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THE SAHARAN AIR LAYER and the FATE of AFRICAN EASTERLY WAVES NASA’S AMMA Field Study of Tropical Cyclogenesis

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THE SAHARAN AIR LAYER and the FATE of AFRICAN EASTERLY WAVES NASA’S AMMA Field Study of Tropical Cyclogenesis THE SAHARAN AIR LAYER AND THE FATE OF AFRICAN EASTERLY WAVES NASA’s AMMA Field Study of Tropical Cyclogenesis BY EDWARD J. ZIPSER, CYNTHIA H. TWOHY,SI-CHEE TSAY, K. LEE THORNHILL,SIMONE TANELLI, ROBERT ROSS, T. N. KRISHNAMURTI, Q. JI, GREGORY JENKINS,SYED ISMAIL, N. CHRISTINA HSU, ROBBIE HOOD, GERALD M. HEYMSFIELD,ANDREW HEYMSFIELD, JEFFREY HALVERSON, H. MICHAEL GOODMAN,RICHARD FERRARE, JASON P. D UNION,MICHAEL DOUGLAS, ROBERT CIFELLI, GAO CHEN,EDWARD V. B ROWELL, AND BRUCE ANDERSON The objectives of an extensive field campaign based from the Cape Verde islands, Africa and Barbados during August–September 2006 included distinguishing disturbances that become tropical cyclones from those that do not. he major hurricanes that affect the United States has become clear that the structure of the waves over T often can be traced back to weak disturbances and near Africa often differs from that in the western moving westward across the tropical Atlantic Atlantic, so they are usually called African easterly Ocean. These are wavelike disturbances in the low- waves (AEWs). They are known to originate over the level tropical easterlies, with a horizontal scale of a few African continent, with their initial growth fueled thousand kilometers and a period of 4–5 days, giving by the temperature contrast between the hot air over rise to the terminology “easterly wave” as described, the Sahara desert and the relatively cooler, humid air for example, by Riehl (1954), mostly based upon ob- to its south. This temperature gradient is persistent, servations in the Caribbean region. In recent years it so it is logical that these AEWs emerge out of Africa AFFILIATIONS: ZIPSER—University of Utah, Salt Lake City, Utah; Maryland, Baltimore County, Baltimore, Maryland; DUNION— TWOHY—Oregon State University, Corvallis, Oregon; TSAY,HSU, AND NOAA/AOML/Hurricane Research Division, Miami, Florida; G. HEYMSFIELD—NASA Goddard Space Flight Center, Greenbelt, DOUGLAS—NOAA/National Severe Storms Laboratory, Norman, Maryland; THORNHILL—Science Systems and Applications, Oklahoma; CIFELLI—Colorado State University, Fort Collins, Inc., Lanham, Maryland; TANELLI—Jet Propulsion Laboratory, Colorado California Institute of Technology, Pasadena, California; ROSS AND CORRESPONDING AUTHOR: Edward J. Zipser, Department of KRISHNAMURTI—The Florida State University, Tallahassee, Florida; Meteorology, University of Utah, Salt Lake City, UT 84112-0110 JI—Earth System Science Interdisciplinary Center, University of E-mail: [email protected] Maryland, College Park, College Park, Maryland; JENKINS—Howard The abstract for this article can be found in this issue, following the table University, Washington, D.C.; ISMAIL,FERRARE, CHEN, BROWELL, AND of contents. ANDERSON—NASA Langley Research Center, Hampton, Virginia; DOI:10.1175/2009BAMS2728.1 HOOD AND GOODMAN—NASA Marshall Space Flight Center, Huntsville, Alabama; A. HEYMSFIELD—National Center for Atmo- In final form 4 February 2009 spheric Research, Boulder, Colorado; HALVERSON—University of ©2009 American Meteorological Society AMERICAN METEOROLOGICAL SOCIETY AUGUST 2009 | 1137 with great regularity throughout the summer and for Medium-Range Weather Forecasts [ECMWF; early fall; however, only a small fraction strengthen 40-yr ECMWF Re-Analysis (ERA-40)] model data into tropical storms or hurricanes, and distinguishing have been widely used to track and analyze tropical those special few from all of the harmless AEWs is disturbances in general and AEWs in particular. of great practical importance and has been a long- Although early studies by Carlson (1969) and Burpee standing scientific mystery. (1974) recognized that there were often two preferred The fate of AEWs is a component of a more general tracks of AEWs emerging from Africa (north and puzzle, that is, why do some tropical disturbances south of 15°N), Thorncroft and Hodges (2001), Fink intensify into hurricanes, whereas most do not? One et al. (2004), and Chen (2006) used ERA-40 to dem- might suppose that the most obvious region to study onstrate that the northern systems were typically to seek a solution would be the east-central Atlantic, at low levels and dry, whereas the southern systems precisely because of the regular progression of AEWs typically were in the midtroposphere and rainy. If through a limited domain. The isolation of this region and how these two systems merge over the Atlantic places it out of range of research aircraft based in the is still questionable, and the central question of how western Atlantic. One must go back in time to 1974, to recognize AEWs that are more likely to evolve into when a massive field program to study tropical con- TCs is still debatable. vection and rainfall covered this region with ships and There have been important recent advances in the aircraft [the Global Atmospheric Research Program capabilities of satellite remote sensing to detect and (GARP) Atlantic Tropical Experiment (GATE); see track the ubiquitous outbreaks of dust that emerge Kuettner and Parker 1976; Houze and Betts 1981]. out of Africa. As early as Carlson and Prospero Indeed, some of the most valuable research on AEWs (1972), these massive dust outbreaks were tracked to uses data from this program (e.g., Reed et al. 1977), the Caribbean, Florida, and beyond. However, their but with little or no attention focused on the question frequency and regularity has stimulated many to ask of which AEWs might evolve into tropical cyclones whether they have an influence on AEWs and tropical (TCs). It would be 32 yr before a field program re- cyclogenesis. The dust outbreaks are usually associated turned to the east Atlantic to seek an answer. with hot, dry air, dubbed the Saharan air layer (SAL). Karyampudi and Carlson (1988) concluded that the BACKGROUND AND MOTIVATION. The SAL is important, if not necessary, in the initial de- African Monsoon Multidisciplinary Analysis velopment of AEWs. Karyampudi and Pierce (2002) (AMMA), a large international project to improve found that the SAL had a positive influence on the the understanding of the West African monsoon genesis of two Atlantic storms through the enhance- (Redelsperger et al. 2006), created the opportunity ment of baroclinic instability. Dunion and Velden for a field program in the east Atlantic in 2006. With (2004) proposed that the SAL can inhibit tropical TC AMMA providing data over a vast area of West Africa, intensity but is primarily due to the stabilizing effect National Aeronautics and Space Administration of the warm, dry air and enhanced vertical wind shear. (NASA) was able to extend that database westward They suggested that any direct influence of the dust on over the Atlantic with their field program [NASA– TC development was not yet fully understood. Others AMMA (NAMMA)] based mainly on Cape Verde’s have proposed that the dust may directly affect cloud islands during August and September, whereas the microphysics by reducing precipitation efficiency National Oceanic and Atmospheric Administration (Rosenfeld et al. 2001) or by enhancing convective in- (NOAA) further extended that east–west coverage tensity (Khain et al. 2005; Jenkins et al. 2008), although with research aircraft operating out of Barbados. it is not at all clear how such changes in microphysics Although the surface-based radiosonde network over would affect tropical cyclogenesis. the Atlantic did not rival that in GATE, three decades Other studies have suggested that African dust of progress in remote sensing from satellites and air- loading over the Atlantic might play an important craft, and in multiscale numerical simulation, led to a role in the activity level of a hurricane season. Evan different but effective comprehensive analysis. At the et al. (2006) conducted a 25-yr study that showed a same time, these improvements have led to significant strong relationship between interannual variations new knowledge that has updated and refocused the in North Atlantic TC activity and atmospheric dust scientific questions. cover over the main development region. This has In recent years, the availability of multidecadal become an active research area, for example, see reanalyses using both National Centers for Envi- Zhang et al. (2007), Evan et al. (2008), and Wong et ronmental Prediction (NCEP) and European Centre al. (2008). Lau and Kim (2007a,b,c) indicated that 1138 | AUGUST 2009 the excess of Saharan dust in the 2006 premonsoon wind shear? Or does the dust play its role through season, as compared to 2005, was largely responsible influencing cloud microphysics, cloud condensa- for chilling the sea surface temperature in the North tion and ice nuclei, and liquid and ice particle size Atlantic, possibly contributing to the big decrease distributions in ways that mitigate or redistribute in TC activity from 2005 to 2006. Daily retrievals precipitation and latent heating in the storm? of aerosol properties over the NAMMA domain of t 8IBUJTUIFDIBSBDUFSJTUJDWFSUJDBMEJTUSJCVUJPO interest are achieved by employing the Deep Blue microphysical and optical properties, and com- algorithm (Hsu et al. 2004) using measurements from position of the African dust, and in what specific the Moderate Resolution Imaging Spectroradiometer ways does the dust affect cloud microphysics and (MODIS) onboard NASA’s Earth Observing System cloud dynamics? (EOS) Aqua satellite. Figure 1 depicts the spatial distribution of Saharan dust loading (aerosol opti- STRATEGY IN THE FIELD. The NAMMA cal thickness τ at 0.55-μm wavelength) from source campaign was
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