Fully guided gravity waves at short periods and gravity wave-critical layer interaction

Gravity waves play a dominant role in establishing the thermal, wind, and constituent structure throughout the middle atmosphere and are especially important in the mesopause region where wave amplitudes are very large. Studies of gravity waves are now receiving increasing attention because of their effects on the global change problems. The Airborne Lidar and Observations of Hawaiian Airglow (ALOHA-93) Campaign was conducted in 1993 to explore the nature of gravity wave activity. Among many observations, the observation of sharp wave front, known as "wall event", on October 10, 1993 and the sudden temperature rise in the mesosphere on October 21, 1993 are most exciting. While some of the initial scientific results were reported in several journal publications, most of the important scientific issues remain open and they are the subjects of this dissertation.; In chapter 2 of this dissertation, an analytical model for gravity waves at short-periods is developed and application to ALOHA-93 data is considered. Gravity waves at short periods ({dollar}<{dollar}10 minutes) are known to be fully guided by the atmospheric thermal structure. The guiding of gravity waves, the interference between waves in adjacent duct and the kissing modes can be more clearly demonstrated and understood in an analytic model. We have developed an analytic model by approximating the Brunt period curve with two rectangular ducts located at the stratosphere and the mesosphere. The analytical model shows how the gravity waves guided by the stratospheric duct interact with the gravity waves guided by the mesospheric duct. It also explains the kissing mechanism occurring when two modal solutions have the same wave number or horizontal phase speed and the same period. An application of our model to the wave in the wall event observed on October 10 during ALOHA-93 and, in particular, to the explanation of the phase reversal of OH and OI green line emission is presented. It is found that the wall event could be explained by a fully-guided gravity wave in the mesospheric duct. By combining the analytical model with the theory of linear response of the density perturbations of minor species under the assumption of chemical equilibrium, the phase reversal of OH and OI green line emissions could also be explained.; In chapter 3 a numerical model for dissipating gravity waves in a critical layer is developed and a simulation is performed to explain the sudden temperature rise observed ALOHA-93 campaign. The temperature change took place in a very narrow vertical region (3-4 km) at about 87 km from the ground. Simultaneously observed radar wind profiles and mesospheric airglow wave structures showing a horizontal phase speed of 35m/s and a period of about half an hour strongly suggest that a critical level may occur in the proximity of that altitude and that the energy dissipation due to the interaction of the gravity wave with the critical level causes the temperature rise. The numerical model makes use of the observed wind profile to solve the gravity wave-mean flow equations with a damping term simulating the energy dissipation from the interaction of the gravity wave and the horizontal wind. The solutions for the temperature variations so obtained show reasonably good agreement with the observed temperature profiles.