Long before meteorology became a national spectator sport, 12-year-old Joshua Wurman built a makeshift weather station in his backyard in a suburb of Philadelphia, erecting a wooden shelter to shield his instruments from the elements as they captured temperature and relative-humidity readings. In 1975, in high school, he spent $220, his savings from a summer job, on a recording thermometer and barometer set. “I was just sort of a nerdy protoscientist,” he recalls.
As a sophomore at MIT, Wurman decided his future would be in meteorology, only to find that there was no program for undergraduates. So he pieced together his own concentration under the heading “interdisciplinary science,” eventually working his way into the school’s Ph.D. program. Restless, he dropped out, only to beg his way back in three years later. It paid off, as did his postdoc position at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. By now he was an expert on radar. It would not be long before Wurman would shift his attention from bistatic radar networks to storm-chasing technologies, a field he would revolutionize.
When Wurman first conceived of the Doppler on Wheels in 1993, he encountered persistent resistance from his colleagues. “I was met with considerable skepticism,” he recalls. “They said, ‘You can’t do that. You can’t take a big radar out there; the things won’t work. You can’t scan while you’re driving. Your computers will crash.’ There were all kinds of reasons, technical and logistical, why people were cautious about doing it.” NCAR would not commit to the idea.
A former competitive debater with an obstinate streak, Wurman went ahead with his plan with the barest support from the University of Oklahoma in Norman, where he soon moved. Jim Wilson, a senior scientist at NCAR, says: “He was the kind of person who has an idea and doesn’t worry very much that there’s no money to buy this or that piece of equipment…. He’d beg, borrow, or steal—whatever. He didn’t let the administration get in the way.” Wurman proceeded to “lash together” a prototype radar truck in a mere four months, using castoffs from NCAR and “military surplus junk.” His total budget was $50,000. “People used to laugh at me because radars are supposed to be fancy, sophisticated systems with lots of good engineering, and there I was Velcroing and duct-taping things together,” he says.
Radar imagery of the tornado that swept through Greensburg, Kansas, in May 2007. Left: Microwaves as reflected by the storm's moisture and debris. Right: A Doppler measurement of velocity, with winds exceeding 135 miles per hour.
Image courtesy Howard Bluestein
The joking subsided in 1995 during the last days of a giant meteorological study called Vortex (Verification of the Origins of Rotation in Tornadoes Experiment), the largest tornado field project to date. Wurman—aided by his wife, Ling Chan, and another meteorologist —maneuvered the DOW, with its full-size radar equipment, close to the outer ring of a tornado rotating at about 170 miles per hour and captured three-dimensional images of these powerful winds. The details of the hidden architecture were breathtaking: an 1,800-foot-high ring of debris, a central eye with a downdraft within it, and evidence of an orderly array of wind speeds across the funnel.
“There was a huge blind area, and we erased that blind area instantly,” Wurman says.
Wilson still marvels at what Wurman achieved. “During Vortex, NCAR had a radar on an airplane called Eldora, and that got some really great data too,” he says. “If you take the DOW radars and airborne radar, boy, a lot of what we know about tornadoes today is pretty much from the data they collected.”
Airborne radar costs about a million dollars to operate per season, so it is rarely employed. Wurman’s mobile-radar trucks have been the single largest source of data over the past decade, including the remarkable capture in 1999 of images of the most powerful wind ever logged: 301 miles per hour.
Four years later, emboldened by his success, Wurman quit a tenured position in Norman and struck out on his own, establishing the Center for Severe Weather Research in Boulder. Unaffiliated, he risked his academic colleagues’ disdain by forming a complex alliance with television’s Discovery Channel to keep his fleet rolling.
Fathoming how monstrous concentrations of wind energy are created and then being able to forecast them are among the biggest, most urgent challenges in disaster science. Meteorologists continually dissect field observations and run computer simulations to isolate the critical variables that contribute to the moment of a tornado’s birth.
The majority of tornadoes take shape within supercells. Unlike hurricanes, which develop over bodies of water, tornadoes tend to incubate over land (water spouts being the exception). The most dangerous kind breed within storms that are themselves intimidating, often delivering hail the size of golf balls, vicious gusts of wind, and rain capable of flooding roads instantly. To qualify as supercells, these storms, which frequently extend to 50,000 feet in altitude, must contain a rotating wind called a mesocyclone. In the 1970s and ’80s, meteorologists identified the three basic ingredients that combine to make a supercell: a source of energy, a source of rotation, and a cap.
“It’s during the transition season of spring that conditions are most often realized,” Wurman says. “That’s when we still have strong waves in the jet stream from the winter, but we’re also beginning to get warm, soupy air up from the tropics, from the Gulf of Mexico.” The rush of the colder, midlatitude westerly air above the southeasterly surface flow creates an environment laden with energy. Differences in direction and speed of airflow provide a source of rotation. Imagine a giant horizontally oriented pinwheel, Wurman says. At the top of the wheel, air is pushing from one side; at the bottom, air is pushing from the opposite side. Together the forces make the pinwheel spin.
The final component in the storm recipe is the cap, a plug of warm air that typically hovers around 10,000 feet up. During morning and midday hours, it traps heat. “The energy is stored up and stored up and stored up during the day as the surface heats, until finally there is enough hot air that it breaks upward through this cap and is released fairly quickly,” Wurman says. At this point, an ordinary thunderstorm develops. But in the already volatile atmosphere, the storm’s updrafts and downdrafts—internal winds that rise and fall—may tilt and stretch the rotation from horizontal to vertical. If that happens, the axle of the pinwheel turns upright, and a potentially dangerous mesocyclone comes into being. The benign storm has graduated into a supercell, its whirling center a possible hatchery for tornadoes.
Yet something big is missing from this picture. “Most severe rotating thunderstorms don’t make tornadoes,” Wurman explains. “Only between one-tenth and one-third of them do. And we don’t really know the differences between the ones that do and don’t, and that’s why the tornado warnings that go out have such a huge false-alarm rate,” he says. “In addition, we don’t know when, during the lifetime of one of these storms, it is going to make a tornado.”
