Oct. 4, 2004 Arlington, Va. -- Newly documented small-scale circulations embedded in thunderstorm squall lines not only spew destructive straight-line winds, but may spawn up to 20% of all U.S. tornadoes. Remnant circulations from large thunderstorm clusters can survive for days, triggering new storm cells.
Scientists expect these and other findings to improve forecasts of damaging winds and heavy rain.
The results emerge from three-dimensional portraits of thunderstorms collected across the stormy Midwest in a field study coordinated by the National Center for Atmospheric Research (NCAR) in Boulder, Colo. Based just east of St. Louis, the Bow Echo and MCV Experiment (BAMEX) employed aircraft and ground-based storm chasers to document a wide range of storm types that prowled the Midwest from May to July 2003. Funding for the $4 million study was provided by the National Science Foundation (NSF), NCAR's primary sponsor.
A summary of BAMEX results will be presented on October 5 in Hyannis, Mass., at the American Meteorological Society's 22nd Conference on Severe Local Storms.
"BAMEX is beginning to yield significant results," says Cliff Jacobs, program director in NSF's division of atmospheric sciences. "The collaborative efforts of many researchers have resulted in new knowledge important to understanding severe storms and the tornadoes they often spawn."
Among the most noteworthy results:
* Size doesn't equal strength when it comes to bow echoes, the arc-shaped squall lines that produce intense straight-line winds and spawn dozens of tornadoes each year. "The greatest damage was typically observed not in the most extensive bow echoes, but in smaller ones spanning 60 miles or less," says NCAR scientist Christopher Davis, co-director of BAMEX. Within these small bows, even smaller circulations appear to focus most of the storm's destructive power and are the highest risk for tornadoes.
* Mesoscale convective vortices (MCVs)--areas of low pressure similar in strength and size to weak tropical depressions--extend through a deep layer of the atmosphere. MCVs, which can trigger several-day rounds of intense thunderstorms, tend to destabilize the atmosphere as they pull warm, moist surface air northward, spawning storm after storm as they go.
Scientists from more than a dozen colleges and universities joined NCAR and the National Oceanic and Atmospheric Administration (NOAA) for BAMEX.
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