The effectiveness of Urban Green Infrastructure for urban heat regulation.

Abstract

Excess urban heat is one of the significant challenges for cities, which combinedly occurs and exacerbates with urban heat islands, heatwaves and climate change impacts. It diminishes the quality of urban life and makes cities intrinsically more vulnerable than rural areas. Extreme heat has claimed more human lives than other weather events. Thus, it requires urgent mitigation and adaptation strategies to cope with current and predicted excess urban heat. Direct heat reduction impacts by remedial actions are considered as mitigation, and the ability of those actions to adjust the preparedness of cities towards current and future severity of heat impacts and, in the long run, climate change is considered as adaptation in this context. Urban Green Infrastructure (UGI) is receiving spotlight attention recently as a mitigation solution for excess heat. UGI provides multiple ecosystem services (ESS) to cities, including excess heat regulation. Cooling potential of UGI is extensively studied in existing literature, but some aspects, including lack of comprehensive evaluations on effectiveness, large-scale, context-specific and realistic assessments and impacts of the spatial configuration of UGI on urban heat mitigation, are yet underexplored, which objectively used to design the thesis. The overall aim of this thesis is to evaluate the effectiveness of UGI as a mitigation strategy for extreme urban heat. This aim was evaluated through 6 chapters in the thesis, with a systematic literature review and 3 content chapters. Overall aim is divided into 3 objectives, which are addressed in each chapter: evaluating the effectiveness of urban surface parameters - USP (Chapter 3), identifying the effectiveness of combined USP to maximise the cooling potential and operationalisability in real-world cities (Chapter 4) and assessing whether the spatial configuration of UGI matters in mitigating excess urban heat at a neighbourhood scale (Chapter 5). To generate high-resolution empirical evidence for Melbourne, Australia, mesoscale and microclimate climate modelling experiments were used. Among the evaluated multiple USP in a scenario analysis, green roofs, cool roofs, and ground-level vegetation ratios showed high correlations (strong, negative, near-linear relationships) with temperature reduction, proving every bit of enhanced green in city-wide matters in urban cooling. Green surfaces exhibit the maximum heat reduction during nighttime, while reflective, cool roofs during daytime. This variation supports the design of the next chapter to combine USPs to achieve a diurnal maximum cooling impact. Realistic ratios and albedos for 3 effective parameters were selected to ensure operationalisability, and 18 scenarios were designed over 10 years. The scenario with maximum combined surface parameters proved to be the most effective with realistic implementation and maximum functionality. This emphasises the necessity of evaluating the effectiveness of heat mitigation solutions considering both functional performance and operationalisability. As results suggest in chapter 5, the same amount of greenspaces with different spatial configurations as big parks or pocket parks matter for a location, but there is no universally superior design. This effectiveness is highly context-specific and influenced by the local climate zone (LCZ), which drives by factors such as surface cover, built-up area, and structural elements such as building height and configuration. Thus, the thesis outcomes will inform the city authorities, urban planners and practitioners to plan with combined USP and UGI solutions in effective spatial configurations to maximise the cooling effectiveness, day and night. It is essential to perform context-specific simulations before planning and designing an effective heat mitigation plan. Show more