The middle atmosphere is the altitude region from the tropopause (10 -18 km) to the mesopause (80 – 100 km). This height region is closely related to processes in the troposphere by dynamical and chemical coupling processes. The dynamical coupling is provided by planetary and internal gravity waves. On the other hand, the impact of photochemically active trace gases can lead to a dynamical coupling by means of diabatic processes.
Internal gravity waves are generated by various atmospheric processes such as flow over topography, convection, and instabilities in upper-level fronts. Internal gravity waves propagate vertically and horizontally over vast distances and they transport energy and momentum. The dissipation of this momentum in the middle atmosphere is a key process driving the global circulation (Brewer-Dobson circulation), which in turn causes drastic effects on the thermal structure. At high latitudes, the temperature of the summer mesopause deviates up to 100K from the radiatively determined equilibrium temperature. Gravity waves in connection with planetary waves also play a key role in large scale circulation changes in the stratosphere such as the quasi-biennial oscillation and sudden stratospheric warming. Latter phenomenon is also accompanied by a strong cooling in the mesosphere and affects, thus, the entire middle atmosphere.
Since gravity waves are mainly generated in the troposphere, the cycle of wave generation, wave propagation and dissipation of waves represents the dominant mechanism coupling the atmosphere from below to above. However, gravity waves also indirectly couple the atmosphere from above to below. As internal gravity waves modify the mean wind through wave dissipation, they change the propagation of planetary waves, thereby affecting the circulation in the troposphere. This two-way coupling is an important finding of recent years. It initiated efforts to extend vertical domains in NWP and Global Circulation Models (GCMs) up to 0.01 hPa and higher to account for these dynamical coupling processes; i.e the UK MetOffice plans to raise the model top up to 400 km altitude.
Although the dynamical linkage has been qualitatively established in the recent ten years, there are still numerous open questions related to the problem of how to consider/represent these coupling processes in current NWP models and GCMs. In the coming years, the following research questions shall be answered:
- Which processes characterize and drive the life cycle (generation, propagation, and dissipation) of internal gravity waves in the troposphere, stratosphere and mesosphere?
- How large are the effects of these processes in the troposphere and in the middle atmosphere?
- Which role does the recovering ozone layer play in the coupling between troposphere and middle atmosphere?
Currently, most observations of the middle atmosphere are carried out by ground–based radars and LIDARs and by instruments carried on sounding rockets. These observations all have in common that they are tied to specific geographical locations. HALO as a long-range and highaltitude platform offers the unique opportunity to observe the atmosphere along extended flight legs in regions where gravity waves actually propagate into the middle atmosphere. Moreover, HALO opens up the possibility to directly sample hotspots of gravity wave generation which are, due to their geographic location, not at all or with great hindrance accessible by ground–based instruments.