Nationwide Local Climate Zone Mapping and Comparative Assessment of Urban Heat Island Effects in South Korea

The pervasive expansion of impervious surfaces in urban areas alters surface thermal properties, resulting in the urban heat island (UHI) effect.Elevated urban temperatures constitute a considerable threat to human health by intensifying thermal stress and exacerbating air pollution.The magnitude and characteristics of UHI are highly sensitive to seasonal, diurnal, morphological, and geographic conditions.Although canopy-level temperature provides a more relevant indicator of human thermal exposure than surface-based measures, most previous assessments have relied on satellite-derived land surface temperature (LST).Motivated by these limitations, this study implemented a nationwide assessment of canopy-level UHI variability across 157 cities in South Korea, combining Sentinel-2 imagery, Korea Meteorological Administration (KMA) gridded air temperature products as a canopylevel measure complementing LST, and a deep-learning local climate zone (LCZ) mapping framework.Using a transfer-learning strategy applied to satellite imagery, we produced a novel nationwide LCZ map that captures the spatial heterogeneity of urban and natural landscapes, with an overall accuracy of 0.78.Leveraging spatially continuous air temperature datasets, we found that canopy-level UHI exhibits significantly higher values at night (0.05C in summer and 0.19C in winter) compared to daytime UHI intensity (0.03C in summer and 0.02C in winter).These effects were amplified in areas with denser and taller building morphologies, highlighting the crucial role of the built environment in shaping urban thermal conditions.Geographical analyses revealed a distinct association between summer UHI intensity and coastal proximity in both day and night (Pearson's r > 0.2 and 0.3, respectively; p < 0.005), while this pattern was attenuated in winter.Our findings provide a valuable scientific basis for evaluating humanrelevant thermal environments across cities and offer essential insights to inform urban planning and climate-resilient design strategies.