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Volume 28 Issue 1

Jan.  2011

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Article >  ADVANCES IN ATMOSPHERIC SCIENCES  > 2011 >  28(1): 16-32

Influence of the Saharan Air Layer on Atlantic Tropical Cyclone Formation during the Period 1--12 September 2003

  • 1.

    Key Laboratory of Mesoscale Severe Weather of Ministry of Education, School of Atmospheric Sciences, Nanjing University, Nanjing 210044,Key Laboratory of Meteorological Disaster of Ministry of Education Nanjing University of Information Science and Technology, Nanjing 210044,Goddard Earth and Technology Center, University of Maryland, Baltimore County, Mesoscale Atmospheric Processes Branch, Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA


doi: 10.1007/s00376-010-9165-5

  • Atmospheric Infrared Sounder (AIRS) data show that the Saharan air layer (SAL) is a dry, warm, and well-mixed layer between 950 and 500 hPa over the tropical Atlantic, extending westward from the African coast to the Caribbean Sea. The formations of both Hurricane Isabel and Tropical Depression 14 (TD14) were accompanied with outbreaks of SAL air during the period 1--12 September 2003, although TD14 failed to develop into a named tropical cyclone. The influence of the SAL on their formations is investigated by examining data from satellite observations and numerical simulations, in which AIRS data are incorporated into the MM5 model through the nudging technique. Analyses of the AIRS and simulation data suggest that the SAL may have played two roles in the formation of tropical cyclones during the period 1--12 September 2003. First, the outbreaks of SAL air on 3 and 8 September enhanced the transverse-vertical circulation with the rising motion along the southern edge of the SAL and the sinking motion inside the SAL, triggering the development of two tropical disturbances associated with Hurricane Isabel and TD14. Second, in addition to the reduced environmental humidity and enhanced static stability in the lower troposphere, the SAL dry air intruded into the inner region of these tropical disturbances as their cyclonic flows became strong. This effect may have slowed down the formation of Isabel and inhibited TD14 becoming a named tropical cyclone, while the enhanced vertical shear contributed little to tropical cyclone formation during this period. The 48-h trajectory calculations confirm that the parcels from the SAL can be transported into the inner region of an incipient tropical cyclone.
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    • Manuscript History

    Manuscript received: 10 January 2011
    Manuscript revised: 10 January 2011
    通讯作者: 陈斌, bchen63@163.com
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    Influence of the Saharan Air Layer on Atlantic Tropical Cyclone Formation during the Period 1--12 September 2003

    • 1. Key Laboratory of Mesoscale Severe Weather of Ministry of Education, School of Atmospheric Sciences, Nanjing University, Nanjing 210044,Key Laboratory of Meteorological Disaster of Ministry of Education Nanjing University of Information Science and Technology, Nanjing 210044,Goddard Earth and Technology Center, University of Maryland, Baltimore County, Mesoscale Atmospheric Processes Branch, Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
    • Manuscript received: 2011-01-10
    • Manuscript revised: 2011-01-10

    Abstract: Atmospheric Infrared Sounder (AIRS) data show that the Saharan air layer (SAL) is a dry, warm, and well-mixed layer between 950 and 500 hPa over the tropical Atlantic, extending westward from the African coast to the Caribbean Sea. The formations of both Hurricane Isabel and Tropical Depression 14 (TD14) were accompanied with outbreaks of SAL air during the period 1--12 September 2003, although TD14 failed to develop into a named tropical cyclone. The influence of the SAL on their formations is investigated by examining data from satellite observations and numerical simulations, in which AIRS data are incorporated into the MM5 model through the nudging technique. Analyses of the AIRS and simulation data suggest that the SAL may have played two roles in the formation of tropical cyclones during the period 1--12 September 2003. First, the outbreaks of SAL air on 3 and 8 September enhanced the transverse-vertical circulation with the rising motion along the southern edge of the SAL and the sinking motion inside the SAL, triggering the development of two tropical disturbances associated with Hurricane Isabel and TD14. Second, in addition to the reduced environmental humidity and enhanced static stability in the lower troposphere, the SAL dry air intruded into the inner region of these tropical disturbances as their cyclonic flows became strong. This effect may have slowed down the formation of Isabel and inhibited TD14 becoming a named tropical cyclone, while the enhanced vertical shear contributed little to tropical cyclone formation during this period. The 48-h trajectory calculations confirm that the parcels from the SAL can be transported into the inner region of an incipient tropical cyclone.

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