Scientific Background

The knowledge of the upper middle atmosphere dynamics is necessary for a number of scientific applications concerning the understanding of global climate and the detection and forecast of its long-term variability, especially with respect to anthropogenically induced climate changes. In this connection, the gross behaviour of mean winds, tides and planetary waves in the upper mesosphere and lower thermosphere is more or less well known from co-ordinated campaigns (e.g. Portnyagin et al. 1994), combined analyses of quasi-routine measurements to obtain hemispheric or even global climatologies, and, more recently, satellite observations (Burrage et al., 1995). Additionally, model simulations (e.g. Forbes and Vial, 1989) provide theoretical climatologies of the mesopause region wind parameters that fit experimental results well (Manson et al., 1989). However, there are still uncertainties about the interannual variability of dynamical properties. Especially the effects of non-linear interaction of mean flow, tides and planetary waves is still not known in much detail, mostly because of the rarity of sufficiently long time series of meteorological data in the mesopause region, in contrast to investigations concerning the stratosphere, where global fields of satellite data are available (e.g. project INTAS-93-2923)

It has been shown from theoretical investigations (e.g. Teitelbaum and Vial, 1991) that the tidal amplitude is modulated by planetary waves by non-linear interaction, and there are some observations that show oscillations of the semidiurnal tide with the quasi 2-day wave period (Sidorov and Fakhrutdinova, 1981; Huuskonen et al., 1991; Mitchell et al., 1996). These, however, refer to selected cases and do not yet deliver detailed information about the extent of the wave-tidal interaction in the course of the year - which would provide additional information about the relation to the mean circulation - and about the interannual variability of the tidal modulation. Only systematic long-term investigations will provide information about the actual degree and importance of the wave-tidal interaction within the mesopause dynamics and will show whether the selected examples reported in literature really can be considered as regular patterns.

Additionally, results from measurements alone generally only provide some additional support to the theory of the non-linear interaction, this means that indeed variability of the tidal amplitude in the period range of planetary waves is found, but from the measurements it cannot be clarified whether these are really due to wave-tidal interaction. However, long-term studies of planetary wave activity and tidal modulation, as they are planned within the project - are able to deliver climatologies of both planetary and tidal waves and thus experimental results for input as well as output of numerical theoretical models. This will enable us to use these climatologies as a very good proof for theoretical considerations.

Both tidal and planetary wave activity are subject to long-term changes (Lastovicka et al., 1994; Bremer et al., 1997; Jacobi et al., 1997), and it is thinkable to interprete these changes as the reaction of the mesopause region to the variability of the atmosphere below, so that changes of the upper middle atmosphere would serve as a signal for climate changes of the atmosphere below. However, this interpretation requires the full understanding of the coupling processes and interaction mechanisms. This can be provided by a combined theoretical and experimental project that takes into account especially the long-term variability of the semidiurnal tidal amplitude modulation by planetary waves.

References:

Bremer, J., R. Schminder, K.M. Greisiger, P. Hoffmann, D. Kürschner, and W. Singer: Solar cycle dependence and long-term trends in the wind field of the mesosphere/lower thermosphere. J. atmos. solar-terr. Phys. 59, 497 - 509, 1997.

Burrage, M.D., D.L. Wu, W.R. Skinner, D.A. Ortland, and P.B. Hays: Latitude and seasonal dependence of the semidiurnal tide observed by the high-resolution Doppler imager. J. Geophys. Res. 100, 11313 - 11321, 1995.

Forbes, J.M., and F. Vial: Monthly simulations of the solar semidiurnal tide in the mesosphere and lower thermosphere. J. atmos. terr. Phys. 51, 649 - 661, 1989.

Huuskonen, A., T.S. Virdi, G.O.L. Jones, and P.J.S. Williams: Observations of the day-to-day variability in the meridional semi-diurnal tide at 70°N. Ann. Geophysicae 9, 407 - 415, 1991.

Jacobi, Ch., R. Schminder, and D. Kürschner: The quasi two-day wave as seen from D1 LF wind measurements over Central Europe (52°N, 15°E) at Collm. J. atmos. solar-terr. Phys., in press.

Lastovicka, J., V. Fiser, and D. Pancheva: Long-term trends in planetary wave activity (2-15 days) at 80-100 km inferred from radio wave absorption. J. atmos. terr. Phys. 56, 893 - 899, 1994.

Manson, A.H., C.E. Meek, H. Teitelbaum, F. Vial, R. Schminder, D. Kürschner, M.J. Smith, G.J. Fraser and R.R. Clark: Climatologies of semi-diurnal and diurnal tides in the middle atmosphere (70 - 110 km) at middle latitudes (40 - 55°). J. Atmos. Terr. Phys. 51, 579 - 593, 1989.

Mitchell, N.J., P.J.S. Williams, A.G. Beard, G.R. Buesnel, and H.G. Muller: Non-linear planetary/tidal wave interactions in the lower thermosphere observed by meteor radar. Ann. Geophysicae 14, 364 - 366, 1996.

Portnyagin, Yu.I., N.A. Makarov, R.P. Chebotarev, A.M. Nikonov, E.S. Kazimirovsky, V.D. Kokourov, V.V. Sidorov, A.N. Fakhrutdinova, G. Cevolani, R.R. Clark, D. Kürschner, R. Schminder, A.H. Manson, C.E. Meek, H.G. Muller, J. Stoddart, W. Singer, and P. Hoffmann: The wind regime of the mesosphere and lower thermosphere during the DYANA campaign. II. Semidiurnal tide. J. atmos. terr. Phys. 56, 1731 - 1752, 1994.

Sidorov, V.V., and A.N. Fakhrutdinova: Quasi two day periods of the atmosphere wind velocity changes for meteor heights. Meteornye issledovanija. N7, Moscow, Radio i svynaz, 83 - 89, 1981.

Teitelbaum, H., and F. Vial: On tidal variability induced by nonlinear interaction with planetary waves. J. geophys. Res. 96, 14,169 - 14,178, 1991.

last change: 9.9.1999