A laboratory study of the effects of rime ice accretion and heating on charge transfer during ice crystal/graupel collisions

In a laboratory study of thunderstorm electrification involving charge transfer between ice crystals and a riming graupel pellet, the effect on charge transfer of rimer heating by droplet accretion has been separated from the associated influence of the vapour flux to the rimer surface. This was accomplished by heating internally a riming target rod whose surface conditions represent those of a falling graupel pellet in thunderstorms while keeping the rate of rime accretion constant. The results show that the positive charging of a rimer may be reversed to negative by artificial heating, with increased heat required at higher rates of rime accretion. It is hypothesised that ice crystals rebounding from riming graupel pellets charge the graupel positively or negatively depending on the cloud and rimer conditions, which influence the relative thicknesses of charge carrying layers on the surfaces of the particles. In the natural case, negative charging of graupel is associated with rime surface heating, which reduces the vapour diffusional growth rate below that of the ice crystals, while positive charging of graupel is associated with vapour provision to the rimer surface from the freezing droplets, which overcomes the rime heating effect. This work compares the results of charge transfer to a riming target obtained in UMIST Manchester, involving multi-crystal interactions, with data from Cordoba Argentina involving single ice sphere interactions. The fact that broadly similar charging behaviour was seen in both studies suggests that it is the rate of growth of the ice surfaces, rather than their particular nature, that is the important factor in controlling the charge transfer during ice particle collisions with a riming ice surface.