The influence of supersaturation at low rime accretion rates on thunderstorm electrification from field-independent graupel-ice crystal collisions

Laboratory cloud chamber experiments have revealed the significance of cloud supersaturation on field-independent graupel-ice crystal charging associated with thunderstorm electrification. At rime accretion rates below approximately 1 g m−2 s−1, altering supersaturation can reverse the polarity of graupel charging when other conditions are maintained. Explanations are given to reconcile variation between many published observations by considering the factors associated with particle surface diffusional growth that contribute to the resulting polarity of charge. Findings are consistent with the Relative Diffusional Growth Rate hypothesis. Variations between observations can be qualitatively accounted for by considering the effects of ice crystal thermal preconditioning, crystal size, and cloud supersaturation. The varieties of observed charge polarity reversal as a function of cloud temperature and liquid water content are also characterised. Recommendations are given for future laboratory experimental measurements to avoid ambiguity during interpretation.