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Spring 2000

High technology meets the High Plains in STEPS-2000

by Bob Henson

Those tapered, bell-shaped thunderstorms that grace so many calendars and posters aren't just photogenic, they're a scientific mystery. Photographers love 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. Based out of the National Weather Service office in Goodland, Kansas, STEPS-2000 will take place between 22 May and 15 July. Key players include NCAR, the NWS, NOAA's National Severe Storms Laboratory, Colorado State University (CSU), the New Mexico Institute of Mining and Technology (NMIMT), and the South Dakota School of Mines and Technology (SDSMT), whose armored T-28 aircraft will probe storms of interest.

STEPS-2000 will be the largest research effort to date focused on lightning. 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, NMIMT has built a network of Global Positioning System-based radio frequency receivers that can trace the VHF radiation from a lightning discharge in three dimensions as the flash wends its way through a storm, between storms, or from cloud to ground. The receivers detect up to 10,000 energy pulses per second. (Each lightning flash includes a number of such pulses; during near-continuous lightning in Oklahoma in 1998, the system recorded 3,000 bursts a second). The system is now being expanded for STEPS.

"We're particularly interested in lightning-free holes in supercells," said William Rison, an NMIMT professor of electrical engineering. On 8 and 13 June 1998, the NMIMT system tracked two supercell storms that each had a lightning-free circle, roughly five kilometers wide, within a doughnut-like ring of lightning. The 13 June storm produced a tornado in the vicinity of the lightning-free hole; the pattern mimicked the hook echo long seen in radar reflectivity patterns near tornadoes. "The hole was almost certainly associated with a very strong updraft in the storm," noted Paul Krehbiel and NMIMT colleagues in the 18 January issue of EOS. At the crown of the updrafts (15-20 km high) in both storms, a separate concentration of lightning was found--to the surprise of researchers. It represented "an entirely new regime of lightning activity," wrote Krehbiel.

A positive approach

Polarity is a key variable in understanding the electrified storms of the High Plains. The main problem with the Oklahoma data, said Rison, is that "we have all this beautiful lightning, but we don't have complete information on its polarity." The latter will be thoroughly charted in STEPS. While the fast-response network tracks lightning location every 100 microseconds, slow- response electric-field antennas will join ground-based and balloon-borne field mills to provide electric field data on the scales of seconds to milliseconds. This should allow the polarity of in-cloud and cloud-to-cloud flashes to be discerned.

Most cloud-to-ground lightning strikes (CGs) deliver negative charge, but 5-10% (in this country) are positive. 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 storm-wide reversal of charge distribution. "The storms appear to be able to reverse polarity relatively easily," said Krehbiel, referring to early results from the NMIMT system. "The $64 question is why and how this occurs."

Scientists do know that LP storms produce more than their share of positive CGs, and STEPS-2000 will be in an ideal place to document them. The study area, along the semipermanent dry line that marks the west edge of Tornado Alley, has one of the nation's highest frequencies of positive flashes.

This research may have forecast applications. For instance, STEPS-2000 investigator Donald MacGorman and colleagues at NSSL have found several cases in which a storm's predominant CG type suddenly shifted from positive to negative within minutes of tornado formation, including the deadly Plainfield, Illinois, tornado of 28 August 1990 (a rare F5 on the Fujita damage scale). Tornadoes don't always accompany such polarity shifts, and vice versa. However, in storms that start out producing frequent positive CGs and do produce tornadoes, it appears that a shift may be a good indicator of when the most violent tornado will appear. 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.

An LP storm looms over eastern Texas. (Photo by Bob Henson.)

What makes an LP?

NCAR's 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. "How much rain or hail comes down is obviously important to the person under the storm,"noted Weisman, "but it's also important in controlling the cold pool [of outflow] that's produced. From one storm, you could get a whole squall line if you get the right cold-pool characteristics, but that depends on the microphysical characteristics of the downdrafts and the precipitation." Sorting out these variables, he said, "could help us be able to predict, given that we see a storm, what's going to happen to that storm or storm complex downstream."

The NCAR and CSU multiparameter radars will provide much-needed detail on water droplets and ice particles. V.N. Bringi, John Hubbert, and V. Chandrasekar (CSU); Jothiram Vivekanandan (NCAR); and Paul Smith (SDSMT) have worked extensively in recent years to learn what can be deduced from the multiparameter data. NCAR's Jay Miller will use Doppler measurements from each radar to construct the three-dimensional wind fields inside each storm. Miller, Weisman and colleagues will then replicate the storms in models, looking for LP-HP differences and tracing the growth and movement of precipitation in three dimensions. According to Miller, "The trajectory analyses will help in unraveling how the storms act to transport electrical charge and grow precipitation, especially hail and its possible correlation with positive CGs."

One of the benefits of this work will be to improve paramaterizations 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.

Neglected for years in the research world, it's time for the LP storm to get its due, said Weisman. "It's such a well-defined storm type: isolated, narrow, small, almost like a laboratory experiment. If you can't understand this simple type of storm--the basic microphysical makeup of one isolated, long-lived steady updraft--then there's less hope of understanding much more complicated types of systems."

Field firsts

STEPS could mark the first time NMIMT lightning data will be processed and made available in real time, said William Rison. His NMIMT team hopes to be able to update the data every minute on the World Wide Web. "We'll be able to follow the development of the storm and get the display to the various radars and the people running aircraft."

The project will also feature a couple of radar firsts. It will offer the first-ever combination of one NWS Doppler radar (based at Goodland) and two multiparameter radars, NCAR's S-Pol (being installed near Idalia, Colorado) and CSU's CHILL (to be placed in Burlington, Colorado). The NWS staff in Goodland are "full partners in the whole experiment," said Morris Weisman (NCAR). "We're dealing with forecast problems that they encounter every day." STEPS will also be the first time CSU has operated CHILL outside its base in Greeley, Colorado. CSU's Steve Rutledge, Larry Carey, and Walt Peterson will be using data from CHILL and other sources to investigate, among other things, why some severe storms produce copious amounts of positive cloud-to-ground strikes.

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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Thu May 4 14:53:14 MDT 2000