Scientific rationale

Atmospheric gravity waves 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 has drastic effects on the thermal structure of the atmosphere. As internal gravity waves 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 gravity waves. Since gravity waves 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 gravity waves used in models yield inadequate results so far and are subject to tuning. For example, state-of-the-art general circulation models show that, in particular, gravity wave 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 gravity waves 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 sparse in the northern hemisphere. Only in the last years airborne high-resolution remote sensing instruments for the middle atmosphere have become available. During the DEEPWAVE field campaign, stratospheric and mesospheric gravity waves were observed over and in the vicinity of New Zealand. Only one flight to high latitudes (to 63°S) was undertaken during DEEPWAVE. The focus of the SouthTRAC campaign in 2019, which was based from Rio Grande in in the southern part of Argentina, was the southern polar vortex at around 60°S, which was unfortunately decreasing in strength in the latter part of the campaign period. Thus, only during few flights gravity waves were observed to reach stratospheric and lower mesospheric altitudes. Recent observations with a ground-based lidar from South Pole indicate the presence of inertia-gravity waves at the center of the polar vortex, which are generated by a stratospheric source not yet understood.

It is hypothesized that an unexplored stratospheric gravity wave source exists at locations of the polar night jet, which are characterized by significant deviations from a balanced state. In addition, the polar night jet acts as a waveguide for gravity waves: orographic and non-orographic gravity waves are refracted into the polar night jet and are able to propagate over vast distances within the jet. These refracted gravity waves interact with locally generated gravity waves, and increase the gravity wave drag.

More observations of gravity waves in the middle atmosphere at high latitudes are needed in order to answer open questions like: Which local conditions relative to the polar night jet are necessary to effectively excite gravity waves? To what extent do theoretical and numerical models predict potential source regions in terms of horizontal and vertical extent, direction of gravity wave emission, temporal coherence, spectral distribution and wave capturing? Which horizontal dimensions do wave packets that are focused into the polar night jet, achieve? What is the intermittency of gravity wave packets in space and time of and what is their relative contribution to momentum deposition in the mesosphere?