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Vortex Rossby Waves and Hurricane Intensification in a Barotropic Model

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  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
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Abstract

In this study the balanced evolution of small but finite as well as large-amplitude asymmetries in a rapidly rotating hurricane-like vortex is investigated. In particular, the wave kinematics and wave–mean flow interaction of vortex Rossby waves in a barotropic nonlinear asymmetric balance (AB) model are examined. By diagnosing the evolution of different asymmetric initial potential vorticity (PV) disturbances and their effect on the symmetric vortex, recent linear and quasi-linear predictions are verified and the proposed AB model is shown to be a viable balance model for azimuthal wavenumbers >1. For disturbance amplitudes that are 40% of the basic-state PV at the radius of maximum wind, a discrete normal mode propagating cyclonically around the vortex is excited as a by-product of the process by which energy is transferred from the asymmetries into the basic state (axisymmetrization). In addition we are able to show that even a strong disturbance axisymmetrizes in a circular flow and is able to intensify the basic state. Side-by-side comparison with some experiments from a primitive equation model show good agreement for both weak and strong asymmetric disturbances. The results raise intriguing questions about the dynamical role of discrete and continuous spectrum vortex Rossby waves in the moist convective dynamics of the hurricane. The application of the results to hurricane intensification will be addressed.

Corresponding author address: Dr. J. D. Möller, Meteorological Institute, University of Munich, Theresienstr. 37, Munich 80333, Germany.

Email: nique@meteo.physik.uni-muenchen.de

Abstract

In this study the balanced evolution of small but finite as well as large-amplitude asymmetries in a rapidly rotating hurricane-like vortex is investigated. In particular, the wave kinematics and wave–mean flow interaction of vortex Rossby waves in a barotropic nonlinear asymmetric balance (AB) model are examined. By diagnosing the evolution of different asymmetric initial potential vorticity (PV) disturbances and their effect on the symmetric vortex, recent linear and quasi-linear predictions are verified and the proposed AB model is shown to be a viable balance model for azimuthal wavenumbers >1. For disturbance amplitudes that are 40% of the basic-state PV at the radius of maximum wind, a discrete normal mode propagating cyclonically around the vortex is excited as a by-product of the process by which energy is transferred from the asymmetries into the basic state (axisymmetrization). In addition we are able to show that even a strong disturbance axisymmetrizes in a circular flow and is able to intensify the basic state. Side-by-side comparison with some experiments from a primitive equation model show good agreement for both weak and strong asymmetric disturbances. The results raise intriguing questions about the dynamical role of discrete and continuous spectrum vortex Rossby waves in the moist convective dynamics of the hurricane. The application of the results to hurricane intensification will be addressed.

Corresponding author address: Dr. J. D. Möller, Meteorological Institute, University of Munich, Theresienstr. 37, Munich 80333, Germany.

Email: nique@meteo.physik.uni-muenchen.de

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