Aspects of Mesospheric Simulation in a Comprehensive General Circulation Model

The simulation of mesospheric circulation in the 40 level GFDL "SKYHI" troposphere-stratosphere-mesosphere general circulation model is examined.The model is shown to produce a "reversed" equator-pole temperature gradient near the upper summer mesosphere with consequent closing off of the summertime easterly jet. This results from the mean flow driving associated with explicitly resolved internal gravity waves. The mean easterlies in the summer upper mesosphere become weaker as the horizontal resolution in the model is improved, presumably reflecting a more complete representation of the gravity wave spectrum. The rate of loss of the eddy kinetic energy in the model due to subgrid scale dissipation is computed. The model results in the mesosphere compare reasonably well with the limited data available concerning the turbulent dissipation rates obtained from in situ rocket experiments.

Also discussed here for the first time are simulations with a diurnally-varying version of the SKYHI general circulation model. The diurnal tidal signal appears to be of realistic amplitude in the model, but it is striking that the other eddy components (gravity waves, Rossby normal modes, quasi-stationary planetary waves) are strong enough to dominate the tide in any instantaneous map of the global mesospheric wind field. The mean flow driving (Eliassen-Palm flux divergence) associated with the diurnal tide is also computed. The present results for this quantity differ considerably in detail from earlier calculations which were based on more idealized tidal theories. The tidal driving of the mean flow in the SKYHI model is an important, though not dominant, effect in the summer mesosphere region.