Comprehensive assessment of daytime, nighttime and compound heatwave risk in East China

An increasing number of heatwaves have seriously threatened human production and life. Since nighttime heatwaves may prevent humans from effectively recovering from the damage to health caused by daytime heatwaves, the combination of daytime and nighttime heatwaves, known as compound heatwaves, may pose even greater risks to human health. Most studies have focused on the impacts of daytime heatwaves, with relatively few examining nighttime and compound heatwaves. Using the entropy weight method and the risk triangle assessment framework, along with heatwave indicators, socio-economic, and ecological data, we assessed the risks of daytime, nighttime, and compound heatwaves in the Yangtze River Delta (YRD) of China for the years 2000, 2005, 2010, 2015, and 2020. The results indicate that (1) the frequency and duration of heatwaves in YRD shows a continuous increasing trend, with an increase of about 20 days in the duration of heatwaves over the 30-year period, (2) most areas experiencing nighttime heatwaves also experience compound heatwaves, and their spatial distribution is essentially the same, (3) in 2020, Anhui had the overall highest heatwave risk, while Shanghai had a relatively lower heatwave risk. The multi-year risk assessment results suggest that nighttime heatwaves may "amplify" or "diminish" the risk of compound heatwaves. This study emphasizes the potential impact of nighttime heatwaves on the risk of compound heatwaves, which is crucial for addressing the risks of compound heatwaves in the context of global warming and accelerated urbanization.