Revisiting the relation between momentum and scalar roughness lengths of urban surfaces
Revisiting the relation between momentum and scalar roughness lengths of urban surfaces
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DOI：
10.1002/qj.3839
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发表时间：
2020-08
影响因子：
8.9
通讯作者：
Qi Li;E. Bou‐Zeid;S. Grimmond;S. Zilitinkevich;G. Katul
中科院分区：
文献类型：
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作者：
Qi Li;E. Bou‐Zeid;S. Grimmond;S. Zilitinkevich;G. Katul
大涡模拟（LESs）中性流在规则长方体阵列上进行，以探索城市环境中动量（z0m）和标量（z0s）粗糙度长度之间的联系，以及它们如何受到表面几何形状的影响。由于LES解决的是障碍物，而不是附着在障碍物上的微尺度边界层，因此上述粗糙度长度在两个不同的空间尺度上进行了分析。在微观尺度上（单个表面的粗糙度，例如屋顶），假设动量和标量传递都受光滑壁面的公认参数控制，这些参数构成了LES壁面模型的基础。在宏观尺度上，粗糙度长度代表了动量和标量传递对整个表面的分解粗糙度元素的综合效应，因此它们是直接从LES计算出来的。结果表明，基于形态学的宏观尺度z0m参数化总体上是足够的。动量和标量宏观粗糙度值之间的关系，通常用log（z0m/z0s）表示，并假设与Re∗n成比例（其中Re∗是粗糙度雷诺数），然后使用表面更新理论（SRT）解释。当只有Kolmogorov尺度涡旋主导标量交换时，SRT预测n = 1/4，而当大涡旋限制更新动态时，SRT预测n = 1/2。发现后者能够更好地捕捉LES结果。然而，两种尺度关系都表明，对于典型的城市几何形状和尺度，z0随着z0m的增加而降低。这与它们的关系通常在城市冠层中建模的方式相反（即，z0s/z0m是一个小于1的固定值）。
Large‐Eddy Simulations (LESs) of neutral flow over regular arrays of cuboids are conducted to explore connections between momentum (z0m) and scalar (z0s) roughness lengths in urban environments, and how they are influenced by surface geometry. As LES resolves the obstacles but not the micro‐scale boundary layers attached to them, the aforementioned roughness lengths are analyzed at two distinct spatial scales. At the micro‐scale (roughness of individual facets, e.g., roofs), it is assumed that both momentum and scalar transfer are governed by accepted arguments for smooth walls that form the basis for the LES wall‐model. At the macro‐scale, the roughness lengths are representative of the aggregate effects of momentum and scalar transfer over the resolved roughness elements of the whole surface, and hence they are directly computed from the LES. The results indicate that morphologically based parametrizations for macro‐scale z0m are adequate overall. The relation between the momentum and scalar macro‐roughness values, as conventionally represented by log(z0m/z0s) and assumed to scale with Re∗n (where Re∗ is a roughness Reynolds number), is then interpreted using surface renewal theory (SRT). SRT predicts n = 1/4 when only Kolmogorov‐scale eddies dominate the scalar exchange, whereas n = 1/2 is predicted when large eddies limit the renewal dynamics. The latter is found to better capture the LES results. However, both scaling relations indicate that z0s decreases when z0m increases for typical urban geometries and scales. This is opposite to how their relation is usually modelled for urban canopies (i.e., z0s/z0m is a fixed value smaller than unity).