Abstract
Wind and temperature observations in the mesosphere and lower thermosphere (MLT) from the Upper Atmosphere Research Satellite (UARS) reveal strong seasonal variations of tides, a dominant component of the MLT dynamics. Simulations with the Spectral mesosphere/lower thermosphere model (SMLTM) for equinox and solstice conditions are presented and compared with the observations. The diurnal tide is generated by forcing specified at the model lower boundary and by in situ absorption of solar radiation. The model incorporates realistic parameterizations of physical processes including various dissipation processes important for propagation of tidal waves in the MLT. A discrete multi-component gravity-wave parameterization has been modified to account for seasonal variations of the background temperature. Eddy diffusion is calculated depending on the gravity-wave energy deposition rate and stability of the background flow. It is shown that seasonal variations of the diurnal-tide amplitudes are consistent with observed variations of gravity-wave sources in the lower atmosphere.
Highlights
Atmospheric solar tides are global-scale waves permanently forced by thermal excitation due to absorption of solar radiation at various levels in the atmosphere
Recent global observations of mesosphere and lower thermosphere (MLT) winds by the High-resolution Doppler imager (HRDI) and Wind-imaging interferometer (WINDII) from the Upper Atmosphere Research Satellite (UARS) (e.g., Hays et al, 1994; McLandress et al, 1994) con®rm the dominant role of tides
The spectrum was tuned to the CIRA-86 empirical zonal mean climatology (Fleming et al, 1990) using a procedure similar to that described by Akmaev (1997)
Summary
Atmospheric solar tides are global-scale waves permanently forced by thermal excitation due to absorption of solar radiation at various levels in the atmosphere. The observed seasonal variations of tidal amplitudes cannot be fully explained by seasonal variations of tidal forcing in the lower atmosphere (Groves, 1982a, b), of the background zonal mean circulation, or of the molecular diusivity and viscosity (e.g., Burrage et al, 1995; Hagan et al, 1995; Hagan, 1996) This suggests that seasonal variations of dissipation induced by gravity waves may play an important role (e.g., Burrage et al, 1995; Khattatov et al, 1997). Chan et al (1994) presented a middle- and upper-atmospheric model based on a spectral formulation that diers in some details from the traditional one (Bourke et al, 1977) but still takes advantage of the readily implementable semi-implicit time-integration scheme Using this model in a twodimensional con®guration Mengel et al (1995) studied equatorial oscillations in the middle atmosphere generated by gravity waves. Further details of the model implementation can be found in the papers by Akmaev et al (1992) and Akmaev (1994)
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