Tropical cyclone spin-up revisited

Authors

  • Roger K. Smith,

    Corresponding author
    1. Meteorological Institute, University of Munich, Germany
    • Meteorological Institute, University of Munich, Theresienstrasse 37, 80333 Munich, Germany.
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  • Michael T. Montgomery,

    1. Dept. of Meteorology, Naval Postgraduate School, Monterey, California, USA
    2. NOAA Hurricane Research Division, Miami, Florida, USA
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    • The contribution of Michael T. Montgomery to this article was prepared as part of his official duties as a United States Federal Government employee.

  • Nguyen Van Sang

    1. Meteorological Institute, University of Munich, Germany
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Abstract

We present numerical experiments to investigate axisymmetric interpretations of tropical cyclone spin-up in a three-dimensional model. Two mechanisms are identified for the spin-up of the mean tangential circulation. The first involves the convergence of absolute angular momentum above the boundary layer and is a mechanism to spin up the outer circulation, i.e. to increase the vortex size. The second involves the convergence of absolute angular momentum within the boundary layer and is a mechanism to spin up the inner core. It is associated with the development of supergradient wind speeds in the boundary layer. The existence of these two mechanisms provides a plausible physical explanation for certain long-standing observations of typhoons by Weatherford and Gray, which indicate that inner-core changes in the azimuthal-mean tangential wind speed often occur independently from those in the outer core. The unbalanced dynamics in the inner-core region are important in determining the maximum radial and tangential flow speeds that can be attained, and therefore important in determining the azimuthal-mean intensity of the vortex. We illustrate the importance of unbalanced flow in the boundary layer with a simple thought experiment. The analyses and interpretations presented are novel and support a recent hypothesis of the boundary layer in the inner-core region. Copyright © 2009 Royal Meteorological Society

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