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Triggering turbulence in clear air

Atmospheric gravity waves could be the main cause

gravity waves

Gravity waves can cause undulating cloud patterns, like these passing over Boulder, Colorado. (Photo courtesy Howie Bluestein, University of Oklahoma.)

January 16, 2013 | Turbulence is the leading cause of injuries to passengers and crew aboard commercial aircraft, and it indirectly increases travel expenses by costing airlines tens of millions of dollars yearly. While much of the rough air occurs within clouds, planes sometimes unexpectedly encounter turbulence while cruising through regions of clear air.

New research by NCAR researchers and collaborators points to gravity waves, which ripple unseen through the atmosphere, as the culprit in many cases of clear-air turbulence. If those waves can be forecast, the research suggests that planes in many cases could be rerouted around them.

“Clear-air turbulence forecasting is one of the last great challenges of numerical weather prediction,” says Bob Sharman, who leads NCAR’s turbulence research team. “As we better understand what causes turbulence, we can begin developing systems to predict it.”

Sharman presented the findings at December’s annual meeting of the American Geophysical Union.

Gravity waves are a common atmospheric phenomenon. They are caused when air is forced upward, generally over mountains or in thunderstorms, and bumps up against the stable floor of the stratosphere. This sets off ripples that can travel hundreds of miles before breaking. (Gravity waves are unrelated to gravitational waves, which are perturbations in the gravitational field.)

For their research, Sharman and colleagues from NCAR; the University of Melbourne in Australia; the University of California, Los Angeles; and the Naval Research Laboratory collected observations of turbulence from more than 100 commercial aircraft. The team then compared those reports to the locations of cloud and other possible sources of gravity waves. The observations, which measure the extent to which rough air causes up-and-down movements of an airplane, were recorded by a special onboard system devised by NCAR several years ago.

Sharman and his team then used the NCAR-based version of the Weather Research and Forecasting model (WRF) to simulate atmospheric conditions associated with observed turbulence events. They found that gravity waves “break” against aircraft, much as ocean waves break on the beach. Although clear-air turbulence has traditionally been thought to be due mainly to areas of high wind shear associated with jet streams, the research indicates that gravity wave breaking events actually account for much of the observed turbulence.

Gravity waves often break within a relatively shallow altitude range, so pilots might be able to avoid them if they knew where the waves were. But the waves cannot be detected by radars aboard commercial aircraft.

For the next phase of their research, Sharman and his colleagues are using a computer model to better understand the initiation and evolution of gravity waves and gravity wave breaking. If they can successfully predict turbulence associated with gravity waves at least 85% of the time, the aviation industry may find it cost effective to reroute aircraft.

Even if the planes did not take action to avoid the waves, pilots could often alert passengers about a bumpy ride ahead. “The goal is to make flying as safe and comfortable as possible,” Sharman says.

The research is supported by NASA and the National Oceanic and Atmospheric Administration.