Numerical simulations on the 120-m resolution wind field in a non-uniform urban canopy layer with a land cover classification and canopy porous medium-based drag model

The wind field in urban canopy layers (UCLs) was of wide concern in air pollution and urban climate studies. If each building would be specifically depicted in modelling the wind field in a non-uniform UCL, the computational task would be excessively large. Thus, methods for urban wind field simulations with reasonable horizontal resolutions and computational tasks are urgently required. In the current work, a non-uniform UCL model was proposed with the land cover classified by local climate zones (LCZs) and wind drag calculated by a porous media model (PMM). With the proposed non-uniform UCL model, the wind field within the UCL of Xi'an, China, was simulated with a grid spacing of 120 m horizontally and 1–3 m vertically. Simulations revealed that the wind field within the UCL was weakened with large-scale vortices observed in areas with higher canopy height (LCZs 1 and 4), which was unfavourable for the outward diffusion of pollutants and waste heat. The height of the roughness sublayer for a non-uniform UCL would be approximately six times the maximum urban canopy height. Airflows were dominated by ejection events within the canopy and sweep events above the canopy. The results revealed that the proposed non-uniform UCL model based on LCZ and PMM was reasonable for wind field simulations with lower computational tasks and higher accuracy (120 m horizontally) to reveal the spatial distribution characteristics of the wind field within the UCL.