Origins of Aircraft-Damaging Clear-Air Turbulence during the 9 December 1992 Colorado Downslope Windstorm: Numerical Simulations and Comparison with Observations
Origins of Aircraft-Damaging Clear-Air Turbulence during the 9 December 1992 Colorado Downslope Windstorm: Numerical Simulations and Comparison with Observations
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1992 年 12 月 9 日科罗拉多下坡风暴期间对飞机造成损害的晴空湍流的起源：数值模拟及与观测结果的比较
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
D. Levinson
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作者：
T. Clark;W. Hall;Robert McDougall Kerr;D. Middleton;Larry Radke;F. M. Ralph;P. Neiman;D. Levinson
本文给出了1992年12月9日科罗拉多锋岭下坡风暴的数值模拟结果。虽然这一事件的特征不是强烈的地面风，但它在高空造成了极端的晴空湍流（CAT），一架DC-8货机在平均海平面以上9.7公里的山区遭受了严重的结构破坏。美国国家海洋和大气管理局/环境研究实验室/环境技术实验室多普勒激光雷达和风廓线仪在这一天的运作和国防气象卫星计划卫星的详细测量允许模拟和观测之间的独特丰富的比较。在模型中使用了四个级别的网格细化。外部域使用国家环境预测中心的初始和边界条件数据。最精细的网格在所有三个维度上使用200米，超过48公里乘48公里的部分。分辨率和域覆盖范围的范围足以解决丰富的各种动态与时间演变的风暴被迫在锋面通道。这种全方位的分辨率和模型复杂性在这种情况下是必不可少的。这场风暴的许多方面本质上都是三维的，并且没有使用2D或所谓的2D‐3D动力学在理想化模型中表示。波浪破碎的时间和位置都与观测结果相吻合。该模型还再现了急流中的横流波动扰动，与卫星和激光雷达观测到的云带的方向和间距进行了比较。模式结果还表明，所观察到的CAT来自这些波浪状急流扰动和山强迫内部重力波之间的相互作用。由于急流的近东西方向，这两种相互作用的波模式彼此正交。与强烈的射流波动产生的水平涡流管（HVT）对齐的平均流量的热梯度。这些HVT保持在高空，而它们在飞机事件的高度向下游传播，激光雷达看到了这种漩涡的证据。模型和观测结果表明，这些强烈的漩涡之一可能导致了飞机事故。在本研究期间，沿锋面山脉沿着有强地面阵风的报告。该模型表明，重力波和流动对齐的急流波动之间的相互作用，导致孤立发生的强地面阵风与观测一致。模拟结果表明，强烈的剪切上，下表面的急流联合收割机结合起来，提供了一个情节“下击暴流的湍流”。在这种情况下，射流的扰动提供了一个与平均流对齐的漏斗形剪切区，该剪切区充当由破碎重力波引起的湍流向下传输的引导。上层的物理图象与克拉克和法利描述的由涡旋倾斜引起的地面阵风相似。进入该漏斗的CAT进料来自喷射流的所有表面，其中超过一半来自喷射流底侧上的垂直倾斜剪切区。从视觉上看，湍流的下击暴流看起来类似于一个雨轴，它降落到地面并在平原上传播，留下相对静止的条件。
Results from numerical simulations of the Colorado Front Range downslope windstorm of 9 December 1992 are presented. Although this case was not characterized by severe surface winds, the event caused extreme clearair turbulence (CAT) aloft, as indicated by the severe structural damage experienced by a DC-8 cargo jet at 9.7 km above mean sea level over the mountains. Detailed measurements from the National Oceanic and Atmospheric Administration/Environmental Research Laboratories/Environmental Technology Laboratory Doppler lidar and wind profilers operating on that day and from the Defense Meteorological Satellite Program satellite allow for a uniquely rich comparison between the simulations and observations. Four levels of grid refinement were used in the model. The outer domain used National Centers for Environmental Prediction data for initial and boundary conditions. The finest grid used 200 m in all three dimensions over a 48 km by 48 km section. The range of resolution and domain coverage were sufficient to resolve the abundant variety of dynamics associated with a time-evolving windstorm forced during a frontal passage. This full range of resolution and model complexity was essential in this case. Many aspects of this windstorm are inherently three-dimensional and are not represented in idealized models using either 2D or so-called 2D‐3D dynamics. Both the timing and location of wave breaking compared well with observations. The model also reproduced cross-stream wavelike perturbations in the jet stream that compared well with the orientation and spacing of cloud bands observed by satellite and lidar. Model results also show that the observed CAT derives from interactions between these wavelike jet stream disturbances and mountain-forced internal gravity waves. Due to the nearly east‐west orientation of the jet stream, these two interacting wave modes were orthogonal to each other. Thermal gradients associated with the intense jet stream undulations generated horizontal vortex tubes (HVTs) aligned with the mean flow. These HVTs remained aloft while they propagated downstream at about the elevation of the aircraft incident, and evidence for such a vortex was seen by the lidar. The model and observations suggest that one of these intense vortices may have caused the aircraft incident. Reports of strong surface gusts were intermittent along the Front Range during the period of this study. The model showed that interactions between the gravity waves and flow-aligned jet stream undulations result in isolated occurrences of strong surface gusts in line with observations. The simulations show that strong shears on the upper and bottom surfaces of the jet stream combine to provide an episodic ‘‘downburst of turbulence.’’ In the present case, the perturbations of the jet stream provide a funnel-shaped shear zone aligned with the mean flow that acts as a guide for the downward transport of turbulence resulting from breaking gravity waves. The physical picture for the upper levels is similar to the surface gusts described by Clark and Farley resulting from vortex tilting. The CAT feeding into this funnel came from all surfaces of the jet stream with more than half originating from the vertically inclined shear zones on the bottom side of the jet stream. Visually the downburst of turbulence looks similar to a rain shaft plummeting to the surface and propagating out over the plains leaving relatively quiescent conditions behind.