Urbanization Magnifies Sub-Daily Precipitation Extremes over Three Vast Urban Agglomerations of China

Against the backdrop of global warming and accelerated urbanization, sub-daily precipitation extremes (SPEs) have become increasingly frequent in China’s megacities, often triggering catastrophic urban flooding. This study comprehensively analyzes the spatiotemporal variations and underlying mechanisms of SPEs across three major urban agglomerations: Beijing–Tianjin–Hebei (BTH), Yangtze River Delta (YRD), and Pearl River Delta (PRD). Using six extreme sub-daily precipitation indices (ESPIs) derived from hourly precipitation data, we demonstrate that both the frequency and intensity of SPEs increased significantly during 1971–2022, with the most substantial changes occurring in the YRD. Moreover, the ESPIs increased more significantly during the rapid urbanization era (2001–2022) relative to the urbanization era (1971–2000) across the three major urban agglomerations. The intensified extreme hourly precipitation almost contributed half of the total precipitation in summers of the 2010s. Urban stations consistently exhibited higher mean values and faster increase rates of ESPIs compared to rural stations. These findings align with the thermodynamic consequences of global warming and localized urban agglomeration effects. Enhanced surface air temperatures, elevated 850-hPa humidity, increased atmospheric instability (e.g., K-index), and strengthened moisture convergence collectively create favorable conditions for more frequent and intense SPEs in the current era of climate change and rapid urban expansion.