Numerical Modeling of Mesoscale Airflow Over Mountains

山区中尺度气流的数值模拟

• 批准号: 9530662
• 负责人: Dale Durran
• 金额: $ 34.15万
• 依托单位: University of Washington
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1996
• 资助国家: 美国
• 起止时间: 1996-05-15 至 1999-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9530662&HistoricalAwards=false
• 关键词: Numerical; Modeling; Mesoscale; Airflow; Over

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. ***

杜兰山脉对地球的天气和气候有着深远的影响。首席研究员将通过一系列数值模拟研究两种地形诱导的大气现象。这项工作的一个重点将是研究跨山流动对山波产生和击穿的三维响应。第二个重点将是确定间隙风和下坡风在产生强地形强迫地面风中的相对重要性。大气科学家早就意识到地形产生的重力波可能对大气产生重要的阻力。对这一现象的早期讨论可以追溯到20世纪50年代末。高分辨率大气环流模式的实验结果表明，在高分辨率大气环流模式中加入重力波阻力参数化可以显著改善纬向平均西风和地面气压场的模拟气候学。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强迫响应的性质在很大程度上仍未被探索。在美国国家科学基金会的支持下，首席研究员最近获得的结果表明，由于重力波阻力而产生的平均流减速分布在山脉上游和下游的一个令人惊讶的大空间域。这些结果还表明，一个不同的诊断变量，即伪动量，可以更好地描述局部对重力波阻力的响应。首席研究员之前的结果是使用高分辨率二维数值模型获得的。这些调查将扩展到三维使用新开发的三维嵌套网格模型。作为拟议研究的一部分，首席研究员还将利用哈密顿流体动力学推导出三维可压缩分层流中伪动量的表达式。天气尺度气流与地形的相互作用可通过两种不同的机制产生强地面风：间隙风和下坡风。当空气被迫穿过山障的狭窄裂缝时，就会产生间隙风。这样的例子发生在哥伦比亚河峡谷和穿过法国罗纳河谷的西北风。当气流在没有明显的跨山间隙的情况下穿过山脉时，可能会产生强烈的下坡风；例如落基山奇努克人和克罗地亚波拉人。在其他情况下，比如从喀斯喀特山脉吹向华盛顿州埃姆克劳镇的强烈东风，间隙风和下坡风之间的区别就不那么明显了。先前的浅水模式研究表明，在大空间尺度上（被间隙穿透的非常宽的山脉），间隙风机制更有效，而在小空间尺度上（狭窄的山脊），下坡风机制最有效。使用新的3D嵌套网格模型，这些早期的结果将扩展到三维和各种更真实的大气配置。和以前一样，目标将是用最少数量的动力和地形参数来描述大尺度强迫，并系统地研究间隙风和下坡风对这些参数变化的敏感性。还将研究两种风况可能的协同组合。＊＊＊