圆形涡旋中心非对称扰动的斜压不稳定研究

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

中文关键词:  圆形涡旋， 不稳定波， 非对称扰动， 重力波

英文关键词:Circular vortex, Unstable wave, Asymmetrical perturbation, Gravitational wave

中文摘要:

      涡旋中的非对称扰动又是一种较为常见的流体运动现象，如不成熟飓风中出现的中尺度深厚对流云区（又称圆形抽气云CEC），热带风暴发展时的不对称性以及环流中心往往位于浓密云区的边缘等，都是涡旋中存在不对称扰动的表现形式。然而对于涡旋中非对称扰动的稳定性问题的研究，目前还比较少。文中从柱坐标下的斜压模式出发，研究了热带气旋等一类涡旋中心的非对称扰动的不稳定问题，结果表明：（1）与平直的基本流相比，涡旋中更容易出现斜压不稳定，扰动更容易发展；（2）不稳定可以使涡旋中的能量由基本场向扰动场转换，涡旋变得不对称。扰动是一种重力内波，其传播速度远小于基本流角速度，有些甚至逆基本流缓慢倒转；（3）在通常的稳定度参数条件下，发展扰动主要集中在高层，从结构上看，扰动的倾斜度大。当稳定度参数很小时，扰动可扩展到整个对流层，倾斜度较小；（4）高层反气旋也可以激发出不稳定内波，这种扰动在低层表现明显。当稳定度参数较小时，扰动发展较快，并且也以很小的速度倒转。

英文摘要:

      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.