The barocline instability of wave in the center of the circular vortex.

The asymmetrical perturbation is an ordinary form of motion in vortices, such as the mesoscale deep convection cloud area in a tropical storm, the dissymmetry of a tropical storm when it develops, the fact that the circulation center of a tropical storm is often located at the edge of a thick/dense cloud system and so on. The researches on asymmetrical perturbations, especially on the development mechanism of perturbation, at present are also quite few. This article studies the asymmetric unstable waves at the center of a circular vortex in the cylindrical coordinates，and the following results are obtained: (1) The baroclinic instability appears more easily in the circular circulation of a vortex than it does in a straight basic flow. (2) The instability causes the energy conversion from the basic flow to the perturbation flow, and at the same time, the vortex becomes asymmetric. The perturbation in the conversion process is one kind of internal gravity waves, whose propagation velocity is far smaller than the angular velocity of the basic flow, such that some propagate reversely and slowly against the basic flow. (3) Under the usual stability parameter condition, the growing perturbation mainly concentrates in upper troposphere, and its tilting angle is large. When the stability parameter is very small, the perturbation may expand to the entire troposphere, and its tilting angle is small. (4) The high-level anticyclone may also induce the unstable internal waves especially in the low level. The perturbation develops quickly and propagates reversely and slowly against the basic flow, when the stability parameter is smaller.