Dynamics of rapid changes of intensity and structure in an eyewall replacement cycle of tropical cyclones remain an open question. To clarify the dynamics of the inner eyewall decaying, a quantitative estimation of inner‐core wind fields based on highly frequent observation images with 2.5‐min temporal resolution in the Himawari‐8 satellite is applied to Typhoon Trami (2018) which had a clear concentric eyewall structure. A high tangential wind of 50 m s−1 is estimated at a radius of 30 km, which is located in the inner edge of the inner eyewall, during an active stage of the inner eyewall. During the decaying stage of the inner eyewall, the estimated tangential wind rapidly decreases to about 20 m s−1 at a radius of 24 km. The satellite‐based tangential winds are validated with dropsondes around the inner core by an aircraft. Vorticity field retrieved by the satellite‐based tangential winds during the decaying stage exhibits a rapid decrease in an outer part of the eye and the inner eyewall, and a slow decrease near the storm center. Examination on an absolute angular momentum coordinate indicates that the rapidly slow‐down rotation in the outer edge of the eye and inner eyewall is faster than a slow‐down rotation explained by surface friction. It suggests that asymmetric eddies transport angular momentum across the moat in the inner eyewall dissipation. This study is the first examination of dynamical contributions of asymmetric eddies to the inner‐eyewall dissipation based on satellite‐estimated tangential winds.
Observations of the winds in tropical cyclones are still limited. We propose a new method for deriving the tangential winds in tropical cyclones, which employs a spectral analysis of high‐frequency cloud imaging by latest‐generation geostationary meteorological satellites such as Himawari‐8. The method was applied to the visible images of boundary layer clouds in the eye of Typhoon Lan (2017) over an 8.5‐hr period. The low‐level tangential winds over the central two thirds of the eye in radius were close to a rigid body rotation and increased with time. On its outside was a region with striating clouds rotating at much higher angular velocities, which may have been supergradient. Asymmetric motions were visualized as the deviation from the inner rotation, and the vorticity of some mesovortices were quantified. These asymmetric motions are suggested to transport angular momentum to accelerate the inner rotation.
Estimation of the radius of maximum wind (RMW) of tropical cyclone (TC) is helpful for the disaster prevention and mitigation. If RMWs are estimated from infrared (IR) imagery taken by geostationary meteorological satellites, their estimation is available densely in time, regardless of the ocean basin. Kossin and coauthors showed that when TCs have clear eyes, the eye radii estimated from IR images have a high correlation with the RMW estimated from aircraft reconnaissance. The regression of the former onto that latter was shown to have a mean absolute error (MAE) of 4.7 km. We revisit the IR-based RMW estimation by using C-band synthetic aperture radar (SAR) sea surface wind estimates. The criteria for selecting clear-eye cases are simplified. The MAE of the Kossin et al. method is found to be smaller than previously suggested: 3.1 km when the proposed relation is used and 2.7 km when the regression is revised with the SAR-measured RMWs. We further propose an improvement of the IR-based method to estimate the eye radii. The resultant MAE is shown to be 1.7 km, which indicates that the IR-based RMW estimation is more accurate than has been suggested.
A strong correlation between eyewall slope and eye size is confirmed. We also investigated cloud features in the eye that may be closely related to RMW and wind structure around RMW. Potential applications of highly accurate RMW estimation are discussed.
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