Unravelling the synergistic effect of urban heat and moisture islands towards healthy cities

Abstract

The synergistic effect of urban heat island (UHI) and urban moisture island (UMI) aggravates summertime heat stress especially in hot and humid cities like Hong Kong. Although UHI effect is widely studied, UMI effect and the synergistic effect of UHI and UMI have not been well understood. In this thesis, we aim to investigate mechanisms and mitigation strategies of UHI and UMI, and investigate their impacts on thermal comfort in different urban settings and for different urban populations. To unravel mechanisms of UHI and UMI, we developed an advanced urban canopy model (UCM) with more robust predictability of urban surface heat and moisture budgets by considering dynamic building-tree-air interactions. With the advanced UCM, we quantified impacts of building emissions and urban vegetation on outdoor air temperature, humidity, and heat stress in different urban forms. Our results showed that as the aspect ratio increases, UHI intensity increases at night, and UMI intensity increases at both day and night. In compact high-rise urban areas, building emissions are nonnegligible heat and moisture sources which can increase air temperature and specific humidity by up to 1.64 °C and 0.89 g/kg, respectively. In contrast, optimal air-conditioning control could alleviate the negative impact on outdoor heat stress by about 30%. On the other hand, the mitigation effects of urban trees vary substantially across aspect ratios and tree species. Specifically, tall trees with large and small leaf areas show optimized heat stress reduction effects for low and high aspect ratios, respectively. Furthermore, building emission reduction by trees enhances the combined mitigation strategy, which effectively reduces heatstroke hour percentages by 11%-35% for all aspect ratios. To investigate street-level thermal stress considering UHI and UMI effects, we developed a new urban climate-human coupling system by integrating the advanced UCM and a human-environment adaptive thermal stress (HEATS) model. The coupled UCM-HEATS model is featured with a state-of-the-art solution to the complex human-street radiative exchanges and incorporates dynamic human thermoregulatory responses to microclimatic changes. We found that UMI effect and impeded urban ventilation exacerbate thermal stress by suppressing human evaporative heat loss, and the combined strategy of urban dehumidification and ventilation shows great potential to reduce sweltering-level (highest-level) heat stress especially in compact cities with weak winds. By explicitly resolving shading effects of buildings and trees on human radiation budgets, our study emphasizes the significant effectiveness of active shade management using green and grey infrastructure on daytime heat mitigation, proposing a “right shade, right place, right time” paradigm by regulating important street canyon geometries (building height, road width, and tree crown radius) and orientations. For personal heat protection, we identified an evident cooling effect of high-albedo clothing and a thermal-comfort-optimal walking speed. Special attentions are paid to heat-vulnerable groups especially older people who suffer from obviously higher heat risks during pandemics with facemask-induced heat burden. This thesis could hopefully deepen understanding of UHI and UMI mechanisms and improve pedestrians’ thermal health via more informed urban design and personal heat protection measures.