SAN FRANCISCO--Data from a 1996 Colorado experiment is illuminating anew class of lightning flashes thousands of times faster than thosepreviously observed. NCAR scientist Eric Defer is analyzing data on aset of short-duration intracloud flashes that can play out as quicklyas 23 millionths of a second, and perhaps even faster than that.(Intracloud flashes average about a quarter of a second in duration.Cloud-to-ground lightning flashes can last more than a second.) Hepresented his findings this week at the annual meeting of theAmerican Geophysical Union.
Defer has been studying a Colorado storm that occurred on July 10,1996, during the deep convection field project of STERAO(Stratosphere-Troposphere Experiment: Radiation, Aerosols and Ozone).Most lightning sensors report only location and polarity, but aFrench VHF interferometer deployed at STERAO can profile singleflashes in three dimensions and judge duration. Out of about 5,400flashes observed in the July 10 storm, only 83 were cloud-to-ground.More than 800 intracloud flashes had durations of less than amillisecond. Many of these lasted no more than 23 microseconds, whichwas the instrument's sharpest resolution.
"Locally strong electric field may explain the ignition of theseflashes, but we don't understand why they don't last longer," saysDefer.
Several other observing systems have spotted these brief flashes inthe past few years, but Defer has produced the first analysisrelating such flashes to radar output. For July 10, 1996, as well asfor several other cases from STERAO, the short-duration lightningtends to occur at heights of 6 to 10 kilometers (4-7 miles) withinthe storm, in close proximity to the strongest updrafts and the mostintense reflectivities (radar returns from water and ice) found atthose heights. Thus, according to Defer, the ultraquick flashes mightsomeday serve as a real-time tool for judging storm severity.
NCAR is managed by the University Corporation for AtmosphericResearch, a consortium of more than 60 universities offering Ph.D.sin atmospheric and related sciences.
Materials provided by National Center For Atmospheric Research (ncar). Note: Content may be edited for style and length.
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