Numerical Simulation of Urban Impacts on Tropical Cyclone Wind Field Structures Over the Yangtze River Delta Region

Abstract

How the complex urban surface heterogeneity influences wind field structures induced by tropical cyclones remains poorly understood despite its importance for disaster mitigation. Here high-resolution numerical simulations using the Weather Research and Forecasting model coupled with the multi-layer urban canopy model (WRF/BEP, Building Effect Parameterization) are conducted to address this issue with the case of landfalling Typhoon Lekima (2019) over the Yangtze River Delta (YRD) urban agglomeration. Results show that the WRF/BEP reproduces the typhoon track and intensity reasonably well and the YRD urban agglomeration only slightly influences the typhoon track and intensity. The BEP significantly improves the overestimation and probability distribution of the 10 m wind speed attributed to the explicit representation of building surface drag effect. Urbanization enhances boundary layer vertical ascending and descending motions, aligning with cross-sectional analyses along urban clusters. 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 (UHI) 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. This study provides new insights into the importance of the urban land surface processes on regulating the wind structures under tropical cyclone backgrounds.