Imran, Hosen M (2018) The Urban Heat Island of Melbourne during Heatwaves: Impacts of Future Urban Expansion and Effectiveness of Green Infrastructure as Mitigation Strategies. PhD thesis, Victoria University.

Abstract

The city of Melbourne in southeast Australia experiences an Urban Heat Island (UHI) effect, which is exacerbated during heatwaves, and the latter are becoming more frequent, intense and longer in southeast Australia. In addition, Melbourne is the fastest growing city in Australia. Therefore, it is urgent to understand the dynamics of UHI and impacts of future urban expansion on the UHI during heatwaves. Based on these issues, there is a crucial need to investigate the effectiveness of potential mitigation strategies to minimize UHI effects during heatwaves. The overarching aim of the thesis is to investigate the impacts of future urban expansion on the UHI during heatwave events in Melbourne, and examine the effectiveness of different Green Infrastructure (GI) scenarios such as green/cool roofs, mixed forest (MF), mixed forest and grassland (MFAG), and mixed shrublands and grasslands (MSAG) in mitigating UHI effects. The Weather Research and Forecasting (WRF) model coupled with the Single Layer Urban canopy Model (SLUCM) was used in simulating the UHI and heatwaves. Since the WRF model is known to be sensitivity to the choice of physical parameterisation options, an initial sensitivity analysis of the model was conducted and the best-possible WRF configuration to simulate the UHI during heatwaves in Melbourne was determined, among a 27-member physics ensemble. This configuration was used throughout the rest of the thesis. Urban expansion increased near surface UHI by 0.75 to 2.80 °C during the night but no substantial impacts during the day. Urban surfaces absorbed more solar heat during the day as compared to vegetated surfaces, and the absorbed heat was released slowly from evening to early morning. The storage heat in urban surfaces was the key driver in increasing UHI during the night. Urban expansion did not substantially affect human health (HTC) comfort in existing and expanded urban areas. Green roofs showed good performance in reducing roof surface UHI (1 to 3.8 °C) and near surface UHI (0.3 to 1.1 °C) during the day but not during the night, while cool roofs showed higher reductions at the roof surface UHI (2.2 to 5.2 °C) and near surface UHI (0.5 to 1.6 °C) during the day. Green roofs increased evapotranspiration and provided shading, and consequently, increased Latent Heat (LH) and substantially decreased storage heat and sensible heat, and as a result, reduced the UHI. Cool roofs reflected a major portion of incoming solar radiation due to higher albedo, and reduced the sensible heat flux and storage heat, and these were the key drivers in reducing UHI during the day. In addition, both green and cool roofs showed good potential in improving HTC from extreme to very strong during the day. Other GI scenarios such as MF, MFAG and MSAG were effective in reducing UHI effects and improving HTC during the night but no substantial reductions were occurred during the day. By increasing GI fractions from 20 to 50 %, the UHI was reduced by 0.6 to 3.4 °C for MF, 0.4 to 3.0 °C for MSAG and 0.6 to 3.7 °C for MFAG. The night time cooling was driven by reductions in storage heat as 20 to 50 % urban areas were replaced by GI, which would have led to even less radiation reaching the ground surface during the day due to their higher LAI and shade factor, and leading to lower storage heat. As the green and cool roofs showed potential in reducing UHI effects during the day while urban vegetated patches showed effectiveness during the night, therefore, a combination of green/cool roofs and urban vegetated patches could be an optimal mitigation strategy in reducing UHI effects and improving HTC during both day and night.

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