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ABSTRACT
This study identifies causative factors of the urban heat island (UHI) and quantifies their relative contributions to the daytime and nighttime UHI intensities using a mesoscale atmospheric model that includes a single-layer urban canopy model. A midlatitude city and summertime conditions are considered. Three main causative factors are identified: anthropogenic heat, impervious surfaces, and three-dimensional (3D) urban geometry. Furthermore, the 3Durban geometry factor is subdivided into three subfactors: additional heat stored in vertical walls, radiation trapping, and wind speed reduction. To separate the contributions of the factors and interactions between the factors, a factor separation analysis is performed. In the daytime, the impervious surfaces contribute most to the UHI intensity. The anthropogenic heat contributes positively to the UHI intensity, whereas the 3D urban geometry contributes negatively. In the nighttime, the anthropogenic heat itself contributes most to the UHI intensity, although it interacts strongly with other factors. The factor that contributes the second most is the impervious-surfaces factor. The 3D urban geometry contributes positively to the nighttime UHI intensity. Among the 3Durban geometry subfactors, the additional heat stored in vertical walls contributes most to both the daytime and nighttime UHI intensities. Extensive sensitivity experiments to anthropogenic heat intensity and urban surface parameters show that the relative importance and ranking order of the contributions are similar to those in the control experiment.
1. Introduction
The urban heat island (UHI) is the most well documented example of anthropogenic climate modification (Arnfield 2003). Numerous studies have reported that the urban air temperature can be 18-38C higher than the rural or surrounding air temperature on average (e.g., Oke 1981; Morris et al. 2001; Bottyan and Unger 2003; Kim and Baik 2004; Grimmond 2007). Some studies have reported cases of very strong UHI intensity (e.g., Klysik and Fortuniak 1999; Fung et al. 2009). The UHI intensity is strongly related both to external influences (e.g., climate, weather, and season) and to the intrinsic characteristics of a city (e.g., city size, building density, and land-use distribution) (Oke 1982). In terms of meteorological conditions, calm, dry, and clear nights near the center of anticyclones are favorable for strong UHI intensity (Gedzelman et al. 2003).
Several factors that cause UHI have been proposed in the literature (e.g., Oke 1982; Grimmond 2007; Rizwan et...