People living in Florida may expect sand from state beaches to blow into the air, and usually do not think of sand and dust from the Sahara Desert swirling around them. However, winds carry desert dust across the Atlantic Ocean, and scientists discovered that these tiny particles from the Sahara Desert can affect thunderstorms in Florida in various ways.
Dust affects the size of a thunderstorm’s anvil, or top; the strength and number of warm updrafts, or rising air, and the amount of rain that builds up and falls from the heat-generated, or convective, thunderstorms.
Findings on the "Impact of Saharan Dust on Florida Storm Characteristics" will be presented at the 2005 annual meeting of the American Meteorological Society on Jan. 11 at the San Diego Convention Center in San Diego, Calif.
Susan C. van den Heever, Gustavo G. Carrió, William R. Cotton, Paul. J. DeMott and Anthony J. Prenni, all from Colorado State University, co-authored the study which will appear in a forthcoming issue of the Journal of Atmospheric Sciences.
Working with her colleagues, van den Heever found that when Saharan dust is in the air, the thunderstorm anvils created by Florida’s convective thunderstorms tend to be a little smaller in area, but they tend to be better organized and thicker. This affects the amount of incoming sunlight and warmth reaching the ground, which can have effects on long-term climate. Over time, more sunlight would warm temperatures, less sunlight would cool temperatures.
The researchers also noticed that the updrafts of warm, moist air, which build into thunderstorms, were stronger, and that there were more of these updrafts produced in the presence of the dust. These updrafts also carry tiny particles of pollution called aerosols up into all levels of the building thunderclouds.
Florida residents not only see more updrafts developing during dust events, but the dust affects the amount of rainfall that reaches the ground. Dust is an aerosol, and aerosols or small particles serve as the center, or nuclei (called a cloud condensation nuclei), for cloud droplets to form around. These cloud droplets then combine to form raindrops that fall to the ground. As such, aerosols affect the production of rainfall.
Nuclei or centers for droplets in a cloud compete for a limited amount of water vapor and liquid water to form raindrops. When there are many particles that act as a center for water vapor, there is less water for each center, resulting in smaller cloud droplets. As such, it is less likely that raindrops will form when droplets combine.
The scientists also found that greater concentrations of Giant CCN (GCCN) as well as ice nuclei initially resulted in more rainfall reaching the surface. However, as the storms continued to develop, the two types of nuclei were removed from the storms by the precipitation and these nuclei then had less of an effect on the amount of rain reaching the surface. The scientists concluded that the overall effect of the Sahara dust on the surface rainfall was to reduce it.
The scientists used data from NASA’s CRYSTAL-FACE (Cirrus Regional Study of Tropical Anvils and Cirrus Layers – Florida Area Cirrus Experiment) field campaign to examine the affects of increased numbers of nuclei from the dust and pollutants. The purpose of the CRYSTAL-FACE mission was to study cirrus clouds to improve forecasts of future climate change.
The scientists concluded that Saharan dust can have a major impact on the amount of rainfall produced by thunderstorms in Florida. Also, because dust affects the size and thickness of thunderstorm anvils, the changes affect the amount of sunlight reaching Earth and being reflected by the clouds, which have implications for a changing climate. Finally, this research can also help answer questions about how tiny particles called aerosols and other pollutants move around the world in the upper atmosphere.
For the extended Abstract of the AMS paper, visit the Web at
http://ams.confex.com/ams/Annual2005/techprogram/paper_83490.htm. For more information about the CRYSTAL-FACE mission, visit the following Web sites: http://www.gsfc.nasa.gov/topstory/20020709crystalface.html, cloud1.arc.nasa.gov/crystalface/