Scientific rationale

Atmospheric gravity waves (GWs) play an important role in the dynamical coupling of the atmosphere. They propagate horizontally and vertically over vast distances and transport energy and momentum. The dissipation of this momentum in the mesosphere drives the global residual circulation which connects both hemispheres and causes drastic effects on the thermal structure of the atmosphere. As internal GWs are mainly generated in the troposphere, wave generation, propagation and dissipation represent the dominant mechanism which couples the atmosphere from below to above. However, the atmosphere is also coupled from above to below by means of GWs. Since GWs modify the background flow through wave dissipation and momentum deposition, they influence the propagation of planetary scale waves, thereby affecting the circulation in the troposphere.

Understanding and quantifying these coupling processes is essential for improving both climate models and numerical weather prediction models. However, the details of the involved processes are not yet understood, and parameterizations of GWs used in models yield inadequate results so far and are subject to tuning. For example, state-of-the-art general circulation models (GCMs) show that, in particular, GW drag is missing in a latitude belt near 60°S. The missing drag contributes to the so-called cold pole bias observed in climate chemistry models. Many ideas and suggestions have been put forward to explain its existence. Proposed generation mechanisms for the missing GWs include fronts and jets in the troposphere, orography from big obstacles (mountain ranges) to small islands in the ocean, secondary wave generation by the multitude of primary waves excited in the troposphere, and stratospheric sources. The latter are least explored due to inherent difficulties in observing the dynamic state of the atmosphere at stratospheric altitudes. Satellite-based instruments have a comparatively low spatial resolution and, most importantly, lack the necessary temporal coverage to detect and track the formation of individual gravity wave packets. On the other hand, ground-based instrumentation is non-existent at southern latitudes near 60°S and very sparse in the northern hemisphere. Only very recently have airborne high-resolution remote sensing instruments for the middle atmosphere become available. During the DEEPWAVE field campaign, stratospheric and mesospheric GWs were observed over and in the vicinity of New Zealand. Only one single flight at high latitudes (to 63°S) was undertaken during DEEPWAVE. More observations of gravity waves in the middle atmosphere at high latitudes are needed in order to solve the puzzle of the missing GW drag which is an important step forward in improving GCMs.