Exploring diurnal thermal variations in urban local climate zones with ECOSTRESS land surface temperature data

Excessive heat in urban areas affects the overall livability of cities and harms the health of urban dwellers. Understanding the spatiotemporal variations of land surface temperature (LST) and the thermal impacts of heterogeneous urban forms is crucial for informing mitigation strategies to improve the urban thermal environment. However, the diurnal thermal dynamics over urban areas has been rarely studied due to the polar-orbiting nature of conventional satellite platforms (e.g., Landsat Series, Terra, Aqua). The National Aeronautics and Space Administration's (NASA) latest Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) provides LST observations at a 70 m resolution for different times of day and night, and thereby offers unprecedented opportunities for exploring the LST diurnal dynamics. We used ECOSTRESS data to explore the diurnal cycling of LST and variations among local climate zones (LCZs) in Xi'an, a megacity in Northwest China. The urban area was segmented into 2733 community units that were classified into seven LCZs. We examined the intra-LCZ variation of LST at different times of day and night to investigate the thermal impact of urban landscapes. Distinct thermal patterns across LCZs were observed during the day: urban landscapes of compact buildings tended to heat up faster than open buildings, and low-rise zones generally had higher LST than high-rise LCZs in both compact and open urban forms. At night, compact buildings had higher LST than the open buildings, but the LST difference was insignificant. Meanwhile, building heights had little effects on LST at night. We also found that vegetation cover, percentage of impervious surface, and population size were related to LST, while the strength of the relationship varied for different times of day and night. Our study demonstrates the great potential of the new ECOSTRESS LST observations in studying the diurnal variations of urban thermal environments with fine spatial resolution. Our findings can provide insight into the impacts of urban form on local climate and can inform municipalities on the development of targeted heat mitigation strategies.