The Co-Evolution of Mesoscale Airflow over Mountains and the Larger Scale Flow

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Mountains exert a profound influence on the weather and climate. Under this award, the Principal Investigator (PI) will study the way that local mountain-induced circulations change in response to the daily changes in large-scale weather patterns, and to study the way in which these local circulations feed back on the large-scale flow. Intellectual Merit:When an air stream encounters a mountain barrier, "mountain waves" may be set up above the barrier and lee vortices may form near the surface downstream. Visible manifestations of the flow patterns in these waves may appear in the form of lenticular clouds. When mountain waves break down they form regions of clear air turbulence that are hazardous to aviation, and they also exert a drag on the larger-scale atmospheric flow. The cumulative effect of the drag exerted by flow over mountains throughout the world is too large to be neglected in models of the global weather and climate; but global models lack sufficient detail to correctly calculate the drag from first principles. Out of necessity, this "gravity wave drag" is parameterized, but the quality and accuracy of these parameterizations are not well established.Most previous investigations have studied the behavior of mountain waves and lee vortices in situations where the large-scale flow did not vary in time or space, however recently the PI, and others, who will also collaborate in the research, have shown that the mountain waves produced in hypothetical horizontally-uniform steady-state environments may be very different from those that occur in slowly varying environments with temporal fluctuations on a period of two days and spatial variations with a wavelength of 2000 km. Preliminary results also suggest that large-scale variability exerts an even more dramatic effect on the structure of lee vortices. The PI will thoroughly examine the influence of realistic large-scale temporal and spatial variations on the development and subsequent decay of mountain waves and lee vortices. He will also examine the interaction of the mountain waves with the larger-scale weather pattern in order to determine how this interaction should be represented in global weather and climate models. Broader Impacts: The research and professional development of two graduate students will be supported under this award. Results may have a positive impact on the forecasting of mountain waves, downslope winds and lee vortices in mountainous regions. Lee vortices can have a major impact on air quality in cities downwind of mountain barriers. In addition to modifying the dispersion of everyday pollutants, lee vortices can recirculate chemical or biological agents over a compact region. This research should also improve computer models for the simulation of global weather and climate by helping to improve the representation of gravity wave drag in those models.