Surface and canopy urban heat islands: Does urban morphology result in the spatiotemporal differences?

Urban heat island (UHI) is a phenomenon wherein urban areas become warmer than surrounding rural areas, which is one of the most important urban issues arising due to human activities. In this study, UHI was investigated at two different layers: canopy urban heat island (CUHI) and surface urban heat island (SUHI). Kitakyushu City, Japan, which is a highly spatially heterogeneous urban area, was utilized as a case study to explore the spatiotemporal differences of the two types of UHIs, both during the daytime and nighttime from January to December 2010. Subsequently, random forest (RF) regression was applied to reveal the correlation between the differences in the two types of UHI intensity (∆UHII) and urban morphology indices. The results showed that (1) the SUHI and CUHI during daytime had similar spatial patterns, while those during nighttime had different spatial patterns; (2) the daytime SUHI intensity (SUHII) could be greatly overestimated compared to the CUHI intensity (CUHII), and this overestimation significantly varied with months; and (3) highly nonlinear correlations existed between ∆UHII and urban morphology indices. Both the internal and external characteristics of urban morphology in a study grid contributed to the generation of ∆UHII, but internal characteristics were more important.