LIGHTNING GENERATION IN A JOVIAN THUNDERCLOUD - RESULTS FROM AN AXISYMMETRICAL NUMERICAL CLOUD MODEL

An axisymmetric cloud model with detailed microphysics was used to cal culate the electrical development of a jovian water cloud. The charge separation mechanism was based on the noninductive interaction between ice crystals and graupel particles in the presence of supercooled liq uid droplets. The results show that for plausible concentrations of cl oud condensation and freezing nuclei in the atmosphere, the electric f ield in a jovian thundercloud builds rapidly and exceeds its breakdown value. Low concentrations of cloud condensation nuclei produced shall ow clouds and did not separate enough charge to produce critical field s. We simulated the lightning discharge by assuming that a flash occur red wherever the electric field, which resulted from the obtained spac e charge configuration, exceeded the local breakdown value. A lightnin g flash was assumed to neutralize a 20-km channel within the cloud. Th e calculated flash rate suggests that the 192-sec exposure time of Voy ager 1 camera, may have captured several superimposed lightning flashe s. The energy calculated for such a simulated lightning discharge was of the order of 10(12)-10(13) J, which translates to 10(9)-10(10) J in the optical energy range, in agreement with Voyager's observations of Jupiter's lightning. (C) 1995 Academic Press, Inc.