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Winter 2000

New category of lightning found

by Bob Henson

The phrase "lightning fast" may take on new meaning through a new class of flashes being explored by scientists in NCAR's Mesoscale and Microscale Meteorology Division (MMM) and their colleagues. A cloud-to- ground lightning flash with multiple strokes can last more than a second, and intracloud flashes average about a quarter of a second long. However, instruments deployed in Colorado in 1996 have documented a separate set of short-duration intracloud flashes that can play out as quickly as 23 millionths of a second, and perhaps even faster than that.

MMM's Eric Defer presented recent findings at the December American Geophysical Union meeting. Defer has been studying a Colorado storm from the deep convection field project of STERAO (Stratosphere-Troposphere Experiment: Radiation, Aerosols and Ozone). Most lightning sensors report only location and polarity, but a French VHF interferometer deployed at STERAO profiles single flashes in three dimensions and judges their durations. Out of about 5,400 flashes observed in a storm on 10 July 1996, only 83 were cloud-to-ground. More than 800 intracloud flashes had durations of less than a millisecond. Many of these lasted no more than 23 microseconds, which is the instrument's sharpest resolution.

Several other observing systems have spotted these brief flashes in the past few years, but Defer has produced the first analysis relating such flashes to radar output. For 10 July 1996, as well as for several other cases from STERAO, the short-duration lightning tends to occur at heights of 6 to 10 kilometers (4–7 miles) within the storm, in close proximity to the strongest updrafts and highest reflectivities (radar returns) found at those heights. Thus, Defer says, the ultraquick flashes might someday serve as a real-time tool for judging storm severity.

What makes the flashes so brief? "A locally strong electric field may explain the ignition of the flashes, but we don't understand why this population of flashes does not last longer," says Defer. He's working with MMM colleagues James Dye and Jessica Hagen, as well as Diana Bartels and Thomas Matejka (both of the National Severe Storms Laboratory), to analyze more STERAO storms and sort out possible causes of the short-duration lightning. "We have an incredible set of data, but it takes time to analyze."

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