Stratospheric gravity-waves over the mountainous island of South Georgia: testing a high-resolution dynamical model with 3-D satellite observations and radiosondes
Stratospheric gravity-waves over the mountainous island of South Georgia: testing a high-resolution dynamical model with 3-D satellite observations and radiosondes
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DOI：
10.5194/acp-2020-465
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发表时间：
2020
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通讯作者：
N. Hindley;C. Wright;A. Gadian;L. Hoffmann;J. Hughes;D. Jackson;J. King;N. Mitchell;T. Moffat‐Griffin;A. Moss;S. Vosper;A. Ross
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作者：
N. Hindley;C. Wright;A. Gadian;L. Hoffmann;J. Hughes;D. Jackson;J. King;N. Mitchell;T. Moffat‐Griffin;A. Moss;S. Vosper;A. Ross
抽象的。大气重力波是地球大气层各层之间能量和动量转移的关键驱动力。然而，这些波在大气环流模式(GCM)中的准确表示被证明是非常具有挑战性的。这是因为大部分重力波谱的尺度接近或低于全球GCM的分辨率。这对于南大洋的南乔治亚州(54°S，36°W)等孤立的山区小岛屿尤为重要。观测显示，该岛是平流层重力波的强烈来源，但由于其体积较小，它们的动量通量在全球模型中可能被低估。这是一个关键的限制，因为冬季60°S附近重力波的不充分表现与长期存在的“冷极问题”有关，即南方平流层极地涡旋在春季分解得太晚，要晚几个星期。这里我们讨论一个基本问题：当一个模型在南乔治亚州上空以非常高的空间分辨率运行时，模拟的重力波与观测值相比有多真实？为了回答这个问题，我们给出了南乔治亚州上空卫星重力波观测和高分辨率模式之间的三维比较。我们使用南乔治亚州上空气象局统一模式的专用高分辨率运行(1.5公里水平网格，118个垂直水平)，以及美国国家航空航天局AIRS/AQUA在2013年7月和2015年6月至7月期间进行的一致3-D卫星观测。首先，用符合的无线电探空仪观测来验证模型风。然后将AIRS观测滤波器应用于模型输出，以使两个数据集具有可比性。在模式和观测中，用三维S变换方法测量了重力波的振幅、波长、方向动量通量和间歇性。我们的结果表明，虽然模式中重力波活动的时间与观测值很接近，但面平均动量通量一般比观测低25%左右。此外，我们发现模式区域总通量的72%位于岛屿的下风向，而AIRS的测量只有57%。在岛的正上方，模式显示出更高的单个通量测量，但这些通量比观测中更具间歇性，90%的总通量只由22%的波浪事件携带，而AIRS的这一比例为32%。观测到的重力波通量随着高度的增加似乎也比模型中消散得更快，这表明波-平均流相互作用在现实中起着更大的作用。最后，波场的频谱分析表明，该模式高估了岛上短水平尺度上的重力波通量，但低估了该地区较大水平尺度的非地形波的通量，导致总体平均值较低。我们的结果表明，虽然提高模式分辨率是重要的，但为了在未来的GCM中实现南大洋上真实的重力波活动，确保准确地模拟背景风矢量的变化和非地形波的作用也是重要的。
Abstract. Atmospheric gravity waves are key drivers of the transfer of energy and momentum between the layers of the Earth’s atmosphere. The accurate representation of these waves in General Circulation Models (GCMs) however has proved very challenging. This is because large parts of the gravity wave spectrum are at scales that are near or below the resolution of global GCMs. This is especially relevant for small isolated mountainous islands such as South Georgia (54° S, 36° W) in the Southern Ocean. Observations reveal the island to be an intense source of stratospheric gravity waves, but their momentum fluxes can be under-represented in global models due to its small size. This is a crucial limitation, since the inadequate representation of gravity waves near 60° S during winter has been linked to the long-standing "cold-pole problem", where the southern stratospheric polar vortex breaks up too late in spring by several weeks. Here we address a fundamental question: when a model is allowed to run at very high spatial resolution over South Georgia, how realistic are the simulated gravity waves compared to observations? To answer this question, we present a 3-D comparison between satellite gravity wave observations and a high resolution model over South Georgia. We use a dedicated high-resolution run (1.5 km horizontal grid, 118 vertical levels) of the Met Office Unified Model over South Georgia and coincident 3-D satellite observations from NASA AIRS/Aqua during July 2013 and June–July 2015. First, model winds are validated with coincident radiosonde observations. The AIRS observational filter is then applied to the model output to make the two data sets comparable. A 3-D S-transform method is used to measure gravity-wave amplitudes, wavelengths, directional momentum fluxes and intermittency in the model and observations. Our results show that although the timing of gravity wave activity in the model closely matches observations, area-averaged momentum fluxes are generally up to around 25 % lower than observed. Further, we find that 72 % of the total flux in the model region is located downwind of the island, compared to only 57 % in the AIRS measurements. Directly over the island, the model exhibits higher individual flux measurements but these fluxes are more intermittent than in observations, with 90 % of the total flux carried by just 22 % of wave events, compared to 32 % for AIRS. Observed gravity wave fluxes also appear to dissipate more quickly with increasing height than in the model, suggesting a greater role for wave-mean flow interactions in reality. Finally, spectral analysis of the wave fields suggests that the model over-estimates gravity wave fluxes at short horizontal scales directly over the island, but under-estimates fluxes from larger horizontal scale non-orographic waves in the region, leading to a lower average value overall. Our results indicate that, although increasing model resolution is important, it is also important to ensure that variability in the background wind vector and role of non-orographic waves are accurately simulated in order to achieve realistic gravity wave activity over the Southern Ocean in future GCMs.