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CX2SA > GPS 11.07.06 02:48l 77 Lines 3895 Bytes #999 (0) @ WW
BID : 21851_CX2SA
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Subj: GPS satellites could help...
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Sent: 060711/0041Z @:CX2SA.LAV.URY.SA #:21851 [Minas] FBB7.00e $:21851_CX2SA
From: CX2SA@CX2SA.LAV.URY.SA
To : GPS@WW
GPS satellites could help predict the weather
=============================================
Weather forecasts should be improved by a technique to track the variable
depth of the atmosphere's lowest layer, using the distortion to signals sent
between satellites.
The atmospheric boundary layer (ABL) is one of the most important layers for
weather forecasters. Its depth is determined by the character and intensity of
the thermodynamic processes going on inside it - such as the convection that
causes cumulus clouds to form - and variations in the energy radiated into the
atmosphere by the Earth.
"Knowing those fluxes is important for weather prediction and climate
monitoring," lead researcher Sergey Sokolovskiy at the University Corporation
for Atmospheric Research (UCAR) in Colorado, US, told New Scientist.
In the Arctic the ABL can be as low as 50 metres, while at temperate latitudes
400 m is more the norm. In the tropics, depths of up to 2000 m are possible.
Researchers have now developed a new way to monitor the ABL globally. It is an
improvement on the patchy information weather balloons currently provide
forecasters, they claim. Balloons only cover well-populated areas in detail,
leaving particularly big gaps over the oceans.
Cut the atmosphere
------------------
The new method exploits the fact that signals sent from GPS satellites to
satellites in lower orbits are bent, or refracted, by the atmosphere. GPS
satellites always transmit standard signals. This means that by examining the
signal that reaches a lower satellite, it is possible to work out how it was
bent by the atmosphere.
As the low-orbiting satellite appears over the horizon (from the GPS
satellite's viewpoint) the direct signal between them cuts through the
atmosphere. As the satellites change their relative positions the signal cuts
increasingly far from the Earth, resulting in a big drop in the amount the
signal bends when it stops passing through the ABL.
In tests of their technique, Sokolovskiy and colleagues found it was
comparable to weather balloons for finding the ABL's depth.
Initial conditions
------------------
Ian Brooks, a meteorologist at Leeds University, UK, says Sokolovskiy's method
could help get forecasts right. "Over the oceans there is little or no
information to provide the initial conditions for forecasting models," he
says.
Weather balloons are expensive while other radar-based methods to measure the
ABL are not so well suited, or have limited range, he says. "Having good
information over the oceans from satellites could make a big difference,"
Brooks says. "Small amounts of information can make large differences to the
predicted outcome of the weather when it reaches land hours later."
Weather forecasters around the world should soon be able to use data on the
ABL gathered by satellite. In April 2006 a constellation of six low-orbit
weather satellites called COSMIC was launched by UCAR. They are equipped to
use the new technique.
"We expect COSMIC to soon provide 2500 measurements a day," says Sokolovskiy.
"Global coverage will be available by a year after launch when the satellites
have been boosted into their final positions."
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