Downdrafts; Deciphering them may make air travel safer
Boulder, Colo. — ''The substance of the winds is too thin for human eyes, their written language is too difficult for human minds.'' So wrote naturalist John Muir.
Although the winds remain invisible, weather scientists are learning to decipher their message. It's a skill that may soon make air travel safer.
In fact, the brightly colored image on a radar display near here could be called a direct translation of wind writing. The display, mounted in a trailer on the plains east of Boulder, represents a special kind of radar which ''reads'' the strength and direction of the winds. Called Doppler radar, it tracks the winds by detecting the motion of airborne water droplets.
Meteorologists are using such radar in a three-year, $2.2 million research project called the Joint Airport Weather Study (JAWS). Its purpose is to uncover the special atmospheric conditions that create so-called microbursts. These are small-scale, explosive downdrafts which have been identified as the cause of several airplane crashes in the last decade.
The occurrence of these short-lived, violent winds was not even suspected in 1975 when an Eastern airliner crashed on landing at the Kennedy International Airport and a Continental jetliner hit the end of the runway while trying to take off from Denver's Stapleton International Airport.
But Theodore Fujita of the University of Chicago, who analyzed the accidents, concluded that violent gusts of wind blowing directly downward were involved. A world authority on severe storms, Dr. Fujita had seen a number of ''star burst'' patterns of wind destruction in the aftermath of severe storms. These were totally unlike the weaving signature of tornadoes. Just as a downward flow of water splatters on the ground, so too would a downward gust of air flow out in all directions. Dr. Fujita suggested that this would create the star burst patterns and explain what had happened to the two airplanes.
When first proposed, the existence of these ''downbursts'' was hotly debated among meteorologists. But once researchers began looking for them, they turned out to be a fairly common phenomenon. Meteorologists documented ten downbursts in Illinois during a large field study in 1977. The current JAWS project is a follow-on to that investigation. It is more narrowly focused on small-scale downbursts, which represent the greatest hazard to aircraft during takeoffs and landings.
The existence of downbursts has been confirmed. But the specific conditions that cause them remain a mystery. According to a report on the current study, ''It is unknown what special physical mechanisms cause downdrafts to reach downburst intensity; it is also unknown if these stronger downdrafts have a different origin.'' Dr. Fujita, one of the principal investigators on JAWS, and his coinvestigators John McCarthy and James W. Wilson of the National Center for Atmospheric Research, published the report in the Bulletin of the American Meteorological Society earlier this year.
Elaborating on this, Dr. McCarthy says: ''We think there are two basic types of conditions which cause microbursts; you might call them wet and dry. In the 'wet' case, microbursts are associated with very strong thunderstorms. In the 'dry' case, they accompany what meteorologists call 'virga.' This is rain that falls from clouds but evaporates before it reaches the ground.
''While pilots know to stay away from thunderstorms, the airlines have no criteria warning them about virga,'' explains Dr. McCarthy, who is both a meteorologist and a pilot. However, ''Just pilot awareness of downbursts has helped greatly,'' Dr. Fujita says.
Dr. McCarthy calls microbursts ''an insidious danger, like a shark lurking out there.'' He says, ''That's why we chose the acronym JAWS.'' It is the strong horizontal winds near the surface that result from a microburst which represent the primary danger to aircraft when taking off and landing. These can be several times stronger than the original downburst.
If a microburst hits ahead of a landing aircraft, it creates a powerful headwind. To keep on the glide path the pilot must cut back substantially on engine power. But within seconds the headwind can change to an equally strong tailwind. This can make the plane stall out before the pilot can increase speed enough to compensate.
For three months this summer, JAWS scientists will be using three Doppler radars, up to six research aircraft, and the United States' most extensive surface weather network to study microburst formation.
''After working on a number of research projects, I wanted to do something applied, which can be put immediately to use,'' says Dr. McCarthy, who is directing JAWS. Thus, besides studying microbursts, the scientists will also be evaluating various detection systems and methods that are being used or have been proposed.
Following Dr. Fujita's original work on downbursts, the Federal Aviation Administration installed detection systems at major US airports. The detectors consist of a series of anemometers spread around an airport. They are connected to a device that tells air traffic controllers when there is a significant variation in wind speed at various locations around an airport. While such a system is effective for spotting gust fronts, it may miss some kinds of microbursts, the JAWS researchers say. They have seen instances where the winds at ground level are low while there are winds 100 to 200 feet aloft.
Another type of detector the JAWS scientists will evaluate is carried on aircraft equipped with an inertial navigation system (INS), which gives an independent measure of aircraft ground speed. By comparing the INS velocity with an airplane's air speed, this detector provides a direct indication of the strength of the wind.
Researchers are concerned that such a system might make pilots overconfident. Downbursts can create winds that are simply too strong for an aircraft to fly safely through them. This system would not warn the pilot of such danger until it was too late to avoid it.
Dr. McCarthy says he believes that, ultimately, the installation of wind tracking Doppler radars at airports will provide the best protection against the danger of downbursts.