Mesoscale Air Flow over Topography: Modeling and Observation

山脉对地球的天气和气候产生了深远的影响。这项研究将研究两种最重要的地形引起的大气现象：强地面风和重力波拖曳。强地面风可能是天气尺度气流与地形通过两种不同的机制相互作用而产生的，即空隙风和下坡风。当空气被强迫通过山障上的狭窄裂缝时，就会产生空隙风。当空气流经具有陡峭背风坡的山脊时，可能会产生下坡风。在极端的空隙风和下坡风事件期间，可能会发生重大的天气灾害和广泛的财产损失。这项关于强地形强迫地面风的研究将集中于空隙风，以及空隙风和下坡风之间的关系和相互作用。建议的空隙风研究的一部分将涉及参与中尺度阿尔卑斯计划(MAP)。当稳定的层结空气流过山障时，通常会产生重力波阻力。在这种情况下，地表压力扰动会产生，对地形施加作用力，从而使山体朝水流方向加速。山体对气流施加相等且相反的力，从而使上方的气流减速。然而，与表面摩擦产生的阻力相反，与重力波产生相关的减速强迫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尺度太小，无法在全球天气和气候模式中解决，因此必须在这些模式中对重力波-重力波-阻力过程进行参数化。全球大气模式产生的天气和气候预报的质量对这种参数化的细节很敏感，目前正在努力开发改进的重力波拖曳参数化。拟议中的重力波阻力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.