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ABSTRACT
Typhoon Winnie (1997) was one of the hurricanes that had extremely large eyewall ever recorded with a diameter of eyewall reaching 370 km. 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 track, intensity, eye and concentric eyewalls. The dynamic and thermal structures of concentric eyewalls were studied based on the model output. It was found that the concentric eyewalls and their surrounding wind fields were asymmetric in observation as well as in simulation. Winnie's outer eyewall was associated with a maximum wind ring, a warm moist ring, and a high vorticity ring. The inner eyewall was associated with a secondary maximum wind ring and a warm moist ring. Upward motion dominated the whole layer of inner eyewall and the area above 2-km altitude of the outer eyewall. Downward motion was found inside the eye and the moat. Radial inflow happened in the boundary layer of the outer eyewall and the moat, but radial outflow dominated the middle and upper levels of the outer eyewall.
Key words: Winnie 1997, typhoon eye, concentric eyewalls
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1. Introduction
On 5 August 1997, Super Typhoon Winnie formed at low latitudes in the Marshall Islands (Fig.l). It intensified continuously during the course of moving westward. The maximum wind speed reached 72 m s^sup -1^ and a clear eye whose diameter was 30 km appeared at 0600 UTC 12 August. After it weakened slowly on 14 August, Winnie showed concentric eyewall clouds. At 0456 UTC 16 August, the inner eyewall cloud began to dissipate and a larger outer eyewall rainband began to circle the eye and preexisting eyewall of Winnie. The diameter of the extremely large eye reached 370 km. Soon after the outer eyewall developed well, the inner eyewall recovered with a diameter about 18 km. This situation had sustained until 1200 UTC 18 August when it landed in Zhejiang Province.
A well-defined eye is one of the most remarkable features of a mature tropical cyclone. In general, the diameter of a hurricane eye over the Atlantic observed by satellite varies from 55 to 85 km (Weatherford, 1984). Eyes with diameters...