Spatio-temporal assessment of urban heat island and its natural and anthropogenic drivers

Abstract

Surface urban heat island (SUHI) shows thermal intensification in urban areas compared to surrounding areas. Land surface temperature (LST) has been widely applied in assessing SUHI. This thesis focuses on the spatio-temporal assessment of SUHI and its natural and anthropogenic drivers based on Landsat series data via Google Earth Engine (GEE). Natural cooling processes of blue spaces have seldom been addressed compared to green spaces. In Chapter 2, I mapped SUHI intensity (SUHII) and blue spaces in order to explore the cooling effect and efficiency of blue spaces in the Pearl River Delta Metropolitan Region. Significant in-situ cooling is observed. The irregular shape of lakes and reservoirs may weaken the cooling effect, but the reshaping of rivers may not. Our results indicated ex-situ cooling spillover of water bodies benefitting the surrounding areas within an envelope of 100-m effective cooling distance. Due to the limitation of LST retrieval method, few researchers attempted to track the long-term dynamic of SUHI and its responses to progressive urbanization based on Landsat series data. In Chapter 3, the dynamics of SUHI composition and configuration and their responses to urbanization was explored. Hangzhou city core was heating up in 1990-2010. The expansion of heated area initially radiated from the original urban core in a linear configuration, and subsequently expanded to infill areas away from the core by replacing neutral and cool area. The urban land had the highest SUHII, whereas forest and water body registered the lowest. The expansion of urban land at the expense of natural land had exceeded a threshold to induce spillover of heat energy from the former to the latter. SUHI of large cities has attracted wide attention but the contradictory findings in previous literature raise the questions about the climatic differences of SUHI drivers. In Chapter 4, SUHIIs of 277 metropolises in different climate zones were calculated, and their primary determinants were investigated. SUHII varied across the sampled cities in different climate zones due to different determinants. In tropical and warm temperate climate zones, the primary determinants were forest fraction and precipitation, and the forest fraction was the most important determinant in arid climate zones. In continental climate zones, the building coverage and continuity of water were the critical drivers of SUHII. The climatic differences induced by SUHII determinants implied the need to have different heat stress strategies in different cities. Based on Chapter 4, Chapter 5 further explored the relationship between the primary determinants and SUHII in the warm temperate climate zone.A mathematic model was applied to explore the component contribution to SUHII. From the perspective of surface energy balance, forest coverage and precipitation were the critical drivers of processes (convection and evapotranspiration), that contributed the most to SUHII changes. In the continental cities, the urban blue infrastructure may be more effective than urban green infrastructure. The findings could inform approaches to protect and enhance natural land-cover as nature-based solutions and rainmaking to mitigate thermal stress in metropolisesin different climate zones.