A possible mechanism for the eye rotation of Typhoon herb

An elliptical eye that rotated cyclonically with a period of approximately 144 minutes in Typhoon Herb 1996 was documented. The elliptical region had a semimajor axis of 30 km and a semiminor axis of 20 km. Two complete perio ds of approximately 144 min were observed in the Doppler radar data. The ro tation of the elliptical eye in the context of barotropic dynamics at three levels were explored: linear waves on a Rankin vortex, a nonlinear Kirchho ff vortex, and with a nonlinear spectral model. The linear wave theory invo lves the existence of both the high (potential) vorticity gradient near the eye edge and the cyclonic mean tangential Bow in the typhoon. The propagat ion of (potential) vorticity waves in the cyclonic mean flow makes the elli ptical eye rotate cyclonically. The rotation period is longer than the peri od of a parcel trajectory moving in the cyclonic mean flow around the circu mference, because the vorticity wave propagates upwind. The nonlinear theor y stems from the rotation of Kirchhoff's vortex. Estimates of the eye rotat ion period from both linear and nonlinear theories agree with observations of the eye rotation period when the observed maximum wind from Herb is used . Nonlinear numerical computations suggest the importance of the interactio n of neutral vorticity waves, which determine the shape and the rotation pe riod of the eye. The calculations also support the rotation of the eye in a pproximately 144 min in the presence of axisymmetrization, vorticity redist ribution, wave breaking, and vortex merging processes.