Evaluating urban heat island mitigation strategies for a subtropical city centre (a case study in Osaka, Japan)

Abstract

Urban heat island (UHI) effects were first observed in London in the 19th century. The urban heat island is identified by developing higher temperatures in urban areas than the surrounding rural areas that directly surround them. However, there are some main mitigation strategies to deal with subtropical UHI, such as increasing the albedo of the urban environment (reflective pavement) and developing the green infrastructure (green roof) in urban areas. This study would analyse the microclimate in a subtropical city by evaluating meteorological parameters with a three-dimensional model simulation software of computational fluid dynamics (CFD) named ENVI-Met.To evaluate Urban Heat Island mitigation strategies for a Subtropical City Centre, subtropical city Osaka, Japan, is selected to investigate UHI with modelling. The simulation has been used to applicate the five scenarios (base scenario, cool pavement scenario, cool roof scenario, increasing vegetation coverage scenario, and integrated scenario) with different albedo and vegetation coverage ratios. In this study, outdoor air temperature, sky view factor, net radiation, mean radiant temperature and thermal radiative power are considered as five criteria for evaluating the efficiency of UHI mitigation strategies. The simulation results of the base model (scenario A) with Osaka's current condition are considered the reference value. The relative percentage differences between each scenario with a base model are determined. The increased albedo of urban fabric material (scenario B Cool pavement model) showed the most efficient to mitigate UHI. The relative percentage differences of the five criteria in the Cool pavement model are more significant than other models. Therefore, the results of this study can provide valuable guidance, both for keeping subtropical residents cooler and informing subtropical climate cities that would be sustainable in the future.