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.