SOME CHARACTERISTICS OF CLOUD-TO-GROUND LIGHTNING IN TROPICAL NORTHERN AUSTRALIA

Peak currents and polarities for the first return strokes of approximately 5000 cloud-to-ground lightning flashes have been studied. The flashes were associated with eight mesoscale convective systems (MCSs) that occurred in tropical northern Australia in 1989 during the second field phase of the Down Under Doppler and Electricity Experiment (Dundee). Statistical analysis of the lightning data produced an average peak current (independent of polarity) of 39 kA. When considered with the average cloud-top heights of the tropical MCSs examined herein, the 39-kA peak current average appears to be in general agreement with Orville's (1990) hypothesis of latitudinal variation in peak current. In six of the Dundee MCSs consisting of both convective and stratiform precipitation components, we found that positive peak current maxima tended to occur in the trailing stratiform region while positive peak current minima were generally situated in convective precipitation. We also observed this pattern in a middle-latitude MCS that occurred on June 10-11, 1985. The magnitude of the positive peak current maxima in the tropical MCSs and the middle-latitude MCS increased coincidentally in time with the growth of the stratiform regions, and reached peak values when the stratiform regions were most intense. Given the apparent dependence of positive peak current magnitude on the life-cycle of the stratiform region and the spatial separation between positive peak current maxima and the convective line, we offer further support for a noninductive charging mechanism being responsible for the electrical charging and subsequent lightning observed in the trailing stratiform regions of MCSs.