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June 19, 2000 -- NASA scientists at the Global Hydrology
and Climate Center are studying a type of radar that uses laser
light instead of microwaves to provide high-quality snapshots
of the winds that travel the globe. Knowing the wind's speed
and direction over large areas could help meteorologists answer
the riddle of tomorrow's weather further in advance, saving lives
and benefiting many areas of the world's economy -- particularly
air travel.
"On-board lidar sensors on each airplane would be valuable
for microburst wind shear and clear-air turbulence warnings.
These are significant threats to passengers' safety," says
Kavaya. Above: The Multi-center Airborne Coherent Atmospheric
Wind Sensor (MACAWS) is an airborne laser radar (lidar) that
remotely senses the distribution of wind velocity within three-dimensional
volumes in the troposphere. MACAWS is presently configured to
fly on the NASA DC-8 research aircraft (above). [more
information from the Global Hydrology and Climate Center]
Image credit: NASA Ames Research Center
"Radar excels at piercing bad weather, but it needs raindrops or hydrometeors (hail or snow) to get a signal," Kavaya said. "Lidar struggles to go through thick clouds or heavy rain, but it can get you wind (measurements) in clear air, because it relies on aerosols."  The reason for this
difference is the frequency of the radiation that each technology
uses. Both emit electromagnetic waves, but while radar typically
uses frequencies in the range of microwaves, lidar uses higher
frequencies in the visible or near-visible light range. Higher
frequency radiation (light) will be reflected by smaller particles
than lower frequency radiation (microwaves). Left: A sodium resonance lidar beam shoots upward into the night sky from the National Astronomy and Ionosphere Center's ground based Lidar Lab in Arecibo, Puerto Rico. Scientists use this facility to study the chemistry and dynamics of the atmosphere above the Caribbean. An Earth-orbiting lidar facility would enjoy a global view for the same types of studies. [more information from the Arecibo Observatory] Lidar also emits a narrower beam than radar, which minimizes interference from ground clutter and improves the resolution of the data. The disadvantage is that lidar has more trouble covering large areas than does radar. Rivers are fair game, too The narrower beam opens up another possible application for lidar: measuring water flow in rivers.  Right:
The Potomac River, upstream from Washington, D.C.Scientists at the Marshall Space Flight Center and the GHCC have been working with the U.S. Geological Survey to see if lidar may be able to replace the manual stream-flow measuring technique that the USGS currently uses. "It turns out that some of their standard methods for measuring stream current involved an element of risk to personnel," said Dr. Jeff Rothermel, a NASA scientist at the GHCC. "In fact, one USGS employee lost his life in the line of duty while making measurements. So there is an interest there to determine whether lidar can be used to measure stream current." Mounted either at the side of the stream or on a satellite in space, a lidar system would measure the speed of the water's surface at several points across the width of the river. Knowing the shape of the river's bottom would allow the volume of water flowing in the river to be calculated from those measurements. "[In addition to U.S. rivers] I imagine that we could contribute to the study of the Earth's hydrology greatly by having improved river flow (data) worldwide," Kavaya said.  Above: GHCC scientists are testing lidar systems as
flow monitors on the Tennessee
River. Lidar's Crystal Ball Predicting hazardous or inclement weather could benefit many
sectors of the U.S. economy. One study estimates saving of about
$110 billion annually if reliable weather forecasts could be
extended to seven days in advance.
Left: During a La Niña phase of the ENSO, strong winds blowing east to west along the equator push the water at the surface toward Southeast Asia. Deep, cold water wells up in the eastern Pacific near South America to replace the water heading toward Asia. When the cycle switches to an El Niño phase, the winds die down, the upwelling weakens, and the eastern Pacific becomes warmer than usual near the equator. [more information] More wind data may help extend weather forecasts, but Robertson cautioned that there is a theoretical limit to how far into the future accurate forecasts will ever be possible. "No matter how good you know your initial conditions, there's a certain amount of chaotic behavior in the atmosphere," Robertson said. "So ... a really deterministic weather forecast (is) only going to be possible for maybe up to two weeks." Dr. James Keesling, a professor of mathematics at the University of Florida who specializes in chaos theory, commented on this theoretical limit. "The lidar system may provide us with unprecedented detailed information about the direction and intensity of winds throughout the globe," Keesling said. "However, we know that unless this data is perfect and the computers using that data in their computations use an impossible number of digits, we will not be able to predict very far into the future. The problem is in the mathematics itself, not the accuracy of the data."  Chaos theory
predicts that systems such as the world's weather that involve
chaotic behavior (in the mathematical sense of "chaotic")
will exhibit a property sometimes called the "butterfly
effect." First identified by a meteorologist named Edward
Lorenz in 1963, the butterfly
effect refers to a situation when very small differences
can lead to very large differences over time. Hence the famous
example of a butterfly flapping its wings in New York City's
Central Park and causing a tornado in Texas. Above: A typical Lorenz Butterfly shows the divergent trajectories of two nearly-identical particles racing around a pair of "chaotic attractors." Visit the San Francisco Exploratorium's web page about Edward Lorenz to learn more about the role of chaos in weather prediction and for a hands-on demonstration of Lorenz Butterflies. Lidar might not be able to sense the gentle breeze of a butterfly in flight, but by forecasting storms and detecting turbulence the technology could save millions of dollars and even human lives.
The Global Hydrology and Climate Center is a joint venture between government and academia to study the global water cycle and its effect on Earth's climate. Jointly funded by NASA and its academic partners, and jointly operated by NASA's Marshall Space Flight Center in Huntsville, Ala., and the University of Alabama in Huntsville, the Center conducts research in a number of critical areas. |
| Web Links |
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Global Hydrology and Climate Center -- a joint venture between government and academia to study the global water cycle and its effect on Earth's climate. The Multi-center Airborne Coherent Atmospheric Wind Sensor -- home page; from the GHCC Why Are Measurements of Winds from Space Needed? -- information from the Global Hydrology and Climate Center |
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The time between the
pulse and the echo determines the distance, and the shift in
the color of the light determines the velocity of the particles
along the laser's line of sight. True wind speed and direction
can be calculated from these results. NASA has employed this
technology on aircraft in meteorological studies, demonstrated
with the Multi-center Airborne Coherent Atmospheric Wind Sensor
(MACAWS) at the Global Hydrology and Climate Center.
Scientists
working on lidar believe that a lidar-equipped satellite in a
polar orbit could bring about such an improvement in weather
forecasting. 
