On the Impact of Middle-Atmosphere Thermal Tides on the Propagation and Dissipation of Gravity Waves

In the middle atmosphere, solar thermal tides cause large variations in the background conditions for gravity-wave propagation. The induced modulation of gravity-wave pseudo-momentum fluxes is responsible for a diurnal force. In past studies, this forcing was derived from gravity-wave parameterizations which neglect time-dependence and horizontal inhomogeneities of the background flow. In our study, we evaluate these assumptions using a highly simplified gravity-wave ensemble. With the help of a global ray-tracing model, a small number of different gravity-wave fields is transported through a time-changing background which is composed of a climatological mean and tidal fields from a general circulation model. Within three off-line experiments, assumptions on horizontal and temporal dependence of the background conditions have been successively omitted. Time-dependence leads to a modulation of gravity-wave observed frequencies and its phase velocities. Transient critical layers disappear. The amplitude of the diurnal forcing is reduced. Horizontal inhomogeneities induce a refraction of the gravity waves into the jet stream cores. Horizontal propagation can lead to large meridional displacements and an inter-hemispheric exchange of gravity-wave energy. With equivalent Rayleigh friction coefficients, it is shown that for the gravity-wave ensemble in use the damping of tidal amplitudes is reduced when horizontal and time dependence of tidal background conditions are taken into account.