Synergistic effects of compound temperature-humidity extremes and urban heat-moisture islands: An observational study in high-density humid subtropical twin cities

Abstract

Research on the synergistic effects of compound temperature-humidity extremes on urban heat island (UHI) and urban moisture island (UMI), particularly in humid subtropical climates, remains limited. This study investigates these synergistic effects in Shenzhen and Hong Kong, two neighboring cities in China's Great-Bay-Area that undergone distinct urbanization processes. Based on the 90th percentiles of daily moving averages of air temperature and specific humidity, three types of extreme events (HotWet, Hot, and Wet events) were identified. Categorical analysis was performed using ten-year observational data across five types of built-up-areas classified by the local climate zones scheme, while spatial analysis drew from 154 weather stations. Results reveal that UHI intensity follows a hierarchy: urban high-rise & urban low/mid-rise areas > urban park & urban seaside > suburban. Extreme events generally intensify UHI and UMI effects, with a stronger impact observed in Shenzhen than in Hong Kong, where UHI and UMI are, on average, 0.22 °C and 0.24 g/kg higher, respectively. Hot events increase UHI by approximately 0.25 °C more than HotWet events, whereas HotWet events enhance UMI by about 0.06 g/kg more than Wet events. Urban vegetation and blue spaces mitigate extreme weather effects, but their effectiveness varies with extreme weather type and urban factors. The frequency of compound extremes is positively correlated with UHI in both summer and winter while positive correlations with UMI intensity are only observed in winter. These findings emphasize the importance of urban resilience enhancements.