The spatial and temporal development of intracloud lightning

A detailed study of the complete sequence of VHF radiation events during intracloud (IC) flashes in Florida has shown that IC flashes often have a bilevel structure connected by a single upward channel. The lower and upper level channels appear to correspond to the main negative and upper positive charge regions of the storm, respectively. The IC flashes are characterized by active and final stages, each comprising about half of the overall flash duration. During the active stage, negative charge is transported upward in the cloud, first as a result of initial breakdown, which establishes the upward channel, and subsequently as a result of repeated breakdown events from the lower to the upper level. For the flashes of this study, the initial breakdown lasted 10 to 20 ms and propagated upward at a speed of 1.5 to 3 x 10(5) m s(-1). It also established the beginning of one or more horizontal branches in the upper positive charge region, which were extended by the subsequent breakdown. Little or no radiation was detected along the upward channel during the subsequent breakdown, indicating that the channel remained more or less continuously conducting following the initial breakdown. After a time delay, the subsequent breakdown extended the lower-level channels in a retrograde manner horizontally away from the flash origin. At the end of the active stage the upward channel ceases to be conducting and the remaining, final stage of a flash is characterized by fast (10(6) to 10(7) m s(-1)), well-defined ''K''-type streamers that begin at successively greater distances along the lower-level channel and transport negative charge toward or into the base of the upward channel. The streamers are identical to those observed during the interstroke intervals and final stages of cloud-to-ground (CG) flashes. Typically, a few late-flash K streamers continue through the flash origin into and along an upper-level channel. The overall results are consistent with other observations of IC hashes and explain several features of the observations. Similar results are obtained for IC flashes in New Mexico storms. Comparison of the initial breakdown of IC and CG flashes shows that the IC breakdown is more intermittent than that of CG flashes, even though both have the same polarity and propagate at about the same average speed.