Weather forecasts should be improved by a technique to track the variable depth of the atmosphere鈥檚 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 鈥� such as the convection that causes cumulus clouds to form 鈥� and variations in the energy radiated into the atmosphere by the Earth.
鈥淜nowing those fluxes is important for weather prediction and climate monitoring,鈥� lead researcher Sergey Sokolovskiy at the University Corporation for Atmospheric Research (UCAR) in Colorado, US, told New 杏吧原创.
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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鈥檚 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.
The technique has been tried before using standard GPS receivers but they had problems tracking the signals without error. A new tracking technique and receivers designed in collaboration with NASA have now fixed that problem. In tests of their technique, Sokolovskiy and colleagues found it was comparable to weather balloons for finding the ABL鈥檚 depth.
Initial conditions
Ian Brooks, a meteorologist at Leeds University, UK, says Sokolovskiy鈥檚 method could help get forecasts right. 鈥淥ver the oceans there is little or no information to provide the initial conditions for forecasting models,鈥� 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. 鈥淗aving good information over the oceans from satellites could make a big difference,鈥� Brooks says. 鈥淪mall amounts of information can make large differences to the predicted outcome of the weather when it reaches land hours later.鈥�
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.
鈥淲e expect COSMIC to soon provide 2500 measurements a day,鈥� says Sokolovskiy. 鈥淕lobal coverage will be available by a year after launch when the satellites have been boosted into their final positions.鈥�
Journal reference: Geophysical Research Letters (DOI: 10.1029/2006GL025955)
