Decrease in urban-rural differences of nighttime humid heatwaves in the Yangtze River Delta, China

Under the combined influence of climate change and human activities, heatwaves have become more frequent and intense globally. Nighttime humid heatwaves (HHWs), frequently occurring in the Yangtze River Delta (YRD), pose greater risks to human health than daytime or dry heatwaves due to reduced relief from daytime heat and difficulty in heat dissipation under hot and humid environment. However, few studies focus on nighttime humid heatwave and the impact of urbanization on it also lacks. This study analyzes the spatiotemporal evolution of nighttime HHWs and evaluate the urban-rural differences in HHWs based on data from 58 meteorological stations across the YRD. The results show that nighttime HHWs became more durable, severe and frequent during 1985-2017. During the rapid urbanization phase of the YRD (2003-2017), while urban regions suffered more severe nighttime HHWs than rural areas, the differences in HHWs between the two decreased over time, as a result of the diminishing urban heat island effect, the differences in climatic characteristic as well as changes in land use and land cover. This study provides insights to evaluate the risk of nighttime HHWs in the YRD and scientific support for urban planning and climate change mitigation.