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Method of predicting clear air turbulence could make flights smoother in the future

A new forecasting method could help pilots chart new courses around patches of rough but clear air that can turn an otherwise unremarkable flight into a nightmare. (Credit: iStockphoto/David Joyner)

It comes blasting out of the blue on your airplane flight: sudden
bumpiness and sometimes even a violent plummeting. It arrives without
warning, and it can be more than frightening, since it causes tens of
millions of dollars in injury claims every year.
It's called clear air turbulence (CAT), and a new forecasting
method, published in the Journal of Atmospheric Sciences and led by a
researcher at the University of Georgia, could help pilots chart new
courses around these patches of rough but clear air that can turn an
otherwise unremarkable flight into a nightmare.

"Our new method allows superior forecasts for CAT beyond the tools
that have been in use," said John Knox, an assistant professor in the
department of geography in UGA's Franklin College of Arts and Sciences.
"Commercial aircraft encounter severe-or-greater turbulence about 5,000
times each year, and the majority of these occur 10,000 feet above the
Earth's surface. This new method gives pilots a way to avoid turbulence
that's not associated with nearby thunderstorms or significant
cloudiness."

Other authors on the paper include Donald McCann of McCann Aviation
Weather Research, Inc., of Overland Park, Kan., and Paul Williams of
the department of meteorology at the University of Reading in Great
Britain.

The new method predicts energy associated with gravity
waves—phenomena in the atmosphere that look like ocean waves but which
can occur in clear air. They can be created by air flow over mountains,
frontal boundaries or other causes. The type of gravity wave that Knox
and his colleagues identified as a possible source of bumpiness comes
from a different source. These waves are spontaneously generated and
associated with jet streams at high altitudes, near cruising levels for
airplanes.

When a plane flies through them, the sensation is like being in a
small boat on a stormy sea. But where a boat's skipper can see rough
sea, gravity waves in the air are usually invisible, and pilots often
don't know they're present until they're flying right into them.

Predicting turbulence caused by nearby storms or low pressure
systems is much easier than knowing when CAT might hit, said Knox.
Still, several hundred significant injuries occur in the U.S. because
of clear air turbulence, and because it occurs in the absence of
obvious weather, wary passengers tend to wonder if they are in danger.

There are predictive models in use now, and an improved version of
the Graphical Turbulence Guidance (GTG) algorithm, currently the best
CAT forecasting method, will soon be online for airline pilots, said
Knox. But he noted that even the GTG doesn't have some of the desirable
features of the method just published in the Journal of Atmospheric
Sciences.

The new method is based on something called the Lighthill-Ford
theory of spontaneous imbalance, developed by a British theoretician in
the early 1990s. Knox and his colleagues spent several years turning
this theory into a forecast tool.

The team first simplified the theory then developed an algorithm to
use the theory to make predictions of turbulence. The algorithm was
next tested on five months' worth of high-resolution weather forecast
model output from 2005-2006. The researchers then compared the
algorithm's prediction of turbulence to actual pilot observations of
it. The results of this statistical analysis demonstrated that the
team's method performed better than the best methods of CAT forecasting
available during that period, said Knox.

"Essentially what we have is a mathematical model that translates
the theory into numbers that describe the gravity waves," said Knox.
"These numbers can then drive an algorithm that gives you a forecast of
the kinetic energy associated with turbulence."

The researchers hypothesize that a clear sequence of events occurs
to create CAT and understanding that sequence is crucial to predicting
the location of the turbulence.

"Gravity waves act upon the environment and then destabilize it," said Knox. "Even weak gravity waves may initiate turbulence."

One problem with current CAT-forecasting models is that they are "at
least partly empirical," said Knox. "Current methods often rely on
rules-of-thumb based on pilot experience that aren't always grounded in
rigorous theory." The new method is based "on a single, consistent
theory of spontaneous imbalance," and thus should at least
theoretically be more reliable, Knox said.

He added that adoption of the new method could potentially create
"major improvements in CAT forecasting." Thousands of passengers who
are fearful of "things that go bump in the flight" hope he's correct.

University of Georgia (2008, October 9). Method Of Predicting Clear Air Turbulence Could Make Flights Smoother In The Future. ScienceDaily. Retrieved November 3, 2008, from http://www.sciencedaily.com­ /releases/2008/10/081001093239.htm