Solar semidiurnal tide in the dusty atmosphere of Mars

Vertical coupling due to the solar semidiurnal tide in Mars's atmosphere, and effects on zonal mean temperature and wind structures, are investigated using a numerical model. The model provides self-consistent solutions to the coupled zonal mean and tidal equations from the surface to 250 km. Breaking (convective instability) of the semidiurnal tide is parameterized using a linear saturation scheme with associated eddy diffusivities. Thermal forcing in the model gives rise to surface pressure perturbations and middle-atmosphere zonal mean winds and temperatures that are consistent with available measurements and general circulation models. Results presented here primarily focus on globally elevated dust levels during Southern Hemisphere summer, conditions similar to those experienced by the Viking 1 and Viking 2 landers during the 1977 global dust storms. Semidiurnal temperature and wind amplitudes maximize in the winter hemisphere and exceed 50 K and 100 m s(-1) above 150 km and are typically 10-20 K and 10-20 m s(-1) at 50 km. Perturbation densities are of order 50%-70% between 90 and 150 km, and thus contribute significantly to variability of the aerobraking regime in Mars's atmosphere. Eddy diffusivities associated with the breaking parameterization reach values of order 10(3)-10(4) m(2) s(-1) between 100 and 150 km, and can be of order 1-10 m(2) s(-1) between 0 and 50 km. Dissipation of the semidiurnal tide induces zonal mean westward winds of order 10-30 m s(-1) below 100 km, and in excess of 200 m s(-1) above 150 km. The corresponding temperature perturbations range between -20 and -70 K over most of the thermosphere, with 10-20-K increases in temperature at high winter latitudes between 50 and 100 km. All of the wave and zonal mean perturbations noted above represent very significant modifications to the thermal and dynamical structure of Mars's atmosphere. Estimates are also provided for the eastward-propagating diurnal tides with zonal wavenumbers s = -1 and s = -2. These waves also have long vertical wavelengths and hence are capable of effectively coupling the lower and upper atmospheres of Mars. However, the perturbation and zonal mean effects of these waves are a factor of 2 or more smaller than those cited above for the semidiurnal tide under dusty conditions.