The Roisters of Tornado Alley

Still think no one is doing anything about the weather? Meet the storm chasers - the roisters of meteorology - who confront nature not in the laboratory, textbook or classroom but in nature's own arena - Tornado Alley. They are more than mere dare-devils, although big storms attract the foolish as well as the wise. The serious and the skilled among them do what they do not because it is bold or because it is brave but because it is science and it needs to be done. Serious storm chasers are scientists at heart, if not by trade.

In spite of recent advances in weather observation using the long eyes of satellite technology and the sensitive fingers of sophisticated ground sensors, forecasters still can't predict the occurrence of tornadoes with any certainty. There is much about tornadoes that remains unknown. Storm chasers are our link to this knowledge. Joshua Wurman, professor of meteorology at the University of Oklahoma (OU), is one of the roisters.

"We still don't know much about their timing and strength," Wurman said about tornadoes. "We've just begun to map the structure of the storms themselves. We don't know enough about maintenance and demise. We don't know much about sub-tornado scale structure. From damage surveys it appears that there are small scale 'suction vortices' that are more intense than the larger tornado."

To find some of these answers meteorologists like Wurman must take their labs into the field, chase down a storm, then perform science in the shadow of a boiling wall cloud the size of an upended Manhattan. His lab these days is a new device called the Doppler on Wheels, or DOW, developed by Wurman in collaboration with OU, the National Severe Storms Laboratory (NSSL) and the National Center for Atmospheric Research (NCAR). (Figure 1.) The DOWs are low, heavy, flat-bed trucks carrying service modules and Doppler radar antennas. The rigs are ready to quarrel, outfitted with roll bars, hail cages for the windows and stabilizing arms which, when extended, brace the vehicle against high winds. Two such units exist, with a third scheduled to be ready during the '97 season. Like conventional radar, Doppler transmits a pulse of energy that bounces off rain or other precipitation, pin-pointing the exact location of the precipitation. Doppler, much more powerful than conventional radar, also measures wind direction within a storm.

"We use 3-cm wavelength radiation and transmit 250 kW pulses that are very short (.25 - .5 microseconds). The dishes are 2.44 m which result in a 0.9 degree beam... When we are successful we are able to map out the three-dimensional structure with 30 m resolution inside the tornado. This helps us understand how tornadoes form, why some are big and some are small, how they function and maintain themselves, how and when they die and why some storms produce tornadoes while others, apparently similar, don't."

Since the radar stream spreads out with distance, like a flashlight beam, reducing the unit's effectiveness, the closer the chasers get to an object the better the results. "If we can get within two or three km we get 20 times better resolution than if we were 50 km from an object. That's 20 times the resolution in two directions, so 400 times better spatial resolution. In close, we get 1,000 times more data points than with a radar 50-100 km away."

In other words, they have to get close to the objects the chasers call "interesting weather", which in actuality are killer storms. These are the subjects of the research.

In Search of the Supercell

Wurman, and other chasers, find these enormous storms by understanding what they observe in the sky and what they interpret from forecasts. (Figure 2.) All forecasting information is online, available on web sites such as OU's http://wwwcaps.ou.edu/Weather.html and Purdue University's http://wxp.atms.purdue.edu. One of the most popular is http://www.rap.ucar.edu/weather.html, a site maintained and built out of raw satellite and weather data by Greg Thompson, an associate scientist at NCAR in Boulder, CO. "There are a lot of weather freaks out there," Thompson said. "We get 11,000 hits a day, more when there's interesting weather. The day Hurricane Fran slammed the Carolina coast I got over 31,000 hits."

Ingredients believed necessary to "fire" a thunderstorm include moisture in the lower to mid levels of the atmosphere; unstable, rising air; and a lifting force, commonly a cold or warm front, dry-line, an outflow from a neighboring thunderstorm, or even topography.

As surface air warms throughout the day it becomes lighter and rises. As the rising air (updraft) cools, it condenses into a cloud. When the lifting force is strong enough and the air contains enough moisture the cloud can rise more than 15,000 m. Chasers look specifically for conditions where the strongest thunderstorms are likely to evolve. Ordinary thunderstorms won't do. There are 100,000 ordinary thunderstorms in the U.S. every year - one somewhere every five minutes. Chasers look for that special one percent of thunderstorms that can potentially spawn tornadoes - the supercell. Supercell thunderstorms occur when the warm updraft reaches sufficient strength and punches through the "cap" of overlying stable air, in effect blowing the lid off. This allows more surface air to rise at speeds approaching 240 km/h, which then condense, releasing enormous amounts of latent heat, which further aggravates the situation. Since tornadoes form in the air rising into a thunderstorm, in the violent updraft, conditions that suggest high storm rotation will send the chasers to gas up the trucks.

Of course, even when all the conditions are there, no one can say for sure when or even if any particular thunderstorm will trigger a tornado. That's the mystery. Classic supercells can take an hour or more to form, developing well-defined wall clouds, inflow regions and rear-flank downdrafts.

To be of most benefit, Wurman's DOW units must be positioned as close along side a storm's path as the chasers dare approach. Experienced chasers never deliberately place themselves on the storm track, directly in front of the storm.

Taking Data

Wurman, who chases with three veteran tornado scientists (OU professors of meteorology Howard Bluestein and Jerry Straka, and NSSL research scientist Erik Rasmussen) approach the largest and most furious supercell storms they can find and intercept, set out their leveling pontoons, aim the radar dish into the maelstrom of the advancing dark wall and "take data". It's a form of brinkmanship a lot like dashing at a band of angered gorillas with a thermometer in an attempt to learn why they're upset. Most chase days end in failure, or "busts". Wurman says he's happy to succeed 20 percent of the time. Most tornadoes last only a few minutes, travel less than 48 km/h and create damage paths about 50 m wide. Extremely destructive tornadoes can roar past at 96 km/h and leave a swath of destuction a mile wide. Using the DOW, Bluestein recorded winds in a 1991 Oklahoma tornado at 428 km/h, believed to be the highest ever measured. Wind that strong can de-bark a tree. (Figure 3.)

From his base in Norman, OK, Wurman is positioned to chase the storms that erupt in Tornado Alley - an area within a box drawn between Denver, Omaha, Waco and Lubbock - the most likely spot on earth to find a tornado. Although tornadoes can occur anywhere, at any time, because of prevailing weather patterns most tornadoes touch ground on the southern Great Plains, and most of those happen between March and June, between noon and sunset.

While Tornado Alley is the usual beat for Wurman, last September he found himself with Charles Edwards and Matt Biddle aiming the DOW into the girth of Hurricane Fran, just as the Category 3 storm made landfall at Wilmington, NC. The results, obtained from a closed runway at Wilmington International Airport 8 km from the coastline, were revealing.

"We saw a series of very intense boundary layer rolls (structures) where the winds were over 160 km/h in one part of the roll but below 80 km/h in the other part of the roll," said Wurman. "This was surprising. These rolls only had short periods of about 600 m to one km, so they may play a significant role in the varied patterns of damage that you see in many landfalling hurricanes and typhoons. We already knew there were differences in damage patterns. Some neighborhoods were destroyed and some were relatively unscathed. There was a lot of speculation about whether this was caused by small tornadoes or microbursts or something else. With these observations we may be beginning to understand what's happening."

The DOW hasn't found anything in tornadoes that wasn't predicted by theory, but it has been useful in confirming what previously had only been theory. The mobile "pencil beam Doppler revealed many previously unresolved structures within tornadic storms and tornadoes," Wurman explained. "Radar data indicate the existence of spiral reflectivity bands and intense radial wind shear zones. We have observed debris shields, clear 'eye' regions, multiple reflectivity bands surrounding the center of the eye and occasional protrusions into the eye. Velocity data from circulations show evidence of axial downdrafts."

Last year, after trying for two years, for the first time the team caught a tornado in its formation process. Unfortunately, the DOW malfunctioned somewhat. This season he plans to use a pair of DOWs in an attempt to assemble complete three-dimensional maps of tornadic wind patterns.

In the Shadow of Nature

Storm research as practiced by storm chasers in the shadow of nature in its most dark and destructive mood is important for two reasons. "First, these are interesting questions about the nature of the world we live in," Wurman said. "Second, a better understanding of tornadoes may lead to improved forecasts and warnings." In the United States between 1950 and 1994, 34,438 tornadoes were reported, an average of 15 every week. No one knows how many were unseen or not reported. Every state has had at least one. Over 4,100 people died during that period, including 69 in 1994.

"The more we know about how and why tornadoes form and which storms cause them," said NCAR's Thompson, a veteran storm chaser, "the better we can warn the public in advance. Most people die because they don't heed warnings or the warnings are not issued in time." Any knowledge, then, that can produce an advantage in tornado forecasting and therefore increase the speed with which warnings are issued, could lead directly to preservation of life.

Against this backdrop chasers have been startled by pending cuts in the National Weather Service (NWS) budget scheduled to eliminate several forecasting positions at both the Tropical Prediction Center and Storm Prediction Center. The relatively few positions cost little money, but which fill jobs of night-shift hurricane and tornado watches - potentially placing the issuance of timely warnings in jeopardy. "People's greatest concern is cuts to staff specialists and the implied shirking of warning responsibility by the government," said storm chaser Cliff Nelson, Research Triangle Park, NC. "Safeguarding the citizenry from surprise disaster goes to the very heart of the purpose of government. Accordingly, NWS has traditionally shouldered final responsibility for disseminating severe weather information. Now we may end up going without tornado watches during winter midnight shifts."

Storm chasers have made significant contributions to the body of storm knowledge obtained in the past 40 years. Dave Hoadley and Neil Ward, who roved about the Great Plains without laptops, GPS or cellphones in the 1950s and 1960s, are considered the pioneers of storm chasing, in the days when little was known about storms and available data was sketchy. The art of chasing accelerated during the 1970s when the University of Oklahoma and the NSSL initiated mobile storm research efforts, the most notable of which was a program designed to deploy an instrument package known as TOTO (the Totable Tornado Observatory) in the path of tornadoes to verify Doppler-observed winds. The popularity of private storm chasing began during the 1970s and a small storm chaser newsletter ("Storm Track") was founded. During the 1980s mobile storm research was featured on television programs "Nova" and "In Search Of". Many of the mobile research programs, including TOTO, were terminated during the late 1980s. With the exception of the recent VORTEX Project in 1994 and 1995 mobile programs in the 1990s has been confined to research into small-scale tornadic structures using instruments like the DOWs. VORTEX, sponsored by the National Science Foundation and the National Oceanic and Atmospheric Administration and led by NSSL's Rasmussen, chased storms across the Midwest in a 12-truck convoy. More than 75 scientists were enlisted.

Contrary to popular belief, storm chasers do not risk their lives in pursuit of the supercell storm. If you believe the chasers themselves, storm chasing isn't as dangerous as Hollywood hyperbole (in the form of movies like Steven Spielberg's popular "Twister") suggests. Chasers maintain the greatest dangers they face are not from the prowling unpredictability of powerful storms but from the road conditions, bad drivers and traffic they encounter. Hydroplaning into a ditch along County Road 30 while dashing through a cloudburst can end a chase; being stranded with a broken axle in the path of a F-3 tornado after being forced off the road by some amateur (chasers call them "yahoos") trying to video tape a twister for the tv news can end a life. Lightning is another constant concern.

"We are experts in storm dynamics and tornadoes," said Wurman. "We know where they are likely to form and move and where to be and not to be. With the DOWs we can see tornadoes at night or through rain and see if the storm is changing course or strengthening. But, we drive thousands of miles, often in rain and often at night. We have to be constantly alert to our driving and to other weary drivers on the road."

In spite of the dangers, or because of them, with or without funding or formal programs, one suspects that most of the storm chasers can be found in the spring of each year in teams of two, three or four (driver, navigator, photographer) churning up a little dust of their own barreling down some dirt road outstripping a Sooner wind in hot pursuit of the unattainable. Once you've stood close to the pulse and sensed the charge of a 40 tera-Watt storm you are changed forever. From then on, one eye remains on the forecast and the other on the horizon, scanning for the telltale signs of helicity and instability. The lure of the chase never leaves you. Matadors never put down the cape.

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