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Characteristics of the Drag Coefficient in the Roughness Sublayer over a Complex Urban Surface

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Abstract

The statistics of momentum exchange in the urban roughness sublayer are investigated. The analysis focuses on the characteristics of the dimensionless friction velocity, \({u_{*}}/U\), which is defined as the square root of the drag coefficient. The turbulence observations were made at a height of 47 m above the ground on the 325-m meteorological tower, which is located in a very inhomogeneous urban area in Beijing. Under neutral conditions, the dependence of the drag coefficient on wind speed varies with wind direction. When the airflow is from the area of densely built-up buildings, the drag coefficient does not vary with wind speed, while when the airflow is from the area covered by vegetation, the drag coefficient appears to decrease with increasing wind speed. Also, the drag coefficient does not vary monotonically with the atmospheric stability. Both increasing stability and increasing instability lead to the decrease of the drag coefficient, implying that the roughness length and zero-plane displacement may vary in urban areas.

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Acknowledgments

This study was supported by the State Key Basic Program under Grant No. 2010CB428501, the National Nature Science Foundation of China under Grant No. 40905002, and the Jiangsu Provincial Collaborative Innovation Center of Climate Change. The authors would like to thank the reviewers whose suggestions greatly helped to improve the manuscript.

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Authors and Affiliations

  1. School of Atmospheric Sciences, Nanjing University, Nanjing, 210093, China

    Zhen Peng & Jianning Sun

  2. Institute for Climate and Global Change Research, Nanjing University, Nanjing, 210093, China

    Jianning Sun

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  1. Zhen Peng
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  2. Jianning Sun
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Correspondence to Jianning Sun.

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Peng, Z., Sun, J. Characteristics of the Drag Coefficient in the Roughness Sublayer over a Complex Urban Surface. Boundary-Layer Meteorol 153, 569–580 (2014). https://doi.org/10.1007/s10546-014-9949-8

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