The energy budget of the middle atmosphere, and especially the Mesopause region, is characterized by a complex interaction of a variety of fundamental physical and chemical processes, such as radiation, photochemistry and dynamic processes.

Gravity waves are excited in the troposphere by a variety of mechanisms (such as geography or convection) and then propagate upwards. Some of these waves grow unstable in the altitude range from 70-110 km where they break and their corresponding kinetic and potential energy is ultimately converted into heat.

The dynamic processes (summarized under the term "wave-mean flow interaction" (WMI)) drive the polar summer mesopause region about 80 K away from radiative equilibrium temperature and turbulent heating rates in the polar summer mesopause region are somewhere in the range of 10 K/day which is on the same order of magnitude as the direct solar radiation energy input. Despite this strong local heating by turbulence, the polar summer mesopause region is the coldest place in Earth's atmosphere with temperatures in the range of <130 K. This happens because the dissipating waves not only lead to a local heating effect but they also transfer their (pseudo)-momentum to the mean flow.

In the northern polar summer, this leads to an eastward acceleration of the mean flow resulting in turn in a meridional flow from the summer- to the winter pole. Mass continuity in turn requires the air masses to rise and expand (and cool adiabatically) over the summer pole and subside (and heat adiabatically) over the winter pole, thereby leading to the cold polar summer and warm polar winter mesopause.

The dramatic effect of this process is illustrated in figure 1. The colored contour lines represent the actual observed temperature structure, where dark blue corresponds to minimum temperatures of 160 K with a contour interval of 10 K. On the left, the white contour lines represent hypothetical radiative equilibrium temperatures. On the right side, the white contour lines show the mass stream function of the wave-driven residual circulation which forms a global circulation cell from the polar summer to the polar winter as described above.

While this qualitative picture appears to be largely understood and has indeed been tested by a variety of direct observations, there are many quantitative details that are still not properly characterized. Prominent examples are the global distribution of gravity wave sources as well as the details of the instability mechanisms which ultimately lead to the wave breakdown and its corresponding momentum and energy deposition in MLT. The latter process determines the overall thermal and dynamical structure of the MLT and is hence is singularly decisive for the structure of the global circulation.


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