Analyzing Cooling Island Effect of Urban Parks in Zhengzhou City: A Study on Spatial Maximum and Spatial Accumulation Perspectives

As a result of urbanization, cities worldwide are experiencing urban heat island (UHI) challenges. Urban parks, which are essential components of urban blue and green landscapes, typically have lower temperatures in providing outdoor comfort than their surroundings with impervious surfaces. This phenomenon, known as the park cooling island effect (PCIE), has been recognized as an effective approach to mitigate the negative effects of the UHI in the context of sustainable development of urban environment. To cope with the serious UHI challenge and to guide urban park planning and design for Zhengzhou City, which is one of the China's new first-tier cities, 35 urban parks in the city were analyzed in this study. Remotely sensed land surface temperature (LST) and reflectance images by Landsat 9 and Sentinel-2 were selected as data sources. A cubic polynomial model that depicts the relationship between the LST and the distance from the park edge was first built for each park. Based on this model, the spatial maximum perspective metrics (including the park cooling area (PCA) and park cooling efficiency (PCE)) and the spatial accumulation perspective metrics (including park cooling intensity (PCI) and park cooling gradient (PCG)) were calculated to quantify the PCIE of each park. The 35 parks were divided into three groups using the hierarchical clustering method for further analysis. For each group, the metrics of the PCIE were statistically analyzed, and the main factors influencing the PCIE were identified by the Spearman correlation coefficient. The results indicate the following: (1) The 35 urban parks exhibit an obvious PCIE. The maximum cooling distance is 133.95 +/- 41.93 m. The mean LST of the park is 3.01 +/- 1.23 degrees C lower than that within the maximum cooling distance range. (2) The PCIE varies among different types of parks. Parks with large areas and covered by certain water bodies generally exhibit higher PCA, PCI, and PCG values. However, parks with small areas and mainly covered by vegetation show higher PCE values, which makes them more economical in exerting the PCIE. (3) Park area and landscape shape index (LSI) were positively correlated with PCA, PCI, and PCG. However, there is a threshold in the relationship between the park area and the PCI. A park area of approximately 19 ha can produce a higher PCI than a smaller one. In central urban areas with limited space, parks with small areas, complex shapes, and predominant vegetation coverage can be designed to achieve higher cooling efficiency.