UNRAVELING THE MYSTERIES OF TWISTERS

By J. MADELEINE NASH/NORMAN

At first, the tornado was just a wispy tendril trailing from a cloud--too fragile, it seemed, to do any harm. But suddenly it spawned three new funnels that spiraled around their parent in a deadly dance. Then, as the car his partner was driving skidded along a mud-slicked road near Hanston, Kansas, Robert Davies-Jones glanced nervously through a rear window and saw that this menacing whorl of dust and debris was following a bit too closely behind. Just as wild animals sometimes turn and track their hunters, Davies-Jones realized with growing alarm, the tornado he had started out chasing was chasing him. No, this is not one of those scary scenes from the movie Twister, which opened in theaters across the country last week (see review). Rather, it is a slice of the real-life science that inspired Michael Crichton and Steven Spielberg, creators of the dinosaur blockbuster Jurassic Park, to make the movie in the first place. For Davies-Jones is not some casual thrill seeker but a serious scientist at the National Severe Storms Laboratory (NSSL) in Norman, Oklahoma. Over the past 2 1/2 decades, he and his colleagues have revolutionized the study of tornadoes, teaching deskbound meteorologists to pack up their instruments and take them into the field.

Already storm chasers like Davies-Jones have acquired a deeper understanding of the spectacular springtime storms that produce the most violent tornadoes. They have played a crucial role in developing a new type of radar that can peer through the darkest clouds and detect areas of rapid rotation as much as half an hour before a twister touches down. And now--thanks to an unprecedented data-gathering effort known as VORTEX (short for Verification of the Origins of Rotation in Tornadoes Experiment)--these daredevils of meteorology are beginning to provide fresh insight into the long-standing mystery of what it is, exactly, that creates a tornado.

Until recently the idea of studying twisters in the field would have struck most people as foolhardy. After all, these monsters of the atmosphere are as unpredictable as sharks, and considerably more dangerous. Just ask the 12,000 residents of Beatrice, Nebraska, whose town was vandalized by a big tornado last week. Or ask Sheriff Donnie Joe Yancey of Izard County, Arkansas, who tangled with the funnel that rampaged through his section of the state last month, killing seven people and injuring 30 more. Yancey was preparing to get out of his jeep, he recalls, when "it got real dark, real still. Then the jeep started rocking and bouncing, I heard a pop, pop, pop, and all the windows blew out." The fierce winds yanked Yancey out of his car, walked him across the road and then, after Yancey anchored himself on some nearby briars, engaged in a tug-of-war for his body and soul. "It was awesome," he exclaims. "I don't believe I ever want to look a tornado in the face again."

Of course the researchers who chase intense storms never want to get that close to a tornado either, and they also try to avoid getting hit by lightning or losing control when driving too fast over slippery roads. And while they are certainly curious about the movie Twister and are well aware of the growing market for tornado videos (see box), scientists like Davies-Jones are worried that the attention their arcane line of work is attracting may inspire people to take unwise risks. They are aware that the romance of tornado chasing has attracted a dangerous number of amateurs. Sometimes the scientists pull up in their instrumented cars to find a mob of folks with video cameras trained on a twister roaring through a farmer's field.

In truth, storm chasing is arduous work that generally entails more common sense than courage and more physical discomfort than danger. Professional chasers often drive 15 hours a day for days at a time, subsisting on junk food and virtually no sleep. "We eat whatever Texaco, Conoco and Citgo are willing to serve up," laughs University of Oklahoma meteorologist Joshua Wurman. Nor do the hazards of the job always come from nature. Last year Wurman stopped during a chase to help extract a car from a ditch. "While I was pushing, the driver gunned his engine and I was covered in mud and cow manure."

Unlike their Hollywood impersonators, real storm chasers try to watch tornadoes from a respectful distance, usually a mile or two, and their vehicles take a lot more punishment than they do. During last year's VORTEX run, for example, Davies-Jones and his partners were charged with the task of placing Turtles--canisters weighted with lead and packed with temperature and pressure gauges--in the path of an oncoming tornado. As it turned out, the twister swerved and missed the Turtles. But the softball-size hailstones that followed found their mark--smashing a windshield and a rear window. Another time, Davies-Jones' partner, concentrating on a tornado that had just been sighted, slammed into a drainage ditch at 30 m.p.h.

Storm-chasing scientists do have a genius for coming up with some pretty wild ideas, however. The University of Oklahoma's Howard Bluestein really did develop an instrument akin to the device called Dorothy in Twister. Bluestein, who was one of the models for meteorologist Bill Harding (Bill Paxton) in the movie, named his device the Totable Tornado Observatory, TOTO for short, and tried to intercept an oncoming funnel. TOTO was a bit unwieldy (it tipped the scales at 400 lbs.), so researchers switched to the more sprightly Turtles, which are cheaper to build and more easily deployed.

In the movie, Dorothy is packed with golf ball-size radio transmitters that are supposed to fly up into the vortex and relay data to ground-based computers. The idea is not all that farfetched, according to real-life storm chasers (some of whom acted as scientific consultants on the film). They speculate, though, that most of the flying transmitters would be blown away from the vortex or destroyed by debris. Over the years, researchers have proposed all sorts of zany schemes for getting instruments into a tornado's heart, including blasting at the twister with instrumented rockets and probing it with pilotless planes. One day, no doubt, someone will try building a Dorothy-like instrument--and may even manage to make it work.

Storm chasing as an organized enterprise began in 1972, when the Tornado Intercept Project was launched to provide what radar meteorologists refer to as "ground truth." At the time, the NSSL was developing a radar capable of detecting areas of strong rotation inside big tornado-producing storms. The chasers provided visual proof that particular radar signatures did indeed precede the formation of tornadoes. The new radar, Doppler radar, made use of the fact that radio waves shift frequency depending on whether the objects they bounce off are advancing or receding. In this case, the objects that create the Doppler shift are water droplets inside storm clouds. As the winds inside these clouds begin to spin, the droplets show up on radar screens as tighter and tighter swirls.

The collaboration between radar developers and storm chasers was immensely productive. It led to the NEXRAD (or Next Generation Radar) system, which the National Weather Service is currently installing nationwide. Already NEXRAD has helped extend the lead time for tornado warnings from three to eight minutes, on average. Sometimes the warning comes even earlier. Last month weather forecasters in Little Rock, Arkansas, called a tornado warning for communities in the Ozark Mountains a full 35 minutes before the twister showed up, giving people who lived in trailer homes time to scurry to friends' basements for safety.

Good as it is, however, the NEXRAD system has not changed the rate of false alarms: 50% to 70% of the warnings forecasters issue are not followed by tornadoes. Why does one threatening-looking storm produce a tornado while others seemingly just like it do not? This question has dogged tornado experts for years, and the VORTEX project was launched to answer it. First in 1994 and again in 1995, VORTEX brought dozens of meteorologists to Oklahoma, Texas and Kansas during May and June--peak tornado season in that part of the country. Every few days for nearly 10 weeks, chase teams piled into planes, vans and cars equipped with every measuring device imaginable--satellite positioning systems, state-of-the-art radar, rooftop weather stations--and raced hundreds of miles to catch up with the storms deemed likely to generate twisters.

In all, the VORTEX scientists managed to encircle 10 tornadoes in their virtual lasso, and the data they recorded--wind speed, temperature, pressure, humidity--have turned out to be extraordinarily rich. "We got more good data out of VORTEX," exclaims Peter Hildebrand of the National Center for Atmospheric Research, in Boulder, Colorado, "than we had collected in the past 30 years." Among other things, the chase teams managed to position a Turtle so it actually caught the sharp pressure drop as a VORTEX passed overhead; tucked into the center of the tornado's swirling interior, a cylinder of down-flowing air that may be the equivalent of a hurricane's eye was spotted by researchers.

Before VORTEX, scientists had a general idea of how tornadoes come to be. They knew, for example, that big twisters are most likely to be generated by what are termed supercell storms--towering cloud structures that sometimes top out at 65,000 ft. and concentrate energy in dangerous ways. Supercells typically form in spring as warm, moist air from the Gulf of Mexico flows north and pushes through colder, dryer layers of air. As it rises, this upwelling of warm air begins to cool, and the moisture it contains condenses first into cloud droplets and then into rain. At that point, the air--now denser because it is colder--starts to sink. But at the same time, the process of condensation that created the rain releases so much latent heat that the air around it warms up again and retains its lift.

The collision between warm and cold air masses sets up conditions that favor the growth of big thunderstorms. A tornado, however, requires something else as well: the presence of what meteorologists call wind shear. This occurs when winds in the so-called boundary layer--the part of the atmosphere closest to earth--blow more gently than winds at higher elevations. These two wind streams push on the layer of air that lies between them as though it were an invisible rolling pin. Then, as the warm updraft that powers a supercell shoots toward the stratosphere, it tilts the rolling pin so that it spins on its end. Soon the updraft starts to spin, giving birth to a mesocyclone, a rotating column of air as wide as six miles.

Mesocyclones are the broad spinning cloud structures from which tightly coiled tornadoes seem to drop. What scientists are trying to find out is how one turns into the other.

The simplest explanation is that tornadoes form when a smaller, even more rapidly rotating updraft descends from the mesocyclone like a vacuum cleaner's nozzle. To the eye, this is exactly what appears to be happening. But while scientists agree that the updraft is essential, many doubt that it provides the sole mechanism for tornado formation. Some scientists think the rapid sinking of colder, dryer air near the rear flank of the storm may be key. If this rush of air encounters wind shear on its way down, then it too will start to rotate. In this scenario, a tornado occurs as air from the downdraft nears the ground, swoops out horizontally and--attracted by the zone of low pressure created by the mesocyclone--spirals back into the storm like smoke curling up a chimney.

Yet another possibility, says UCLA meteorologist Roger Wakimoto, is that the tornadoes typical of supercell storms are formed by the same mechanism that creates the smaller, less destructive funnel clouds known as waterspouts, landspouts and dust devils. These twisters all build their vortexes not from the clouds down but from the ground up. They are triggered, Wakimoto says, by low-lying eddies of air that are perturbed by a fast-moving front or some other local disturbance. When a supercell storm passes over such an area, swirling air near the ground could easily be sucked into the updraft and spun up into a tornado. "Why," he asks, "should a supercell tornado be different from all the rest?"

Until VORTEX, the competing hypotheses about tornado formation could not be rigorously tested. The downdraft theory, for example, was bolstered by storm chasers' sightings. Observes Erik Rasmussen, field coordinator for VORTEX: "What storm chasers see first is a big dark cloud, then a bright spiral slicing into the base." The problem is that the flow of air within a big storm is so complex that what the eye sees cannot always be trusted. Hence the need for measurements.

Rasmussen and his colleagues are just beginning to work their way through the VORTEX data, and they cannot yet say how well the different models are faring. Like all successful experiments, VORTEX has produced more questions than answers, and later this month chase teams will take to the road to try to answer some of them. For instance, how important is it that the tornadoes observed by VORTEX all hit areas that had been visited by storms earlier the same day? No one knows. "Right now," says University of Oklahoma researcher Jerry Straka, "VORTEX has confused the hell out of us."

Scientists like Straka are puzzled because above all else, they value simplicity. Gyres of wind, they believe, must be subject to Ockham's razor, a principle first stated by 14th century philosopher William of Ockham. According to this principle, the theories most likely to prove true are those shorn of unnecessary embellishments. But, says Texas A&M meteorologist Louis Wicker, the process of tornado formation now looks more complicated than ever. In fact, the more VORTEX data sets he feeds into his computer models, the more convinced he is that there could be several ways to make a tornado. "Nature," he laughs, "has kicked us in the pants."

But then tornadoes have long been known for their capricious behavior. The same twists of wind that can derail trains and rip up pavement can be surprisingly gentle. Says National Weather Service meteorologist Donald Burgess: "I've seen a phonograph record driven through a telephone pole, and the record wasn't broken. I've seen a fridge thrown several hundred yards, while glasses on a nearby table weren't touched." Last month Betty Lou Pearce, a 64-year-old clerk from Pilot, North Carolina, hid from a tornado in her bathtub and moments later found herself sliding into the woods in a ceramic sleigh. She returned from her Wizard of Oz-like journey scratched and bruised but otherwise miraculously unharmed.

Scientists love to swap these stories almost as much as they enjoy debunking oft-repeated twister myths (like the one about tornadoes driving bits of straw through fence posts--what may actually happen, scientists suggest, is that a sudden drop in air pressure forces the wood to expand, allowing the straw to lodge in newly opened cracks). But even with all they've learned about the physical forces that power the creation of twisters, meteorologists still cannot say beforehand what path a particular tornado is likely to take or how much damage it is likely to do. That's what makes these fearsome creatures so fascinating and so deadly. And such natural Hollywood stars.