HR: 08:45h
AN: AE31A-03    [PDF]
TI: Laboratory Investigation of Positive Streamer Discharges From Simulated Ice Hydrometeors and Possible Relevance to the Problem of Lightning Initiation
AU: * Petersen, D
EM: danyalpetersen@hotmail.com
AF: Department of Physics and Astronomy, University of Oklahoma, Norman, OK 73019 United States
AU: Bailey, M
EM: bailey@dri.edu
AF: Division of Atmospheric Sciences, Desert Research Institute, Reno, NV 89512 United States
AU: Hallett, J
EM: hallett@dri.edu
AF: Division of Atmospheric Sciences, Desert Research Institute, Reno, NV 89512 United States
AU: Beasley, W
EM: whb@ou.edu
AF: School of Meteorology, University of Oklahoma, Norman, OK 73019 United States
AB: Observations have demonstrated that lightning can begin at very high altitudes at which environmental temperatures are well below -18$\deg$C, yet previous laboratory studies of positive streamer discharges from ice hydrometeors have been taken as evidence that ice hydrometeors at these cold temperatures likely do not contribute significantly to lightning initiation. The results of a new laboratory investigation indicate that simulated ice hydrometeors at temperatures as low as -38$\deg$C can initiate positive streamer discharges; however, relatively strong electric fields are required. Among many hypotheses for lightning initiation, we are concerned with two possibilities: 1) the hydrometeor-initiated intensifying positive streamer system, whereby a relatively weak (but still stronger than observed) background electric field can be locally intensified to levels believed sufficient for lightning leader formation, and 2) runaway breakdown, an alternative means of local electric-field enhancement that requires background electric-field strengths comparable with observational evidence. During the course of the laboratory investigation, a third possibility came to mind: perhaps reality is better explained by a combination of these two hypothesized processes. Perhaps an initial runaway electron avalanche need only result in a local electric-field enhancement sufficient to support initiation of positive streamer discharges from nearby hydrometeors. If so, any initial positive streamers should intensify rapidly and branch, resulting in significant electric-field enhancement at their origin. The positive feedback on the electric field should cause a succession of intensifying positive streamer systems, further enhancing the local electric field to strengths sufficient for the formation of a bipolar leader in a manner similar to that of the negative stepped-leader 'step'. Then perhaps this small bipolar leader could serve as the embryonic lightning discharge, proceeding to develop in a bipolar fashion up to the scale of typically observed lightning flashes. This paper presents results of the laboratory investigations.
DE: 0320 Cloud physics and chemistry
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
DE: 3354 Precipitation (1854)
DE: 3394 Instruments and techniques
SC: Atmospheric and Space Electricity [AE]
MN: 2003 Fall Meeting