A modelling study of the effect of cloud saturation and particle growth rates on charge transfer in thunderstorm electrification

The diffusional growth rates of ice crystals and riming graupel particles are calculated for various cloud saturation ratios, then the sign of electric charge transferred during crystal/graupel collisions is determined according to the concept of the relative vapour diffusional growth rates, according to Baker et al., 1987 [Baker, B., Baker, M.B., Jayaratne, E.J., Latham, J., Saunders, C.P.R., 1987. The influence of diffusional growth rates on the charge transfer accompanying rebounding collisions between ice crystals and soft hailstones. Quart. J. Roy. Met. Soc. 113, 1193-1215]. It is found necessary, in order to account for the observation of positive charging of riming graupel at high accretion rates, to modify the assumptions of Baker et al. in order to increase the flux of vapour to the graupel surface. The variable growth parameters available may be adjusted to represent the environmental saturation conditions in various laboratory experiments, including the mixing of clouds from regions having different growth conditions, and are used to determine charge sign sensitivity to cloud saturation ratio, temperature and accretion as measured by the cloud effective liquid water content.