Models of Tropical Cyclone Wind Distribution and Beta-Effect Propagation for Application to Tropical Cyclone Track Forecasting

Lester E. Carr III Department of Meteorology, Naval Postgraduate School, Monterey, California

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Russell L. Elsberry Department of Meteorology, Naval Postgraduate School, Monterey, California

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Abstract

A model of the tangential wind speed in the outer regions of tropical cyclones is proposed based on approximate conservation of angular momentum. The purpose is to derive an operationally useful model of the beta-effect propagation (BEP), which barotropic numerical models have shown to be primarily related to the outer wind structure. The functional dependence of the predicted BEP speeds over a range of latitudes and the radii Ro at which the cyclonic winds are reduced to zero is determined from dimensional analysis. Given the empirical nature of the profile and imprecise estimates of Ro, only four tropical cyclone sizes or outer wind structures are defined based on the magnitude of the BEP speeds and their relative contributions to the total motion. A second aspect of BEP from the barotropic model integrations is the development of a trailing anticyclone to the southeast of the tropical cyclone as a result of Rossby wave dispersion. The four tropical cyclone size categories or outer wind structures are also characterized in terms of the potential for the trailing anticyclone to be part of a change in the tropical cyclone environment structure. Whereas small tropical cyclones have a small BEP speed and do not change their environment, large tropical cyclones have BEP speeds exceeding 2 m s−1 and have a large amplitude peripheral anticyclone that may introduce significant changes in the environment structure, which can indirectly contribute to a track change.

Corresponding author address: L. E. Carr, III, Department of Meteorology (Code MR/Cr), Naval Postgraduate School, 589 Dyer Rd., Room 254, Monterey, CA 93943-5114.

Email: Carrle@nps.navy.mil

Abstract

A model of the tangential wind speed in the outer regions of tropical cyclones is proposed based on approximate conservation of angular momentum. The purpose is to derive an operationally useful model of the beta-effect propagation (BEP), which barotropic numerical models have shown to be primarily related to the outer wind structure. The functional dependence of the predicted BEP speeds over a range of latitudes and the radii Ro at which the cyclonic winds are reduced to zero is determined from dimensional analysis. Given the empirical nature of the profile and imprecise estimates of Ro, only four tropical cyclone sizes or outer wind structures are defined based on the magnitude of the BEP speeds and their relative contributions to the total motion. A second aspect of BEP from the barotropic model integrations is the development of a trailing anticyclone to the southeast of the tropical cyclone as a result of Rossby wave dispersion. The four tropical cyclone size categories or outer wind structures are also characterized in terms of the potential for the trailing anticyclone to be part of a change in the tropical cyclone environment structure. Whereas small tropical cyclones have a small BEP speed and do not change their environment, large tropical cyclones have BEP speeds exceeding 2 m s−1 and have a large amplitude peripheral anticyclone that may introduce significant changes in the environment structure, which can indirectly contribute to a track change.

Corresponding author address: L. E. Carr, III, Department of Meteorology (Code MR/Cr), Naval Postgraduate School, 589 Dyer Rd., Room 254, Monterey, CA 93943-5114.

Email: Carrle@nps.navy.mil

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