GRAVITY

The Interaction between Gravity Waves and Solar Tides in the Middle Atmosphere

Prof. Ulrich Achatz and Dr. Bruno Ribstein
Goethe-Universität Frankfurt
Institut für Atmosphäre und Umwelt
Frankfurt am Main

Gravity waves (GWs) and solar tides (STs) are main constituents of the dynamical coupling between the atmospheric layers. To the largest part they are generated in the lower atmosphere. STs are large-scale waves that modulate all dynamical fields in the mesosphere and thus leave a strong imprint on the dynamics of the latter. Predominantly via momentum deposition GWs to a large extent control the mesospheric mean circulation. GWs and STs interact strongly with each other: STs modulate the propagation of GWs, while the momentum deposition by breaking GWs influences the amplitude and phase structure of STs in the mesosphere. So far general circulation models (GCMs) describe this interaction process with insufficient accuracy. Conventional GW parameterizations (GWP), although indispensable for capturing the effect of mesoscale GWs, neglect both the impact of the transience of the ST fields and that of horizontal gradients in the large-scale wind fields in the atmosphere. As shown recently with a newly developed ray-tracing code (Senf and Achatz 2011) this leads to significant errors in the estimated GW impact on STs (Fig. 1). The first part of the project is to investigate the interactive development of GWs and STs under correct consideration of the processes mentioned above. For this, it is intended to couple the ray tracer of Senf and Achatz (2011) with a linear tidal model (Achatz et al. 2008). The second part is to focus on the impact of the diurnal cycle in the convective GW generation on STs. Since the effect of meridional GW propagation, neglected in conventional GWPs, can be considerable this could allow for an impact on extratropical STs. Differences between present modeling results on extratropical STs and corresponding observations could thus possibly be explained. Since present-day GWPs are tuned strongly to past and present climate, the project could give important contributions to better constraining these tunings and thereby increase confidence in the performance of GWPs in a variable climate.