THE EFFECT ON THUNDERSTORM CHARGING OF THE RATE OF RIME ACCRETION BY GRAUPEL

Analysis of laboratory data concerning the charging of small graupel p ellets in thunderstorms has shown that the charge transferred to a rim ing target during collisions with ice crystals is affected by the rate of rime accretion. Earlier, we found that the magnitude of the charge s 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 thi s effect and a set of equations are presented describing the charge tr ansfer as a function of ice crystal size, velocity, temperature and ri me accretion rate. As a first step in determining the importance of th e new formulations on thunderstorm electrification, they have been tes ted in a one-dimensional numerical model. The model results indicate t hat the normal thunderstorm positive dipole can be generated in the ob served time by means of ice crystal/graupel interactions. (C) 1997 Els evier Science B.V.