Some Aspects of Vortex Structure Related to Tropical Cyclone Motion

Some effect of tropical cyclone structure on the vortex motion are examined in a nondivergent, barotropic numerical model with no basic current. As suggested earlier by DeMaria, the initial maximum wind speed has little effect on the track. Vortex translation associated with the beta effect depends sensitively on the strength of the flow between 300 and 1000 km from the center. If the flow in this annulus is made more cyclonic, the track will turn cyclonically and move more toward the west in the Northern Hemisphere. The dynamics of this beta-drift is studied via a decomposition into symmetric and asymmetric circulations. The symmetric flow experiences a slight weakening of the maximum wind speed and an anticyclonic circulation is induced beyond 600 km. The asymmetric circulation is dominated by an azimuthal wavenumber one circulation with an anticyclonic gyre east of the center, a cyclonic gyre to the west and a nearly uniform, broad-scale ventilation flow between the gyres. The vortex translation speed and direction are almost equal to the averaged of this ventilation flow over the area of significant cyclonic circulation in the vortex. Analysis of the model streamfunction tendency equation demonstrates that the linear beta term is responsible for the initial formation of the asymmetric gyres. Nonlinear advection of the asymmetric circulation by the symmetric vortex flow twists the interior region between the gyres and orients the ventilation flow toward the northwest rather than toward the north. Because this term nearly balances the linear beta forcing, the stream-function time tendency (and storm motion) is predominantly due to the advection of the symmetric vortex by the ventilation flow between the gyres.