Baroclinic Dynamics of Simulated Tropical Cyclone Recurvature

Greg J. Holland Bureau of Meteorology Research Centre, Melbourne, Victoria, Australia

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Yuqing Wang Centre for Dynamical Meteorology and Oceanography, Monash University, Melbourne, Victoria, Australia

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Abstract

The mechanisms associated with tropical cyclone recurvature are investigated using a five-level primitive equation model and an idealized environment with characteristics observed in cyclone recurvature conditions. All cyclones moved generally with the flow in the lower and middle troposphere, but the precise motion occurs by a combination of divergence and of advection in both the horizontal and the vertical. The horizontal advection arises from a combination of the initial environmental flow and local changes from rearrangement of the potential vorticity field by cyclone-environment interaction (the so-called,β effect). The balance between these mechanisms changes as the vortex recurves. Since the gradients of potential vorticity increase sharply poleward of the subtropical ridge, this is the preferred region for development of an anticyclonic gyre. This gyre is advected eastward and becomes the dominant anticyclonic system. Recurvature is aided by horizontal deformation of the cyclone in the vicinity of this gyre, and by the manner in which the vertical tilt of the vortex and local divergence fields vary as it moves through a changing vertical wind shear of the environment. Recurvature is sensitive to the degree of diabatic heating and to small meridional changes in the initial vortex location.

It is shown that recurvature can occur through an initially unbroken subtropical ridge, but that the presence of a midlatitude trough substantially enhances the potential for recurvature. However, while changes in the upper troposphere are indicative of recurvature potential, recurvature is accomplished largely by lower-tropospheric changes. An important component of this change is the development of a major anticyclone poleward and eastward of the cyclone. A recent observational study by Ford et al. concurs with this finding.

Abstract

The mechanisms associated with tropical cyclone recurvature are investigated using a five-level primitive equation model and an idealized environment with characteristics observed in cyclone recurvature conditions. All cyclones moved generally with the flow in the lower and middle troposphere, but the precise motion occurs by a combination of divergence and of advection in both the horizontal and the vertical. The horizontal advection arises from a combination of the initial environmental flow and local changes from rearrangement of the potential vorticity field by cyclone-environment interaction (the so-called,β effect). The balance between these mechanisms changes as the vortex recurves. Since the gradients of potential vorticity increase sharply poleward of the subtropical ridge, this is the preferred region for development of an anticyclonic gyre. This gyre is advected eastward and becomes the dominant anticyclonic system. Recurvature is aided by horizontal deformation of the cyclone in the vicinity of this gyre, and by the manner in which the vertical tilt of the vortex and local divergence fields vary as it moves through a changing vertical wind shear of the environment. Recurvature is sensitive to the degree of diabatic heating and to small meridional changes in the initial vortex location.

It is shown that recurvature can occur through an initially unbroken subtropical ridge, but that the presence of a midlatitude trough substantially enhances the potential for recurvature. However, while changes in the upper troposphere are indicative of recurvature potential, recurvature is accomplished largely by lower-tropospheric changes. An important component of this change is the development of a major anticyclone poleward and eastward of the cyclone. A recent observational study by Ford et al. concurs with this finding.

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