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-21-7695-2021
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发表时间：
2021-05
影响因子：
6.3
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
摘要。大气重力波在大气动力学中起着重要的作用，但在大气环流模式中准确地表示重力波具有一定的挑战性。这对于由南大洋小山地岛屿上的风流产生的地形gw来说尤其如此。目前，这些岛屿处于全球模式分辨率的“灰色地带”，既没有完全解析，也没有完全参数化。预计随着gcm接近当前高分辨率局域模式的空间分辨率，可以在不需要参数化的情况下解析小岛屿GW源。但是，与观测结果相比，这些高分辨率模拟中分辨出的gw有多真实呢？在这里，我们在多山的南乔治亚岛（54°S, 36°W）上测试了一个高分辨率（1.5公里水平网格，118个垂直高度）的气象局统一模型的局部配置，运行时没有GW参数化。模型很好地解析了该岛的地形，并在2013年7月和2015年6月至7月两个时间段使用了实时边界条件。我们将模型模拟的GWs与Aqua上大气红外探测仪（AIRS）的同步三维卫星观测结果进行了比较。通过使用AIRS分辨率和测量足迹对模型进行仔细采样（以下表示为AIRS采样的模型），我们首次使用三维s变换方法对岛上模拟和观测的三维GW振幅、波长和定向GW动量通量（GWMF）进行了类似的比较。将AIRS采样和分辨率应用到模型中，我们发现模拟的南乔治亚岛GWMF的时间、震级和方向与观测值基本一致。这2个月的区域平均纬向GWMF在AIRS和模式数据集中分别为5.3和5.6 mPa左右，但直接在岛上的数值可超过50 mPa。然而，在AIRS中，高达35%的GWMF实际上是在岛屿的逆风处发现的，而在作为AIRS采样的模型中，这一比例仅为17%，这表明在我们的模型配置中，AIRS观测到的非地形gww可能被低估了。经向GWMF结果显示，在以AIRS采样的模式中，有一个小的北偏（~ 20%），与重合的无线电探空仪测量结果相比，这可能对应于南风偏。最后，我们给出了一个在岛屿上空的大振幅（T′≈15-20 K，海拔45 km）短水平波长（λH≈30-40 km）的gw的例子，它与作为AIRS采样的模式非常吻合。这表明在T′≈45 K， λH≈30-40 km的全分辨率模式下，地形gw在现实中是可以发生的。我们的研究表明，高分辨率的局部区域模型不仅可以模拟小山区岛屿上真实的平流层GWs，而且将卫星采样和分辨率应用于这些模型也可以成为验证这些模型的有效方法。
Abstract. Atmospheric gravity waves (GWs) play an important role in atmospheric dynamics but accurately representing them in general circulation models (GCMs) is challenging. This is especially true for orographic GWs generated by wind flow over small mountainous islands in the Southern Ocean. Currently, these islands lie in the “grey zone” of global model resolution, where they are neither fully resolved nor fully parameterised. It is expected that as GCMs approach the spatial resolution of current high-resolution local-area models, small-island GW sources may be resolved without the need for parameterisations. But how realistic are the resolved GWs in these high-resolution simulations compared to observations? Here, we test a high-resolution (1.5 km horizontal grid, 118 vertical levels) local-area configuration of the Met Office Unified Model over the mountainous island of South Georgia (54∘ S, 36∘ W), running without GW parameterisations. The island's orography is well resolved in the model, and real-time boundary conditions are used for two time periods during July 2013 and June–July 2015. We compare simulated GWs in the model to coincident 3-D satellite observations from the Atmospheric Infrared Sounder (AIRS) on board Aqua. By carefully sampling the model using the AIRS resolution and measurement footprints (denoted as model sampled as AIRS hereafter), we present the first like-for-like comparison of simulated and observed 3-D GW amplitudes, wavelengths and directional GW momentum flux (GWMF) over the island using a 3-D S-transform method. We find that the timing, magnitude and direction of simulated GWMF over South Georgia are in good general agreement with observations, once the AIRS sampling and resolution are applied to the model. Area-averaged zonal GWMF during these 2 months is westward at around 5.3 and 5.6 mPa in AIRS and model sampled as AIRS datasets respectively, but values directly over the island can exceed 50 mPa. However, up to 35 % of the total GWMF in AIRS is actually found upwind of the island compared to only 17 % in the model sampled as AIRS, suggesting that non-orographic GWs observed by AIRS may be underestimated in our model configuration. Meridional GWMF results show a small northward bias (∼20 %) in the model sampled as AIRS that may correspond to a southward wind bias compared to coincident radiosonde measurements. Finally, we present one example of large-amplitude (T′≈15–20 K at 45 km altitude) GWs at short horizontal wavelengths (λH≈30–40 km) directly over the island in AIRS measurements that show excellent agreement with the model sampled as AIRS. This suggests that orographic GWs in the full-resolution model with T′≈45 K and λH≈30–40 km can occur in reality. Our study demonstrates that not only can high-resolution local-area models simulate realistic stratospheric GWs over small mountainous islands but the application of satellite sampling and resolution to these models can also be a highly effective method for their validation.