THE SPATIAL AND TEMPORAL DEVELOPMENT OF INTRACLOUD LIGHTNING

A detailed study of the complete sequence of VHF radiation events duri ng intracloud (IC) flashes in Florida has shown that IC flashes often have a bilevel structure connected by a single upward channel. The low er and upper level channels appear to correspond to the main negative and upper positive charge regions of the storm, respectively. The IC f lashes are characterized by active and final stages, each comprising a bout half of the overall flash duration. During the active stage, nega tive charge is transported upward in the cloud, first as a result of i nitial breakdown, which establishes the upward channel, and subsequent ly as a result of repeated breakdown events from the lower to the uppe r level. For the flashes of this study, the initial breakdown lasted 1 0 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 branche s in the upper positive charge region, which were extended by the subs equent 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 br eakdown. After a time delay, the subsequent breakdown extended the low er-level channels in a retrograde manner horizontally away from the fl ash origin. At the end of the active stage the upward channel ceases t o be conducting and the remaining, final stage of a flash is character ized by fast (10(6) to 10(7) m s(-1)), well-defined ''K''-type streame rs that begin at successively greater distances along the lower-level channel and transport negative charge toward or into the base of the u pward channel. The streamers are identical to those observed during th e interstroke intervals and final stages of cloud-to-ground (CG) flash es. 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 f eatures of the observations. Similar results are obtained for IC flash es 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.