Mesoscale Air Flow over Topography: Modeling and Observation

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Mountains exert a profound influence on the earth's weather and climate. This research will investigate two of the most important terrain-induced atmospheric phenomena: strong surface winds and gravity-wave drag.Strong surface winds may be can be generated by the interaction of the synoptic-scale flow with topography through two different mechanisms, gap winds and downslope winds. Gap winds are produced when air is forced through a narrow break in a mountain barrier. Downslope winds may be generated when air flows across mountain ridges with steep lee slopes. Significant weather hazards and extensive property damage can occur during extreme gap-wind and downslope-wind events. This study of strong oro-graphically-forced surface winds will focus on gap winds, and on the relation and interaction between gap winds and downslope winds Part of the proposed research on gap winds will involve participation the Mesoscale Alpine Programme (MAP).Gravity wave drag is often produced when stably stratified air flows over a mountain barrier. Under these conditions surface-pressure perturbations develop that exert a force on the topography that tends to accelerate the mountain in the direction of the flow. An equal and opposite force is exerted by the mountain on the airstream that tends to decelerate the overlying flow. In contrast to the drag produced by surface friction, however, the decelerative forcing associated with gravity-wave generation can be exerted at elevations far above the ground. Since the gravity waves that transport the majority of the drag aloft are too small in scale to be resolved in global weather and climate models, gravity-wave-drag processes must be parameterized in these models. The quality of the weather and climate forecasts generated by global atmospheric models is sensitive to the details of this parameterization and efforts are underway to develop improved gravity-wave-drag parameterizations. The proposed research on gravity wave drag falls within one of the crucial research areas identified for study under the U.S. Weather Research Program.