A MODELING STUDY ON THE EARLY ELECTRICAL DEVELOPMENT OF TROPICAL CONVECTION - CONTINENTAL AND OCEANIC (MONSOON) STORMS

Numerical modeling studies of continental tropical and maritime tropic al convection were conducted using the two-dimensional, nonhydrostatic , cloud electrification model developed at the South Dakota School of Mines and Technology. The model contains six classes of water (water v apor, cloud water, cloud ice, rain. snow, and graupel) and a full set of ion equations. All hydrometeors are permitted to exchange charge. C harge transfer between microphysical species is accomplished through a noninductive charging parameterization following Takahashi. The goal of the numerical experiments was to examine the kinematic and microphy sical differences that lead to marked differences in observed electrif ication between the break (continental) and monsoon (oceanic) convecti ve regimes observed near Darwin, Australia. The break regime is associ ated with deep, intense convection that forms in high-CAPE (convective available potential energy) environments. Normally, copious amounts o f lightning accompany break period convective events. Monsoon conditio ns are associated with heavy rain and relatively weak convection that forms in moderate to low-CAPE environments. Very little lightning acti vity is normally observed in the monsoon. Three numerical simulations ranging from high- to low-CAPE conditions are presented. The results i ndicate that the electrification of the simulated storm critically dep ends on the juxtaposition of the level of charge reversal (LCR), which is in turn dependent on temperature and liquid water contents, and th e particle interaction region, which is the level where ice particle c ollisions occur and thus where noninductive charging can take place. I n the high-CAPE (break period) case, the LCR is located several kilome ters below the interaction region, and strong in-cloud electric fields develop as a consequence. In the low- to moderate-CAPE (monsoon) case s, the LCR and interaction region are closely located in the vertical. As hydrometeors move across the LCR in both directions, the charge on their surfaces continually changes sign, thus preventing the developm ent of a significant in-cloud electric field. It is further hypothesiz ed that in conditions of zero to extremely low CAPE, the particle inte raction region would be situated below the LCR, leading to the develop ment of an inverted dipole (positive charge underlying negative charge ), such as may occur in the stratiform regions of mesoscale convective systems.