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ABSTRACTThe flow of wind over mountain ranges is believed to be one of the primary generation mechanisms of atmospheric gravity waves. Mountain gravity waves contribute to the production of stratospheric turbulence, a potential cause of accidents in high-altitude aircraft flights. In an effort to advance the current capabilities of forecasting stratospheric turbulence, theoretical models have been developed for the generation and breakdown of mountain gravity waves under realistic conditions, including effects not captured by existing forecasting tools. Specifically, short-scale oscillations in the background buoyancy-frequency profile, often seen in balloon measurements, but normally ignored in theoretical models, are found to cause significantly increased gravity-wave activity, resulting in wave breaking above and upstream of the mountain. The effect of temporal variations in the wind velocity is also studied for a range of amplitudes and periods typical of those encountered in the field. Transient gravity waves resulting from such variations can be significant, and steady flow states predicted on the assumption of uniform wind may not be attainable. Finally, a study is made of the nonlinear evolution of a gravity-wave packet as it propagates upwards in the atmosphere, generating low-frequency inertial-gravity waves of the type seen in recent field observations.
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