The Sensitivity of an Intermediate Model of the Midlatitude Troposphere's Equilibrium to Changes in Radiative Forcing*

Abstract

The sensitivity of the equilibrated state of a dry high-resolution quasigeostrophic β-plane channel model, coupled to both a simplified model of the atmospheric boundary layer and an interactive static stability, to changes in forcings is investigated. An earlier study with the same model found that with standard parameter values, the potential vorticity in the center of the channel just above the atmospheric boundary layer was homogenized. The new experiments show that this result is robust does not vary strongly with variations in forcing over a wide range of forcing parameters. This is so even though the meridional temperature gradients and static stability are generally sensitive to the forcing; that is, the changes in these cooperate to keep the meridional potential vorticity gradient zero. The potential vorticity gradients at higher levels are also robust although nonzero. The homogenization in the lower troposphere does disappear if the differential diabatic heating is decreased sufficiently or if the tropopause level is lowered sufficiently. The model results are also used to assess proposed parameterizations of eddy effects. Stone's parameterization of the meridional eddy heat flux is most successful at reproducing the model's results for most of the experiments. However, no parameterizations of the eddy heat flux captured the results of the experiments in which the diabatic heating timescale was varied. In these experiments, changes in the eddy heat fluxes kept the tropospheric temperature structure essentially unchanged even though the timescale changed from 5 to 80 days.