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Walter A. Lyons, Thomas E. Nelson, Earle R. Williams, Steven A. Cummer, and Mark A. Stanley

Abstract

During the summer of 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) program deployed a three-dimensional Lightning Mapping Array (LMA) near Goodland, Kansas. Video confirmation of sprites triggered by lightning within storms traversing the LMA domain were coordinated with extremely low frequency (ELF) transient measurements in Rhode Island and North Carolina. Two techniques of estimating changes in vertical charge moment (M q) yielded averages of ∼800 and ∼950 C km for 13 sprite-parent positive polarity cloud-to-ground strokes (+CGs). Analyses of the LMA's very high frequency (VHF) lightning emissions within the two mesoscale convective systems (MCSs) show that +CGs did not produce sprites until the centroid of the maximum density of lightning radiation emissions dropped from the upper part of the storm (7–11.5 km AGL) to much lower altitudes (2–5 km AGL). The average height of charge removal (Z q) from 15 sprite-parent +CGs during the late mature phase of one MCS was 4.1 km AGL. Thus, the total charges lowered by sprite-parent +CGs were on the order of 200 C. The regional 0°C isotherm was located at about 4.0 km AGL. This suggests a possible linkage between sprite-parent CGs and melting-layer/brightband charge production mechanisms in MCS stratiform precipitation regions. These cases are supportive of the conceptual MCS sprite-production models previously proposed by two of the authors (Lyons and Williams).

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Walter A. Lyons, Thomas E. Nelson, Russell A. Armstrong, Victor P. Pasko, and Mark A. Stanley

A variety of storm top electrical discharges have been observed using several types of low-light imagers, film, and the human eye. Recently, a video recorded an unprecedented, bright blue upward discharge from a tropical thunderstorm top near Puerto Rico. The event reached the base of the ionosphere. The horizontal dimensions of cloud top discharges can range from 100 m to several kilometers. Upward extents vary from 100 m to 70 km. Shapes include “points” of light, upwardly flaring trumpets, and narrow, vertical, lightning-like channels, some topped with expanding blue, flame-like features. 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 night vision whereas some upward discharges have been clearly seen during daylight. Cloud top “pixies” last no longer than 16.7 ms, whereas upward lightning-like channels are often characterized as long lasting (2.0 s or more). To date, optical measurements have not associated cloud-top events with specific lightning flashes. There is a strong tendency for all such events to occur above the convective dome of rapidly intensifying thunderstorms. It is possible that the great diversity of forms illustrates the complexity inherent in the upward streamer mechanism for blue jets. 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 should focus on rapidly growing convective storm tops, including supercells and intense oceanic storms, as opposed to the stratiform regions of large mesoscale convective systems that have characterized sprite observations to date.

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Walter A. Lyons, Steven A. Cummer, Mark. A. Stanley, Gary R. Huffines, Kyle C. Wiens, and Thomas E. Nelson

Over a decade of monitoring mesospheric transient luminous events (TLEs) above U.S. high plains storms confirmed sprites are almost exclusively associated with positive polarity cloud-to-ground lightning (+CGs). Following C. T. R. Wilson's theory proposed in 1925, only those +CGs lowering large amounts of charge to ground should induce sprites. The key metric, the charge moment change, generally must exceed ~600 C km to initiate the electric breakdown at 75 km, which evolves into the sprite. High plains storms generate the highest percentage, the largest average peak current, and highest density of +CGs in the nation. Various storm types generate +CGs, and especially supercells are often dominated by positive strokes. Few sprites observations above supercells have been obtained (and usually during their decaying phase), while thousands of sprites have been imaged above mesoscale convective system (MCS) stratiform regions and some squall lines. During the 2000 Severe Thunderstorm Electrification and Precipitation Study (STEPS), two supercells were examined. One storm contained >90% +CGs, but none exceeded the sprite charge moment change threshold. A second nocturnal supercell did produce sprites from the last two +CGs of the storm as a stratiform region developed, more favorable for significant continuing currents to follow the +CG return stroke. Unexpectedly, three sprites occurring during the most intense phase of the storm were triggered by unusually intense and impulsive +CGs, which lowered sufficient charge in the return stroke alone for sprite initiation. Such +CGs, and thus sprites, are probably relatively rare events during the supercell mature stage.

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Stephen Baxter, Gerald D Bell, Eric S Blake, Francis G Bringas, Suzana J Camargo, Lin Chen, Caio A. S Coelho, Ricardo Domingues, Stanley B Goldenberg, Gustavo Goni, Nicolas Fauchereau, Michael S Halpert, Qiong He, Philip J Klotzbach, John A Knaff, Michelle L'Heureux, Chris W Landsea, I.-I Lin, Andrew M Lorrey, Jing-Jia Luo, Andrew D Magee, Richard J Pasch, Petra R Pearce, Alexandre B Pezza, Matthew Rosencrans, Blair C Trewin, Ryan E Truchelut, Bin Wang, H Wang, Kimberly M Wood, and John-Mark Woolley
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