Modelling the Propagation of Atmospheric Tides from the Lower to the Middle and Upper Atmosphere

Upward propagating tides make an important contribution to the momentum budget of the lower thermosphere, and significantly modify ionized and neutral structures at higher levels, accounting for such phenomena as the midnight collapse of the ionosphere over Arecibo, and the midnight temperature anomaly. Modelling the thermospheric penetration of upward propagating tides excited in the lower atmosphere can be approached by (a) numerical solution of the governing tidal equations from ground level, taking into account realistic thermal forcing and the non-classical effects of turbulent diffusion of momentum and heat, zonal mean winds, and latitudinal temperature gradients; or (b) the utilization of meteor, M.F. and Thomson scatter radar data to specify tidal oscillations at the lower boundary ( ca. 95 km) of a thermosphere general circulation model (TGCM). In this paper the main physical processes affecting the propagation of tides from the lower to middle and upper atmosphere are reviewed, as well as existing models used to simulate tidal propagation. In addition, new theoretical work is presented which quantifies the effects of mean winds and dissipation on the diurnal propagating tide in the mesosphere and lower thermosphere, and a new method is proposed whereby the lower boundary specification of the semidiurnal perturbation geopotential for TGCM's can be analytically specified through utilization of radar wind data and classical tidal theory.