Gravity wave - tidal and gravity wave - planetary wave interactions in the mesosphere and lower thermosphere

This dissertation is motivated by two observational results: (1) Oscillations in the thermosphere and ionosphere (i.e. {dollar}>{dollar}100 km altitude) at periods close to those associated with normal modes (2-days, 5-days, 10-days, 16-days) in the lower atmosphere. (2) The semiannual variation of the diurnal propagating tide in the mesosphere and lower thermosphere (MLT) region (ca. 80-150 km). Gravity waves play a major role in determining the mean flow in the MLT. It is shown that the interaction between internal gravity waves and planetary waves and tides represents a plausible mechanism for explanation of both of these phenomena. A 2-D linear steady-state tidal model is used in conjunction with hybrid gravity wave parameterization to examine the role of gravity wave interactions.; Diurnal tidal analyses indicates that annual variations of the background atmosphere result in annual variations in the eddy diffusion and the diurnal harmonic of the momentum flux divergence produced by breaking gravity waves. The seasonal variability in these two mechanisms can explain the observed semi-annual variability in the diurnal tide: amplitude minima at solstice and maxima at equinox. The relative roles of wave stress and diffusion are examined and it is shown that both play important roles in tidal modification by damping its amplitude. The effects of varying inputs to the gravity wave parameterization are explored.; Numerical simulations involving the quasi 2-day wave indicate that inclusion of the wave stress produced by breaking gravity waves increases the magnitude of the wave in the mesosphere. In addition the periodic nature of the resulting wave stress is shown to produce a separate secondary amplitude peak in the lower thermosphere near 110 km. Horizontal winds associated with the modified 2-day wave extend well into the thermosphere. The phase structure of the modified wave is consistent with the concept of gravity wave filtering by a longitudinally varying planetary wave, with an anti-correlation between the phase of the mesospheric and thermospheric amplitude peaks. The strength and vertical extent of the thermospheric wave signature is found to depend primarily on the strength of the meridional gravity wave source.