© Author(s) 2010. This work is distributed under the Creative Commons Attribution 3.0 License. Atmospheric Chemistry and Physics

Abstract. In these experiments, the electric charge carried by single particles ejected from the surface of a graupel par-ticle growing by riming was measured. Simulated graupel pellets were grown by accretion of supercooled water drops, at temperatures ranging from −2 to −10 ◦C in a wind tun-nel at air velocities between 5 and 10 m s−1, with the goal of studying the charging of graupel pellets under conditions of secondary ice crystal production (Hallett-Mossop mecha-nism). The graupel, and induction rings upstream and down-stream of the graupel, were connected to electrometers and analyzing circuits of sufficient sensitivity and speed to mea-sure, correlate and display individual charging events. The results suggest that fewer than 1 % of the ejected particles carry a measurable electric charge (>2 fC). Further, it was observed that the graupel pellets acquire a positive charge and the average charge of a single splinter ejected is −14 fC. This mechanism of ejection of charged particles seems ad-equate to account for a positive charge of around 1 pC that individual precipitation particles of mm-size could acquire in the lower part of the cloud, which in turn could contribute to the lower positive charge region of thunderstorms