A triggering mechanism of the axisymmetric spontaneous intensification of tropical cyclones

Tropical cyclones (TCs) often experience two intensification phases with different intensification rates (e.g., Sitkowski and Barnes 2009): an initial slow intensification phase and rapid intensification phase (RI, Kaplan and DeMaria 2003). As a RI mechanism, the previous studies showed that a TC intensifies through a positive feedback between diabatic heating around the rotation center and vortex-scale motion (Charney and Eliassen 1964; Ooyama 1969; Emanuel 1989) provided that a TC has an axisymmetric structure and eyewall. The acceleration of tangential velocity by the heating in the eyewall can be understood as a dynamical response of TC (Schubert and Hack 1982; Shapiro and Willoughby 1982). Nolan et al. (2007) found that the axisymmetric component of heating plays a primary role in accelerating the tangential velocity. In recent years, the initiating processes of RI have been intensively studied in recent years (Rogers 2010; Guimond et al. 2010; Barnes and Fuentes 2010; Miyamoto and Takemi 2013). Although the previous studies have suggested that the thermodynamic and kinematic properties of the TC core play an important role in initiating RI, it is not clear yet when TC vortex changes its phase to the RI or what physical processes work for the trigger of RI.

We analyzed the results of the idealized numerical experiment of Miyamoto and Takemi (2013). The time evolutions of the boundary-layer mass convergence and diabatic heating around the RMW showed that the dibatic heating increases half hour before the onset of RI. It was also shown that the axisymmetric component of diabatic heating enhances 2 hours before the RI. Therefore, the results strongly suggest that the eyewall formation is the triggering process of the TC. The temporal change in the cloud fields showed that one of the cloud cells generated inside the RMW developed into the eyewall. We investigated the physical quantities of all the convective cells generated inside the RMW after the TC becomes nearly axisymmetric. It was found that the cell was accompanied with stronger axisymmetric component of boundary-layer convergence than the other cells, whereas the other quantities are not so different from the other cells.

Further investigation of the temporal change in the radius of maximum convergence (RMC) revealed that the RMC well reaches the RMW when the cell generates. Hence, it is suggested that the correspondence between the RMW and RMC makes the flow field favorable for the positive feedback process or for the formation of the eyewall. Since the location of RMC depends on Rossby number (Ro) determined by the tangential velocity, RMW and Coriolis parameter (Eliassen and Lystad 1977; Kepert 2001), a series of sensitivity experiments to these quantities was conducted. The results showed that the onset of RI becomes early with increasing Ro, which supports the results of present study. Therefore, it is hypothesized that initial vortices with large Ro more likely experience RI.