Effects of urbanization on heat waves based on the wet-bulb temperature in the Yangtze River Delta urban agglomeration, China

The Yangtze River Delta (YRD) urban agglomeration is one of the six major urban agglomerations in the world and is also one of the world's three most frequent high-temperature areas. The rapid urbanization process significantly affects meteorological air temperatures and relative humidity in urban and rural areas. Therefore, this study investigates the spatiotemporal evolution of heat waves (HWs) based on the wet-bulb temperature (TW) and urbanization effects. The results indicated that (1) the annual maximum TW exhibiting a significant decreasing trend is distributed in the Yangtze River valley and Lake Taihu Basin. The number of HW days showed a significant decrease during both 1980–1999 and 2000–2009. (2) The HW characteristics showed an insignificant decreasing trend in the Yangtze River valley, while it showed an insignificant increasing trend in the YRD. Further, the HW duration, intensity, and peak values showed significant upward trends in the coastal areas of the YRD urban agglomeration. (3) The proportion of HW duration increased from 28% in 1962–1990 to 55% in 1991–2019. The contribution of urbanization to the duration of the high-temperature HWs was 47.8%. Moreover, the impact of urbanization on HWs increased after 1990. (4) Although urban areas had a higher probability of extreme temperatures during 1991–2019, decreases in relative humidity have caused a decreasing trend of HW occurrence. Further, urban with low RH on the HW in urban areas were lower than those in rural and suburban areas. This research provides important evidence and recommendations for assessing the impact of heat waves related to urbanization.