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The seasonal and interannual variability of the quarterdiurnal tide is analysed using meteor radar wind observations at the two midlatitude sites Collm and Obninsk. Generally tidal amplitudes increase with height. Maximum tidal amplitudes are found in winter. Meridional amplitudes are smaller than zonal ones on an average. Phases mainly differ between summer and winter. Zonal and meridional phases differ by slightly less than 90°. The vertical wavelengths are very long in winter, but shorter and on the order of 20 km in summer. Collm and Obninsk amplitudes and phases agree well, indicating that the migrating quarterdiurnal tide may be responsible for a major part of the observed waves. Observations since 1980 show that the tidal amplitudes have increased on a whole, although the increase is not linear but mainly happening during the late 1990s and the early 2000s.

Solar tides such as the diurnal and semidiurnal tide, are forced in the lower and middle atmosphere through the diurnal cycle of solar radiation absorption. This is also the case with higher harmonics like the quarterdiurnal tide (QDT), but for these also non-linear interaction of tides such as the self-interaction of the semidiurnal tide, or the interaction of terdiurnal and diurnal tides, are discussed as possible forcing mechanism. To shed more light on the sources of the QDT, 12 years of meteor radar data at Collm (51.3∘ N, 13∘ E) have been analyzed with respect to the seasonal variability of the QDT at 82–97 km altitude, and bispectral analysis has been applied. The results indicate that non-linear interaction, in particular self-interaction of the semidiurnal tide probably plays an important role in winter, but to a lesser degree in summer. Numerical modelling of 6 h amplitudes qualitatively reproduces the gross seasonal structure of the observed 6 h wave at Collm. Model experiments with removed tidal forcing mechanisms lead to the conclusion that, although non-linear tidal interaction is one source of the QDT, the major forcing mechanism is direct solar forcing of the 6 h tidal components.

Radar observations of mesosphere/lower thermosphere (MLT) winds at middle latitudes obtained from meteor radar 2004–date and the low-frequency drift method 1979–2008 have been combined to obtain a homogeneous long-term data set of MLT dynamics at 90 km altitude. The data are analyzed to obtain tendencies of prevailing winds during the last three decades. On a seasonal time scale, the zonal winds show positive trends towards more westerly winds throughout the year, while the meridional wind trends are negative in autumn/winter and positive in spring/summer. There is a tendency that the trends of both zonal and meridional summer prevailing winds are changing after 1995. Using a numerical circulation model with realistic ozone and carbon dioxide fields and with tropospheric/lower stratospheric circulation taken from ERA reanalyses, the observed trend changes can be reproduced qualitatively. The MLT wind trends are, according to the model, mainly due to the influence of changes in tropospheric and lower stratospheric dynamics.