Axisymmetric and asymmetric processes of tropical cyclone-like vortex during the rapid intensification phase

Axisymmetric and asymmetric processes during the rapid intensification phase of tropical cyclone (TC)-like vortex are examined using a non-hydrostatic three dimensional model, in order to understand which process is dominant for the development of TC. Although the theories for the intensification of TC (e.g., Ooyama 1964; Charney and Eliassen 1964; Emanuel 1997; Craig and Gray 1998; Kieu and Zhang 2009) are based on the vital assumption; the vortex is azimuthally symmetric (i.e., axisymmetric), few studies have examined how this assumption works. Since the theories provide us a lot of insights on the basic physical features of TC-development, it is important to investigate the axisymmetricity of TC-structure during the rapid intensification. On the other hand, especially in these days, many studies have pointed out the importance of the asymmetric processes. It is also interesting to investigate the contribution of the asymmetric processes to the intensification. The asymmetric processes are dominant especially when TCs are in the genesis phase, while the structure of mature TC is almost axisymmetric. Therefore, there must be a time when a vortex finishes its axisymmetrization, thus the axisymmetric vortex structure becomes dominant and this has to be done during the intensification period. Because of the difficulty to recognize the vortex-structure during the early stage of TC-life, it is unclear when the genesis phase finishes and TCs go into the intensification phase. In addition to the above two purposes, we will try to distinguish the genesis and development phases of a TC physically. Similar to the suggestions of the earlier studies, the results indicate that the vortex-intensification started from the homogeneous sounding with the axisymmetric vorticity field, can be divided into two phases. One is the asymmetric phase that convections are organized randomly and the other one is the axisymmetric phase that the vortex intensifies by shrinking of a high-wind ring characterized by the concentration of a lot of convections around the RMW. The numerical results indicate that, during the former phase, the TC-vortex is intensified by a number of mergers of cyclonic vortices generated by the strong convections with 10-km horizontal scale. They are recognized as the vortical hot towers (VHTs) suggested by Montgomery et al. (2006). These vertical vortices are originally generated from the tilting of the horizontal component of vorticity produced by the vertical shear of cyclonic velocity, and intensified by the vertical stretching due to the presence of strong upward velocity associated with the convection. Since the vortices rotating counter-clockwise can go up to the gradient of vorticity, they go inward, merge with other vortices. After the axisymmetric structure becomes significant, the vortex has a few spiral bands characterized by the large amount of water content. The horizontal vorticity generated by the convective updraft in the bands is provided from outside of the core region through the bands, and it is tilted and stretched by the convection around the RMW. This intensification process is dynamically similar to the case of very small vortices such as dust devil (Ohno and Takemi 2009). Based on the analysis carried out by dividing the physical variables into the azimuthal average (axisymmetric value) and the deviation from the average (asymmetric one), it is found that the contributions of vorticity budgets composed of the asymmetric component of velocity to the change in the intensity of vortex, are smaller than the one by the axisymmetric component. Therefore, the intensification of the vortex results mainly from the axisymmetric processes after the TC-like structure is well organized. On the other hand, based on the results of this study, a parameter standing for the degree of axisymmetricity of the vortex as the ratio of the variance of the azimuthally-averaged Ertel's potential vorticity (PV) to the sum of axisymmetric value of PV and asymmetric one. This parameter also indicates that the latter phase of the rapid intensification can approximately be regarded as the axisymmetric phase.