Quantifying threshold and scale response of urban air and surface temperature to surrounding landscapes under extreme heat

Studies have shown that tree canopies contribute to the reduction of both air temperature (Ta) and surface temperature (Ts) in urban areas, while impervious surfaces contribute to an intensified thermal burden. However, rare studies have investigated the threshold effect of surrounding landscapes (tree canopy and impervious surface) - the specific level of landscape coverage at which its influence on temperature is most significant, with specific attention to the scale-dependency, especially under extreme heat. Here we analyzed the influence of the surrounding landscapes on Ta and Ts and their respective threshold effects across five spatial scales, with radii of 20, 40, 60, 80 and 100 m. We obtained Ta through mobile measurements and Ts from data collected by the Unmanned Aerial Vehicle (UAV) on three typical streets in Shanghai at dawn, noon, and sunset during extreme heat. The results indicated that: (1) For most scales of analysis, Ta at noon exhibited a decrease with the increase in tree canopy coverage and an increase with the rise in impervious surface coverage; (2) Across all scales, the thresholds of tree canopy and impervious surface coverage were in the ranges of 20%-45 % and 40%-70 % respectively; (3) For large scales of analysis (60-100 m), the threshold coverage of impervious surface that impacts Ts gradually decreased with the increase of scale. The findings of this study provide insightful implications for climate mitigation and adaption under extreme heat.