Short Numerical Integrations of a Three-Level Spectral Quasi-Geostrophic Model

Abstract

Two sets of initial conditions are used to integrate a three-level quasi-geostrophic model in spectral form. After a maximum perturbation kinetic energy is reached, a barotropic exchange in established between the zonal flow and the perturbation with an apparent periodicity from 2 to 4 days. The initial state includes a finite amplitude baroclinic mode which is highly unstable in the linear sense. This mode exhibits a negative growth rate for about the first two days of the integration due to barotropic exchanges with other modes. Spectra of kinetic and available potential energies, enstrophy, and omega2 are presented. The kinetic and available energy display a ?3 slope for intermediate, scales for the initial integration period when perturbation kinetic energy is actively growing at the expense of the mean flow available energy. The effect of the variation of stratification with height, dissipation, heating, mountains, and truncation of the spectral system is discussed.