Numerical simulation of urban impacts on wind field structures over the Yangtze River Delta under typhoon conditions

How the complex urban surface heteorogeneity influences wind field structures induced by tropical cyclones remains poorly understood despite its importance for disaster management. Here high-resolution numerical simulations using the Weather Research and Forecasting model coupled with two urban canopy models (WRF/UCMs) are conducted to address this issue with the case of landfalling Typhoon Lekima (2019) over the Yangtze River Delta (YRD) urban agglomeration. Detailed building morphology data is employed to capture urban heterogeneity. Results show that the WRF/UCMs reproduce the typhoon track and intensity reasonably well and the YRD urban agglomeration only slightly influences the typhoon track and intensity. The multi-layer UCM (BEP) significantly improves the overestimation and probability distribution of the 10 m wind speed compared to the single layer UCM (SLUCM) attributed to the explicit representation of building surface drag effect. The simulations accurately reproduced jet-like wind speeds near 1 km height, with urbanization enhancing vertical ascending and descending motions. During the typhoon primarily influenced period, the attenuation rate of the daytime wind speed due to urbanization at the lowest level reaches 56.6%, which is about 15.5% larger than that during pre-typhoon or post-typhoon periods, leading to a more pronounced vertical gradient in near-surface wind speeds. This is attributed to the almost vanished urban heat island effect during the typhoon influenced period, which stabilizes the daytime boundary layer conditions. As a result, the vertical mixing is reduced and consequently the vertical downward transport of momentum to the surface is weakened. Urban effects on the boundary layer asymmetries of the wind field structures and evolution during landfall are also explored. This study provides new insights into the importance of the urban land surface processes on regulating the wind structures under tropical cyclone backgrounds.