Satellite identification of atmospheric-surface-subsurface urban heat islands under clear sky

The urban heat island (UHI) is a major topic in the study of urban climates. However, comprehensive research on the atmospheric UHI (UHI_Atm), surface UHI (UHI_Surf), and subsurface UHI (UHI_Sub) simultaneously has yet not been reported. Using the MODIS land surface temperature and atmospheric profile data during the 2010–2016 period, we investigated the diurnal, seasonal, and vertical variations of UHI_Atm, UHI_Surf, and UHI_Sub in Beijing. The major findings include but are not limited to the following: As the altitude increases from 1000 hPa (near the surface) to 700 hPa (~3 km), the daytime UHI_Atm intensity in summer decreases piecewise linearly with a rapid decline above 850 hPa (~1.5 km), while in spring and autumn it decreases after a slight increase in the lower atmosphere. The nighttime UHI_Atm intensity decreases approximately linearly in all seasons with a rapid (gradual) decline in winter (summer). As the depth increases from the surface to ~0.2 m, the daytime intensity from UHI_Surf to UHI_Sub decreases in summer but increases in the other seasons, while the nighttime trends are opposite to the daytime ones. The diurnal (seasonal) variation of UHI_Sub intensity converges to the daily (annual) mean as the depth reaches ~0.5 m (10 m). These new findings have theoretical and practical implications for in-depth understanding of the urban thermal environment from the boundary layer to the subsurface.