The Beta Drift of Three-Dimensional Vortices: A Numerical Study

Abstract

The beta effect on translation of cyclonic and anticyclonic vortices with height-dependent circulation (the beta-drift problem) is investigated via numerical experiments using a dry version of a multilevel primitive equation model (Florida State University model). The vertical structure of vortex circulation influences steady translation in a manner similar to that of the horizontal structure. Both spatially change the mean relative angular momentum (MRAM) of the vortex. The translation speed and its meridional component are both approximately proportional to the square root of the magnitude of MRAM of the initial (or quasi-steady-state) symmetric circulation. The latitude is another important factor controlling the speed of the beta drift. The meridional component decreases by about 45% when the central latitude of the vortex increases from 10° to 30°N. The beta-drift speed is intimately related to the axially asymmetric pressure field. During quasi-steady vortex translation the asymmetric pressure field maintains a stationary wavenumber 1 pattern in azimuthal direction with a high in the northeast and a low in the southwest quadrant of a Northern Hemisphere cyclone. The beta-drift velocity is approximately equal to the geostrophic flow implied by the asymmetric pressure gradient at the vortex center. If the Rossby number associated with the asymmetric flow is small, to the lowest order, the asymmetric pressure gradient force at the vortex center is balanced by the Coriolis force associated with the beta drift of the vortex.