DYNAMICAL AND RADIATIVE FORCING OF THE SUMMER MESOPAUSE CIRCULATION AND THERMAL STRUCTURE .1. MEAN SOLSTICE CONDITIONS

We present here a simple eigenvalue model which illustrates the sensitivity of the mean circulation and thermal structure near the high-latitude summer mesopause to dynamical and radiative processes under solstice conditions. Dynamical forcing is provided by gravity wave energy and momentum fluxes and their divergence which contribute both and zonal body force and implied turbulent heating and transport in the mesopause region. These profiles are chosen to be consistent with measurements of gravity wave fluxes and energy densities at high latitudes. Radiative forcing represents the effects of solar heating, non-LTE CO2 cooling, and LTE O-3 cooling below 70 km and is parameterized as a height-dependent relaxation toward an equilibrium temperature profile accounting for both radiative and chemical influences. Gravity wave momentum flux divergence forces a mean vertical motion related to the meridional gradient of the density-weighted momentum flux through the ''downward control principle'' and a mean meridional circulation required to balance the zonal body force. The model exhibits strong sensitivity to dynamical forcing and implies significant constraints on wave influences at mesopause altitudes. For plausible wave fluxes and energy densities, the model yields a mean vertical motion of similar to 0.05 m s(-1), a mean meridional jet of similar to 10 to 15 m s(-1), and a mesopause temperature of similar to 130 K at Arctic latitudes, all of which are in good agreement with observations. The eigenvalue solution also yields estimates of the induced turbulent heating and vertical diffusion that may pose useful guidelines in modeling studies. Seasonal variations in the mesopause structure ase the subject of a companion paper.