AIRFLOWS OVER MOUNTAINS

Abstract

In this thesis, a two-dimensional hydrostatic numerical model is designed and employed to the study of mountain waves and the associated downslope wind­ storms. Simulation studies were carried out for two atmospheric conditions: (1) a two-layer structure with a very stable lower layer under a less stable upper layer, and (2) a sheared mean wind with a critical level. Our results clearly demonstrated a fundamental similarity between mountain waves and associated disturbances in such atmospheric conditions and the classical hydraulic flows over obstacles. Strong downslope winds were found to occur as the flow over mountain develops features similar to the transitional hydraulic flow with downstream hydraulic jump. Dynamic factors such as the height of the mountain, the depth and stability of the lower layer, and the height of the mean­ wind critical level, were found to strongly influence the occurrence of downslope windstorms. The studies of flows under a mean wind critical level confirmed the wave-break theory of Smith (1985,1989). Mountains of bell shape and plateau shape with variom; degrees of asymmetry were used in our model to investigate the effects of mountain profiles on downslope windstorms. No significant effects were found for flows under a mean wind critical level. However, in atmosphere with a two-layer stability structure, the critical mountain height for the occurrence of downslope windstorms was shown to depend on the shape of the mountain. Asymmetry in mountain shape also showed pronounced effects, especially for the case of plateau-shape mountains.