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THUNDERSTORMS
ARE AFFECTED BY POLLUTION
A
NASA-funded researcher has discovered that tiny airborne particles
of pollution may modify developing thunderclouds by increasing
the quantity and reducing the size of ice crystals within
them. These modifications may affect the cloud's impact on
the "radiation budget," the amount of radiation
that enters and leaves the Earth.
"I've
found that aerosols depress the size of ice crystals in thunderclouds
over land and oceans - and as a result may reduce precipitation,"
said Steven Sherwood of Yale University. Because smaller ice
crystals are lighter, they don't fall out of the cloud as
easily and evaporate instead of falling as rain.
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Using
several satellites and instruments including NASA's Total
Ozone Mapping Spectrometer (TOMS), Sherwood looked at how
airborne pollution particles (aerosols) affect large thunderstorms,
or cumulonimbus clouds, in the tropics.
Common
aerosols include mineral dust, smoke, and sulfates. An increased
number of these particles increase the number of ice crystals
in a cumulonimbus cloud, but they're just smaller in size.
As a result of their smaller size, the ice crystals evaporate
from a solid state directly into a gas, instead of falling
as rain. Sherwood noted that this effect is more prevalent
over land than open ocean areas.
Previous
research by Daniel Rosenfeld of Hebrew University revealed
that aerosols and pollution reduced rainfall in shallow cumulus
clouds of liquid water, which do not have the capability to
produce as much rainfall. Sherwood expanded on that research
by looking at cumulonimbus clouds, which contain ice particles.
Previous
studies have proven that ice particles are smaller in the
upper reaches of thunderclouds when there is more pollution,
and when the rising air in the clouds (convection) is stronger.
Aerosols seem to have the most influence on seasonal and longer
timescales, such as during the warmer months when plants and
undergrowth are burned to clear fields.
Over
areas where biomass burning occurs, such as South America,
aerosols have been found to reduce the diameter of ice crystals
in the clouds by as much as 20 percent. Areas over deserts,
such as Africa's Sahel Region where dust is a primary aerosol,
there was a 10 percent decrease in the diameter of ice crystals
in cumulonimbus clouds.
Aerosol
particles are necessary for clouds to form, and it has been
suspected that clouds might be altered by large concentrations
of them, from a fire for example. "I was able to show
by looking at 10 years of aerosol data and statistically analyzing
many thunderclouds in the tropics that they are definitely
affected," Sherwood said.
Clouds
play an important role in regulating heat in the atmosphere
by reflecting the Sun's rays back to space.
Sherwood
found that ice crystals are smaller in clouds over continents
than oceans, which could be attributed to the amount of pollution
generated over land. The highest values occur widely over
Northern Africa, where desert dust and smoke from agricultural
burning occur. Intermediate values prevail over much of Asia,
through the Indonesia region and into the south Pacific. The
largest ice crystal sizes were found over the eastern Pacific
and southern Indian Oceans.
Sherwood
used aerosol data from TOMS to verify pollution levels. He
also used cloud reflectivity data from NASA's Tropical Rainfall
Measuring Mission (TRMM) satellite, and radiance (reflected
light generated from aerosols) data from the Advanced Very
High Resolution Radiometer (AVHRR) aboard a NOAA satellite.
Sherwood's
article "Aerosols and Ice Particle Size in Tropical Cumulonimbus"
appears in the May 1, 2002 issue of the American Meteorological
Society Journal of Climate.
This
work was performed under the NASA Earth Observing System/Interdisciplinary
Science (IDS) program under the Earth Science Enterprise (ESE).
The mission of NASA's ESE is to develop a scientific understanding
of the Earth System and its response to natural or human-induced
changes to enable improved prediction capability for climate,
weather and natural hazards.
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