Thunderstorm graupel charging in the absence of supercooled water droplets

Abstract

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.