REGIONS WHERE LIGHTNING FLASHES BEGAN

Regions where 773 flashes began during 13 thunderstorms were located by calculating centroids of the sources of the first six or 10 VHF pulses that were emitted by each flash. Sources were located by measuring differences in the times at which their pulses arrived at five widely spaced receivers stationed on the ground. We found that the distribution or origin heights was bimodal with peaks at 5.3 and 9.2 km above mean sea level (amsl). Standard errors in the coordinates of flash origins were estimated to be 20-80 m in X and Y and 160-240 m in the height coordinate Z. There were 431 flashes in the lower group and 342 in the higher. Flashes in the lower group were more numerous in 10 storms; in three storms, high flashes were in the majority. There was evidence to suggest that this condition depended partly on the phase of the host thunderstorm. Recorded E field changes produced by 165 flashes whose paths we had mapped convinced us that the vast majority of the 773 flashes, including the 342 high flashes, had also been negative. Flash origins tended to cluster in regions that were a few kilometers or less in horizontal diameter. Densities of flash origins in these regions ranged from 1.25 to 25 km-3. The origins of 658 flashes were mapped onto radar precipitation patterns of their host storms. We found that 66% of the flashes began within one picture element, approximately 270 m, of the contours for 20 dBZ; 27% began inside these contours, and most of these began at edges of high-reflectivity cores. The remaining 7% began outside the 20-dBZ contours. Similar results (73% : 19% : 8%) were obtained for 276 high flashes that began at heights above 7.4 km amsl, but 195 ground flashes scored 54% : 36% : 9% and showed a greater tendency to begin inside the 20-dBZ contours. The distribution of distances between origins and their nearest 20-dBZ contours showed a marked peak near zero. We concluded that charge-density in thunderclouds was affected by the presence of heavy precipitation and that 20-dBZ surfaces enclosed regions that carried excess negative charge.