Mountain waves and wakes generated by South Georgia: implications for drag parametrization
Mountain waves and wakes generated by South Georgia: implications for drag parametrization
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南乔治亚岛产生的山浪和尾流：对阻力参数化的影响
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发表时间：
2014
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通讯作者：
S. Vosper
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作者：
S. Vosper
利用南乔治亚岛（南大西洋）上空的高分辨率气流模拟，增加了对小孤立的山地岛屿对大尺度气流可能产生的影响的认识，并确定了参数化方案在多大程度上可以解释模型中缺少的阻力，因为这些岛屿只有部分得到了解决。长期（1个月）的南方冬季预报，水平网格间距为1.5 km，用于量化由岛屿产生的山波动量通量和与流动阻塞动力学相关的低层阻力。考虑了阻力的特征，如高阻力状态和低阻力状态的发生，其对风向的依赖以及对山波动量通量的谱贡献。流动分裂和低层波浪破碎被证明是造成从岛屿延伸数百公里的尾迹区域的原因。由于山波的耗散，平流层中的减速区域也很明显。地形阻力参数化方案在多大程度上可以再现阻力，通过与粗分辨率（15公里网格间距）模拟的比较进行了研究。在粗分辨率（15公里网格间距）模拟中，地形分辨率很差，很大一部分阻力被参数化。结果表明，在这些模拟中，分解后加上参数化阻力的总和与高分辨率时的总和非常接近，尽管在高阻力情况下它被低估了。对该方案进行简单的修改，当低层气流与子网格地形的长轴近似垂直时，增加阻力，可以纠正这一问题。研究表明，至少对于相对简单的孤立山脉，阻力和山波动量通量可以通过适当复杂性的调整好的参数化方案在确定性意义上进行预测，尽管包含随机效应可能会导致进一步的改进。
High‐resolution simulations of flows over South Georgia (South Atlantic) are used to increase understanding of the likely influence of small isolated mountainous islands on the large‐scale flow and to ascertain the extent to which parametrization schemes can account for the missing drag in models where such islands are only partially resolved. Long‐duration (1 month) austral winter forecasts with a horizontal grid spacing of 1.5 km are used to quantify the mountain‐wave momentum fluxes generated by the island and the low‐level drag associated with flow‐blocking dynamics. The characteristics of the drag, such as the occurrence of high and low drag states, its dependence on wind direction and the spectral contributions to the mountain‐wave momentum flux, are considered. Flow splitting and low‐level wave breaking are shown to be responsible for wake regions that extend for hundreds of kilometres from the island. Regions of deceleration are also evident in the stratosphere, due to mountain‐wave dissipation. The extent to which an orographic drag parametrization scheme can reproduce the drag is investigated by comparison with coarse‐resolution (15 km grid spacing) simulations in which the orography is poorly resolved and a large proportion of the drag is parametrized. It is demonstrated that the total of the resolved plus parametrized drag in these simulations closely resembles that at high resolution, although it is underpredicted during instances of high drag. Simple modifications to the scheme, which enhance the drag when the low‐level flow is approximately normal to the major axis of the subgrid orography, are shown to rectify this. The study demonstrates that, at least for relatively simple isolated mountain ranges, the drag and mountain‐wave momentum fluxes can be predicted in a deterministic sense by a well‐tuned parametrization scheme of suitable complexity, although inclusion of stochastic effects might lead to yet further improvements.