Evaluation of a mechanical drag coefficient formulation in the complex urban area of Beijing
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ORIGINAL PAPER
Evaluation of a mechanical drag coefficient formulation in the complex urban area of Beijing Miao Yu 1
&
Jorge González 2 & Shiguang Miao 1
Received: 20 May 2020 / Accepted: 7 August 2020 # Springer-Verlag GmbH Austria, part of Springer Nature 2020
Abstract The constant building drag coefficient was replaced by a mechanical drag coefficient formulation in a multilayer urban canopy system coupled with the Rapid-refresh Multiscale Analysis and Prediction System to improve the wind speed prediction performance in complex urban areas. Although this formulation has been assessed in idealized urban areas by computational fluid dynamic simulations, the applicability of this formulation has not been sufficiently evaluated, especially in complex urban areas with inhomogeneously distributed tall buildings. The main objective of this study is to evaluate the performance of the coupled system in the Beijing urban area. The results show that the average wind speed model performance is improved by 40% in summer and 36% in winter by introducing a drag coefficient into the regional forecast model.
1 Introduction In the 1980s, scientists recognized the significant impacts of rough urban terrain on micro- and mesoscale flow fields and began to develop urban parameterizations in numerical mesoscale models to determine how cities change meteorological fields and the structure of the boundary layer (Bornstein 1987; Hosker 1984). In particular, Raupach (1992) analyzed the treatment of drag and drag partitions on rough surfaces and provided simple predictive expressions for practical applications. The turbulent transport of energy depends considerably on the wind profile in the urban boundary layer. Accordingly, roofs and canyon floors induce a frictional force and an associated loss of momentum; furthermore, the mean wind profile above rooftops follows the common log law, but the wind profile varies within a canyon (Masson 2000). Over the last 30 years, many observational and numerical experiments have been conducted to study flow fields in the urban boundary layer. However, more realistic urban parameterizations were
* Miao Yu [email protected] 1
Institute of Urban Meteorology, China Meteorological Administration, Beijing, China
2
Department of Mechanical Engineering and NOAA-CREST Center, City College of New York, New York, NY, USA
presented in the 2000s (Masson 2000; Kusaka et al. 2001; Martilli et al. 2002). An urgent problem to be solved in simulating winds in urban areas is how to estimate the drag induced by a group of buildings. Many atmospheric models use the roughness approach and modify the parameters in rural areas to represent the aerodynamic and thermodynamic characteristics of urban surfaces (Grell et al. 1994). For example, Monin-Obukhov similarity theory was used to calculate the gridded roughness length and the surface exchange coefficients in rough urban surfaces (Monin and Obukhov 1954). More recently, simplified urban canopy models (UCMs) have been developed to give more accurate descripti
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