Scientists using NASA satellite data have discovered tiny particles of dust
blowing across the Atlantic Ocean from the Sahara Desert can affect Florida

Dust affects the size of the top or “anvil” of a thunderstorm, the strength and
number of updrafts of warm winds. It also affects the strength of convective
(heat generated) thunderstorms by influencing the amount of rain that builds up
and falls.

Susan van den Heever, Gustavo Carrio, William Cotton, Paul DeMott and Anthony
Prenni, all of Colorado State University, Fort Collins, Colo., co-authored the
study presented today at the 2005 annual meeting of the American Meteorological
Society in San Diego.

The researchers found when saharan dust is in the air, the anvils produced by
Florida’s convective thunderstorms tend to be a little smaller in area, but
better organized and thicker. This affects the amount of incoming sunlight and
warmth reaching the ground, potentially affecting long-term climate. If occurring
over time, more sunlight and warmer temperatures would mean a warmer climate.

The researchers also noticed the updrafts of warm moist air, which build into
thunderstorms were stronger, and there were more updrafts produced in the
presence of the dust. The updrafts also carry tiny particles of pollution, called
aerosols, up into all levels of the building thunderclouds.

Florida residents see more updrafts developing during dust events, and the dust
affects the amount of rainfall that reaches the ground. Dust is an aerosol.
Aerosols serve as the cloud condensation nuclei around which droplets form. These
cloud droplets then combine to form raindrops, which fall to the ground. In this
way aerosols affect the production of rainfall.

Saharan dust can act as cloud condensation nuclei, giant cloud condensation
nuclei and ice nuclei. Van den Heever ran two types of computer model
simulations, one that included saharan dust and another without the desert dust.

She then compared the results and found something unusual. The increased
concentrations of cloud condensation nuclei due to the dust decreased the amount
of rainfall at the Earth’s surface.

The scientists also found that greater concentrations of giant cloud condensation
nuclei, as well as ice nuclei, initially resulted in more rainfall at the
surface. However, as storms continued to develop, the two types of nuclei were
removed from them by the precipitation. These nuclei then had less effect on the
amount of rain reaching the surface. Van den Heever concluded the overall effect
of the saharan dust on the surface rainfall was to reduce it.

The scientists used data from NASA’s 2002 CRYSTAL-FACE (Cirrus Regional Study of
Tropical Anvils and Cirrus Layers — Florida Area Cirrus Experiment) field
campaign to examine the impact of increased nuclei concentrations. The purpose of
the mission was to study cirrus clouds, to improve forecasts of future climate

Modeling results were compared to airborne radar data. The researchers also used
sounding data from two ground sites, as well as Geostationary Operational
Environmental Satellite data specifically tailored by NASA’s Langley Research
Center, Hampton, Va., to compare their simulation results.

This research has several implications. Saharan dust can have a major impact on
the amount of precipitation produced by thunderstorms in Florida, and it can help
answer questions about the transport of aerosols and other pollutants in the
upper atmosphere.
The study will appear in a future issue of the Journal of Atmospheric Sciences.

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