Effect of Baroclinicity on Vortex Axisymmetrization. Part II: Baroclinic Basic Vortex

The effect of baroclinicity on vortex axisymmetrization is examined within a two-layer dynamical model. Three basic state vortices are constructed with varying degrees of baroclinicity: (i) barotropic, (ii) weak baroclinic, and (iii) strong baroclinic. The linear and nonlinear evolution of wavenumber-2 baroclinic disturbances are examined in each of the three basic state vortices. The results show that the radial propagating speed of the vortex Rossby wave at the lower level is larger with the stronger baroclinicity, resulting in a faster linear axisymmetrization process in the stronger baroclinic vortex. It is found that the nonlinear axisymmetrization process takes the longest time in the strongest baroclinic vortex among the three different basic vortices due to the weaker kinetic energy transfer from asymmetric to symmetric circulations at the lower level. A major finding in this study is that the same initial asymmetric perturbation can have different effects on symmetric vortices depending on the initial vortex baroclinicity. In numerical weather prediction models, this implies that there exists a sensitivity of the subsequent structural and intensity change solely due to the specification of the initial vertical shear of the tropical cyclone vortex.