Mesoscale Air Flow over Topography: Modeling and Observation

山脉对地球的天气和气候有着深远的影响。这项研究将调查两个最重要的地形引起的大气现象：强地面风和重力波阻力。天气尺度气流与地形的相互作用可通过间隙风和下坡风两种不同的机制产生强烈的地面风。当空气被迫穿过山障的狭窄裂缝时，就会产生间隙风。当气流穿过背风坡陡峭的山脊时，可能会产生下坡风。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。对强地形强迫地面风的研究将侧重于间隙风，以及间隙风与下坡风之间的关系和相互作用。关于间隙风的部分拟议研究将涉及参与中尺度高山计划（MAP）。重力波阻力通常是在稳定的分层空气流过山障时产生的。在这种情况下，地表压力扰动产生，对地形施加一种力，倾向于加速山向水流的方向移动。山对气流施加了一个大小相等、方向相反的力，使上覆气流减速。然而，与表面摩擦产生的阻力相反，与重力波产生有关的减速力可以在离地面很远的高度施加。由于将大部分阻力输送到高空的重力波的尺度太小，无法在全球天气和气候模式中解决，因此这些模式必须将重力波-阻力过程参数化。由全球大气模式生成的天气和气候预报的质量对这种参数化的细节很敏感，人们正在努力开发改进的重力波阻力参数化。关于重力波阻力的拟议研究属于美国气象研究计划确定的关键研究领域之一。

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.