The effect on thunderstorm charging of the rate of rime accretion by graupel

Analysis of laboratory data concerning the charging of small graupel pellets in thunderstorms has shown that the charge transferred to a riming target during collisions with ice crystals is affected by the rate of rime accretion. Earlier, we found that the magnitude of the charges transferred depend on velocity; however, the velocity also controls the rate of target rime accretion which affects the sign of the charge transfer with higher accretion rates favoring positive rimers during crystal collisions. Experiments have now been conducted to confirm this effect and a set of equations are presented describing the charge transfer as a function of ice crystal size, velocity, temperature and rime accretion rate. As a first step in determining the importance of the new formulations on thunderstorm electrification, they have been tested in a one-dimensional numerical model. The model results indicate that the normal thunderstorm positive dipole can be generated in the observed time by means of ice crystal/graupel interactions. (C) 1997 Elsevier Science B.V.