Structural Changes of Growing Baroclinic Waves

Abstract

The nonlinear interaction between a single zonal wave and the zonal mean flow is simulated with a primitive equation model. It is determined that as the wave evolves and modifies the zonal flow, the wave growth rate diminishes more rapidly near the earth's surface than it does aloft, allowing the upper portions of the wave to grow to a larger amplitude than the surface disturbance. The more rapid reduction in growth rate near the surface is accomplished primarily by an increase of static stability. It is proposed that this mechanism accounts for some of the differences in wave structures between linear baroclinic instability theory (where the, maximum amplitudes of the geopotential perturbation for wavenumbers 5-7 is at the earth's surface) and the eddies in a general circulation model (where the maximum amplitude for wavenumbers 5?7 is at the tropopause) that were noted by Gall (1976). In addition, the amount of kinetic energy within an individual wave at the time that the increase of kinetic energy ceases through nonlinear interaction with the zonal mean flow is a function of wavenumber. This is because the short wavelengths are primarily surface disturbances, and therefore the increase of the zonal mean static stability near the earth's surface by the wave and frictional dissipation cause wave growth to cease sooner than it does for the long waves which extend up to the tropopause. It is this mechanism that may explain why the short wavelengths do not dominate the general circulation statistics, even though the shorter wavelengths were found by Gall to have the highest growth rates.