Analysis of the forcing mechanism of the terdiurnal solar tide in the middle atmosphere
Final Report Abstract
This project allowed to extensively study the excitation mechanisms of the TDT using a numerical circulation model. Besides the widely known direct solar forcing, also secondary mechanisms such as nonlinear tidal interactions and GW-tide interactions have been taken into account. The latter ones have only been rarely analyzed before. For the rst time, the local forcing regions of the secondary mechanisms have been identified. Solar heating contributes to TDT forcing mainly in the troposphere and stratosphere, nonlinear interactions of tides contribute in the mesosphere, while GW-tide interaction contributes to TDT forcing in the mesosphere/lower thermosphere region. The contribution of the different forcing terms to the total TDT amplitude was determined by removing and enhancing the wavenumber 3 excitation mechanism. In contrast to the proposal, we did not implement the removal/enhancement of the 8 h-component since this method turned out to result in a numerically expensive algorithm. However, this analysis is not necessary when only considering migrating tides in the model. Although secondary mechanisms turned out to be much smaller than the primary one on a global average, they were detected to significantly contribute to the total TDT amplitude in certain regions and seasons. It was observed, that removing one of the secondary mechanisms may enhance the TDT rather than reduce it. This unexpected feature was investigated in more detail and destructive interferences between the tides of different origins were found. Furthermore, the wavenumber 3 component of GW-tide interactions was found to strongly influence the neutral zonal mean dynamics of the lower thermosphere. This underlines the importance of tides and their excitation mechanisms for the middle and upper atmosphere. In this project, we mainly focused on numerical simulations. The analysis of satellite data was not included. The climatology of TDTs from radar data could be taken from earlier publications, while possible nonlinear interactions have not been analysed. However, the numerical simulations suggest that the origin of nonlinear tidal interactions is usually located at lower altitudes than the MLT region and therefore, a local analysis of nonlinear interaction processes would lead to doubtful results.
Publications
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(2019) Nonlinear forcing mechanisms of the migrating terdiurnal solar tide and their impact on the zonal mean circulation. Ann. Geophys. (Annales Geophysicae) 37 (5) 943–953
Lilienthal, Friederike; Jacobi, Christoph
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(2015). Long-term variability of mid-latitude mesosphere-lower thermosphere winds over Collm (51° N, 13° E). J. Atmos. Sol.-Terr. Phys., 136, 174-186
Jacobi, C., F. Lilienthal, C. Geiÿler, and A. Krug
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(2015). The terdiurnal tide in the MUAM circulation model. Rep. Inst. Meteorol. Univ. Leipzig, 53, 33-44
Krug, A., F. Lilienthal, and C. Jacobi
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(2016). Influence of the spatial distribution of gravity wave activity on the middle atmospheric dynamics. Atmos. Chem. Phys., 16 (24), 15755-15775
Šácha, P., F. Lilienthal, C. Jacobi, and P. Pišoft
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(2016). The Role of Solar Heating in the Forcing of Terdiurnal Tides. Rep. Inst. Meteorol. Univ. Leipzig, 54, 57-66
Lilienthal, F. and C. Jacobi
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(2018). Forcing mechanisms of the terdiurnal tide. Atmos. Chem. Phys., 18 (21), 15725-15742
Lilienthal, F., C. Jacobi, and C. Geißler
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(2019) Tidal wind shear observed by meteor radar and comparison with sporadic E occurrence rates based on GPS radio occultation observations. In: Adv. Radio Sci. 17 213–224
Jacobi, Christoph; Arras, Christina
