Charge transfer during crystal-graupel collisions for two different cloud droplet size distributions

Laboratory experiments of graupel charging during ice crystal collisions reveal a charge sign dependence on temperature, liquid water concentration as well as the size distribution of the cloud droplets used for riming. The measurements were performed with an impact velocity of 8.5 m s(-l), the ambient temperature was varied in the range -5 to -30 degreesC and the effective water content up to 4 g m(-3) Charge diagrams of the sign of the electric current on the graupel as a function of the ambient temperature (T-alpha) and the effective liquid water content (EW) for each of the two cloud droplet spectra used are presented. The results indicate that broadening the droplet spectrum leads to negative graupel charging and the implications of these findings to thunderstorm electrification are discussed.