Extreme heatwaves exacerbate canopy urban heat islands: Multi-city observational evidence from eastern China

Abstract

Heatwaves (HWs) combined with the urban heat island (UHI) effect pose a significant threat to urban systems and public health. Understanding how UHI intensity (UHII) varies during HWs is critical for cities to prepare for extreme heat events. However, existing research has provided inconsistent findings. Some studies reported a substantial increase in UHII during HWs, whereas others observed minimal change or a decrease. To address this discrepancy, we analyzed three years of observational data from 15 cities in Jiangsu Province, China, using a network of dedicated monitoring stations. Our study focused on canopy-layer UHII variations during HWs, defined as periods of at least three consecutive days (May–September) with daily maximum and minimum temperatures exceeding the 90th percentile of historical records. HWs were classified into weak, moderate, and strong intensity based on the average temperature. We used non-heatwave hot summer days (within 15 days before and after a HW event) as a reference baseline for comparison. Our findings reconcile prior contradictions by demonstrating that the UHII is intensity-dependent. The UHII was on average of 0.53 °C lower during weak HWs and 0.61 °C higher during strong HWs than in the reference periods, whereas no significant differences were observed during moderate HWs. Mechanism analysis revealed that strong HWs increased UHII through higher anthropogenic heat release (e.g., air conditioning use) and lower latent heat flux. These findings advance our understanding of the interaction between UHIs and HWs, offering valuable insights into urban heat adaptation and mitigation strategies.