Quantifying the main and interactive effects of the dominant factors on the diurnal cycles of land surface temperature in typical urban functional zones

The urban heat island (UHI) effect exists both during daytime and nighttime and varies with urban characteristics, such as 2D/3D urban morphology and socio-economics. However, there is a lack of quantitative understanding of the roles of these characteristics in influencing land surface temperature (LST) variations in different urban functional zones (UFZs) throughout the diurnal cycle. In this study, we examined the responses of diurnal LSTs in different UFZs to 2D/3D urban morphology and socio-economic variables. Results showed the following: (1) During daytime, the main drivers of LST varied with not only the UFZs but also the observation times; during nighttime, the LST variations across different UFZs were largely controlled by 3D urban morphology and socio-economic factors. (2) At 10:37, LST declined most rapidly when the percentage of tree cover (PER_Tree) exceeded a certain threshold. The threshold values of PER_Tree were 85%, 70%, 50%, and 60% for the residential, industrial, commercial, and public service zones, respectively. Irrespective of the UFZs, a nighttime cooling effect occurred only when sky view factor (SVF) exceeded 0.8. (3) For locations with high population density (Pop_Den) in the residential zone, whether urban trees induced a cooling effect depended on both the observation time and PER_Tree during daytime; however, a higher SVF tended to result in an increased LST during nighttime. In the public service zone, when Pop_Den exceeded 50, urban trees with high height contributed to nighttime LST cooling, whereas a warming effect occurred with trees with low height. The direct implications of this study suggest that 3D urban morphology and socio-economics are more efficient mitigation strategies for all UFZs at night, and the interactive effects between the dominant drivers of diurnal LSTs should be considered to cool the city most effectively.