Archive of Spotlight Feature Articles

Taking the Next STEPS

By Andrew Freedman

A convoy of NOAA vehicles with their yellow lights flashing and anemometers spinning pull into a Conoco gas station in Goodland, Kansas to fill up before the storms begin to fire for the day. "Watch this," says Shannon Myers, a National Severe Storms Laboratory student researcher. "Get ready to answer some questions." Sure enough, gas station patrons abandon their gas pumps and begin flocking around the weather vehicles. "What are these instruments for?" one person asks. "Neat car," a child comments. Mostly the people are eager to know basic and potentially life-saving weather information. "Are we going to get a bad storm today?" "Any chance for tornadoes around here?"

"Well, that is why we are here, to better predict these events for people like yourself," one researcher tells them

The pseudo-celebrity status NOAA researchers are enjoying in the tri-state region of Kansas, Colorado and Nebraska is a testament to the sheer size and expertise of the weather research team. In fact, there is so much weather brainpower and instrumentation located on the Kansas/Colorado border that the region has been dubbed the weather research capital of the world for the spring and summer of 2000.

The project NOAA scientists are taking part in is called STEPS, for Severe Thunderstorm Electrification and Precipitation Study. STEPS builds upon the findings of previous thunderstorm studies and aims to unlock the tantalizing secrets of severe thunderstorms over the High Plains.

STEPS is focusing on the processes of electrification and precipitation inside severe thunderstorms, two areas that are not well understood, particularly when it comes to a certain kind of thunderstorm: the Low Precipitation Supercell. The "LP" Supercell is most common over the High Plains and is known for producing little in the way of rainfall, an unusually high rate of positively charged lightning strikes, large hail and occasionally, tornadoes.

The difference between the capabilities of STEPS versus previous storm studies is akin to the difference between a typical movie and a three-dimensional IMAX film. In fact, much of the data that is being gathered during STEPS is in the form of 3-D displays. Polarimetric radars from Colorado State University and the National Center for Atmospheric Research show a vertical cross-section of the storm as well as the typical 2-D picture. This gives scientists valuable data about the internal structure of the storm. Lightning mapping systems from the New Mexico Institute of Mining and Technology allow scientists to view a cross-section of the lightning distribution in a thunderstorm, revealing various levels of electrical activity within certain parts of the storm. STEPS is even flying an armored aircraft through the storms to get an inside view. The highly-modified T-28 from the South Dakota School of Mines and Technology penetrates the storm and records data on everything from the electric field to the size of cloud particles.

NOAA's scientists are leading the electrification portion of STEPS. David Rust and Don MacGorman, NSSL researchers who co-authored a definitive text on lightning, are launching sophisticated balloons into thunderstorms to get a better idea of the electrical characteristics of the storm. They are especially interested in why storms on the High Plains have a tendency to lower a positive charge to the ground, whereas the vast majority of thunderstorms in the U.S. and worldwide lower mainly negative charges earthward through cloud to ground lightning strikes. Rust and MacGorman have analyzed some preliminary data and have come up with some intriguing results.

Although they have yet to intercept a low precipitation supercell, they have succeeded in recording the first-ever sounding through a low precipitation thunderstorm. Rust and MacGorman have found that some thunderstorms in the STEPS region have an inverted polarity structure. Instead of having a negatively charged region in the lower part of the storm, these storms show a positively charged region there. Some researchers are also noticing a correlation between the strength of a thunderstorm's updraft and the polarity of cloud to ground strikes. That is, the lightning seems to switch from negatively charged to positively charged when a strong updraft is present. This finding holds promise for improving warnings for tornadoes and hail, since both can be the result of a particularly strong updraft.

The electrification results of STEPS will also shed light on the mysterious "Transient Luminous Events," or TLE's. These lightning discharges are emitted from the top of thunderstorms and reach the outer layers of the earth's atmosphere. The Yucca Ridge Observatory near Fort Collins, Colorado is keeping an eye out for TLEs during STEPS. Research done at Yucca has shown that TLEs are derived from discharges within the parent thunderstorm, particularly positively charged discharges. Until STEPS, no datasets have been available to characterize the parent lightning strike. Now scientists will be able to examine both the TLE and the parent lightning bolt. TLE research is important for ensuring the safety of air travel in the outer reaches of the earth's atmosphere. It is also thought that TLEs have something to do with the maintenance of a global electrical circuit, an intriguing area of research.

Joining Rust and MacGorman on the ground have been six mobile mesonets, a joint product of NSSL and the University of Oklahoma. These vehicles are used to sample the atmosphere in and around a storm, and to verify or "ground truth" the conditions on the ground with what the radars are depicting. The mesonets have been working on characterizing boundaries, such as cold fronts and dry lines, to get a better understanding of how storms develop along these boundaries. The mesonet team is also interested in why there is a tendency for severe storms to produce tornadoes when crossing a boundary. These findings will ultimately lead to a better forecast, as both "convective initiation" and tornado formation are key challenges for National Weather Service forecasters.

[6/23/00]

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