Jan. 11, 2005

Gretchen Cook-Anderson/Dolores Beasley 
Headquarters, Washington 
(Phone: 202/358-0836/1753) 

RELEASE: 05-015

SAHARAN DUST AFFECTS THUNDERSTORM BEHAVIOR IN FLORIDA

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

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 Carrió, 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 change. 

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.

For more information and images about this research on the Web, visit: 

http://www.nasa.gov/vision/earth/lookingatearth/florida_dust.html 

For information about NASA and agency programs on the Web, visit: 

http://www.nasa.gov 

	
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