Exploring the effect of urbanization on hourly extreme rainfall over Yangtze River Delta of China

Short-term extreme rainfall events (EREs) often produce urban flooding and result in the loss of life and property. In this study, three high spatial-temporal resolution precipitation products were used to analyze hourly EREs during May-October from 2001 to 2018 over the Yangtze River Delta (YRD). To quantify the possible link between EREs and environmental factors (such as urbanization, climate change, and orography), the Hovmoller diagrams and Generalized additive model for location, scale, and shape (GAMLSS) nonstationary model were used. The results showed that the eastward propagating precipitation systems created a heavy rainfall center in the southwest mountainous of YRD due to orographic lifting, and enhanced in the plains over urbanized areas, especially in the vicinity of Nanjing, Changzhou, and Nantong metropolitan regions. The variability of hourly rainfall extreme in the urbanized regions is high correlated to the multivariate ENSO index (MEI), and shows a greater magnitude in the urban than rural regions. This phenomenon was intensified by large-scale circulation patterns due to strong-ENSO over the NanjingChangzhou-Nantong (NCN) urban regions, but weakened over the Shanghai-Hangzhou-Ningbo (SHN) urban regions. An analysis of nonstationary GAMLSS also suggests the large-scale circulation presents a greater impact on the hourly extreme rainfall than the urbanization, while the hourly rainfall extremes in the urban areas are still higher than that in rural regions under the nonstationary scenario. Those results suggest intense rainfall occurred more frequently in the urban region under observed climate change and urbanization in YRD.