Gray–Green Synergy Reduces Heat Exposure in Expanding Cities: Interactive Thresholds of Diurnal and Seasonal Land Surface Temperature

Continuous urban expansion and the resulting land use and land cover (LULC) changes significantly exacerbate the urban heat island effect and intensify heatwaves. While the cooling effects of blue–green spaces are widely documented, most studies focus on single landscape types or specific time frames. Few investigations systematically explore the comprehensive thermal regulation mechanisms of gray–green spaces, or their nonlinear driving factors and interactive effects across coupled seasonal and diurnal scales. To address these gaps, this study focuses on Chengdu, a typical expanding city in China, to establish a comprehensive indicator system for urban gray–green spaces. This system encompasses four key dimensions: coverage, fragmentation, aggregation, and morphological spatial pattern. After evaluating 12 machine learning models, the optimal model was selected for further analysis using SHapley Additive exPlanations (SHAP) and Partial Dependence Plots (PDP). This research investigates the nonlinear thresholds and interactive effects of composite gray–green space indicators on land surface temperature (LST) across varying seasonal and diurnal cycles. The results indicate that: (1) The impact of gray–green spaces on LST varies significantly across seasonal and diurnal contexts. Green spaces primarily exert a cooling effect during spring, summer, and autumn, whereas gray spaces dominate heat retention during winter and across all nocturnal periods. (2) The morphological spatial pattern of green spaces (GMSPA) outperforms traditional coverage indicators (G1) in providing cooling benefits across multiple scenarios. (3) The cooling efficiency of GMSPA peaks between −0.8 and −0.5, reaching saturation at 0.53. Conversely, LST exhibits a sharp, step-like increase when gray space aggregation (B3) exceeds −0.58. (4) Optimizing areas with high GMSPA can significantly mitigate heat exposure risks in expanding cities. These findings offer robust theoretical insights and actionable guidelines for spatial planning aimed at thermal resilience, urban thermal environment management, and building energy conservation in rapidly growing urban areas.