Numerical Modeling of Mesoscale Airflow Over Mountains

9530662都兰山对地球天气和气候产生了深远的影响。首席调查员将通过一系列数值模拟研究两种由地形引起的大气现象。这项工作的重点之一将是调查跨山流动对山波产生和破裂的三维响应。第二个重点将是确定空隙风和下坡风在强地形强迫地面风产生中的相对重要性。大气科学家很早就意识到，地形产生的重力波可能会对大气造成重要拖累。对这一现象的早期讨论可以追溯到20世纪50年代末的S。高分辨率大气环流模式的试验表明，在高分辨率大气环流模式中加入重力波阻力参数可以显著改善纬向平均西风和地面气压场的模拟气候学，从而推动了对重力波阻力的持续兴趣。GWD参数化现在是大多数高分辨率大气环流和中期天气预报模式的标准特征，但中尺度大气对GWD强迫的响应在很大程度上仍未被探索。首席调查员在之前NSF的支持下获得的最新结果表明，由于重力波阻力而产生的平均流减速分布在山的上下游一个令人惊讶的大空间域上。这些结果还表明，一个不同的诊断变量，即伪矩，可以更好地描述局部对重力波阻力的反应。首席调查员之前的结果是使用高分辨率的二维数值模式获得的。这些研究将使用新开发的3D嵌套网格模型扩展到三维。作为拟议研究的一部分，首席调查员还将利用哈密顿流体动力学推导出三维可压缩分层流动中的伪矩的表达式。强地面风是天气尺度气流和地形通过两种不同的机制相互作用产生的：缝隙风和下坡风。当空气被强迫通过山障上的狭窄裂缝时，就会产生空隙风。这样的例子出现在哥伦比亚河峡谷和横穿法国罗纳河谷的西北风中。当空气流经没有明显跨山间隙的山脉时，可能会产生强烈的下坡风；例如落基山脉奇努克山脉和克罗地亚博拉山脉。在其他情况下，比如从瀑布吹向华盛顿州埃努姆克劳小镇的强劲东风，缝隙风和下坡风之间的区别并不那么明显。以前的浅水模式研究表明，空隙风机制在大空间尺度(被空隙穿透的很宽的山脉)上更有效，而下坡风机制在小空间尺度上(狭窄的脊线)最有效。使用新的3D嵌套网格模型，这些早期结果将扩展到三维和各种更现实的大气配置。和以前一样，目标将是用最少的动力和地形形状参数来描述大范围强迫的特征，并系统地调查空隙风和下坡风对这些参数变化的敏感性。还将研究这两种风场可能的协同组合。***

9530662 Durran Mountains exert a profound influence on the earth's weather and climate. The Principal Investigator will investigate two types of terrain-induced atmospheric phenomena through a series of numerical simulations. One focus of this effort will be to investigate the three-dimensional response of the cross-mountain flow to mountain wave generation and breakdown. The second focus will be to determine the relative importance of gap winds and downslope winds in the generation of strong orographically-forced surface winds. Atmospheric scientists have long been aware that orographically generated gravity waves may exert an important drag on the atmosphere. Early discussions of this phenomena date back to the late 1950's. Continued interest in gravity wave drag (GWD) has been fueled by experiments with high-resolution general circulation models, which suggest that the inclusion of GWD parameterizations in such models can significantly improve the simulated climatology of the zonally averaged westerly winds and the surface pressure fields. GWD parameterizations are now standard features in most high-resolution general circulation and medium-range weather forecast models, yet the nature of the mesoscale atmospheric response to GWD forcing remains largely unexplored. Recent results obtained by the Principal Investigator under prior NSF support suggest that the mean-flow deceleration that develops in response to gravity wave drag is spread over a surprising large spatial domain upstream and downstream from the mountain. These results also suggested that a different diagnostic variable, the pseudomomentum, could provide a better description of the local response to gravity wave drag. The Principal Investigator's previous results were obtained using a high-resolution two-dimensional numerical model. These investigations will be extended to three dimensions using a newly developed 3D nested-grid model. As part of the proposed research, the Principal Investigator will also derive expressions for the pseudomomentum in 3D compressible stratified flow using Hamiltonian fluid dynamics. Strong surface winds can be generated by the interaction of the synoptic-scale flow and 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. Examples of this occur in the Columbia River gorge and in the mistral, which blows through the Rhone valley of France. Strong downslope winds may be generated when the air flows across mountains without significant cross-mountain gaps; examples include the Rocky Mountain Chinook and the Croatian Bora. In still other cases, such as the strong easterly winds that blow from the Cascades toward the town of Enumclaw, Washington, the distinction between gap winds and downslope winds is not so clear. Previous research with a shallow-water model has suggested that the gap wind mechanism is more effective on large spatial scales (very broad mountain ranges pierced by a gap) whereas the downslope wind mechanism is most effective on small spatial scales (narrow ridges). These early results will be extended to three dimensions and to a variety of more realistic atmospheric configurations, using the new 3D nested grid model. As before, the goal will be to characterize the large-scale forcing with a minimal number of dynamical and terrain-shape parameters and to systematically investigate the sensitivity of gap winds and downslope winds to variations in these parameters. Possible synergetic combinations of the two wind regimes will also be investigated. ***