(2014), Continuous broadband digital interferometry of lightning using a generalized cross-correlation algorithm, J. Geophys. Res. Atmos., 119, 3134-3165, doi:10.1002 MHz signals received at three orthogonally located antennas are continuously digitized over multisecond intervals, as opposed to sequences of short-duration triggers. Because of the coherent nature of the measurements, radiation sources are located down into the ambient receiver and environmental noise levels, providing a quantum leap in the ability to study lightning discharge processes. When postprocessed using cross correlation, the measurements provide angular uncertainties less than 1 • and time resolution better than 1 μs. Special techniques have been developed to distinguish between actual lightning sources and noise events, with the result being that on the order of 50,000-80,000 radiation sources are located for a typical lightning flash. In this study, two-dimensional interferometer observations of a classic bilevel intracloud flash are presented and combined with three-dimensional Lightning Mapping Array observations to produce a quasi 3-D map of lightning activity with the time resolution of the interferometer. As an example of the scientific utility of the observations, results are presented for the 3-D progression speed of negative leaders associated with intracloud K-leaders.

[1] The Lightning Research Group of Osaka University has been developing and improving the VHF broadband digital interferometer, which locates the impulsive VHF radiation sources caused by lightning discharges with extremely high time resolution in three dimensions. As a result of the VHF observations during the 2006-2007 monsoon season in Darwin, Australia, cloud flashes accompanied by K processes are clearly visualized in three dimensions with high time resolution. In the late stage of the cloud flashes, negative recoil streamers accompanied by K changes propagate along the same channels as that of preceding negative breakdowns in the early stage. The speeds of the negative recoil streamers are 2 orders of magnitude faster than the preceding negative breakdown. The negative recoil streamers are supposed to initiate from the tips of positive breakdowns that have progressed along the same path as previous negative breakdowns in the opposite direction.

[1] We examine VHF interferometric images, channel-base currents, and broadband electric field waveforms of the initial stage (IS) in two rocket-and-wire triggered lightning flashes. Two types of negative leaders, termed "long-duration" and "short-duration" leaders, were imaged by the VHF interferometers during the IS of the two flashes. There were three leaders that had relatively long durations of more than a few milliseconds. These three leaders were not accompanied by a significant change of channel-base current during their early stages of development, indicating that they corresponded to intracloud (IC) discharges that were not connected to the grounded triggered-lightning channel. Two of these three leaders eventually connected to the triggered-lightning channel and initiated initial continuous current (ICC) pulses. The third long-duration leader apparently developed from the vicinity of an isolated negative charge region toward an upper-level positive charge region and toward a branch of the grounded channel; it served to bridge the positive charge region and the triggered-lightning channel, resulting in the opposite polarity portion of the bipolar ICC. The short-duration negative leaders had durations of some hundreds of microseconds. These negative leaders apparently recoiled along the conductive channels created by branches of the upward positive leader (UPL); they initiated ICC pulses when the grounded channel was sufficiently conductive. It follows that ICC pulses can be initiated either by recoil leaders or via interception of separate in-cloud leaders by a grounded current-carrying channel.

The upgraded VHF digital interferometer (VHF DITF) system is introduced which can continuously sample the radiation associated with lightning. A new processing technique was implemented which uses the distribution of slopes of the phase difference versus frequency to locate the radiation source. By using this technique, frequency components which are not due to lightning can be excluded and low as well as high amplitude sources are located. As a result, both positive breakdown and negative breakdown are located, and negative recoil leaders (recoil leaders) are visualized in great detail. The recoil leaders which continue into the positive charge region are seen to slow their propagation and dim their radiation as they cross the flash initiation region. Analysis of the relative received power of the different breakdown types, negative leaders, recoil leaders, and positive leaders, also can be made. In both the intracloud and cloud-to-ground flash, the modes of the distributions of received power for negative leaders, recoil leaders, and positive leaders were approximately the same. The brightest emissions seen from the positive leader were substantially lower than the brightest emission seen from the negative leader. The results also indicate that positive leaders as well as lower elevation negative leader emit more low frequency radiation than recoil leaders and high-elevation negative leaders. By continuously sampling the VHF waveform, the upgraded VHF DITF locates many weak sources which the previous system was not capable of locating.

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The charge distributions in a thundercloud play an important role in the initiation and propagation of lightning discharges. To further understand the effects of charge distributions on lightning discharge, the authors conducted a very high-frequency (VHF) lightning observation campaign during the 2006/07 monsoon in Darwin, Australia, using a VHF broadband digital interferometer (DITF). A C-band polarimetric weather radar to estimate the precipitation profiles such as hydrometeor classification was operated by the Bureau of Meteorology (BOM) Research Centre. Cloud-to-ground (CG) and intracloud (IC) flashes were initiated from the outer and the inner parts of the upper side of the graupel regions, respectively. In the cases of CG flashes, the negative leaders travel first about 10 km horizontally through positive charge regions and then begin to bend toward the ground when they reach the edge of the positive charge regions where there is no graupel region underneath. In contrast, in the cases of the IC flashes the negatively charged graupel regions block the downward developments of negative leaders. It is noted that positive charge regions could facilitate the extension of the horizontal negative leader. These results may suggest that lightning flash types are closely dependent on their initiation locations and the surrounding charge distributions. The experimental results are consistent with other previous observation results and charge model simulations.

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