Turbulent Transport of Momentum and Scalars Above an Urban Canopy

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• 作者：Linlin Wang (1)
  Dan Li (2)
  Zhiqiu Gao (1)
  Ting Sun (3)
  Xiaofeng Guo (1)
  Elie Bou-Zeid (2)
  
• 关键词：Ejections and sweeps ; Quadrant analysis ; Reynolds analogy ; Scalar similarity ; Transport efficiency ; Urban canopy
• 刊名：Boundary-Layer Meteorology
• 出版年：2014
• 出版时间：March 2014
• 年：2014
• 卷：150
• 期：3
• 页码：485-511
• 全文大小：2,005 KB
• 作者单位：Linlin Wang (1)
  Dan Li (2)
  Zhiqiu Gao (1)
  Ting Sun (3)
  Xiaofeng Guo (1)
  Elie Bou-Zeid (2)
  
  1. State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  2. Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ?, 08540, USA
  3. State Key Laboratory of Hydroscience and Engineering, Department of Hydraulic Engineering, Tsinghua University, Beijing, China
  
• ISSN：1573-1472

文摘

Turbulent transport of momentum and scalars over an urban canopy is investigated using the quadrant analysis technique. High-frequency measurements are available at three levels above the urban canopy (47, 140 and 280?m). The characteristics of coherent ejection–sweep motions (flux contributions and time fractions) at the three levels are analyzed, particularly focusing on the difference between ejections and sweeps, the dissimilarity between momentum and scalars, and the dissimilarity between the different scalars (i.e., temperature, water vapour and $\hbox {CO}_{2})$ . It is found that ejections dominate momentum and scalar transfer at all three levels under unstable conditions, while sweeps are the dominant eddy motions for transporting momentum and scalars in the urban roughness sublayer under neutral and stable conditions. The flux contributions and time fractions of ejections and sweeps can be adequately captured by assuming a Gaussian joint probability density function for flow variables. However, the inequality of flux contributions from ejections and sweeps is more accurately reproduced by the third-order cumulant expansion method (CEM). The incomplete cumulant expansion method (ICEM) also works well except for $\hbox {CO}_{2}$ at 47?m where the skewness of $\hbox {CO}_{2}$ fluctuations is significantly larger than that for vertical velocity. The dissimilarity between momentum and scalar transfers is linked to the dissimilarity in the characteristics of ejection–sweep motions and is further quantified by measures of transport efficiencies. Atmospheric stability is the controlling factor for the transport efficiencies of momentum and heat, and fitted functions from the literature describe their behaviour fairly accurately. However, transport efficiencies of water vapour and $\hbox {CO}_{2}$ are less affected by the atmospheric stability. The dissimilarity among the three scalars examined in this study is linked to the active role of temperature and to the surface heterogeneity effect.