Monthly simulations of the lunar semi-diurnal tide

Abstract

Although having a much smaller amplitude than solar tides, lunar tides are also present in the atmosphere. Lunar tides are attractive for theoretical and observational studies because their frequencies and forcing are better determined than for any other atmospheric waves. Lunar tides are generated by the lunar tidal potential which itself is very well determined. However, this potential also affects the Earth and oceans, modifying their mass distributions and elevations, and creating secondary tidal potentials as well as periodic movements of their interfaces with the atmosphere. The periodic load of ocean over the Earth crust due to the tides also induces a secondary modification of tidal potential as well as movement of the atmosphere interface. In our present work we seek to provide a comprehensive model of atmospheric M2 lunar tidal oscillations from the surface to the lower thermosphere ( c. 105 km), taking account of the above-mentioned effects i.e. Earth, ocean and load tides. This study is motivated by two facts. First, we now have good determinations of ocean tides using satellite altimetry whereas previous studies of lunar tides were based only on numerical simulations of these ocean tides. Second, some reliable analyses of lunar tides in the lower thermosphere are now available as radars are now operating over a sufficiently long period. Their number should increase in the near future leading to a need for a theoretical study of lunar tides. In this paper, we develop a numerical model and analyse the effects of the different primary and secondary forcings on the lunar tide. Monthly simulations are discussed, and some comparisons with available data (at ground level as well as in the lower thermosphere) are presented.