VORTEX AXISYMMETRIZATION - DEPENDENCE ON AZIMUTHAL WAVE-NUMBER OR ASYMMETRIC RADIAL STRUCTURE CHANGES

Intense vortices in the atmosphere and ocean exhibit a high degree of axisymmetry despite persistent asymmetric forcing from their environme nt. To further elucidate vortex axisymmetrization a variety of idealiz ed initial-value models for barotropic non-divergent now is considered . To ensure basic understanding, disturbance evolution is first examin ed in a rectilinear system of simple shear. Particular emphasis is pla ced on identifying how inviscid disturbance-evolution depends on the z onal wave-number and on the meridional structure of the initial condit ions. Insight acquired from the rectilinear problem is then applied to a bounded Rankine vortex. Here, the dependency of disturbance evoluti on on the azimuthal wave-number is of special interest. Recent develop ment of a low-frequency balance theory for rapidly rotating (large Ros sby number) vortices has provided observational evidence that the low- azimuthal-wave-number asymmetries, especially wave number one, are dom inant in the near-vortex region. The results of this work provide furt her theoretical evidence of an inviscid wave-number-selection mechanis m that preferentially damps the high-wave-number asymmetries. The radi al structure and location of the initial conditions are found to be im portant factors in determining how rapidly a disturbance is compressed or elongated. This in turn controls the rate of disturbance growth or decay. For swirling flows, a definition of an effective shear is prop osed that accounts for both the radial variations of the initial condi tion and the radial variation of the angular velocity. Using the recip rocal of this effective shear, time-scales for a disturbance to decay to half its initial energy, the half-life, are calculated for initial conditions and symmetric wind-profiles that are found in hurricanes. S imple-shear flow and the bounded Rankine vortex do not admit discrete modal solutions since there is no mean-state vorticity-gradient to sup port them. The unbounded Rankine vortex is examined to investigate how the presence of discrete modes (Rossby edge-waves) associated with th e radial vorticity-gradient of the Rankine swirl modifies the continuo us spectrum solutions presented here.