Seasonal and Diurnal Variability of Surface Urban Heat Island Intensity and the Influence of Local Climate Zone Mix: A Case Study of Suwon, South Korea

Urbanization increases impervious surfaces and intensifies the surface urban heat island (SUHI).Satellite-based land surface temperature (LST) is widely used to assess SUHI, but limited high-resolution nighttime observations have led many studies to focus on daytime conditions.In this study, using 70-m ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) LST, we quantify seasonal (summer vs. winter) and diurnal (day vs. night) variability in surface urban heat island intensity (SUHII) in Suwon, South Korea, based on ten clear-sky acquisitions (2022)(2023)(2024)(2025).A two-way repeated measures ANOVA confirms that SUHII is significantly stronger and more spatially heterogeneous in summer than in winter, and during the day than at night.To explain these time-dependent SUHII patterns in relation to urban form while minimizing scale-mismatch information loss, we aggregate 30-m local climate zone (LCZ) compositions within each 70-m SUHII pixel and estimate time-specific effects via compositional regression.Natural LCZs are generally associated with cooling, with tree-related classes showing cooling that varies with canopy density.Water (LCZ G) exhibits a pronounced day-night contrast, shifting from daytime cooling to nighttime warming.Built-up LCZs increase SUHII, with high-density development (LCZ 1-3) consistently producing warming across all times.Vertical structure effects are time-dependent, with stronger warming in low-rise areas during the day and in high-rise areas at night.This suggests that shading and solar exposure dominate daytime surface heating in low-rise buildings, whereas nocturnal warming in high-rise areas is more strongly shaped by radiative trapping and heat storage within urban canyons.These results quantify time-dependent patterns in SUHII and LCZ-specific heat responses, supporting a basis for identifying season- and time-specific heat-vulnerable areas and tailoring mitigation by urban form.