Nonlinear Equilibration of Baroclinic Eddies and Its Sensitivity to Boundary Layer Dissipation

The planetary boundary layer is characterized by turbulent momentum and heat transports, and strong surface friction as well as heat exchange with the ground. With the existence of large scale baroclinic eddies in the extratropics, it is still an open question as to what determines the boundary layer thermal structure and how these boundary layer processes influence the eddy equilibration. A β plane multilevel quasi-geostrophic channel model with interactive static stability and simplified parameterization of the atmospheric boundary layer physics is used to study the nonlinear saturation of baroclinic eddies and the role of different boundary layer processes in eddy equilibration.

The standard run simulation, in which realistic values are chosen for the coefficients of the boundary layer parameterization, shows that even though the flow illustrates fully nonlinear flow characteristics, the process of saturation and the equilibrated state are primarily maintained by wave-mean interaction. Although wave-wave interaction is of secondary importance during eddy equilibration, it can still influence the equilibrated state by selecting the dominant wave scale. But this wave scale is not far from the linearly most unstable mode.

The sensitivity of the equilibrium state to individual boundary layer processes is studied. Under fixed SST boundary conditions, surface heat exchange is the major factor that determines the surface air temperature gradient. Vertical thermal diffusion, which acts to couple the free troposphere with the surface air, can largely suppress the mixing of potential vorticity as well as the mixing of potential temperature by baroclinic eddies in both the boundary layer and the interior layer. The distribution of the equilibrium state temperature gradient is also influenced by surface friction, but its response to enhanced surface friction is not monotonic. The equilibrium state is insensitive to the vertical momentum dissipation in the boundary layer.