The Maintenance of the Zonal Mean State of the Upper Atmosphere as Represented in a Three-Dimensional General Circulation Model Extending to 100 km

Abstract

A primitive equation general circulation model has been developed to simulate the atmosphere from the surface to 100 km. The model was based on the semispectral technique and has 15 zonal wavenumbers and 40 grid points between poles. The simulation was for fixed January conditions, was nondiurnal, had land-sea contrast with specified sea surface temperatures but omitted both orography and the hydrologic cycle. For convenience and brevity only the zonal mean state of the model is presented in this paper. The overall simulation revealed several interesting and novel features of the atmosphere. The mean meridional wind exhibited a distinctly layered structure which was confined to the tropics throughout the stratosphere and mesosphere. This was attributed to inertial instability in the winter hemisphere resulting from the intense mesospheric jet: the actual mesosphere also may experience such an instability to a much lesser extent. Some observational evidence exists to support layering in the synoptic meridional velocities. A single mean meridional circulation in the mesosphere extending from pole to pole was not obtained, rather the principal component was only between the midlatitudes. This circulation was an extension of the winter tropospheric Hadley cell and it was the most dominant feature of the stratosphere and mesosphere. The mesospheric easterlies generated by this circulation were confined to the summer hemisphere of the model by an intense southward flux of relative angular momentum owing to the inertial instability in the tropics centered on 60 km altitude. The extension of this mean circulation into the winter hemisphere was the primary cause of the westerly jet. A detailed explanation of the angular momentum balance of the mesosphere and stratosphere, with particular emphasis on the role of the troposphere is presented. A leakage of wave energy from the troposphere to 100 km occurred in both hemispheres of the, model, with the winter hemispheric energy flux being about three times the larger at 100 km compared with 10 times at 25 km. The tropospheric energy flux into the upper atmosphere by large-scale eddies alone was almost three times the local generation of available potential energy. This energy flux was sufficiently strong to reverse the baroclinic cycle over most of the upper atmosphere. Regions with large eddy kinetic energy were found to exist in the tropical mesosphere and throughout most of the lower thermosphere of the model.