SMLTM simulations of the diurnal tide: comparison with UARS observations

Abstract

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 speci®ed 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 modi®ed to account for seasonal variations of the background temperature. Eddy di usion is calculated depending on the gravitywave energy deposition rate and stability of the background¯ow. It is shown that seasonal variations of the diurnal-tide amplitudes are consistent with observed variations of gravity-wave sources in the lower atmosphere.

Simulation of large‐scale dynamics in the mesosphere and lower thermosphere with the Doppler‐spread parameterization of gravity waves: 2. Eddy mixing and the diurnal tide

“…This behavior is to be expected for a migrating tide that may locally be considered as an "equivalent gravity wave" [e.g., Lindzen, 1970] It should be noted that in other implementations of the Lindzen-type parameterizations, no substantial damping of the diurnal tide has apparently been detected [Akmaev et al, 1996[Akmaev et al, , 1997McLandress, 1998;Norton and Thuburn, 1999]. It appears that the tidal damping due to GW momentum deposition obtained by Miyahara and Forbes [1991] may be an artifact of an incorrect implementation of the Lindzen GW scheme (see appendix).…”

“…The model has been integrated for 30 days in a full three-dimensional mode with tidal forcing for perpetual January and April conditions. As in the previous SMLTM tidal studies [Akmaev et al, 1996[Akmaev et al, , 1997, the lower-atmospheric tidal forcing has been imposed at the model's lower boundary (100 hPa or approximately 16 kin). It has been found sufficient to only specify the tidal variations of geopotential [cf.…”

“…Numerically, this means that the eddy mixing coefiqcient may abruptly drop to 0 from relatively large values that it typically attains in the lower thermosphere. One can argue [Akmaev et al, 1997] that as molecular viscosity increases with height, the inertial range of turbulence that is primarily responsible for eddy mixing in the form (1) does not have to collapse abruptly. As the Kolmogorov [1941] April.…”

“…(2) where K is calculated by (1), u is molecular viscosity, and/• is a numerical coefficient presumably of the order of 1. As in the study by Akmaev et al [1997],/• = I has been chosen in the present simulations. Of course, the molecular dissipation processes themselves (viscosity and thermal conductivity) have been fully retained in the model.…”

“…This paper presents SMLTM simulations of the diurnal tide using the DSP by Hines [1997a, b]. A procedure for calculation of the eddy diffusion coefiqcient is outlined that essentially follows the recommendations by Hines [1997a, b] with some modification in the thermosphere [Akmaev et al, 1997] to account for a gradual cessation of turbulence due to enhanced molecular viscous dissipation. The diurnal tide is found to be sensitive to specification of eddy mixing in the MLT.…”