Equilibration in an Eddy Resolving Model with Simplified Physics*

The role of waves in maintaining the midlatitude tropospheric climate is investigated in a dry high-resolution quasigeostrophic b-plane channel model coupled to both a simplified model of the atmospheric boundary layer and an interactive static stability. The climate of the model’s equilibrated state is found to be separated into two dynamical regimes, one within the boundary layer and the other within the free troposphere. Thermal diffusion in the atmospheric boundary layer prevents the eddies from modifying the mean temperature structure there by damping temperature fluctuations. The potential vorticity gradients are essentially eliminated in the lower troposphere above the boundary layer, in agreement with observations. The homogenization of potential vorticity occurs in the region where the baroclinic waves have a critical layer, and is accomplished mainly by an increase in the static stability in the lower troposphere due to the vertical eddy heat fluxes. Even though the model has kinetic energy and enstrophy spectra characteristic of a fully turbulent flow, the equilibrated state of the model is essentially maintained by wave–mean flow interaction, primarily by the interaction between wave 5 and the zonal mean state. The zonal mean of the equilibrated state is found to be linearly stable to all waves. The largest-scale wave in the fully nonlinear state, wave 4, is found to be maintained by an energy cascade from the higher wavenumbers. However when wave 4 is large, stability analysis indicates that it is unstable, with the growing mode dominated by wave 6. This instability appears to saturate quickly and hand its energy over to wave 5. The result is that the amplitude of waves 4 and 5 in the equilibrated state are strongly anticorrelated, but the fluctuations in total eddy kinetic energy are strongly correlated with the fluctuations in the sum of the energy in waves 4 and 5.