Spatio-temporal variations in urban heat islands effects of Nanjing, China derived from the dense Landsat imagery (1992–2011)

The magnitude and pattern of the urban heat island (UHI) effects have been major concerns of many urban climatology studies and land use planning programs. Taking Nanjing, China as the case study, based on the dense Landsat thermal observations (8 scenes) from 1992 to 2011, geometric correction and radiometric calibration were implemented to refine the thermal data, followed by the retrieval and mapping of the surface brightness temperatures. Via identifying a suite of PIFs (Pseudo-Invariant Features) in concert with 6 near simultaneous measurements collected from the local meteorological observatories, the ground truth temperatures maps were derived and normalized to make all the images comparable directly. Then, grading the heat island patterns was employed through a fixed brightness temperature mean and standard error to characterize the spatio-temporal variations in the UHI. Results suggested that conversions from large-scale low temperature and sub-low temperature areas into medium temperature and high temperature areas were frequently witnessed in Nanjing, and the expansion route of the heat islands mainly ran along north to south. The 1990's increase rates of the heat island areas were smaller than those in the 21st century and the peak values of the areas were observed in 2007 and 2011. The average intensity of the heat island remained relatively stable and the extreme temperatures varied greatly. The reasons responsible for the changes in spatio-temporal characteristics of the UHI were the sharp increase in impervious surfaces, increased use of fossil fuels and greening policies. Ultimately, increase in the naturally vegetated surfaces, effective control of urban air pollution, accelerating urban ventilation, strengthening environmental philosophy of the nationals were recommended as the countermeasures mitigating the heat island effects, which may enable the establishment of a harmonious, livable and low-carbon urban space for living.