Contrasting frequency of global canopy and surface urban heat Island

The frequency of urban heat island (UHI) is a critical metric for assessing the temporal dynamics of urban heat islands and holds significant value for mitigation strategies. The investigation of the frequency of canopy UHI and surface UHI (CUHIF and SUHIF) is of great importance, as it represents different aspects of urban thermal environments. However, studies examining global CUHIF and SUHIF variations remain limited, and the mechanisms and relative contributions of various driving factors to these two types of heat island frequencies are poorly understood. This study calculated CUHIF and SUHIF for 1162 cities worldwide using global near-surface air temperature ($T_a$) and seamless land surface temperature (LST) datasets. The UHI frequency calculation thresholds were defined using global mean UHI intensity, specifically 0.5 K for canopy UHI and 1.0 K for surface UHI. The contributions of background climate (BGC), surface properties (SUP), and overall urban metric (OUM) were evaluated using the LightGBM model and SHAP algorithm. Results show that global daytime CUHIF and SUHIF are 72 and 217 days, respectively, whereas the nighttime values are 92 and 161 days. The tropical regions exhibit the highest daytime CUHIF and SUHIF (91 and 298 days), while arid regions have the highest nighttime values (135 and 200 days). The background climate exerts a dominant influence on daytime CUHIF and SUHIF (38% and 45%, respectively), while nighttime UHI frequencies are influenced by a combination of background climate and surface properties (52% and 36%, respectively). Meanwhile, CUHIF and SUHIF reach their annual maximums in summer and minima in winter. These findings enhance understanding of UHI frequency patterns and drivers, providing scientific evidence for mitigation strategies.