Rapid-Scanning Doppler on Wheels Keeps Pace with Twisters
June 1, 2005
BOULDER—A multibeam Doppler radar that can scan tornadic storms every 5 to 10 seconds is prowling the Great Plains through June 30 in search of its first close-up tornado. Engineers at the National Center for Atmospheric Research in Boulder helped build the Rapid-Scan Doppler on Wheels (DOW).
Together with a powerful analysis technique pioneered by NCAR scientist Wen-Chau Lee, the radar--newly enhanced for its first full spring of thunderstorm tracking--promises the most complete picture to date of tornado evolution.
The radar is being deployed this spring, along with another DOW unit, by NCAR scientific visitor Joshua Wurman (Center for Severe Weather Research, or CSWR) from a temporary base in Hays, Kansas.
Most Doppler radars transmit only a single beam, which takes about 5 minutes to make the vertical and horizontal scans needed for a three-dimensional storm portrait. But tornadoes can develop or dissipate in a minute or less. With its 5- to 10-second resolution and close range, the Rapid-Scan DOW can detail these critical steps in tornado behavior.
"The development of the Rapid-Scan DOW is an important advancement for meteorological research," said Steve Nelson, director of NSF’s physical and dynamic meteorology program, which funded the radar's development. "The new radar will result in unique measurements of rapidly evolving meteorological phenomena such as tornadoes."
The first DOW was deployed in 1995. Since then, Wurman’s group has collected data on roughly 100 tornadoes, intercepted the eyes of eight hurricanes, and profiled forest fires. On May 3, 1999, DOW measured a world-record wind speed of 301 miles per hour just above ground level in an Oklahoma tornado.
As part of a $1.6 million NSF grant, Wurman and Curtis Alexander (University of Oklahoma) are analyzing the entire DOW data set on tornadoes to uncover new information, such as how closely tornado diameters are correlated with top wind speeds. Other scientists at OU and Pennsylvania State University will also carry out DOW analyses through the grant.
"We can’t answer even the basic questions about ‘typical’ tornadoes right now, such as how strong their winds are," says Wurman. "By looking at these 100 cases, we hope to understand the distribution of features across many types of tornadoes." These findings could be compared to storm types to help produce better warnings, Wurman adds.
Wurman and Lee plan to select a few tornadoes for more in-depth study. They’ll use a technique called velocity track display (VTD), originally developed by Lee for hurricane studies, that allows scientists to extract three-dimensional winds using data from a single Doppler radar. The two scientists have already used VTD with DOW data to analyze a large and intense tornado that struck Mulhall, Oklahoma, in 1999. They discovered a central downdraft, similar to the eye of a hurricane, surrounded by a ring of updrafts blowing at near-hurricane force, with multiple small vortices rotating around this ring.
The structure found in the Mulhall tornado had been observed for many years in lab experiments and computer models, but it had never been verified by radar data. Lee expects to find a simpler structure in weak tornadoes, without the central downdraft observed in Mulhall.
"We want to use DOW data to analyze more tornadoes of different sizes and intensities and see how they compare to our laboratory work and our model results," Lee says.
Dubbed ROTATE-05, the field work is supported by the National Geographic Society. Design and construction of the Rapid-Scan DOW is funded by the National Science Foundation, which is also NCAR’s primary sponsor.
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