Vertical Development of Lightning Activity Observed by the LDAR System: Lightning Bubbles

Abstract

In some Florida thunderstorm cells, impulsive very high frequency (VHF) radiation from lightning channels begins abruptly in a layer that is typically 3?6 km in diameter, 1?3 km tall, and initially located just above the freezing level. In 208 cases described here, 58% of the lightning bubbles ascended with a velocity 11?17 m s?1. Of the 1060 1-h periods with lightning that were examined, approximately 10% had one or more ascending concentrations of lightning activity. Often in summer, as one region ascended, a new lightning bubble would abruptly begin near the freezing level. This subsequent region would be horizontally displaced 1?2 km from the starting point of the previous region. In winter, no more than one ascending region was seen in any one storm. A detailed examination of the structure of lightning associated with the rising layers of lightning activity indicates that these layers were composed of negative leaders, which tend to propagate through positive charge. This suggests that the rising layer of lightning activity is due to the ascent of an upper positive charge in storms.