In this work we present the results of experimental measurements of the charge transferred to simulated graupel under wet growth conditions. The range of temperature was between −7 and −18 °C. The speeds of collision employed in the measurements were between 8 and 13 m/s with the goal of representing graupel of different initial densities. The aim of this study was to corroborate the existence of charge transfer under wet growth. The results show the presence of positive charge when graupel reaches partial wet growth, and the magnitude of this charge is comparable to the charge transferred under dry growth conditions. On the other hand, when total wet growth is reached no charge transfer is registered. This leads to the conclusion that the non‐inductive mechanism could be working under partial wet growth conditions when the ice crystals collide with the dry regions of the graupel surface and then bounce off.

This study reports new measurements of the charge separation during ice crystal-graupel collisions in the absence of supercooled water droplets. Experimental measurements of the average charge acquired by a simulated graupel, under condition of subsaturation with respect to water and supersaturation with respect to ice, were performed. The relative humidity of the air was controlled and measured during the experiments. The experiments were carried out in the temperature range between −7 and −21 • C, with a relative velocity of 3 m s −1 between the ice particles and the graupel. The aim of the measurements was to study the dependence of the non-inductive mechanism on the presence/absence of supercooled water droplets. Present results together with previous findings indicate that the sign of the graupel charging depends on the presence/absence of supercooled water droplets and on the relative humidity (subsaturation/supersaturation) of the environment in the case of absence of water droplets. This laboratory study is the first to map the non-riming conditions with low velocity impact and these new results confirm the negative charging of graupel under these conditions. It was found that the magnitude of the charge separated per collision in non-riming conditions is of the same order of magnitude as in riming conditions. This suggests that the non-inductive mechanism could be operating in the storms even in the absence of water droplets, emphasizing the importance of knowledge of the environmental thermodynamic conditions in order to know the charging behaviour of the graupel particles.

This work reports a new laboratory study of the electric charge separated in collisions between a spherical target of 1 cm in diameter growing by riming and vapor‐grown ice crystals, with the objective of studying the charging behavior of the larger ice precipitation particles in thunderstorms in terms of the noninductive mechanism. A series of experiments was conducte d for a wide range of environmental conditions; the measurements were performed for effective liquid water content between 0.5 and 5 g m−3, for ambient temperatures between −5 and −30 °C and at air speed of 11 m s−1. The magnitude and sign of the electric charge transfer on the ice sphere as a function of the ambient temperature and the effective liquid water content is presented. The results show a charge reversal temperature for the riming target, which is roughly independent of liquid water concentration in the measured range. The simulated graupel charges negatively for temperatures below −15 °C, and positively at temperatures above −15 °C.