Exploring synergies between urban heat islands and heat waves and their effects on heat stress in Dhaka, Bangladesh

This study examines the synergies between urban heat islands (UHIs) and heat waves (HWs) and their quantitative effect on urban heat stress in Dhaka, Bangladesh under HWs. For this, total 51 days in the pre-monsoon seasons of 2020, 2021, and 2022 are simulated using the Weather Research and Forecasting model. During the HW days, the synergies between UHIs and HWs are weak in the daytime (0.07 °C) and pronounced in the nighttime (1.28 °C). Due to HWs, the heat storage and the release of stored heat increase greatly in the urban area, which is responsible for the pronounced synergies in the nighttime. To quantitatively evaluate the effect of the synergies on urban heat stress, individual effects of HWs, UHIs, and their synergies on urban heat stress are quantified and the normalized relative impact index is defined. The HW effect plays the most important role in daytime urban heat stress under HWs, and the UHI effect is most important for aggravating nighttime urban heat stress under HWs. The synergistic effect on urban heat stress is minor in the daytime, whereas it plays roles in worsening nighttime urban heat stress, its influence being 37–44% of the UHI effect. • Synergies between urban heat islands (UHIs) and heat waves (HWs) are studied. • The synergies are pronounced in the nighttime, being 1.28 °C. • The increase in urban heat storage due to HWs is important for the synergies. • The HW (UHI) effect is most important for daytime (nighttime) urban heat stress. • The synergistic effect worsens urban heat stress particularly in the nighttime.