Introduction

Concentric eyewalls have been observed in the life cycle of strong tropical cyclones (TC). Willoughby et al.1 identified a double eyewall structure for hurricane Gilbert with the inner eyewall in the radius of 8-20 km and the outer eyewall between 55 and 100 km. A more detailed analysis of Gilbert1 showed that the primary eyewall appeared first. During a weakening stage of the storm, the outer eyewall formed. Later on, the outer eyewall strengthened and contracted while the inner eyewall weakened.

Fig. 1. Flight-level tangential wind speed from south to north traverses through the center of Hurricane Gilbert. Bold 'I' and 'O' denote the location of the inner and outer eyewall wind maximum, respectively. Times at the beginning and end of each radial pass are plotted at the top of the panels (refer to 1).

Fig. 1. Flight-level tangential wind speed from south to north traverses through the center of Hurricane Gilbert. Bold 'I' and 'O' denote the location of the inner and outer eyewall wind maximum, respectively. Times at the beginning and end of each radial pass are plotted at the top of the panels (refer to 1).

Finally, the outer eyewall replaced the inner eyewall and completed an eyewall replacement cycle (Fig. 1).

Several studies have devoted to understand mechanisms through which concentric eyewalls form. Willoughby et al.10 and Willoughby11 suggested a symmetric instability that might contribute to the formation of the outer eyewall. They, however, could not develop a causal relation between the location of the outer eyewall and the instability. Montgomery and Kallenbach3 implied that the TC concentric eyewalls could result from radially propagating linear vortex Rossby waves that are dynamically constrained near a critical radius. Since the development and propagation of the vortex Rossby waves are attributed to the TC basic state radial vorticity gradient, the vortex Rossby waves are confined near the radius of the maximum wind (RMW). Nong and Emanuel4 studied the formation of the concentric eyewalls in an axisymmetric model. Their simulations showed that the secondary eyewall might result from a finite-amplitude WISHE instability, triggered by external forcing.

Black and Willoughby1 noted that the outer eyewall formed during the TC weakening stage (Fig. 1). Shapiro and Willoughby7 and Willoughby et al.9 used a symmetric model (hereafter SW model) to diagnose the secondary circulation induced by a point heat source in balanced, axisymmetric vortices. For a heat source near RMW, a maximum of the tangential wind tendency lay just inside of RMW, so that the maximum wind propagated inward in response to the heating, which provided a plausible physical explanation for the contraction of the outer wind maximum. However, their simulations did not reproduce a double-peak structure. The fact that an outer eyewall forms during TC weakening stage suggests that a rapid decrease of convective heating may play a role in the formation of double eyewalls. Peng et al.5 examined this idea by introducing a negative heat source in a simple TC model.

Theoretically, concentric eyewalls may be formed as two cyclonic vortices with different sizes and intensities interact without merging into a monopole.2 The criteria for the concentric eyewall formation is that (1) the core vortex must be at least six times stronger in vorticity than the neighboring weaker vortex, (2) the neighboring vortex is larger in size than the core vortex, and (3) a separation distance is within three to four times of the core vortex radius. Note that in this scenario, a symmetric positive vortcity belt has been given initially. The interaction between the two vortices just redistributes the vorticity of the outer vortex. In this study, we present a different, wave-mean flow interaction scenario. We examine a new concentric eyewall formation scenario in which a core vortex interacts with an asymmetric perturbation that has a wave-like structure and zero symmetric vorticity component in the outer region. We will examine how the symmetric flow gains energy from the asymmetric perturbation in the outer region, and how the second peak of the symmetric tangential wind is induced.

The outline of this paper is as follows. A brief description of the model and the experimental design is given in Sec. 2. Results from the nonlinear simulations are discussed in Sec. 3. Finally, a summary is given in Sec. 4.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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