STRUCTURAL CHANGES OF GROWING BAROCLINIC WAVES.

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 R. Gall.