artificial heating, with increased heat required at higher rates of rime accretion. It is hypothesis ed that ice crystals rebounding from riming graupel pellets charge the grau pel positively or negatively depending on the cloud and rimer conditions, w hich influence the relative thicknesses of charge carrying layers on the su rfaces of the particles. In the natural case, negative charging of graupel is associated with rime surface heating, which reduces the vapour diffusion al growth rate below that of the ice crystals, while positive charging of g raupel is associated with vapour provision to the rimer surface from the fr eezing droplets, which overcomes the rime heating effect. This work compare s the results of charge transfer to a riming target obtained in UMIST Manch ester, involving multi-crystal interactions, with data from Cordoba Argenti na involving single ice sphere interactions. The fact that broadly similar charging behaviour was seen in both studies suggests that it is the rate of growth of the ice surfaces, rather than their particular nature, that is t he important factor in controlling the charge transfer during ice particle collisions with a riming ice surface. (C) 1999 Elsevier Science B.V. All ri ghts reserved.