Quantifying spatial morphology and connectivity of urban heat islands in a megacity: A radius approach

Urban heat island (UHI) effect is an important ecological consequence of rapid urbanization. Although the spatio-temporal evolution of urban heat islands (UHIs) and their driving forces have been discussed in previous studies, the accurate identification of the spatial morphology and connectivity of UHIs is currently lacking. Taking Beijing City as an example, the radius approach (RA) was applied to identify the thresholds of UHIs, and multiple indexes were calculated to analyze the changing connectivity of UHIs from 2000 to 2015. The results showed that the UHIs in Beijing City formed archipelagos, which composed of single main heat island that occupied >79.85% of the total area, and small heat islands scattered in the centers of surrounding districts. In 15 years, the total area occupied by UHIs increased by 30.04%, indicating that the UHI effect became worse. As for landscape patterns of UHIs, aggregation index (AI) increased by 1.6%, landscape shape index (LSI) decreased by 4.1%, and probability of connectivity (PC) increased by 69.1%, all indicating that the distribution of UHIs became more compact, and the connectivity between islands increased. Different expansion types had different influences on the landscape patterns of UHIs: the edge-expansion reduced the fragmentation of UHIs and increased connectivity between islands, the infilling expansion made the boundaries of UHIs regular, and the leapfrog expansion made the AI slight decrease. Based on the radius approach, identifying the range of multi-center UHIs and their spatial expansion type can provide an effective planning guideline for mitigating the negative UHI effect.