High technology meets the High Plains in STEPS-2000

May 03, 2000

Those tapered, bell-shaped thunderstorms that grace so many calendars and posters aren't just photogenic, they're a scientific mystery. Photographers love these so-called low-precipitation (LP) supercells because of their spectacular cloud formations and sparse rainfall. Yet researchers have generally ignored them: they seldom produce tornadoes or flooding, and they tend to occur across the High Plains instead of over more densely populated areas.

Scientists are now realizing that the unsung LP storm may hold a key to understanding the microphysics and electrification of other kinds of thunderstorms. That's why a group of researchers will set up shop in northwest Kansas and eastern Colorado for the Severe Thunderstorm Electrification and Precipitation Study (STEPS), funded in part by the National Science Foundation (NSF). Based in Goodland, Kansas, STEPS-2000 will take place between May 22 and July 15. Involved organizations include the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. NSF is NCAR's primary sponsor. NSF will support a STEPS radar system operated by Colorado State University and an airplane used for microphysical and electrical measurements in thunderstorms, operated by the South Dakota School of Mines and Technology, serving as national facilities for the study.

"This is our first opportunity to bring together several new techniques for observing the electrical properties of storms and the rain and hail they produce," says Rod Rogers, program officer in NSF's division of atmospheric sciences, which funds STEPS-2000. "We are expecting a greatly improved understanding of the relationships between thunderstorm electrification and precipitation production -- and maybe some surprises."

STEPS-2000 will be the largest research effort to date focused on lightning. Data from a national network that tracks the location of cloud-to-ground strikes has hinted at intriguing evolutions in lightning behavior over the course of a storm. The STEPS-2000 study area, along the semipermanent dry line that marks the west edge of Tornado Alley, has one of the nation's highest frequencies of lightning flashes. If STEPS-2000 can follow a storm as it produces a tornado, the link between a storm's electrical behavior and microphysics should become more clear, say atmospheric scientists.

"We're particularly interested in lightning-free holes in supercells," says William Rison, an New Mexico Institute of Mining and Technology (NMIMT) electrical engineer. NMIMT is also involved in STEPS-2000. A system developed by researchers at NMIMT tracked two supercell storms that each had a lightning-free circle, roughly five kilometers wide, within a doughnut-like ring of lightning. One storm produced a tornado in the vicinity of the lightning-free hole. "The hole was almost certainly associated with a very strong updraft in the storm," notes Paul Krehbiel and NMIMT colleagues. At the crown of the updrafts (15-20 kilometers high) in both storms, a separate concentration of lightning was found--to the surprise of researchers.

NCAR scientists Morris Weisman and Charles Knight are especially interested in how embryonic storms become LPs instead of taking a different route. "In the back of everybody's mind, this has been a continuing mystery," said Weisman. Right now, he said, "The distinction between HP [high-precipitation] and LP storms is largely a visible one. Very often an LP will look weak on radar, but if you're watching it in person you see a tremendous updraft."

Another basic question is why the LP storms don't produce much rain. They may contain as much water vapor as their wetter counterparts, but they are far less efficient at producing precipitation. Scientists also don't know how these storms evolve into larger storm systems later on.

Other agencies and institutions involved in STEPS-2000 include the National Weather Service, NOAA's National Severe Storms Laboratory, the University of Oklahoma, and FMA Research, Inc.
Program contact: Rod Rogers

NSF is an independent federal agency which supports fundamental research and education across all fields of science and engineering, with an annual budget of about $4 billion. NSF funds reach all 50 states, through grants to about 1,600 universities and institutions nationwide. Each year, NSF receives about 30,000 competitive requests for funding, and makes about 10,000 new funding awards.

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