Dependence of vortex axisymmetrization on the characteristics of the asymmetry

Abstract

AbstractThis study investigates how different characteristics of initial asymmetries, including their positions and profiles, can impact on the vortex axisymmetrization process for barotropic vortices. When an initial disturbance is placed near the core of a vortex, a new asymmetry is generated inside the original asymmetry and grows due to its upshear tilt. Differential basic‐state rotation then shifts the phase to a downshear tilt and the asymmetry weakens. As the initial radius of the imposed asymmetry is increased, the initial upshear tilt of the asymmetry decreases. There is also a decrease in the efficiency with which the differential rotation shifts the phase tilt from upshear to downshear. The latter is related to differential radial propagation of the asymmetry in the form of vortex Rossby waves. These two mechanisms that are position‐dependent act against each other. There is an optimal radius at which the energy exchange between the symmetric and asymmetric flows is maximized. For a range of very different basic‐state profiles examined here, the optimal radius is around 1.5 to 2 times the radius of the maximum wind. The initial growth of asymmetries with higher azimuthal wavenumbers is weaker than their lower‐wavenumber counterparts due to a smaller upshear phase tilt with their smaller azimuthal length‐scales.Nonlinearity reduces the magnitude and multiple perturbations of the newly induced inner asymmetry, and also limits the radial propagation of the asymmetry. The further the asymmetry is away from the core, the slower the axisymmetrization is. Depending on the position of the initial asymmetry, the basic state can have an increase of the maximum wind, a double‐peak profile, or an increase of its outer wind profile through axisymmetrization. Copyright © 2008 Royal Meteorological Society