CONSEQUENCES OF THE GEOSTROPHIC MOMENTUM APPROXIMATION ON BAROTROPIC INSTABILITY

A subsynoptic instability with dominant barotropic signature has been found to be a possible mechanism for the development of frontal waves. Linear semigeostrophic calculations of the growth rate of waves devel oping along a front exhibiting a low-level potential Vorticity band ar e here compared with the results of similar experiments made with a pr imitive equation model. The growth rates of the subsynoptic instabilit y are significantly underestimated by the semigeostrophic system, a re sult which is the opposite of what has been found by previous semigeos trophic-primitive equation comparisons in the case of baroclinic insta bility calculations. Analytical solutions of the Rayleigh model of pur e barotropic instability confirm this behavior of the geostrophic mome ntum approximation (GMA) in the case of barotropic instability. The in accuracy of the equations with GMA is very sensitive to the basic wind shear amplitude. Such sensitivity seems to be directly linked with th e definition of the semigeostrophic vorticity. It should be noted, how ever, that the frontal wave instability is not filtered by the balance d system but rather it is described in a quantitatively inaccurate way .