On the dynamics of the secondary eyewall genesis in Hurricane Wilma (2005)

In this study, the multiple nested Weather Research and Forecast (WRF) model is used to simulate the genesis and evolution of the secondary eyewall in hurricane Wilma (2005). The simulated track and minimum central pressure compare favorably to those from the best track analysis. The structure and the time evolution of the secondary eyewall and the eyewall replacement cycle were well captured. The theory of empirical normal modes (ENMs) is then applied to study the genesis of the secondary eyewall. For azimuthal wavenumber 1 anomalies, the wave activity spectra indicate that the leading modes (1 and 2), which explain most of the variance in a 24-hour analysis period, are vortex Rossby waves (VRWs). The Eliassen-Palm (EP) theorem is used to diagnose the impact of the propagating waves on the formation of the secondary eyewall. Analysis of the EP flux and its time-mean divergence show that in the lower troposphere the VRWs propagate outward outside the primary eyewall. The fact that the critical radius of the leading modes is located close to the region of secondary eyewall genesis and coincides with a positive maxima of EP flux divergence, supports the notion of an important role played by VRWs in the formation of the secondary eyewall. Sensitivity experiments were performed to generate a weaker “Wilma”. The results indicate that in weaker storms, the critical radius is located too far and the wave activities are too weak to generate a secondary eyewall.