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...

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Bibliographic Details
Published in:Journal of geophysical research. Atmospheres 2014-03, Vol.119 (6), p.3134-3165
Main Authors: Stock, M. G., Akita, M., Krehbiel, P. R., Rison, W., Edens, H. E., Kawasaki, Z., Stanley, M. A.
Format: Article
Language:English
Subjects:
Algorithms
Broadband
Digital
Geophysics
interferometer
Interferometers
Interferometry
K-leader
Lightning
Lightning flashes
Noise levels
positive breakdown
Radiation sources
Three dimensional
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Summary: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° 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. Key Points Cross‐correlation algorithm for continuously sampled DITF data Quasi 3‐D conversion of 2‐D interferometric maps of lightning Measured 3‐D velocities of K‐events as a function of time
ISSN:2169-897X
2169-8996
DOI:10.1002/2013JD020217