A NUMERICAL STUDY OF THUNDERCLOUD ELECTRIFICATION BY GRAUPEL-CRYSTAL COLLISIONS

A three-dimensional numerical study to evaluate the electrification ca used by only the ice crystal-graupel collisions mechanism is presented . The electrification model includes the dependencies of the magnitude of the separated charge with the crystal size and the relative veloci ty. In addition, the dependence of the reversal temperature (T-r) with the liquid water content is taken into account following two differen t experimental data sets: Takahashi [1978] and Saunders et al. [1991]. The particle spectrum is discrete with eight categories. The results show that using the first data set, the tripolar structure is obtained producing magnitudes of the charge densities in accordance with in si tu measurements, whereas using the second data set, an inverted dipole (negative on positive) is obtained. Possible causes for these discrep ancies are discussed. Also, the charge on individual particles is disc ussed in the frame of experimental, data reported by airborne in situ measurements.