STEPS: Severe Thunderstorm Electrification and Precipitation Study
Photo taken during STEPS by Jeremy Smith.
STEPS had two broad goals concerning storm electricity:
- to improve understanding of how severe storms become electrified and
- to better understand how variations in the type and flash rate of lightning relate to the type of severe storm and its evolution. Of particular interest were severe storms that produce unusual lightning activity. Unusual lightning includes ground flashes that lower positive charge to the ground instead of the usual negative charge and cloud flashes whose polarity of electric current is reversed from the polarity normally found in a given storm region.
STEPS collected data from May 22-July 16, 2000 near the Colorado-Kansas border, a region of the country with the highest incidence of positive ground flashes. Besides wanting to learn what electrical characteristics a storm must have to produce positive ground flashes, scientists wanted to confirm and learn why positive ground flashes often are associated with the production of large hail. They also wanted to study how changes in the dominant polarity of ground flashes are associated with changes in the structure of a storm relative to its potential for tornadoes and other severe weather.
STEPS also collected data to discover if inverted-polarity cloud flashes are caused by storms having unusual electrical structures. Inverted-polarity cloud flashes have only recently been discovered by new technology for mapping lightning inside clouds, but an extraordinary number appear to have occurred during STEPS. An initial assessment of STEPS observations is that the electrical structure of at least a few storms was completely inverted throughout the entire storm depth. Though scientists have searched for electrically inverted storms for many years, the few observations supporting their existence have been inconclusive. STEPS data are capable of providing conclusive evidence. To verify that such storms occur and to try to explain why, scientists are beginning a detailed intercomparison of electrical structure, lightning location and structure, wind structure, and the distribution of various types of precipitation inside storms.
STEPS dissects storm for over two hours
NSSL researchers evaluate their storm intercept strategy during a gas stop in Canadian, Texas on the evening of June 13, 2000, while a supercell thunderstorm capable of producing a tornado approaches the town. Photo by Daphne Zaras
NSSL researchers involved with the Severe Thunderstorm Electrification and Precipitation Study (STEPS) field program held operations on 27 days during an eight-week project near the Colorado-Kansas border. Scientists from nine institutions made a wide range of meteorological and electrical observations of supercell thunderstorms to better understand supercell physics. The most significant cases included two major downburst storms, two low-precipitation storms, two tornadic storms, and three supercells.
One of the top cases of the project occurred on June 29 when STEPS observed a supercell thunderstorm for more than two hours. The storms were scrutinized by Doppler radar, four electronic field meter launches, T-28 aircraft penetrations, visual observations of precipitation and a tornado, mobile mesonet instrument readings, and supporting sounding measurements by several mobile sounding systems. The Lightning Mapping Array reported flash rates of one per second at times.
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