Turbulence in the mesosphere
Turbulence measurements at 69°N during MIDAS/MacWave 2002
In summer 2002 an international research campaign was conducted from the Andoya Rocket Range during which coordinated detailed measurements with sounding rockets and ground based experiments were performed. Besides the majority of ground based measurements, IAP was responsible for three major sounding rocket payloads for the measurement of turbulent energy dissipation rates (see figure below). These observations showed that for the first time in about a decade of mesospheric turbulence observations with sounding rockets in the polar summer, turbulence was observed below an altitude of about 82 km. At the same time, complementary measurements of mesospheric winds, temperatures, and polar mesosphere summer echoes all revealed a distinctly disturbed dynamical and thermal state of the middle atmosphere as compared to previous years.
In the right Figure we have attempted to summarize the different observed properties of the middle atmosphere and its underlying physical mechanism in one simple schematic. Assuming an atmosphere in radiative equilibrium, we would expect a westward oriented monotonously increasing zonal wind in the polar summer middle atmosphere. In reality, however, the zonal wind is accelerated to the East above an altitude of ~ 80 km due to the momentum deposition of breaking gravity waves. This wave mean flow interaction ultimately results in the observed wind reversal at altitudes between 80 and 90 km. If, however, the breaking level of the gravity waves shifts to lower altitudes (and hence the altitude region of turbulence production), the corresponding momentum deposition shifts to lower altitudes, and hence the wind reversal is expected to be found well below its typical height. All these expectations could be verified in the frame of the MIDAS/MacWave campaign including its implications for the meridional wind, the temperature profile, and the properties of polar mesosphere summer echoes. Hence, the MIDAS/MacWave observations turned out to be a perfect test case for our understanding of mesospheric dynamics.