Mastering the microburst; these elusive winds drop from clouds and sweep planes out of the sky, but scientists are developing systems to detect them

Science News, March 21, 1987 by Richard Monastersky

MASTERING THE MICROBURST

On Aug. 2, 1985, Delta flight 191 wasdescending through scattered thunderstorms at 6:04 p.m. on a routine approach to the Dallas-Ft. Worth Airport. At 6:06, the plane was a ball of flames, lying less than a mile from the runway. In the interim, flight 191 had flown through the treacherous winds of a microburst.

Microbursts "are the largest source ofair carrier death in the United States,' says John McCarthy, a meteorologist at the National Center for Atmospheric Research (NCAR) in Boulder, Colo. Over the last 12 years, this small, short-lived pattern of intense winds has been implicated in three of the most catastrophic weather-related air accidents, which were responsible for a combined total of 398 deaths.

Currently airports are ill equipped todetect these significant but infrequent threats to air travel. However, in the wake of these crashes, a concert of meteorologists, computer specialists and aerodynamic engineers is developing systems to detect microbursts and warn pilots of their deadly presence.

Last summer in Huntsville, Ala., scientistsconducted the most recent in a series of large-scale field experiments designed to learn more about microbursts. These experiments indicate that a combination of Doppler radar and computers may be effective in detecting microbursts, although the widespread use of these systems is several years away.

A microburst is a wind pattern thatdescends from rain clouds during spring and summer months. When this stream of falling air, or downflow, hits the ground, the wind fans out horizontally into an outflow. In this way it resembles the spray pattern that water from a kitchen faucet makes when it hits the bottom of the sink. By producing a strong divergence of wind, the microburst outflow causes a condition known as wind shear, a quick change in the wind's speed or direction.

For airplanes on takeoff or landing, anintense wind shear can be particularly hazardous. When an airplane enters a microburst, it first runs into a headwind, which increases the speed of the air that is rushing over the wings and gives the plane additional lift. After the plane passes through the downdraft in the center of the microburst, it is swept by a tailwind, which robs the plane of lift.

This dramatic change from headwindto tailwind poses an insidious combination of forces. A plane on a landing approach is usually at 70 to 80 percent of full power. Upon entering the headwind of a microburst, a pilot may mistake this for an ordinary headwind, and will not expect a sudden wind shift. "Then you are caught in a big surprise' when the airspeed drops dramatically and the plane begins to fall, says T. Theodore Fujita of the University of Chicago. It takes several seconds to reach full power, and by that time the plane may have lost too much altitude to pull out of its fall.

Fujita is generally credited as being thefirst to deduce the existence of microbursts, during an aerial survey of tornado damage in 1974. Instead of seeing a typical swirling pattern of fallen trees he noticed hundreds of trees blown outward like the spokes of a wheel. Since then, he and NCAR have conducted several large-scale field experiments, including the 1982 Joint Airport Weather Studies (JAWS) and last summer's Microburst and Severe Thunderstorm (MIST) project. The aim is to understand the mechanics of the microburst event and develop strategies to avert wind-shear-related air disasters.

Although the mechanics are not totallyunderstood, says Fujita, it appears that the evaporation of raindrops is critical to the microburst formation. This evaporation cools a parcel of air, which will begin to fall as it gets heavier, thereby producing a downdraft. Humid areas like Huntsville, where the downdraft is almost always associated with rain, usually spawn the so-called wet microbursts. Dry microbursts, on the other hand, frequent more arid places like Denver, where cloud bases are higher and the precipitation will often totally evaporate before the downdraft reaches the ground.

These factors conspire to make themicroburst a dangerously elusive phenomenon. Wet microbursts will produce a visible rain shaft, but this can often be obscured within a benign rain shaft. Conversely, without any associated precipitation, dry microbursts present almost no visual clues of their presence. Pilots and controllers hoping to spot a microburst cannot even rely on the character of the clouds for clues because both thunderheads and small, seemingly innocuous rain clouds can produce these hazardous wind shears.

From the JAWS and MIST projects,scientists have learned that microbursts are small, typically 0.6 to 1.9 miles across, and short-lived--on the order of 5 to 15 minutes. The average difference between the headwind and the tailwind is 60 miles per hour, but Fujita has documented a case with a differential in excess of 172 mph.

Presently, 90 airports across the UnitedStates are fitted with a Low-Level Wind Shear Alert System (LLWAS)--an array of 5 anemometers, or wind detectors, at the boundary of the airport surrounding an anemometer at the center field position. A processor compares differences in wind speed and direction, to determine if there is any shear within the LLWAS area.