photograph, acquired in February 1984 by an astronaut aboard the space
shuttle, shows a series of mature thunderstorms located near the Parana
River in southern Brazil. With abundant warm temperatures and
moisture-laden air in this part of Brazil, large thunderstorms are
commonplace. A number of overshooting tops and anvil clouds are visible
at the tops of the clouds. Storms of this magnitude can drop large
amounts of rainfall in a short period of time, causing flash floods.
However, 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 the ice crystals
within them. These modifications may affect the clouds’ impact on the
Earth’s “radiation budget,” or the amount of radiation that enters and
leaves our planet.
Steven Sherwood, a professor at Yale University, found that airborne
aerosols reduce the size of ice crystals in thunderclouds and may
reduce precipitation as well. Using several satellites and instruments
including NASA’s Total Ozone Mapping Spectrometer (TOMS) and NASA’s
Tropical Rainfall Measuring Mission (TRMM) satellite, Sherwood observed
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
create a larger number of smaller ice crystals in cumulonimbus clouds.
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
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 with more ice particles. Studies have also 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.
By looking at ten years of aerosol data and statistically analyzing
many thunderclouds, Sherwood was able to confirm that they were
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’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).
Image STS41B-41-2347 was provided by the Earth Sciences and Image Analysis
Laboratory at Johnson Space Center.
Additional images taken by astronauts and cosmonauts can be viewed at
the NASA-JSC Gateway to Astronaut Photography of Earth
- Space Shuttle - 35mm Camera