Turbulent Transport of Momentum and Scalars Above an Urban Canopy

Linlin Wang, Dan Li, Zhiqiu Gao, Ting Sun, Xiaofeng Guo, Elie R. Bou-Zeid

Research output: Contribution to journalArticle

38 Scopus citations

Abstract

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 CO2. 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 CO2 at 47 m where the skewness of CO2 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 CO2 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.

Original languageEnglish (US)
Pages (from-to)485-511
Number of pages27
JournalBoundary-Layer Meteorology
Volume150
Issue number3
DOIs
StatePublished - Mar 2014

All Science Journal Classification (ASJC) codes

  • Atmospheric Science

Keywords

  • Ejections and sweeps
  • Quadrant analysis
  • Reynolds analogy
  • Scalar similarity
  • Transport efficiency
  • Urban canopy

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