Interpreting GCMs with Mixed Layer Theory

Mixed layer theory is used to evaluate the representation, and state, of the subtropical marine boundary layer as represented by two general circulation models, and the NCEP reanalysis data. For the former, both present day, and CO_2 warming scenarios are investigated. Initial analyses are performed in terms of the steady state solutions with diurnally averaged radiative forcings, and a decoupling criterion based on the relative magnitude of surface versus cloud top cooling. The mixed layer theory is used both to interpret the effective entrainment rates of the various models, and the extent to which changes in the simulated boundary layers consistently reflect changes in the climatological conditions. On a qualitative level the mixed layer solutions show skill in predicting regions of stratiform cloudiness, and provide a basis for more quantitatively discriminating amongst the varied representations of the large-scale as represented by the general circulation models, and the reanalysis data. Comparing mixed layer equilibrium solutions from present day simulations and doubled CO_2 simulations also provides a basis for diagnosing cloud feedbacks from stratocumulus.