|
Gravity waves (GWs) and thermal tides are important phenomena in middle-atmosphere dynamics. Breaking GWs have a major impact on the mean flow by driving the residual circulation in the mesosphere / lower thermosphere.
Moreover, a major part of mesospheric variability is made up by these types of wave motion. In former investigations of the GW-mean flow interaction columnar GW parameterizations (e.g. Lindzen 1981) have been used. Note, that simple GW parametrizations are also in use in high resolution wheather forecast models. In these approximations the impact of temporal and horizontal variations of the background atmosphere on the GW propagation is neglected. However, in the middle atmosphere highly transient tidal perturbations are always present and dominate diurnal variations through which the GWs propagate. To obtain an improved understanding of GW motion we use a ray tracing technique to solve the time-dependent dynamical equations of GWs in WKB approximation. Both, the horizontal gradients of the background (including the tides) and its time dependence are properly taken into account. It is shown that tidal transience leads to a strong modulation of the frequency. Horizontal gradients induce refraction of horizontal wave vector, thus refraction of pseudo-momentum. In comparison with the conventional columnar approach lower breaking heights and reduced mean flow accelerations above the onset of breaking are obtained. |
|