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Why Do Model Tropical Cyclones Intensify More Rapidly at Low Latitudes?

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  • 1 Meteorological Institute, Ludwig-Maximilians University of Munich, Munich, Germany
  • 2 Department of Meteorology, Naval Postgraduate School, Monterey, California
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Abstract

The authors examine the problem of why model tropical cyclones intensify more rapidly at low latitudes. The answer to this question touches on practically all facets of the dynamics and thermodynamics of tropical cyclones. The answer invokes the conventional spin-up mechanism, as articulated in classical and recent work, together with a boundary layer feedback mechanism linking the strength of the boundary layer inflow to that of the diabatic forcing of the meridional overturning circulation.

The specific role of the frictional boundary layer in regulating the dependence of the intensification rate on latitude is discussed. It is shown that, even if the tangential wind profile at the top of the boundary layer is held fixed, a simple, steady boundary layer model produces stronger low-level inflow and stronger, more confined ascent out of the boundary layer as the latitude is decreased, similar to the behavior found in a time-dependent, three-dimensional numerical model. In an azimuthally averaged view of the problem, the most prominent quantitative differences between the time-dependent simulations at 10° and 30°N are the stronger boundary layer inflow and the stronger ascent of air exiting the boundary layer, together with the much larger diabatic heating rate and its radial gradient above the boundary layer at the lower latitude. These differences, in conjunction with the convectively induced convergence of absolute angular momentum, greatly surpass the effects of rotational stiffness (inertial stability) and evaporative-wind feedback that have been proposed in some prior explanations.

Corresponding author address: Prof. Roger K. Smith, Meteorological Institute, Ludwig-Maximilians University of Munich, Theresienstr. 37, 80333 Munich, Germany. E-mail: roger.smith@lmu.de

Abstract

The authors examine the problem of why model tropical cyclones intensify more rapidly at low latitudes. The answer to this question touches on practically all facets of the dynamics and thermodynamics of tropical cyclones. The answer invokes the conventional spin-up mechanism, as articulated in classical and recent work, together with a boundary layer feedback mechanism linking the strength of the boundary layer inflow to that of the diabatic forcing of the meridional overturning circulation.

The specific role of the frictional boundary layer in regulating the dependence of the intensification rate on latitude is discussed. It is shown that, even if the tangential wind profile at the top of the boundary layer is held fixed, a simple, steady boundary layer model produces stronger low-level inflow and stronger, more confined ascent out of the boundary layer as the latitude is decreased, similar to the behavior found in a time-dependent, three-dimensional numerical model. In an azimuthally averaged view of the problem, the most prominent quantitative differences between the time-dependent simulations at 10° and 30°N are the stronger boundary layer inflow and the stronger ascent of air exiting the boundary layer, together with the much larger diabatic heating rate and its radial gradient above the boundary layer at the lower latitude. These differences, in conjunction with the convectively induced convergence of absolute angular momentum, greatly surpass the effects of rotational stiffness (inertial stability) and evaporative-wind feedback that have been proposed in some prior explanations.

Corresponding author address: Prof. Roger K. Smith, Meteorological Institute, Ludwig-Maximilians University of Munich, Theresienstr. 37, 80333 Munich, Germany. E-mail: roger.smith@lmu.de
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