Abstract
Abstract. 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.
Highlights
The mesosphere and lower thermosphere (MLT) dynamics are strongly influenced by atmospheric waves, including the solar tides with periods of a solar day and its harmonics
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
Bispectral analysis of observed MLT winds at Collm indicate that non-linear interaction of tides may play a role in forcing the quarterdiurnal tide (QDT)
Summary
The mesosphere and lower thermosphere (MLT) dynamics are strongly influenced by atmospheric waves, including the solar tides with periods of a solar day and its harmonics. Liu et al (2006) noted a 6 h signature in medium frequency radar data over Wuhan, China, but mainly in their upper height gates above 90 km They found from bispectral analyses that there are indications for non-linear interaction of tides as a possible forcing mechanism, but only in the upper height gates. In a further study, Liu et al (2015), again using TIMED/SABER data, analyzed the migrating QDT between 50◦ S and 50◦ N in the middle atmosphere From their analyses they considered both direct heating and tidal interaction as possible sources of the QDT. We use a mechanistic numerical model and analyze the most likely forcing mechanism for the QDT through removing either solar heating or non-linear interaction for the model
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