How to optimize local spatial attributes of heterogeneous core street spaces in built-up areas to enhance thermal comfort during heatwaves? A case study of Hiroshima City, Japan in the summer of 2023

During heatwaves, the thermal comfort of pedestrians in high-density built-up areas is seriously threatened. However, few studies have systematically examined how thermal comfort and the effectiveness of optimization strategies vary across street spaces with different local spatial attributes. This study develops a structured research framework—comprising identification, simulation, and verification—designed to reveal the diversity and effectiveness of thermal comfort optimization strategies across street types. First, core street spaces were identified using local climate zones classification, and ENVI-met was used to simulate thermal comfort variations under heatwave conditions. Then, random forest regression was applied to identify key local spatial attributes influencing thermal comfort and to formulate differentiated optimization strategies. Finally, a comparative analysis of thermal comfort before and after optimization was conducted to assess effectiveness. Results indicate that local climate zones 1, 3, 4, 6, and 8 constitute core street spaces. During heatwaves, street spaces dominated by high-rise buildings exhibit better thermal comfort than those with low-rise buildings. Enhancing thermal comfort across all core street spaces requires building surface optimization and increased vegetation coverage. Moreover, targeted optimization strategies tailored to each local climate zones type, considering building and vegetation configuration, soil properties, and spatial openness, are essential. The implementation of these strategies effectively improves thermal comfort, though their effectiveness during peak daytime temperatures remains limited. To address this, a more integrative approach incorporating diverse cooling measures is recommended. The findings offer valuable support for climate-adaptive planning in urban built-up areas and support to mitigate the rising risks of frequent heatwaves. • Used the local climate zones classification to identify urban core street spaces. • The street spaces of local climate zones 1, 3, 4, 6 and 8 form core street spaces. • The thermal comfort of street spaces with high-rises is better during heatwaves. • Different types of core street spaces require targeted optimization strategies. • Integrated cooling measures are advised for daily heatwave peaks.