A mesoscale numerical model was used to investigate the generation mechanism of a mesoscale gravity wave event observed during the Convective Cooperative Precipitation Experiment (CCOPE). It is shown that the interaction between terrain-induced leeside upslope flow and a mountain barrier leads to a multistage generation of the gravity waves. The wave generation process was largely due to a low-level thermal and dynamic adjustment process rather than upper-level geostrophic adjustment. From the simulation, a distinct four stage conceptual model of the nighttime orographic wave generation conceptual model is presented. During stage I, shortly after sunset, the up-branch of the remnant daytime terrain-induced thermal circulation was driven back toward the mountain by a thermally forced upslope wind. This circulation system propagated westward with the characteristics of a density current. The density current lifted the isotherms and strengthened the low-level stable layer, necessary for wave ducting. In the transition period of stage II, which lasted an hour or two, this westward propagating density current was blocked before the steepest mountain slope. A meso-beta scale cold ridge developed above the original warm lee trough by extensive adiabatic cooling of the stronger upward motion. During stage III, the cold ridge produced/increased eastward horizontal pressure gradient force, which generated a new and even stronger upward motion center to the east; in the meantime, the original upward motion center in the cold ridge was suppressed, and the downward motion developed to the west. Thus, a gravity wave developed. In stage IV, the newly generated wave drifted eastward with the mean wind, and propagated in a well-defined duct.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics