Contrasting interactions of urban heat islands with dry and moist heat waves and their implications for urban heat stress

The synergistic interactions between urban heat islands (UHIs) and heat waves are of great concern due to their adverse impacts on urban residents and scientific interest. Here, we examine the interactions of UHIs with two types of heat waves, dry heat waves (DHW) and moist heat waves (MHW). For this, Daegu, a large city in South Korea, is selected and observational and reanalysis data from 2001 to 2022 are analyzed. DHW (MHW) is defined as the heat waves with daily mean relative humidity lower (higher) than its 10th (90th) percentile. The mean nighttime UHI intensity calculated using daily minimum 2-m temperatures is stronger under DHW by 1.07 °C and weaker under MHW by 0.24 °C than under non-heat waves. This indicates synergistic interactions between UHIs and DHW, but negative interactions between UHIs and MHW. Evaluation of four thermal discomfort indices shows that the prominent nighttime synergistic UHI-heat wave interactions can result in similar or even greater nighttime heat stress under DHW compared to MHW for urban residents, despite lower humidity. This study highlights the necessity of considering heat wave characteristics and the consequent UHI-heat wave interactions in assessing and managing urban heat-related risks.