Assessment of morphometric methods for estimating aerodynamic roughness parameters using a real-city LES turbulence database

Aerodynamic roughness parameters are key parameters in momentum exchange in atmospheric surface layer. Various morphometric methods have been proposed for use in urban environments based on wind tunnel measurements and LES simulations for idealized or urban-like morphologies. While these estimation methods have been widely used in various studies, their quantitative evaluation requires further investigation. This study aims to assess different morphometric methods that are widely used in urban environmental simulations. In doing so, a real-city LES simulations database was established, each of which includes flow, turbulence, and drags for 852 urban surface types extracted from the Seoul metropolitan area in Korea. From the LES database, total surface drags (pressure and frictional drags) were explicitly obtained. Then, we assessed the performance of six morphometric methods (Kanda et al., 2013; Millward-Hopkins et al., 2013; Zhu et al., 2016; Macdonald et al., 1998; Grimmond et al., 1999; Shao and Yang, 2005) in simulating total surface drags at various different morphometric environments. In addition, we compared the aerodynamic roughness parameters estimated by the morphometric methods. Using these parameters and the LES dataset, friction velocity was calculated based on the logarithmic law and quantitatively evaluated using statistical indices. Among them, three methods suggested from homogeneous urban morphology underestimated the LES-resolved friction velocity, showing relatively large errors in high momentum drag conditions. Meanwhile, the methods by Millward-Hopkins et al. (2013) and Kanda et al. (2013) relatively well compared against the LES database. They also showed large errors increasing with the magnitude of total surface drags. Additionally, relatively large errors were found in low building-density areas.