Abstract
Nested simulations of a downslope windstorm over Cangshan mountain, Yunnan, China, have been used to demonstrate a method of topographic smoothing that preserves a relatively large amount of terrain detail compared to typical smoothing procedures required for models with terrain-following grids to run stably. The simulations were carried out using the Met Office Unified Model (MetUM) to investigate downslope winds. The smoothing method seamlessly blends two terrain datasets to which uniform smoothing has been applied–one with a minimum of smoothing, the other smoothed more heavily to remove gradients that would cause model instabilities. The latter dataset dominates the blend where the steepest slopes exist, but this is localised and recedes outside these areas. As a result, increased detail is starkly apparent in depictions of flow simulated using the blend, compared to one using the default approach. This includes qualitative flow details that were absent in the latter, such as narrow shooting flows emerging from roughly 1–2 km wide leeside channels. Flow separation is more common due to steeper lee slopes. The use of targeted smoothing also results in increased lee side temporal variability at a given point during the windstorm, including over flat areas. Low-/high-pass filtering of the wind perturbation field reveals that relative spatial variability above 30 km in scale (reflecting the background flow) is similar whether or not targeting is used. Beneath this scale, when smoothing is targeted, relative flow variability decreases at the larger scales, and increases at lower scales. This seems linked to fast smaller scale flows disturbing more coherent flows (notably an along-valley current over Erhai Lake). Spatial variability of winds in the model is unsurprisingly weaker at key times than is observed across a local network sampling mesoscale variation, but results are compromised due to relatively few observation locations sampling the windstorm. Only when targeted smoothing is applied does the model capture the downslope windstorm’s extension over the city of Dali at the mountain’s foot, and the peak mean absolute wind.
摘要
使用英国气象局统一模型(MetUM), 在对中国云南苍山的下坡风暴嵌套模拟中应用了一种新的地形平滑方案, 与要求模式具有地形跟随格点才能稳定运行的传统平滑方案相比, 该方案保留了大量的地形细节. 该方案对可能引起模式不稳定的地形梯度作平滑处理, 同时尽可能地保留其他区域的地形信息; 换言之, 该方案针对性地平滑最陡峭的斜坡, 而在这些区域外, 平滑效应逐渐衰减. 结果表明, 与传统方案相比, 使用该平滑方案的气流模拟中增加的细节非常明显. 例如, 新方案定性地模拟出了在宽约1–2 km的背风峡谷中出现的狭窄喷射流. 同时, 由于背风坡更陡, 气流分离也更常见. 采用新方案的模拟试验中, 背风侧的风场在风暴期间的时间变率更大. 对风场扰动的低通/高通滤波表明, 在主要反映背景流的30 km以上尺度, 是否使用目标平滑方案对相对空间变率的影响不大; 而在更精细的尺度, 当采用目标平滑方案, 相对流动的变率将在较大尺度上减小, 在较小尺度上增大, 这可能与快速的小尺度气流对连续气流的干扰有关(特别是洱海上空沿山谷的气流). 由于此次风暴涉及的观测站点较少, 与当地中尺度气象站网的观测结果相比, 模式模拟的风场空间变率较弱; 但只有应用了新方案的试验模拟出了下坡风暴在大理市上空的延伸, 以及其平均绝对风速的峰值.
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Acknowledgements
This work and its contributors were supported by the UK–China Research & Innovation Partnership Fund through the Met Office Climate Science for Service Partnership (CSSP) China as part of the Newton Fund. The Met Office authors would like to thank their Chinese counterparts and the staff of Dali National Climate Observatory and CMA for an excellent visit to the site.
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Article Highlights
• 100 m horizontal grid spacing NWP simulations reproduce mountain wave and rotor flow.
• Spatially selective smoothing targeted only at very steep slopes results in enhancement of the fine scale flow detail.
• As a result, observed wind, and its spatial and temporal variability, are better reflected.
This paper is a contribution to the 2nd Special Issue on Climate Science for Service Partnership China.
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Sheridan, P., Xu, A., Li, J. et al. Use of Targeted Orographic Smoothing in Very High Resolution Simulations of a Downslope Windstorm and Rotor in a Sub-tropical Highland Location. Adv. Atmos. Sci. 40, 2043–2062 (2023). https://doi.org/10.1007/s00376-023-2298-0
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DOI: https://doi.org/10.1007/s00376-023-2298-0