The Response of a Spectral General Circulation Model to Refinements in Radiative Processes

The authors present results and analyses of a series of numerical experiments performed with a spectral general circulation model (GCM). The purpose of the GCM experiments is to examine the role of radiation/cloud processes in the general circulation of the troposphere and stratosphere. The experiments were primarily motivated by the significant improvements in the GCM zonal mean simulation as refinements were made in the model treatment of clear-sky radiation and cloud-radiative interactions. The GCM, with the improved cloud/radiation model, is able to reproduce many observed features, such as a clear separation between the wintertime tropospheric and polar night jets, winter polar stratospheric temperatures of similar to 200 K, and interhemispheric and seasonal asymmetries in the zonal winds. In a set of sensitivity experiments, the authors stripped the cloud/radiation model of its improvements, the result being a significant degradation of the zonal mean simulations by the GCM. Through these experiments, it is possible to identify the processes responsible for the improved GCM simulations: 1) careful treatment of the upper boundary condition for O sub(3) solar heating; 2) temperature dependence of long-wave cooling by CO sub(2) 15- mu m bands; 3) vertical distribution of H sub(2) O which minimizes the lower stratospheric H sub(2) O long-wave cooling; and 4) dependence of cirrus emissivity upon cloud liquid water content. Comparison of the GCM simulations, with and without the cloud/radiation improvements, reveals the nature and magnitude of the following radiative-dynamic interactions: 1) the temperature decrease (caused by errors in radiative heating) within the winter polar stratosphere is larger than can be explained by purely radiative adjustment; 2) the role of dynamics in maintaining the winter polar stratospheric thermal structure is greatly diminished in the GCM with the degraded treatment of radiation; 3) the radiative and radiative-dynamic response times of the atmosphere vary from periods of less than 2 weeks in the lower troposphere to similar to 3 mo in the polar lower stratosphere; and 4) within the stratosphere, the radiative response times vary significantly with temperature, with the winter polar values larger than the summer polar values by as much as a factor of 2.5. Cirrus clouds, if their emissivities are arbitrarily prescribed to be black, unrealistically enhance the radiative cooling of the polar troposphere above similar to 8 km.