Abstract. The process of thunderstorm electrification by charge transfers between ice crystals and riming graupel pellets (the noninductive process) has been the subject of extensive study in the laboratory in Manchester. Quantitative dependencies of the sign and magnitude of charge transfer have previously been determined as functions of ice crystal size, graupel/crystal relative velocity, temperature, and the effective liquid water content (EW) in the cloud experienced by the riming graupel pellets. We now present results of laboratory studies of thunderstorm charging in terms of the rime accretion rate (RAR = EW x V), which combines into one variable the velocity and EW dependence of the sign of graupel charging on temperature. The magnitude of the charge transfer can be determined from its dependence on the crystal size and graupel velocity, while the sign of the timer charging can now be determined from a new figure showing the dependence of the charge sign on RAR and temperature. This figure may be used to compare charge transfer results from other laboratories obtained over a range of graupel/crystal velocities. These new experiments extend the temperature range of the previous studies and indicate that negative charging of graupel can occur at temperatures as high as -2øC in conditions of low RAR, while at temperatures below-30øC, more positive graupel charging is noted than in the earlier work.

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Avila and Caranti [1994] measured electric charge transferred when 100 /zm frozen ice spheres collide with a timing target. This work is relevant to thunderstorm electrification caused by the interactions of ice crystals and small graupel with falling graupel pellets. Avila and Caranti note that charges are transferred in two modes, one in which ice fragments from the timing target carry charge away and one in which no fragments are observed. We argue here that there is no requirement for fracturing of a rimer surface or fragment production in thunderstorm electrification processes involving the collisions of ice crystals with a timing target as studied by Jayaratne et al., [1983] and Saunders et al., [1991]. Furthermore, the results to be discussed here lead to the conclusion that ice crystals impacting with frost growths on a timer are unable to remove these surface features. A consequence of this analysis is that To find out whether charging could be associated with fragment production during ice crystal/rimer collisions, further experiments have been performed. A metal mesh was mounted in a cloud chamber and rotated at about 3 m s -l through a continuously supplied cloud of supercooled water droplets until the mesh became covered in rime ice of a similar density to that used in the charge transfer experiments, typically 0.3 g m -3. Experiments were performed at -17øC to -22øC with a range of liquid water contents up to 2.6 g m -3, which was well below that required for wet growth on the target mesh. To test whether fragmentation of the rime can occur during particle impacts, sand grains in the diameter range 200-500/zm were dropped into the path of the moving rimed target, while the cloud was illuminated and studied visually. Shortly a•er the sand/rime collisions, ice crystals appeared in the cloud confirming the production of tiny ice fragments that grew in the supersaturated environment. This result adds weight to the suggestion of Avila and Caranti that particle collisions may give rise to secondary ice production.Charge transfers during crystal/rimer collisions have been observed with small ice crystals as shown in Figure 1, and so an important question is whether small ice crystals can break off rime surface features. Using ice crystals to check this is complicated by the crystals themselves because they can be mistaken for ejected rime fragments. So, lycopodium spores of about 20-#m diameter were dropped in the path of the rotating 9533