The role of vortex structure in tropical cyclone motion
Elsberry, Russell L.
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The role of vortex structure in tropical cyclone motion is studied using a moving-grid, nondivergent barotropic model on a b plane in a no-flow environment. Initial condition sensitivity tests reveal that the northwestward "beta" drift of the vortex is controlled by the symmetric circulation in the r = 3000 - 80 km "critical" annulus. Enhanced cyclonic or anticyclonic flow in the critical annulus leads to long-term cyclonic or anticyclonic turning motions. The dynamics of the motion process is examined in terms of the symmetric and asymmetric circulations. When the vortex is moving in a quasi-steady manner, the asymmetric flow appears as a pair of large-scale, counter-rotating gyres with a broad "ventilation" flow through the vortex center. A second much smaller pair of gyres is also found near the center. It is the interaction between these two sets of gyres and the symmetric slow that governs the motion process as revealed by a streamfunction tendency analysis and dynamically sensitivity tests in which the model equation is modified during the integration. Beta drift can be described as a balancing process between linear Rossby dispersion which generates the asymmetric gyres and nonlinear advection that moved the vortex the limit gyre development. Vortex structure is the key to this balance as it determines both the linear generation of the asymmetric forcing and the nonlinear interaction between the symmetric and symmetric circulations.
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