Tracking Tornadoes
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rumble in the night, like the sound of a freight train rushing past, jolted Mary Ann Brudos awake. By the time she woke Galen, her husband, the sound—and the tornado—were gone. The next morning Mary Ann and Galen easily spotted the path of the tornado. Like a bulldozer it had cut a 200-m-long swath down a hill, blasting trees in its path. The tornado missed the barn on their Wisconsin farm by just 150 m and their valley home by about 300 m. No other signs of damage were visible for kilometers. Few natural events are as unpredictable and violent as tornadoes. They strike quickly, leaving a well-defined path of destruction behind. Meteorologists are determined to understand and predict their formation, but they have few techniques to directly measure these sudden events (2). Tornadoes also hold a special place in American lore, and for good reason. Intense tornadoes strike the central United States more frequently than any other location in the world. Tornado-like phenomena may also occur on other planets. For instance, dust devils, which are much smaller and weaker than tornadoes, have been photographed on Mars. On Earth, tornadoes are often associated with severe thunderstorms. As the storm cell advances, warm, moist air is drawn up into the clouds. Drier air flowing around the updraft at higher altitudes is cooled by evaporating pre-
cipitation and spirals toward the ground. The sinking air is pulled in a cyclonic direction (counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere) around this developing column of air. Tornadoes rarely form with an anticyclonic spin. A gust front appears along the leading edge of this air movement. As the air column moves forward it picks up more warm air, continuing the cycle. Films of tornadoes reveal that the air flowing just outside the funnel surface is generally spiraling upward. The advancing front is marked by rapid cool-
FOCUS ing, dramatic pressure changes, and dangerously high winds. Tornadoes evolve through as many as five discrete stages: the first signs of updraft, a descending funnel shape, a vertical funnel, a shrinking funnel diameter and increasing funnel tilt, and decay into a rope-like shape as wind shear and surface drag dissipate the tornado. Some large tornadoes just spread out and eventually disperse. Winds inside the most intense tornadoes have been estimated to reach as high as 125 m s - 1 . The kinetic energy of the winds is packed into a column of air that can measure just 100 m in diameter. Estimates of the energy in tornado
winds range in the order of 103 megawatts, which is far less than the total energy of the parent storm that spawns the tornado. However, how the storm generates the tight funnel vortex of a tornado remains poorly understood (2). Tornadoes that form in conjunction with severe thunderstorms often develop along the right rear flank (in the Northern Hemisphere) where new storm cells appear. Where the tornado forms is often several kilometers from the storm's rain front. Many of the most intense tornadoes are associated with a different phenomenon known as a mesocyclone, a cyclonically rotating vortex that forms over an updraft. Mesocyclones are low-pressure areas that range in diameter from a few kilometers to ~ 1 5 km and display heights t h a t stretch from about ground level to almost the tropopause (>10 km). Fortunately, only about 3% of the approximately 900 tornadoes that annually form in the United States lead to fatalities and large-scale property damage. Most tornadoes are shortlived (a few minutes), inflict damage within a narrow range (2 km long by just 50 m wide), and move quickly (15 m s - 1 )· An extremely intense tornado can last more than 1 h, generate damage over a 150-km-long by 3-km-wide path, and move with a surprisingly fast translational speed that may reach 30 m s - 1 . Storms can even produce multiple tornadoes, which will form succes-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 5, MARCH 1, 1991 • 297 A
FOCUS sively and even overlap in time but which rarely coexist. Over water, tornado-like systems are labeled waterspouts. These appear over the ocean or large lakes and occur more frequently than tornadoes. For instance, in favorable locations such as the Florida Keys more than 400 water spouts form annually per 105 sq km of ocean surface. That compares with an annual frequency of two to four per 10 s sq km for tornadoes in likely land regions. Fortunately, waterspouts are usually less intense than their landbound relatives. As can be imagined, collecting data on these natural phenomena isn't easy. One means to gain data on tornadoes is to film them in action. Like a problem in trigonometry, detailed analysis re quires a knowledge of the camera loca tion, distance to the tornado, azimuth and elevation of landmarks in the film, and camera framing rate. Debris being whirled around the tornado offers markers to calculate wind speeds. For instance, film of an April 1963 tornado that landed near Kankakee, IL, was used to calculate a maximum vertical wind speed of 80 m s _ 1 at a point more than 200 m from the axis and ~200 m above the ground. Useful film sequences of tornadoes, though, are rare. Dust and poor light ing generally obscure any images. Fur thermore, to extract velocity informa tion from these images, simplified as sumptions about the motion of the debris and the air must be made. Complicating the analysis, film stud ies occasionally must also contend with asymmetric and complex motions about the tornado's axis. Large torna does actually consist of several vortices packed into one column. Thus visual data must delineate the tornado's hori zontal motion, translation of the vari ous vortices about the axis, and rota tion about each individual vortex. When large tornadoes touch down, they leave telltale cycloid-shaped marks along the ground. These can be used to directly calculate the velocity of the individual (or suction) vortices that make up the larger tornadoes. However, these marks are found only on certain surfaces such as corn fields. Often the tracks are erased by the tor nado. Another type of track, damage along the tornado's path, has been used to estimate wind speeds. Most meteorolo gists cite the Fujita (F) scale that runs from F0—wind speeds of 18-32 m s - 1 , which are attested to by broken branches and a few missing roof shin gles, to F5—violent winds of 117-142 m s - 1 distinguished by completely disin tegrated houses, heavy debris thrown
considerable distances, and other in credible phenomena (3). The F scale requires some familiarity with construction quality, knowledge of structure orientation with respect to the powerful winds, and an under standing of how structures fail. For tor nadoes that strike across open country, this type of damage trail obviously does not exist. Direct measurements within torna does have also been tried. During 198183, determined researchers from the National Oceanic and Atmospheric Administration (NOAA) in Denver and from the University of Oklahoma at tempted to place the totable tornado observatory (appropriately named TOTO) directly in the path of a torna do (4). TOTO's recording instruments were secured inside a hardened canister. Several inkless impact chart recorders collected data at the rate of one impact per second. A T-shaped arm rising out of the canister held the observatory's instruments: a wind direction vane, a thermistor to measure temperature, a static pressure sensor with a range of ±20 mbar, a corona discharge probe consisting of a hypodermic needle wired to an operational amplifier cir cuit to measure leakage currents to ±10 μΑ, and a wind speed sensor. The last instrument contained no moving parts and measured wind speed by sudden changes in air pressure that are propor tional to the horizontal wind speed squared. To deploy the instruments, research ers would rush to the vicinity of a tor nado and unload the 400-lb. TOTO in the cyclone's path within about 20 s. To minimize time and risk, TOTO was carried on its side so that when it was tilted upright at the sampling site, mer cury contact switches automatically ac tivated the observatory. Although the researchers were reasonably successful at intercepting tornadoes in progress, TOTO failed to collect measurements inside a tornado. Furthermore, in wind tunnel tests, TOTO was disabled by gusts of
