HR: 0830h
AN: AE11A-05    [Abstracts]
TI: On the Initiation of Upward Lightning Discharges Above Thunderstorms
AU: * Krehbiel, P
EM: krehbiel@ibis.nmt.edu
AF: Langmuir Laboratory, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801
AB: A simple cylindrical charged disk model has been used to study the occurrence of lightning in thunderstorms. The model uses three-dimensional lightning mapping data to estimate the heights, thicknesses, and diameters of the charge regions in an actual storm, and assumes an exponential decrease in the lightning breakdown threshold with altitude. The storm charging currents and charge amounts are estimated by matching the predicted and observed flashing rates for both intracloud and ground flashes. The model was developed as a means of quantitatively interpreting and simulating balloon-borne electric field soundings and lightning mapping data in storms but provides additional insights into the occurrence of lightning discharges. For example, the model simulations clearly show the importance of lower positive charge in initiating cloud-to-ground (CG) lightning; the presence of such charge enhances the electric field to breakdown values between the main negative and lower positive charge regions of normal polarity storms, whereas without lower positive charge the electric field strengths do not reach breakdown values in the presence of ongoing intracloud activity. A similar, surprising result of the model calculations concerned the effect of the negative screening charge attracted to the cloud top by the upper positive charge region of the storm. The screening charge enhances the electric field between it and the upper positive charge to the point that would initiate a discharge between the two regions. Because the magnitude of the positive charge is greater than that of the screening charge, the discharge would develop vertically upward above the storm top, just as a CG flash usually travels on through the lower positive charge to ground. Observations show that such discharges usually do not occur, implying that the upper screening charge is somehow removed, probably by being dissipated into the upper positive charge. In the absence of such dissipation, or in the presence of some extraordinary triggering event (such as an energetic cosmic ray), upward discharges would or could be initiated. This is likely to be the cause of blue jets, blue starters, and other luminous phenomena observed at and above the tops of thunderstorms. The conditions for triggering upward discharges are enhanced by the occurrence of CG discharges, which remove negative charge from storm mid-levels and suddenly enhance the electric field between the upper positive charge and the screeing charge. Upward discharges are therefore likely to occur relatively soon after negative polarity CG flashes in a storm. Upward discharges from normal polarity storms would transport positive charge upward and be generative of the atmospheric electric potential.
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
DE: 3362 Stratosphere/troposphere interactions
SC: Atmospheric and Space Electricity [AE]
MN: 2005 Joint Assembly