The contribution of symmetrization to the intensification of Tropical Cyclones
Miller, Henry A.
Williams, Roger T.
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Operational ability to forecast tropical cyclone motion is much better than the ability to forecast intensity change. Several recent works have studied the mechanisms that bring about the symmetrization of various types of asymmetries in tropical cyclones. This study was conducted to add to that knowledge by examining the transfers of kinetic energy between scales and how those energy transfers alter the wind structure of the cyclone. Adding to the understanding of how this process can alter winds is a step toward increasing ability to forecast these changes. A non-divergent barotropic spectral model was used to integrate annular bands of enhanced potential vorticity, simulating hurricane eyes, with varying degrees of offset from the center of the vortex. Offset monopoles of vorticity, simulating asymmetric convection in tropical storms, were also integrated. As discovered by previous researchers, these unstable eyes broke down into a series of mesovortices, which merged and eventually relaxed to monopolar or tripolar final states. The offset monopoles formed spiral bands and became symmetric as well. Kinetic energy was transferred from the mean flow to the asymmetries as mesovortices formed and then transferred back to the mean flow as symmetrization occurred. These energy transfers occurred very quickly. As energy was transferred from asymmetry to mean flow, the azimuthally averaged wind increased in a band of about 70 km from the center of the vortex, even though the maximum wind decreased. Azimuthally averaged wind in the monopole cases also increased, but the change was confined to a smaller radial band near the radius of maximum wind.
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