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Turbulence in the AirMost of us are all too familiar with the occasional bumps and lurches that come with flying on an airplane. Turbulence, the stealthiest of weather hazards, can strike from a sky that is literally clear and blue. It costs U.S. airlines an estimated $100 million each year because of injuries and operational disruptions, such as delays and rerouting. Turbulence has also stymied some of the greatest minds in physics. Turbulence refers to an irregular or disturbed flow in the atmosphere that produces gusts and eddies. It may be caused by many factors. These include
Up to 60% of aircraft encounters with this phenomenon are due to turbulence associated with thunderstorms. Often pilots can avoid turbulence because they can see bad weather on weather radars. However, turbulence is not always related to what the pilot sees on their onboard radar. NCAR scientists have worked with NASA to develop new software that uses existing airborne Doppler radars to detect thunderstorm turbulence —even in regions that appear benign. However, when a sudden change in wind velocity occurs in the absence of clouds, it is invisible to radar. Such “clear air” turbulence can buffet a plane without warning. Clear air turbulence occurs at high altitudes, frequently in small pockets of the atmosphere. It is associated with a rapid change in wind that causes disturbances, or eddies, near the edge of the otherwise smooth flow of the jet stream .
Thanks to ever-more-powerful computers, it's now possible for some simulations to depict turbulence more directly. Scientists are also gathering more observations of turbulence. Starting in 1997, dozens of passenger planes have been fitted with software that converts every bump and jostle of the aircraft into a numeric reading of turbulence. It's the first time airlines and scientists have had a quantitative, comprehensive picture of this persistent hazard. These plane-based sensors were the brainchild of an NCAR scientist. Taking on the challenge of clear-air turbulence, scientists at NCAR and colleagues fly research aircraft equipped with lidar, which uses lasers to detect minute particles that are invisible to radars. The team has successfully tracked air motions—and thus turbulence—up to several miles ahead of the aircraft. Placing lidars on board the nation's commercial aircraft would be a major undertaking, but the day may come when radar and lidar are combined to forge a complete detection-and-warning system. To this end, NCAR researchers are working on algorithms, or mathematical formulas, that can produce a measure of turbulence severity. Improved computer forecasts can help pilots plan ahead to avoid turbulence. In 2003 an NCAR-developed system called Graphical Turbulence Guidance became available for airline use. GTG provides aviation forecasters, dispatchers, and pilots with online displays of expected regions of clear-air turbulence, updated hourly. The online displays combine turbulence estimates from a weather prediction model and pilot reports of turbulence encounters to forecast clear-air turbulence up to 12 hours ahead. NCAR researchers successfully tackled a different hazard to aircraft in the 1980s. Microbursts, which are strong winds in a concentrated area that blast down from a rain shower or thunderstorm, used to pose a deadly threat to aircraft. In 1983, the Federal Aviation Administration asked NCAR to embark on a study with other organizations to better understand this dangerous type of wind shear and then create an airport system that could detect microburst threats in time to alert nearby aircraft. This work resulted in development of the Low Level Windshear Alert System, which is in place at major airports across the United States as well as several other countries. The system uses remote sensors located across the airport to measure wind speed and direction, determining the strength of microbursts and pinpointing their location. Since the widespread adoption of the system, microbursts have not caused a fatal crash. NCAR researchers also design custom systems for airports with challenging wind shear and turbulence problems.
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Due to its highly complex nature, turbulence remains an enigma to
atmospheric science. Nobel laureate Richard Feynmann called it the
last great unsolved problem in classical physics. Researchers use
a technique called large-eddy simulation to depict turbulent eddies
in computer models. Instead of trying to model every swirl down to
the smallest scales, the approach is to numerically describe the
larger, energy-containing eddies and then model the smaller-scale
motions using statistics.
