HR: 09:45h
AN: AE31A-06    [PDF]
TI: Lightning Modes in Thunderstorms
AU: Krehbiel, P
EM: krehbiel@ibis.nmt.edu
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 United States
AU: * Rison, W
EM: rison@ee.nmt.edu
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 United States
AU: Thomas, R J
EM: thomas@ee.nmt.edu
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 United States
AU: Hamlin, T
EM: thamlin@nmt.edu
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 United States
AU: Harlin, J
EM: jharlin@nmt.edu
AF: Geophysical Research Center, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 United States
AB: The Lightning Mapping Array (LMA) shows the complete sequence of lightning discharges in convective storms and has expanded our understanding of the types of lightning discharges that can occur in the storms. In addition to the intracloud (IC) and cloud-to-ground (CG) discharges of normal-polarity storms, which occur between the mid-level negative and upper positive charge regions, and between the negative charge and ground, the LMA data show with surprising clarity the presence of lower positive charge in the storm, with some CG flashes discharging lower positive charge on their way to ground and others going straight to ground. In addition, the data also show the occurrence of `low-altitude' ICs between the mid-level negative and lower positive charges, that do not go to ground. Finally, the observations delineate the mechanism of `bolt from the blue' type flashes, which begin as normal-polarity upward-developing IC discharges in which the upper positive charge appears to be weak in comparison to the mid-level negative charge, resulting in the negative breakdown exiting the side of the cloud (through apparent positive screening charge along the radar cloud boundary) and going to ground as a -CG up to 5 or 10 km away from the storm center. Such discharges are surprisingly common and are even sometimes the dominant CG lightning type of a storm. The above behaviors of the lightning can be explained in terms of the relative amounts of the positive and negative charges in the different parts of the storm. The question as to what types of discharges will be occurring can be explained in terms of the energy of the storm charge distribution and how the energy would be changed by the different types of discharges, with the discharge types tending to be the most energetically preferred ones as time progresses. In addition to the above `normal-polarity' lightning types, the LMA observations show where positive CG (+CG) discharges originate and how they develop inside a storm. They have also revealed the surprising occurrence of inverted-polarity IC discharges between main mid-level positive charge and upper negative charge. Inverted polarity ICs are the dominant lightning type in anomalously electrified storms and provide a good indicator of the anomalous electrification. The anomalous storms tend to be supercell or severe storms, but not all such storms are anomalously electrified. The discharges sometimes indicate a complex, rapidly evolving, multilayer charge structure but often indicate simple, inverted-type dipolar structures in which relatively shallow negative charge is above a dominant, deeper positive charge. The anomalous storms can go for long periods of time (or for their entire lifetime) without producing CG discharges, something that does not occur in normally electrified storms. The inverted polarity ICs can be bi-level in nature or can propagate large vertical distances downward through the storm precipitation, something never seen in normal-polarity ICs. Finally, the LMA has provided greatly expanded observations of short-duration (sub-ms) discharges in storms. The short duration discharges are temporally isolated and can occur as precursor events to full-fledged lightning or as spatially limited or attempted breakdown events. They occur much more commonly in anomalous than in normally electrified storms, and are associated with the upper negative charge region and convective surges in the anomalous storms. A more complete understanding of the short-duration discharges should provide important clues about the processes by which lightning is initiated in storms.
DE: 3304 Atmospheric electricity
DE: 3324 Lightning
DE: 3360 Remote sensing
SC: Atmospheric and Space Electricity [AE]
MN: 2003 Fall Meeting