| A quantitative understanding of the middle atmospheric dynamics is crucial in evaluating the impact of human activities on global climate change and to comprehend the effects of natural disturbances. Two wave perturbations, the diurnal tide and the quasi 2-day wave, are the most important dynamical disturbances in the upper mesosphere and lower thermosphere. The primary objective of this research is to analyze extensively the dynamical structure and long-term behavior of these two waves and to investigate the mechanisms governing their generation, development and propagation. This goal is accomplished by comparing the HRDI observations to the results from the 3-dimensional National Center for Atmospheric Research (NCAR) Thermosphere-Ionosphere-Mesosphere-Electrodynamics general circulation model (TIMEGCM) (30-500km) and some 2-dimensional models.; The solar migrating diurnal tide is a global oscillation excited by tropospheric water vapor, latent heat release and stratospheric ozone. It propagates from the source region to the upper atmosphere, where long-term global dynamical observations are available from HRDI on the Upper Atmosphere Research Satellite (UARS). The measurements in the present study show that the tidal amplitude grows exponentially with height up to 100km, with a mean vertical wavelength of 25km, and decays quickly above that altitude. The vertical wavelength decreases with height to a value of 23km in the mesopause region, which possibly results from the interaction between the tide and gravity waves. The tidal structures are seasonally-dependent and appear more symmetric about the equator at equinox than at solstice. A clear semiannual variation is observed in the amplitudes of the diurnal tide at all altitudes between 50 and 115km. The agreement with TIMEGCM simulations for an equinox condition is excellent. The model simulations also show that the gravity wave forcings can interact strongly with the tide and induce other wave modes in the mesopause region.; The two-day wave is observed as a transient disturbance in the mesosphere and lower thermosphere. The HRDI winds show that the strongest event occurs in January with an amplitude of 60ms{dollar}sp{lcub}-1{rcub}{dollar} in the meridional component and two other weak events occur during September-October and in July-August. The January two-day wave begins with a small disturbance at southern hemisphere mid-latitudes and advances northward changing to a larger disturbance centered at the equator. The dynamical structures during the maximum amplitudes are consistent with the normal mode theory. However, the small perturbation in the southern hemisphere is possibly related to the baroclinic instability above the stratospheric easterly jet. The HRDI observations reveal that there exists a large vertical shear in the mean zonal wind in the southern hemisphere during January, which is a necessary condition for baroclinic instability to occur. |
