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Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida
Area Cirrus Experiment (CRYSTAL-FACE), a scientific mission By Veronika
Soukhovitskaya To make a more accurate computer model of Earth's climate,
about 450 scientists from NASA and many other organizations are studying
tropical cirrus clouds in the Florida region during July 2002. Researchers
are using six aircraft equipped with state-of-the-art instruments
to measure characteristics of clouds and their influence on temperatures
on Earth. Satellites and ground-based instruments also will play a
role in the experiment.
Experimenters are conducting studies in the tropics because solar
energy absorption there is the heat engine that drives the world's
atmospheric circulation. The clouds under study are cirrus clouds
that form in the tropics. Cirrus clouds are high, cold clouds made
of ice crystals. They form at altitudes of 30,000 to 60,000 ft. (9-18
km).
Cirrus-cloud ice crystals trap and scatter incoming sunlight, reducing
the amount that reaches the Earth's surface. This results in surface
cooling. Cirrus clouds also absorb upwelling infrared radiation emitted
by the surface and lower atmosphere.
Indirectly, this produces a surface-warming effect by trapping heat,
energy that might otherwise escape into outer space. Infrared radiation
is that part of the electromagnetic spectrum that ranges from 0.7
to 1,000 micrometers. Often, this radiation results from heat exciting
matter, vibrating its molecules.
The warmth of the planet's surface also causes liquid water to evaporate
resulting in an increase of water vapor in the atmosphere, leading
to additional surface warming. Water vapor, like carbon dioxide and
ozone, are 'greenhouse gases.' These gases are strong absorbers of
infrared radiation, and therefore trap heat in Earth's atmosphere.
(Nitrogen and oxygen gas, on the other hand, do not absorb much infrared.)
The CRYSTAL-FACE mission results also will help scientists to better
understand the processes controlling water vapor concentrations in
the atmosphere, which have increased in the last decade.
The net effect of tropical cirrus clouds on a region's surface temperature
depends on several factors, including cloud height, thickness and
ice crystal sizes.
The uncertainty of climate prediction by current computer 'models'
of Earth's weather, called 'general circulation models,' results from
a lack of knowledge of exactly how clouds affect temperature and other
atmospheric conditions. Accurately representing clouds in computerized
models of weather is challenging today because of our limited knowledge
of tropical cirrus cloud physical properties and processes.
CRYSTAL-FACE scientists will study tropical cirrus clouds using a
combination of measurements and computer modeling.
Researchers will examine the effects of increased greenhouse-gas concentrations
on the formation of tropical cirrus clouds. Increased greenhouse-gas
concentrations cause higher temperatures that, in turn, cause more
intense convection. Convection in the atmosphere is the up or down
circulation of air that plays a role in cloud formation. Scientists
also will study the role of convection within clouds.
Researchers plan to characterize convective systems in terms of structure,
'mass fluxes' and updraft velocities.
A mass flux, or flow, of cloud circulation can represent 'moist convection,'
and includes movement of several cloud layers, one of which is a middle
layer where rain and droplets of water often form.
Scientists will use airborne and ground-based Doppler radar to help
study convection. Doppler radar is radar that can very accurately
determine the velocity of moving objects, including cloud particles.
Remote sensing of the conditions of the clouds and atmosphere will
also take place from other instruments, both aboard satellites and
on the ground.
Of the six aircraft to be used during the study, two are NASA airplanes.
The high-flying ER-2 is based at NASA Dryden Flight Research Center,
Edwards, Calif.; and the WB-57 is from NASA Johnson Space Center,
Houston. |
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