THUNDERSTORM CHARGING - LABORATORY EXPERIMENTS CLARIFIED

Further laboratory experiments of thunderstorm charging by ice crystal collisions with riming graupel pellets have led to results that may h elp account for a major difference between two independent studies rep orted earlier. The problem concerns values of the cloud liquid water c ontent required to change the sign of graupel charging. The present wo rk has noted and investigated differences in the techniques of cloud n ucleation used in the two studies. I? one set of measurements by Takah ashi, ice crystals are nucleated continuously in a supercooled water d roplet cloud, while in the other experiments by Jayaratne et al. and S aunders et al., the droplet cloud is nucleated only briefly. These two nucleation methods lead to differences in the mass of cloud particles collected on a sampling probe used to determine the liquid water cont ent of the cloud. In the continuous nucleation case, the probe collect s sufficient numbers of ice crystals to lead to a substantial overesti mate of the cloud liquid water content.