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July 2000

Drought puts no damper on STEPS

The spigots were shut down during the first half of the two-month project, but an NCAR team studying severe weather on the High Plains of Colorado, Kansas, and Nebraska isn't shedding any tears. The Severe Thunderstorm Electrification and Precipitation Study (STEPS) will continue through 15 July. Scientists based in Burlington, Colorado, and at the National Weather Service in Goodland, Kansas, are hoping the usual storms of early summer will prowl through their study area before that date.

Morris Weisman (MMM) is coordinating much of STEPS from the CHILL radar base near Burlington, Colorado. (Photo by Carlye Calvin.)

In the meantime, the project has gotten a surprising amount of good data, says Morris Weisman (MMM), a principal investigator and the field coordinator for STEPS. "Despite the fact that it's been the worst drought in recent history, we've actually had a series of good cases from the radar and electrification perspective," says Morris. "In terms of the coordination of all of the components–the mobile units, the aircraft, and so forth–it's all working very, very well." Dave Rust (NOAA National Severe Storms Laboratory, or NSSL) agrees: "This weather is tough, but we have taken advantage of every opportunity and have retrieved some valuable data. So far it has been a success."

Rainfall has indeed been scarce across the STEPS area. Goodland recorded its second driest May ever, with just a half-inch (1.2 centimeters) of rain compared to an average of 3.49 inches. The first 20 days of June provided only 0.88 inches more. It's been so quiet that some are dryly calling the study STEPSS–the Severe Thunderstorm Electrification and Precipitation Suppression Study. As it turns out, though, one of the prime goals of STEPS is to study low-precipitation storms, so the relative dryness of the storms they've seen isn't a particular problem. (Of course, more supercells–classic, long-lived severe storms–would be welcome).

ATD's Ned Chamberlain readies a radiosonde for launch. (Photo by Carlye Calvin.)

STEPS-2000 is the largest research effort to date focused on lightning. Key players include NCAR, the NWS, NSSL, Colorado State University (CSU), the New Mexico Institute of Mining and Technology, and the South Dakota School of Mines and Technology (SDSMT), whose armored T-28 aircraft is probing storms of interest.

Data from a national network that tracks the location and polarity of cloud-to-ground strikes has hinted at intriguing evolutions in lightning behavior over the course of a storm. More recently, New Mexico Tech has built a network of radio frequency receivers, based on the Global Positioning System, that can trace the VHF radiation from a lightning discharge in three dimensions. The receivers detect up to 10,000 energy pulses per second.

The scientists at STEPS are especially eager to find positive cloud-to- ground (CG) lightning. Most CGs deliver negative charge, but in the U.S., 5—10% are positive, and the STEPS area has one of the nation's highest concentrations of these rare CGs. The percentage varies widely by storm, season, latitude, and the like. It's still not known exactly what goes on inside a storm to produce positive CGs. One possibility is a stormwide reversal of charge distribution. According to Morris, "It may be that the actual polarity of these storms is reversed. We've gotten one case where the anvil is predominantly negatively charged. We're getting unique data on the electrification of these storms–maybe unprecedented data."

Charlie Knight (MMM) is spending the summer in eastern Colorado, where the S-Pol radar (background) is scanning the skies. (Photo by Carlye Calvin.)

Sixty miles northwest of Goodland, Charlie Knight (MMM) and a team from ATD is staffing the S-Pol multiparameter radar near the town of Idalia. S-Pol is working with two other Dopplers, the NWS Goodland radar and the CHILL radar, stationed near Burlington and operated by CSU. The NCAR and CHILL radars will provide much-needed detail on water droplets and ice particles. One of the benefits of this work will be to improve parameterizations of storms in real-time forecast models. Also, since the national network of NWS Dopplers is scheduled to be converted to multiparameter status in the next few years, STEPS-2000 will provide valuable input on what can be gleaned from the upgraded radars.

Drought or no drought, STEPS will persevere. The infrequency of storms has prodded the researchers to pay extra-close attention to what does drop by, says Morris. "People have been surprised at how intriguing these results have been. We've had to open our minds to anything that comes within the STEPS domain."

• Bob Henson

This smoke is a good sign: The venerable T-28 is gearing up for another flight into rain, hail, and wind. Film crews documented the T-28 at the STEPS media day, 1 June. (Photo by Zhenya Gallon.)

On the Web:
For more background on STEPS, see the article in the Spring 2000 issue of the UCAR Quarterly.


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Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Wed Jul 19 13:56:38 MDT 2000