What Makes a Tornado?
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SNAPSHOTS What makes a tornado? The biggest swarm of tornadoes ever recorded—148 in all—rum- the first research-based bled across the U.S. Midwest and South on 3–4 April 1974. A storm chase projects, network of radars operated by NOAA’s National Weather Ser- launched in the 1970s vice caught many of the tornadic circulations, and dozens of in Texas and Okla- watches and warnings issued by the service gave many people homa, while data from ample time to seek cover. Still, more than 300 people died in the hail project helped what became known as the Jumbo Outbreak. scientists develop con- Severe weather research was still in its infancy at the ceptual models that time, and NCAR was one of its incubators. A small group underscored the 3-D of researchers, some of them involved with the National Hail nature of storms. No- Research Experiment, or NHRE (see page 20), turned their body had yet tried to attention to supercells, the long-lived thunderstorms that turn these concepts This tornado caused 3 deaths and 210 spawned the most violent tornadoes. Would it be possible to into software, though. injuries in Xenia, Ohio, during the 1974 zero in on the storm scale and generate a supercell inside a To model with a do- Jumbo Outbreak. computer model? main a supercell real- Two young scientists—Joseph Klemp (NCAR) and istically, Klemp and Wilhelmson would need to build a 3-D Robert Wilhelmson (University of Illinois at Urbana- model with a domain large enough to encompass the storm Champaign)—gave it a try. Until then, most weather modeling and its environs, but detailed enough to capture important had simulated motions and processes spanning thousands of features within the thunderstorm. miles. On a smaller scale, modelers at NCAR and elsewhere The computational challenge was immense. Tem- had made some progress in simulating severe thunderstorms perature, moisture, and wind near thunderstorms can vary in two dimensions, an approach that worked well for the lin- sharply across small distances and short time scales. Klemp ear nature of nontornadic squall lines. and Wilhelmson pared the computing demands in carefully However, supercells were more complex and three-di- chosen ways, drawing on basic research in such areas as tur- mensional, a portrait that emerged from a wealth of satellite bulence and cloud physics. Technology also gave Klemp and and radar observations as well as still Wilhelmson a major boost with the 1977 arrival of NCAR’s and moving imagery. Photos first supercomputer, the Cray-1 (see page 23). and film came from Among other things, the Klemp-Wilhelmson model captured the development of a splitting supercell, one that breaks into left- and right-moving pieces under the influ- ence of powerful upper-level winds. Over the subsequent years, as universities gained computing prowess, dozens of scientists adopted the Klemp-Wilhelmson techniques, and the field of severe-storm modeling came into its own. Today, both scientists serve as senior leaders at the same institutions from which they forged a classic NCAR-uni- versity collaboration. Wilhelmson looks forward to tornado simulation us- ing models with resolutions as fine as 10 meters (33 feet), compared to the 2 kilometers (3.2 miles) used in his earliest simulations with Klemp. “We didn’t understand at the time Modeling by Joseph Klemp, Robert Wilhelmson, and colleagues the impact we would have on the modeling community,” Wil- showed the evolution of a severe thunderstorm in strong wind helmson observes. “What we did know, however, is that we shear that splits into two cells (right) within 90 minutes. had a great time working together.” 28 | UCAR AT FIFTY VORTEX2: Rounding up severe weather he spring of 2009 saw More thanTODAY 50 vehicles program, the teams formed 140 tornadoes before V2. “If what was perhaps the were on the road from early a dense set of observing there were low-hanging fruit, largest array of vehicles May to mid-June, spanning platforms as the storm it would have been plucked ever deployed for an a study area that stretched passed. Each evening the already. The features we’re Tatmospheric study. Although from Texas to southern groups reconvened, typically looking for are subtle, and the Great Plains were oddly Minnesota. On each “go” hundreds of miles from the they’ll only come out through devoid of severe weather for morning, participants headed day’s starting point. As NCAR an arduous and complicated weeks, the second Verification out to a potentially stormy participant Timothy Lim synthesis of data.” of the Origins of Rotation area, eventually zeroing in observed, “It’s hard to get your in Tornadoes Experiment on a county-sized target laundry done when you don’t (VORTEX2 or V2) captured where a tornadic supercell know what state you’ll be in by one twister in unprecedented was deemed possible over nightfall.” detail, as well as a number a several-hour window. As Major advances in of potentially tornadic they stayed in touch through technology pushed V2 far thunderstorms that never multiple cellphone networks beyond the bounds of the first made the grade. and a Web-based chat VORTEX study, conducted in 1994–95. Several sets of mobile Doppler radars were deployed, each with their own specifications and strengths, along with an array of other “I think we observing systems. A tornado on 5 June in Goshen County, were all Wyoming, was sampled at close range as frequently as surprised by every few seconds, yielding an unprecedented data set. how well the Nontornadic cases also drew keen interest. Models communica- These radar-derived depictions of velocity (winds, left) and have trouble depicting the tions worked.” reflectivity (precipitation, right) from a Doppler on Wheels unit small pools of cool, moist show circulation around the 5 June 2009 tornado in Goshen air that descend from severe —Paul Markowski, storms and appear to help County, Wyoming, at a height of about 200 meters (660 feet). Pennsylvania State trigger tornadoes. V2 gathered University wind, temperature, and moisture data from a number of these cold pools, including several from storms that developed strong rotation but no tornadoes. Even after a second season of sampling in 2010— and a total of more than 20 tornadoes documented—much more work lies ahead. “We’re not expecting ‘Eureka!’ data,” says NCAR visiting scientist Joshua Wurman (Center for A W-band radar deployed by the universities of Oklahoma and Severe Weather Research), Massachusetts captured the end stages of the 5 June 2009 whose mobile radars had tornado in Goshen County, Wyoming. already sampled more than THE SEVENTIES | 29.