A New Twist in Forecasting

Color-coded Doppler radar for earlier tornado warnings

Seated at a video console, the meteorologist intently watches the splashes of color as they flash across the screen. Spotting some possibly ominous patches, he zeroes in on one of the red and yellow areas. Then, fiddling with the controls, he orders up another display, showing tiny arrows circling counterclockwise and swirling ever closer in a tightening loop. After checking the coordinates on a map superimposed on his screen, the operator telephones an alert for the threatened area to the National Weather Service: a tornado may be about to strike.

As the U.S. endures its annual trial by tornado during the next two months, this scenario is likely to be repeated at the National Severe Storms Laboratory in Norman, Okla. The lab operates two new research radars that could be the prototypes for a tornado early-warning system. A sophisticated offspring of the familiar Doppler radar used by police to nab highway speeders, these electronic marvels for the first time are letting scientists peer into the very heart of storm systems in which tornadoes are born. Forecasters figure that should allow them to predict, more than 20 minutes in advance, when and where the terrifying funnels will hit the ground. Says Severe Storms Lab Director Edwin Kessler: "We can dissect a thunderstorm just like a warm-blooded animal. We can see air flows in great detail. We can see raindrops as well as flies or bees."

Tornadoes are the most violent of storms. Striking with little warning, they cut a narrow swath of almost complete destruction. Cars are hurled into the air and houses are splintered into kindling wood. By last week several outbreaks of tornadoes had already occurred in the South and Midwest, causing injuries and extensive damage. Indeed, about three-quarters of the world's tornadoes strike in the U.S., most of these in a belt nicknamed Tornado Alley that extends from Texas to Ohio and reaches as far east as the Appalachians and as far west as the Rockies. Since 1950 an average of 114 people in the U.S. have lost their lives to the deadly whirlwinds each year. Last week the Weather Service warned that if the 1980s prove to be a normal decade, at least 7,000 tornadoes will hit the U.S., killing as many as 1,000 people.

That toll could be sharply reduced if enough Doppler weather radar stations are built in the tornado-prone regions. In fact the Norman lab's Doppler, propitiously located in the heart of the tornado belt, has already shown its future value. On April 10 last year, the radar spotted the evolving tornado-producing storm that devastated Wichita Falls, Texas.

Conventional radar stations on which Tornado Alley still depends are capable of picking up certain telltale signs that can presage tornadoes, including so-called hook echoes--reflections from areas of heavy precipitation surrounding a rotating parcel of air. But they cannot detect actual movement within the storm. By contrast, Doppler radar not only measures the direction of air flows, but actually clocks the speeds of raindrops and ice particles as they are whirled about in a brewing tornado.

The radar relies on a phenomenon first explained by the 19th century physicist Christian Doppler, who observed that sound waves from an approaching source--say the whistle on a speeding train--rise in pitch (or frequency), while the waves from a departing source decrease in pitch. Applying the same principle to radio waves, Doppler radar uses shifts in frequency of the electromagnetic echoes from a storm system to measure motions within it. The research station in Oklahoma is capable of making almost 1 million measurements in a minute. These data are digested by a powerful computer and displayed as color-coded patterns on screens in the center's control room.

The first important revelation about tornadoes from Doppler radar was identification of the characteristic patterns of large, rotating air masses, known as mesocyclones, which frequently spawn twisters. Forecasters have since learned to spot even smaller areas of rapid rotation that represent actual incipient tornadoes, and have predicted within a kilometer (0.6 mile) where they will touch down. Since 1977, when they began sustained testing of their skills, the Oklahoma scientists spotted tornadoes aborning an average of 21.4 minutes before they struck. By contrast, warnings issued by the Weather Service's regular Oklahoma City office on the basis of ordinary radar preceded the twisters by an average of only 2.2 minutes.

Still, all of the new technology is useless if the public does not know how to respond properly to a warning. Many people in Wichita Falls, hearing advance alerts, made for their cars instead of shelters, hoping to outrace the funnel. As a result, the majority of the storm deaths were associated in one way or another with motor vehicles. Noted a federal report: "In 16 of the vehicle-related deaths, the victims had got into their cars or trucks specifically to escape the tornado's path. The homes of eleven of these victims did not have major damage." qed

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