AN ASSESSMENT OF THE BALANCE APPROXIMATION IN HURRICANES

The validity of the traditional balance approximation for the asymmetr ic flow above the boundary layer generally in hurricanes is examined h ere. Scaling considerations of the divergence equation show that the v alidity of the balance approximation hinges on the smallness of the no ndimensional product (delta(n)'/zeta(n)')x(n (v) over bar/<(eta)over b ar>r). The first term represents the ratio of asymmetric horizontal di vergence to asymmetric vertical vorticity for azimuthal wavenumber n, while the second term represents a Rossby number based upon the azimut hal mean tangential wind and absolute vertical vorticity of the hurric ane vortex. Wind observations of Hurricane Gloria (1985) indicate that this product is not at all small in the near-vortex region (several h undred kilometers beyond the radius of maximum tangential winds) where asymmetric convergence forced by surface friction and cumulus convect ion is typically large. Although the Gloria observations represent onl y a single case, there are dynamical reasons to expect this product to be O(1)just above the hurricane boundary layer in steadily translatin g hurricanes. The meteorological relevance of these results to the pro blem of balance dynamics in hurricanes is briefly discussed.