Tide/gravity-wave/mean-flow interactions in the mesosphere and lower thermosphere

Interactions between the solar diurnal tide, semidiurnal tide, internal gravity waves and mean zonal winds in the mesosphere and lower thermosphere are investigated using a quasi-nonlinear time dependent numerical model under solstice conditions. In the model, a coupled system of zonally-averaged nonlinear equations and perturbation equations for tides, including a parameterization for gravity wave propagation, is integrated simultaneously making allowance for mutual interactions. It is shown that the propagation of internal gravity waves is affected not only by the mean zonal wind but also by the tidal waves, and the tidal wave fields as well as the mean zonal winds in the upper mesosphere and lower thermosphere weaken as a result of the momentum deposition due to gravity waves. It is also shown that the solar tides make a significant contribution to the generation of the zonal mean fields, especially an equatorial easterly jet and cellular meridional circulation structures in the upper mesosphere and lower thermosphere. Preliminary results of full nonlinear calculations of the solar tides using a general circulation model, in which the tidal wave breakdown is simulated by a convective adjustment, are also presented.