Interaction between concentric eye‐walls in super typhoon Winnie (1997)

Abstract

AbstractSuper typhoon Winnie had one of the largest eye‐walls ever recorded in the western Pacific Ocean. The diameter of the outer eye‐wall reached 370 km when Winnie passed through Okinawa on 16 August 1997. Observations revealed a 24‐hour intensity cycle of concentric eye‐walls after the large eye‐wall formed. Using the Penn State University/National Center for Atmospheric Research mesoscale model MM5 with 3 km grid horizontal spacing on the finest nested mesh, Winnie was successfully simulated in terms of concentric eye‐walls, spiral rain bands in the moat and the cycle of concentric eye‐walls.The life cycle of concentric eye‐walls is found to be associated with vortex Rossby wave propagation in the moat. The propagation properties of the PV (rainfall) spiral bands are found to be consistent with predictions using vortex Rossby theory. The initial inward vorticity gradient at the inner edge of the outer eye‐wall and outward vorticity gradient at the outer edge of the inner eye‐wall support inward and outward PV (rainfall) spiral bands, respectively, in the moat at the lower level. At the middle level, only an outward vorticity gradient exists at the outer edge of the inner eye‐wall, which permits the development of an outward PV band in the moat at initial time. The inward spiral PV (rain) band moves faster than the outward PV (rain) band and both of them propagate cyclonically. At 09 h, when the fast inward PV band catches up to the slow outward PV band, the slow outward rain band is replaced by the fast inward rain band, resulting in a well‐developed and independent inner eye‐wall at 14 h. After the vorticity gradient near the inner eye‐wall becomes stronger than that near the outer eye‐wall at lower level, the whole convective core is attracted to the outer eye‐wall. It finally merges with it at 37 h, leaving a weak vortex in the centre. The concentric eye‐walls of Winnie interact with each other through vortex Rossby waves. Copyright © 2005 Royal Meteorological Society.