Evolution of lightning and the possible initiation//triggering of lightning discharges by the lower positive charge center in an isolated thundercloud in the tropics

The evolution of lightning and the shape of recovery curves after multiple-discharge flashes in a thundercloud have been studied from the surface measurements of electric field and Maxwell current near a tropical thundercloud. Observations suggest a tripole structure of the cloud and that its lower positive charge center (LPCC) plays a dominant role in initiating/triggering an intracloud (IC) or cloud-to-ground (CG) lightning discharge. IC discharges in the initial stage of thundercloud are followed by CG discharges from the LPCC and then by two distinct groups of multiple-discharge flashes. Each flash in the first group consists of an IC discharge triggered by a CG discharge and in the second group a CG discharge triggered by an IC discharge. Flashes in each group are bunched together for similar to15-20 min and occur with almost a regular periodicity of 1-1.5 min. The Maxwell current during every such flash in both groups has a bipolar transient and a positive overshoot that subsequently relaxes back to its predischarge value. The magnitudes of overshoot for the flashes in the first group are found to be much lower than those for the flashes in the second group. From a small portion of the recovery curves of such multiple-discharge flashes, one can conclude that the rate of charge buildup in the main negative charge center is higher than that in the LPCC.