Extended Abstract (544K)10A.1
Electrical Discharges from Thunderstorm Tops
Walter A. Lyons, FMA Research, Inc., Fort Collins, CO; and T. E. Nelson, V. P. Pasko, and M. A. Stanley
Since their confirmation using low-light television (LLTV) in 1989, over 10,000 sprites have been imaged high above thunderstorm systems in response to unusual positive polarity cloud-to-ground lighting flashes. Sprites initiate near 70-75 km and propagate streamers both downwards and upwards, extending typically from between 90 km to 40 km. Initially misnamed “cloud-to-stratosphere lightning,” sprites occur largely in the mesosphere, are not lightning in the conventional sense of the word, and there is no conclusive evidence they have a physical connection with the cloud tops below. Yet sprite research campaigns have also uncovered distinctly different classes of electrical phenomena. A growing variety of storm top electrical discharges have been observed using several types of LLTV imagers, film and even the human eye. During STEPS, the first ground-based LLTV images of such unusual cloud-top discharges were obtained and have undergone extensive analysis and comparison with other photographic and anecdotal eyewitness reports. The variety of sensors makes comparison of these observations difficult. The horizontal dimensions of the events can range from 100 m to several km. Upward extents vary from 100 m to 70 km. Shapes includes “points” of light to upwardly flaring trumpets to narrow vertical lightning-like channels, some topped with flaring blue flame-like featrues. Some, such as a recent video obtained above a storm near Puerto Rico, appear to develop considerable upward branching structure. Visual appearances range from brilliant white lightning-like channels to a grainy, almost particulate appearing jets of dim blue light, and sometimes as a blue flame within which a brilliant white channel appears. The classical blue jet is at the lower limit of human vision whereas some upward discharges have been clearly seen during full daylight. The cloud top “pixies” last no longer than 16 ms, whereas the upward lightning-like channels are often characterized as long lasting (order of 0.5 s to 2.0 s). None of the events, however, have been associated with a specific IC or CG discharges. There is a strong tendency for all such events to occur above the convective dome of rapidly developing intense thunderstorms. It is possible that the great diversity of forms taken by these discharges illustrates the complexity inherent in the upward positive streamer mechanism. Appearances may be modulated by ambient pressures ranging from a large as 350 hPA to less than 5 hPA. It is also possible that the basic blue jet is only one of several distinct classes of discharges from highly electrified storm cloud tops. Future research will need to focus on the regions atop rapidly growing convective storms, including supercells, as opposed to the stratiform regions of large MCS which has characterized sprite observations to date.
Supplementary URL: http://pasko.ee.psu.edu/Nature
Session 10A, Results from the Severe Thunderstorm Electrification and Precipitation Study (STEPS) Experiment
Wednesday, 14 August 2002, 1:30 PM-3:00 PM
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