Diurnal Asymmetry in Nonlinear Responses of Canopy Urban Heat Island to Urban Morphology in Beijing during Heat Wave Periods

Currently, the diurnal asymmetric mechanism by which urban morphology affect canopy urban heat island (CUHI) during heat wave (HW) periods has not received sufficient attention. This study took the area within the Fifth Ring Road in Beijing as the research object, integrating XGBoost machine learning model and ENVI-met microclimate simulation technology to quantitatively analyze the non-linear response characteristics of CUHII to urban morphology during HW periods. The results show that CUHI intensity (CUHII) during HW periods is significantly enhanced compared with nonheat wave (NHW) periods, with a daytime increase of 91.3 % and a nighttime increase of 52.7 %. The analysis of the XGBoost model indicates that the building coverage ratio (BCR) is the core driving factor of CUHII during the day, while the sky view factor (SVF) plays a more prominent dominant role at night. The regulatory effects of 2D/3D morphological indicators during HW periods are significantly stronger than during NHW periods. ENVI-met simulations further reveal the nonlinear regulation mechanism of building height on diurnal thermal environments: as SVF decreases, daytime thermal environments are collaboratively driven by short-wave radiation shading and ventilation resistance, while nighttime thermal environments are dominated by the reflection and accumulation of long-wave radiation by buildings. Furthermore, this study explores the regulatory effect of the wind environment on CUHII and its diurnal differences. The findings of this study provide new insights into the formation mechanisms of diurnal differences in CUHII and offer a scientific basis for the optimal design of urban morphological indicators under extreme high-temperature conditions.