Simple Models of Thermally Driven Mesopheric Circulation

Abstract

Possible thermally driven symmetric circulations of the mesosphere and upper stratosphere are considered, subject to the following major assumptions: the motions are in hydrostatic and geostrophic balance; the motions are small perturbations about a state of rest which is defined by the horizontal average of the radiative equilibrium temperature; the eddy fluxes of momentum and heat act only as Rayleigh friction and Newtonian conductivity, or as vertical eddy viscosity and conductivity. The assumption of small perturbations is justified by consideration of photochemical and radiative processes. The derived meridional circulations have a principal flow branch from summer pole to winter pole with maximum amplitude less than 1 m sec⁻¹. There is good agreement with the observed amplitude, distribution, and seasonal phase of the mean zonal winds if eddy viscosity and eddy conductivity parameters are assumed to be about 500 m² sec⁻¹. Radiative and photochemical processes are found to have an important damping effect on motions of this type; a radiative-photochemical damping parameter can be derived having the order of magnitude 10⁻⁶ sec⁻¹.