The Heat Balance of a Riming Graupel Pellet and the Charge Separation during Ice–Ice Collisions

Abstract

It has been suggested that the sign of charge acquired by a riming graupel pellet during ice crystal interactions depends on its surface state being negative when it is evaporating and positive when growing by vapor diffusion. Experiments were conducted to determine the surface states of riming cylinders in two previous laboratory studies of thunderstorm electrification. Calculations of the heat balance of the simulated graupel pellets are considered in terms of experimental measurements of the charge transferred under similar conditions. The results show that the contribution from in-cloud supercooled droplets is not always adequate to enable the graupel surface to grow by vapor diffusion in all laboratory cloud conditions where it acquires a net positive charge. It is suggested that factors such as the vapor fields around droplets freezing on its surface and the shape and surface roughness of the graupel may play important roles in determining the surface state and hence the overall sign of the charge transfer. Furthermore, the graupel charges negatively under specific conditions where it is clearly growing from the vapor, confirming that it is imperative to consider the relative growth rate of the two interacting surfaces rather than that of the graupel alone. It is concluded that, during the interaction of two ice particles, the positive charge is generally acquired by the surface that is growing faster or evaporating slower at the point of impact. A suitable physical mechanism may be provided by the liquidlike layer hypothesis of Baker and Dash, and it is suggested how it may be extended to include the cases when one or both of the surfaces are evaporating.