Gravity-Wave Dynamics in the Atmosphere

Motivated by reports of accidents in high-altitude aircraft flights due to unforecast turbulence caused by gravity waves over mountain ranges, this research program aimed to advance the theoretical understanding of physical mechanisms responsible for the generation of gravity waves by wind over orography. Specifically, a systematic study was made of the Earths rotation under various atmospheric conditions. It was found that rotation can contribute to significantly increased gravity-wave activity when the tropopause the border between the troposphere and the stratosphere is located at certain heights over mesoscale mountains. Under such tuned conditions, the combined effect of rotation and nonlinear interactions is to drive inertia-gravity waves that propagate far downstream from the mountain. The effect of temporal variations in the wind velocity was also studied for time scales typically encountered in the field. It was found that small-amplitude oscillatory wind components play an important part, and the wave response to unsteady wind can be quite stronger than that predicted by assuming an average steady wind profile. Finally, a preliminary study was made of the radiation of gravity waves by weakly nonlinear wavepackets arising from shear-flow instabilities.