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Frontiers
Tracking Tornadoes, Nature's Most Powerful Winds
March/April 1998
In the late afternoon on the lonely roads of the
Texas panhandle, an armada of vans, trucks and airplanes bristling with
scientific instruments races toward a rendezvous with the weather. As
the storm experts converge on the little Texas town of Dimmit, one of
their vehicles, a truck topped by a radar with a six-foot diameter antenna
called DOW, trains its electromagnetic beam on a violent twister stripping
away highway asphalt and tossing aside automobiles as if they were tumbleweeds.
The truck gets within two miles of the tornado and records detailed images,
including spiraling bands of clouds and concentric rings of debris.
Storm chases such as this one are more than just adventures in bravery.
They are serious scientific experiments made possible by the use of mobile
Doppler radars. One of the goals of these chases is to solve a mystery
that has stumped meteorologists for decades–why some thunderstorms
give rise to deadly twisters. Such a breakthrough was one of the hopes
for the Verification of the Origins of Rotation in Tornadoes Experiment
(VORTEX), funded in part by NSF.
"The big issue that's still unresolved with tornadic storms is
at the lower levels," says Stephan P. Nelson, Director of NSF's Mesoscale
Dynamic Meteorology Program, which funds DOW, VORTEX and other weather
research. "We have a good understanding of the structure of storms,
and good information on the initiation of rotation in mid to upper levels
of the storms. But what finally causes a tornado to touch down on the
ground is still a mystery."
Until about 10 years ago, these critical low-level circulations
were largely out of reach of Doppler radars, which were too
big to haul on a storm chase. In 1987, engineers at Los Alamos
National Laboratory designed a break-through portable Doppler
radar. Further advancement came in 1995 with Doppler-on-Wheels
(DOW), built by Joshua Wurman and Jerry Straka of the University
of Oklahoma. DOW has a wider beam than earlier versions, and
can penetrate further into the dense water and ice of a thunderstorm.
While ground-based radars such as DOW effectively show features of the
tornadoes, the Electra Doppler Radar (ELDORA) reveals the nature of the
broader thunderstorm. ELDORA was developed jointly by the National Center
for Atmospheric Research and French scientists, and yields dual-Doppler
imagery to map 3-D wind fields, resulting in highly detailed cross-sections
of thunderstorms.
Data collected by these radars are used to produce advanced computer
simulations, which have in turn prompted a variety of hypotheses on tornadoes.
Morris Weisman, a computer modeler working on VORTEX, is undaunted by
the continuing mystery. "VORTEX observations should show if our results
are reasonable or just a fantasy model," he says.
Meanwhile, as modelers are busy simulating events and testing hypotheses,
the mobile radars still have plenty of data to gather. Their objectives
remain clear: to increase our understanding of hurricanes and tornadoes,
to better predict their behavior, and to know when to get out of their
way.
[February 1997]
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