Continuous broadband digital interferometry of lightning using a generalized cross-correlation algorithm

The VHF Broadband Digital Interferometer developed by Osaka University has been improved to allow continuous sampling over the entire duration of a lightning flash and to utilize a generalized cross-correlation technique for determining the lightning source directions. Time series waveforms of 20-80 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 degrees and time resolution better than 1 mu s. 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.