Mesoscale Airflow over Mountains: Modeling and Observational Analysis

山脉对天气和气候有深远的影响。这项研究的目的是调查山地引起的环流直接影响人类活动的两种重要方式：强烈的地面风和猛烈的低层旋涡。大尺度气流与地形的相互作用可能通过间隙风和下坡风两种不同的机制产生强地面风。在极端的间隙风和下坡风事件中，可能会发生重大的天气灾害和广泛的财产损失。当空气被迫穿过山脉屏障的缝隙时，就会产生间隙风。当间隙风吹起时，山顶以上的风不需要有垂直于山脊线的分量。另一方面，下坡风只有在山脊顶部有显著的跨山风分量时才会发生。当气流从山顶下降时，下坡风达到了很高的速度。在某些情况下，通过山口的间隙流和靠近山口的斜坡上的下坡风之间可能存在协同作用。本研究的目的是为了更好地了解间隙风的动力学，确定给定大尺度流在哪些条件下更适合产生间隙风或下坡风，最终提高山区大风事件的预报水平。经过长脊的气流经常产生水平轴平行于脊线的低层涡。这些被称为旋翼的水平涡旋对商业、军事和民用航空构成严重危害。转子流动通常是非常湍流的。据报道，一些飞机经历了极端的滚转运动（滚转角接近90度），一些飞机在据信存在强旋翼的情况下失踪。首席研究员和合作者最近的工作已经确定了典型转子内旋转的基本来源。正如龙卷风包含嵌入的高涡度区域（吸力涡）一样，地形诱导旋翼似乎也包含非常高涡度的局部区域。目前，关于这些副旋翼的最大强度或动力学几乎一无所知，但它们可能对飞机构成最大的危险。本建议的目标是更好地了解这些子转子的可能强度、发生频率和动力学。这项研究的成功完成可能会更好地预测由山区引起的大风灾害。

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.