Realistic representation of city street-level human thermal stress via a new urban climate-human coupling system

Urban overheating aggravated by climate change and rapid urbanization poses a severe threat to thermal health of urban residents. To more realistically represent street-level heat stress, we propose a new urban climate-human coupling system by integrating an advanced urban canopy model (UCM) with a new human-environment adaptive thermal stress (HEATS) module. The coupled UCM-HEATS system features a state-of-the-art solution to complicated human-street radiative exchanges and incorporates dynamic human thermoregulatory responses to microclimatic changes. The UCM-HEATS system was evaluated in a typical hot and humid city, Hong Kong, and then applied to investigate street-level thermal stress in various urban settings and under different personal conditions. By explicitly resolving shading effects of buildings and trees on human radiation budgets, our study emphasizes the marked effectiveness of active shade management using green and gray infrastructure on daytime heat mitigation, proposing a "right shade, right place, right time" paradigm for regulating important street canyon geometries (building height, road width, and tree crown width) and orientations. Additionally, human evaporative heat dissipation can be hindered by urban moisture islands and wind impediments; thus, a detailed urban ventilation strategy is suggested considering different temperature-humidity combinations. For personal heat protection, we identified an evident cooling effect of high-albedo clothing and a thermal-comfort-optimal walking speed. Special attention is paid to heat-vulnerable groups, especially older people who suffer from notably higher heat risks during pandemics with facemask-induced heat burden. Bridging urban climate and human ergonomics, this study aims to advance human-centric urban design toward future smart, resilient, and inclusive cities.