Some Aspects of Vortex Axisymmetrization: Nonlinearity and positions of initial asymmetries

The effects of different positions of the initial asymmetries and the nonlinearity on the vortex axisymmetrization are investigated for barotropic vortices. In the linear simulations, when the initial disturbance is placed at the radius of the maximum wind (RMW), the disturbance tilts downshear quickly due to the sheared basic-state angular velocity and transfers its energy to the basic state while propagating outward. Meanwhile, a weak asymmetry is generated inside the original asymmetry but it has only a small effect on the axisymmetrization process. When the initial disturbance is placed outside the RMW, a much stronger asymmetry is generated inside the RMW due to the separation between vorticity change by the basic state and by the disturbance. The newly induced inner asymmetry tilts upshear and extracts energy from the basic state at the early stage. As tilting of the asymmetry changes to a downshear phase, it loses energy back to the basic state. A critical radius where the exchange of energy between the asymmetry and the symmetric part is the largest is established. In general, nonlinearity reduces the intensity of the newly induced inner asymmetry and limits the multi- channel radial propagation of the asymmetry. When an asymmetry is placed at an outer distance, the symmetric wind weakens substantially in early stage (consistent with the linear simulations) and does not restore its intensity back to its original level or not until much later time. Diagnostics of the kinetic energy indicates the energy transfer between the symmetric and asymmetric flows can be explained by the eddy momentum flux and its gradient, both of which depend on the phase tilt of the disturbance. The results suggest that asymmetries such as those forced by convection can have very different effects on the symmetric wind, depending on their locations.