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.