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The Saharan Air Layer and the Fate of African Easterly Waves—NASA's AMMA Field Study of Tropical Cyclogenesis

Edward J. Zipser
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Cynthia H. Twohy
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Si-Chee Tsay
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K. Lee Thornhill
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Simone Tanelli
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Robert Ross
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T. N. Krishnamurti
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Q. Ji
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Gregory Jenkins
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Syed Ismail
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N. Christina Hsu
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Robbie Hood
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Gerald M. Heymsfield
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Gao Chen
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Edward V. Browell
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Bruce Anderson
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In 2006, NASA led a field campaign to investigate the factors that control the fate of African easterly waves (AEWs) moving westward into the tropical Atlantic Ocean. Aircraft and surface-based equipment were based on Cape Verde's islands, helping to fill some of the data void between Africa and the Caribbean. Taking advantage of the international African Monsoon Multidisciplinary Analysis (AMMA) program over the continent, the NASA-AMMA (NAMMA) program used enhanced upstream data, whereas NOAA aircraft farther west in the Atlantic studied several of the storms downstream. Seven AEWs were studied during AMMA, with at least two becoming tropical cyclones. Some of the waves that did not develop while being sampled near Cape Verde likely intensified in the central Atlantic instead. NAMMA observations were able to distinguish between the large-scale wave structure and the smaller-scale vorticity maxima that often form within the waves. A special complication of the east Atlantic environment is the Saharan air layer (SAL), which frequently accompanies the AEWs and may introduce dry air and heavy aerosol loading into the convective storm systems in the AEWs. One of the main achievements of NAMMA was the acquisition of a database of remote sensing and in situ observations of the properties of the SAL, enabling dynamic models and satellite retrieval algorithms to be evaluated against high-quality real data. Ongoing research with this database will help determine how the SAL influences cloud microphysics and perhaps also tropical cyclogenesis, as well as the more general question of recognizing the properties of small-scale vorticity maxima within tropical waves that are more likely to become tropical cyclones.

University of Utah, Salt Lake City, Utah

Oregon State University, Corvallis, Oregon

NASA Goddard Space Flight Center, Greenbelt, Maryland

Science Systems and Applications, Inc., Lanham, Maryland

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California

The Florida State University, Tallahassee, Florida

Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, Maryland

Howard University, Washington, D.C.

NASA Langley Research Center, Hampton, Virginia

NASA Marshall Space Flight Center, Huntsville, Alabama

National Center for Atmospheric Research, Boulder, Colorado

University of Maryland, Baltimore County, Baltimore, Maryland

NOAA/AOML/Hurricane Research Division, Miami, Florida

NOAA/National Severe Storms Laboratory, Norman, Oklahoma

Colorado State University, Fort Collins, Colorado

CORRESPONDING AUTHOR: Edward J. Zipser, Department of Meteorology, University of Utah, Salt Lake City, UT 84112-0110, E-mail: ed.zipser@utah.edu

In 2006, NASA led a field campaign to investigate the factors that control the fate of African easterly waves (AEWs) moving westward into the tropical Atlantic Ocean. Aircraft and surface-based equipment were based on Cape Verde's islands, helping to fill some of the data void between Africa and the Caribbean. Taking advantage of the international African Monsoon Multidisciplinary Analysis (AMMA) program over the continent, the NASA-AMMA (NAMMA) program used enhanced upstream data, whereas NOAA aircraft farther west in the Atlantic studied several of the storms downstream. Seven AEWs were studied during AMMA, with at least two becoming tropical cyclones. Some of the waves that did not develop while being sampled near Cape Verde likely intensified in the central Atlantic instead. NAMMA observations were able to distinguish between the large-scale wave structure and the smaller-scale vorticity maxima that often form within the waves. A special complication of the east Atlantic environment is the Saharan air layer (SAL), which frequently accompanies the AEWs and may introduce dry air and heavy aerosol loading into the convective storm systems in the AEWs. One of the main achievements of NAMMA was the acquisition of a database of remote sensing and in situ observations of the properties of the SAL, enabling dynamic models and satellite retrieval algorithms to be evaluated against high-quality real data. Ongoing research with this database will help determine how the SAL influences cloud microphysics and perhaps also tropical cyclogenesis, as well as the more general question of recognizing the properties of small-scale vorticity maxima within tropical waves that are more likely to become tropical cyclones.

University of Utah, Salt Lake City, Utah

Oregon State University, Corvallis, Oregon

NASA Goddard Space Flight Center, Greenbelt, Maryland

Science Systems and Applications, Inc., Lanham, Maryland

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California

The Florida State University, Tallahassee, Florida

Earth System Science Interdisciplinary Center, University of Maryland, College Park, College Park, Maryland

Howard University, Washington, D.C.

NASA Langley Research Center, Hampton, Virginia

NASA Marshall Space Flight Center, Huntsville, Alabama

National Center for Atmospheric Research, Boulder, Colorado

University of Maryland, Baltimore County, Baltimore, Maryland

NOAA/AOML/Hurricane Research Division, Miami, Florida

NOAA/National Severe Storms Laboratory, Norman, Oklahoma

Colorado State University, Fort Collins, Colorado

CORRESPONDING AUTHOR: Edward J. Zipser, Department of Meteorology, University of Utah, Salt Lake City, UT 84112-0110, E-mail: ed.zipser@utah.edu
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