Numerical Study of a Typhoon with a Large Eye: Model Simulation and Verification

Abstract

Typhoon Winnie (1997) was the fourth supertyphoon in the western North Pacific in 1997. In its mature stage, an outer eyewall, consisting of deep convection with a diameter of 370 km, was observed by satellite and radar. Within this unusually large outer eyewall existed an inner eyewall, which consisted of a ring of shallow clouds with a diameter of ∼50 km. In this study, Typhoon Winnie is simulated using a nested-grid version of the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with an inner grid length of 9 km. The model reproduces an outer cloud eyewall with a diameter of ∼350 km. The simulated radar reflectivity and hourly precipitation are verified with satellite microwave, infrared, and cloud brightness temperature images. Analysis of the model results indicates that the large outer eyewall in many ways possesses the structure of a typical hurricane eyewall. This includes strong tangential winds and radial inflow outside the eyewall as well as an extremely large horizontal wind shear right at the eyewall. The outer eyewall is characterized with a ring of high vorticity (RHV). This RHV is closely related to a ring of high convergence (RHC). This RHC is caused by organized convective systems along the eyewall. The eye simulated by Winnie is characterized by a broad region of warm, dry slowly sinking air. The factors determining the diameter of eyes in tropical cyclones are discussed by considering the scale of the environmental angular momentum and the maximum kinetic energy achieved by parcels of air originating in the environment and reaching the radius of maximum wind. It is hypothesized that the formation of a large eye is favored by large circulations in which parcels of air are drawn in toward the center of the storm from great distances, and trajectories of air in Winnie that support this hypothesis are shown.