Charge separation in non-riming conditions *

Laboratory measurements have shown that the magnitude and sign of the charge transfer to graupel particles during interactions with ice crystals is a function of the cloud microphysical conditions. In fact, it depends on: the cloud temperature, supercooled water concentration, cloud droplet size distribution, ice crystal size and impact velocity. Few experiments were conducted under non riming conditions and under zero liquid water content because the low magnitude of the charging current but ice particles with significant charge is observed in stratiform cloud regions where the ambient is subsaturated respect to liquid water. New laboratory measurements of the charge transfer in collisions between vapor-grown ice crystals and a graupel particle (2 mm diameter) in non-riming conditions are presented in this work. The experiments were all performed for a supersaturated-atmosphere with respect to ice and subsaturated with respect to liquid water; which ensures the environment free of supercooled liquid water droplet. The relative humidity of the air inside the chamber was controlled and measured during the measurements. The experiments were conducted for ambient temperatures between –7°C and –20°C and air velocity around 3 m/s. The results show that the charging acquired by the graupel depends on the temperature and supersaturation.