Mesoscale Airflow over Mountains: Modeling and Observational Analysis

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Mountains exert a profound influence on the weather and climate. This research is aimed at investigating two important ways in which mountain-induced circulations directly impact human activities: strong surface winds and violent low-level rotors.Strong surface winds may be generated by the interaction of the larger-scale flow with the topography through two different mechanisms, gap winds and downslope winds. Significant weather hazards and extensive property damage can occur during extreme gap-wind and downslope-wind events. Gap winds are produced when air is forced through gaps in a mountain barrier. The winds above mountain top level need not have a component perpendicular to the ridge line while the gap winds are blowing. Downslope winds, on the other hand, only occur if there is a significant cross-mountain wind component at ridge-top level. Downslope winds attain their high velocities as the flow descends from the mountain crest. In some cases there may be synergistic interactions between the gap flow through a mountain pass and downslope winds on the slopes adjacent to the pass. A goal of this research is to better understand the dynamics of gap winds, to determine those conditions under which a given large-scale flow is more suitable for the generation of gap winds or downslope winds, and to ultimately improve the forecasting of high wind events in mountainous regions.Airflow over long ridges often produces low-level vortices with horizontal axes parallel to the ridge line. These horizontal vortices, known as rotors, pose a serious hazard to commercial, military and civilian aviation. The rotor flow is often very turbulent. Some aircraft have reported undergoing extreme rolling motions (with roll angles approaching 90 degrees), and some aircraft have been lost under conditions when strong rotors were believed to have been present. Recent work by the Principal Investigator and collaborators has identified the basic source of rotation within a typical rotor. Just as the tornadoes contain embedded regions of high vorticity (suction vortices), terrain induced rotors also appear to contain localized regions of very high vorticity. At present, almost nothing is known about the maximum strength or the dynamics of these subrotors, yet they may pose the most danger to aircraft. A goal of this proposal is to develop a better understanding of the probable strength, frequency of occurrence, and the dynamics of these subrotors.Successful completion of this research can potentially lead to better forecasts of mountain-induced severe wind hazards.