Mesoscale Airflow over Mountains: Wave Drag and Orographic Precipitation

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Mountains exert a profound influence on the weather and climate. This project will investigate two important aspects of the interaction between mountains and the atmosphere. The first effort will be to examine how the drag exerted by mountains on an air stream is distributed among different vertical levels in the atmosphere and how that drag feeds back on the larger scale flow. The second effort will examine the way that mountains enhance precipitation and dry the air downstream.Intellectual Merit:When an air stream encounters a mountain barrier, "mountain waves" may be set up above and downstream of the barrier and the low-level flow may be blocked and diverted around the flanks of the mountain. These disturbances are too small in scale to be resolved in global weather and climate models, but they produce an important drag on the large-scale flow that must be parameterized. This project will attempt to place such parameterizations on a firmer theoretical footing and also determine the large-scale responses to orographic drag through the analysis of high-resolution numerical simulations.Although the precipitation over mountains is largely driven by the episodic passage of large-scale weather systems, prototypical theoretical studies of the interactions of an isolated ridge with such systems have not yet been undertaken, due in part to the difficulty of simultaneously modeling complex large and small-scale flows. We will take advantage of advances in computing power and model architecture to investigate the nature of the synergistic interactions of the large-scale lifting and precipitation with the more localized ascent forced by the topography. Such interactions are likely to generate more intense precipitation than that produced in previous simulations of simple upslope flows. The strength and downstream distribution of the dry air in the lee will also be determined and compared to that suggested by models forced by horizontally uniform cross-mountain flows.Broader Impacts:Two graduate students will be supported under this NSF award for their research and professional development. Research results should help improve the parameterization of low-level gravity wave drag in large-scale models essential for global weather forecasting and much climate research. Orographic precipitation is a key source of water for many of the world's civilizations. By developing a better understanding of how precipitation is enhanced when large-scale weather systems pass over topography, we can provide guidance to water resource managers and hydrologists trying to maintain water supplies in the current climate and planning for the effect of possible future shifts in mid-latitude storminess.