The general field of research of the department ‘Theory and Modelling’ is the coupling of the tropo-, strato-, and mesosphere. A specific goal is to explore the role of the upper mesosphere/lower thermosphere for the whole atmosphere and the general circulation. Presently the department concentrates on the excitation, propagation and breakdown of internal gravity waves and the associated turbulence. Other fields of research comprise the dynamics of Rossby waves, atmospheric thermal tides, and racer transport, as well as macroturbulence and the interaction of radiative transfer with gravity graves.
The vertical coupling of the atmosphere is dynamically controlled by atmospheric waves, which are found on vastly differing temporal and spatial scales. The most important wave types are Rossby waves, internal gravity waves, tides and equatorial waves. Generally, these waves originate in the troposphere, propagate vertically and dissipate at higher altitudes, giving rise to wave-mean flow interaction and zonal acceleration or deceleration. By this mechanism, atmospheric waves drive the residual circulation which accounts for the strong departures from radiative equilibrium seen in the middle atmosphere. In particular, gravity waves drive a summer-to-winter-pole circulation in the upper mesosphere/lower thermosphere, thus suggesting a strong dependence of the mesopause region on tropospheric variability. Planetary Rossby waves drive the residual circulation from low to high latitudes in the winter stratosphere, giving rise to phenomena like stratospheric warmings or the arctic oscillation. The residual circulation also determines the transport of chemically and radiatively active trace species, inducing vertical coupling via chemistry and radiative transfer.
Internal gravity waves are characterized by short-period oscillations in the temperature and wind fields. Breaking gravity waves generate turbulence. This process can be described using linear stability analysis (optimal perturbations, normal modes) and direct numerical simulations. Due to the interaction between gravity waves and the mean circulation the upper mesosphere becomes baroclinically unstable in summer. This leads to the in-situ-formation of planetary Rossby-waves. The picture on top of the text shows an explicit global simulation of both wave phenomena using the Kühlungsborn Mechanistic Circulation Model (KMCM).