Data from a 1996 Colorado experiment is illuminating a new class of lightning flashes thousands of times faster than those previously observed. NCAR scientist Eric Defer is analyzing data on a set of short-duration intracloud flashes that can play out as quickly as 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.) He presented his findings this week at the annual meeting of the American 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 a French VHF interferometer deployed at STERAO can profile single flashes in three dimensions and judge duration. Out of about 5,400 flashes observed in the July 10 storm, 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 was the instrument's sharpest resolution.
"Locally strong electric field may explain the ignition of these flashes, but we don't understand why they don't last longer," says Defer.
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 July 10, 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 the most intense reflectivities (radar returns from water and ice) found at those heights. Thus, according to Defer, the ultraquick flashes might someday serve as a real-time tool for judging storm severity.
NCAR is managed by the University Corporation for Atmospheric Research, a consortium of more than 60 universities offering Ph.D.s in atmospheric and related sciences.
Writer: Bob Henson