Urbanization intensified predecessor rainfall redistribution associated with Typhoon Khanun (2017) in the lower Yangtze River region

The Yangtze River Delta region (YRD) is one of the most important regions with rapid urbanization in China. The research employs the Weather Research and Forecasting Model (WRF) to investigate the influences of urbanization on precipitation patterns and intensities of predecessor rain events (PREs) induced by Typhoon Khanun (2017) in the YRD. The results indicate that using land use data during the precipitation event can improve the model simulation performance. Although the total regional precipitation remains relatively unchanged, urbanization significantly alters the spatiotemporal distribution of rainfall, resulting in enhanced precipitation in the coastal urban regions near the heavy precipitation center regions, with a maximum increase up to 52 %. In contrast, rainfall near the inland outer region of the heavy precipitation center is reduced, with a maximum decrease up to 50 %, exhibiting a prominently uneven distribution pattern, which requires adaptive zoning in flood management. Mechanistically, around the coastal urban regions near heavy precipitation centers, urbanization enhances water vapor convergence and atmospheric instability, necessitating revised stormwater infrastructure design standards for these high-risk zones; while it suppresses water vapor transport, enhancing atmospheric stability in the inland outer region. Further analysis of the cloud microphysics reveals that urbanization significantly strengthens the processes of raindrop accretion of cloud water and water vapor condensing into raindrops, thereby promoting increased rainfall in the coastal urban region; in contrast, it may weaken the processes of snow melting and water vapor condensing into raindrops, leading to reduced precipitation in the peripheral inland outer region. These findings offer scientific support for climate-resilient urban planning, demonstrating how urbanization shifts precipitation patterns, necessitating coordinated regional adaptation for flood resilience.