Midlatitude Static Stability in Simple and Comprehensive General Circulation Models

The static stability of the extratropical troposphere is examined in two atmospheric general circulation models (GCM’s) over idealized boundary conditions, with emphasis on the role of moisture in determining the midlatitude stability. The determination of the static stability is compared within two models: an idealized moist model with simplified representations of radiative transfer and other physical processes, and a comprehensive GCM with full physics. The GCM’s are run over a zonally symmetric, fixed sea surface temperature (SST) aquaplanet surface, with a multitude of SST distributions to study the response of the extratropical static stability over a wide parameter range. In both models, the dry static stability averaged over the midlatitudes increases both with increases in the meridional temperature gradients, and with increases in the mean SST. These changes in static stability are compared with both moist theories and dry theories. Dry baroclinic eddy theories are invalid for the entire parameter range in the idealized GCM, and for much of the parameter range considered in the comprehensive GCM. A moist theory, on the other hand, works remarkably well in predicting the midlatitude stability over the entire parameter range for both models. These simulations give strong support for the influence of moisture on the thermal structure of the midlatitudes.