Orographic drag on islands in the NWP mountain grey zone
Orographic drag on islands in the NWP mountain grey zone
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DOI：
10.1002/qj.2894
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发表时间：
2016-10
影响因子：
8.9
通讯作者：
S. Vosper;A. Brown;S. Webster
中科院分区：
文献类型：
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作者：
S. Vosper;A. Brown;S. Webster
在“灰色地带”分辨率的山区岛屿上的流动模拟中，研究了由分辨率和次网格气流阻塞和重力波引起的阻力，其中过程既不是很好的分辨率，也不是完全的亚网格尺度。通过对狭窄(南乔治亚)和宽阔(新西兰)山障的流动模拟，确定了分解的和参数化的压力、阻力和重力波动量通量作为水平网格长度的函数的行为。对谱空间气压阻力的分析表明，来自8-10个网格长度以下的波长的贡献的分辨率很差，参数化方案应该补偿这些尺度上的过程所丢失的阻力，而不是严格地补偿次网格尺度。南乔治亚州的模式性能是这样的，总的(分解的加上参数的)压力阻力在一个分辨率范围内近似不变。当特征岛的波长大约是网格长度的8倍时，参数化(分解的)阻力随着网格长度的增加而增加(减小)，并且分解的和参数化的阻力变得相似。如果将相同的阻力参数化调整应用于新西兰，则情况并非如此，在这种情况下，参数化阻力会随着栅格长度的增加而过快地增加。然而，当重新调整方案时，得到了令人满意的结果，结果表明，对这两个岛屿的最优调整与将亚网格地形高度缩放为代表更长长度尺度上的地形是一致的，最大可达10个网格长度。结果表明，拖曳方案需要这些较长尺度上的地形信息，而不仅仅是次网格尺度。在最粗的分辨率下，岛屿波长和网格长度相似，参数化阻力太小，因为网格框越来越多地被视为海点。讨论了这一问题的可能解决方案。
The drag due to resolved and subgrid flow blocking and gravity waves is examined in simulations of flow over mountainous islands at ‘grey zone’ resolutions, in which the processes are neither well resolved or fully subgrid‐scale. Simulations of flows over narrow (South Georgia) and broad (New Zealand) mountain barriers are used to determine the behaviour of resolved and parametrized pressure drag and gravity‐wave momentum fluxes as a function of horizontal grid length. Analysis of the pressure drag in spectral space suggests that contributions from wavelengths shorter than 8–10 grid lengths are poorly resolved and that parametrization schemes should compensate for the missing drag from processes on these scales, rather than strictly the subgrid scales. The model performance for South Georgia is such that the total (resolved plus parametrized) pressure drag is approximately invariant across a range of resolutions. The parametrized (resolved) drag increases (decreases) as the grid length increases and the resolved and parametrized drag become comparable when the characteristic island wavelength is approximately 8 times the grid length. This is not true when the same tuning of the drag parametrization is applied to New Zealand, in which case the parametrized drag increases too rapidly with increasing grid length. However, satisfactory results are obtained when the scheme is re‐tuned and it is shown that the optimal tuning for the two islands is consistent with scaling the subgrid orographic heights to be representative of the orography on longer length‐scales, of up to 10 grid lengths. The results suggest that drag schemes require orographic information on these longer scales, rather than only the subgrid scale. At the coarsest resolutions, where the island wavelength and grid length are similar, the parametrized drag is too small because the grid boxes are increasingly treated as sea points. Possible solutions to this problem are discussed.