How Lightning is Initiated in Storms

How lightning is initiated in storms in the absence of electrical conductors
is a long-standing, unanswered 'holy grail' question of lightning studies.
The question has recently been addressed by Rison et al. (2016) from studies at
Langmuir Laboratory of an equally mysterious and unusual type of lightning
discharge called narrow bipolar events (NBEs). NBEs are high speed, prolific
transient radiators of high power (>10-100 kW) VHF radiation that often occur
at the beginning of intracloud (IC) flashes (Rison et al., 1999, Smith et al.,
1999). Utilizing a newly-developed flash-continuous broadband VHF
interferometer (INTF) in combination with Langmuir's 3-D Lightning Mapping
Array (LMA) and fast electric field sferic measurements, NBEs have been found
to be produced by a previously unrecognized type of discharge process called
fast positive breakdown. The breakdown is found to occur with a wide range of
sferic and VHF strengths, and to be the initiating event of many and potentially
all in-cloud (IC and cloud-to-ground (CG)) lightning discharges.

The fast positive breakdown is inferred to consist of a system of repeated,
cascading positive streamers that are initiated on ice hydrometeors in a
localized region of intense electric field. The streamers do not produce a
conducting channel, but self-intensify the electric field at their starting
points to values sufficient for initiating conventional negative air breakdown
and an ensuing flash. In this presentation we summarize observational data in
support of these findings.

References:

Rison W., P.R. Krehbiel M.G. Stock, H.E. Edens, X-M. Shao, R.J. Thomas,
M.A. Stanley, and Y. Zhang, Observations of narrow bipolar events reveal
how lightning is initiated in thunderstorms, Nature Comms. 7, 2016.
doi:10.1038/ncomms10721.

Rison, W., Thomas, R. J., Krehbiel, P. R., Hamlin, T. and Harlin, J.,
A GPS-based three-dimensional lightning mapping system: initial observations
in central New Mexico. Geophys. Res. Lett. 26, 3573–3576 (1999).

Smith, D. A. et al., A distinct class of isolated intracloud lightning
discharges and their associated radio emissions. J. Geophys. Res. 104,
4189–4212 (1999).