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Urban landcover differentially drives day and nighttime air temperature across a semi-arid city

April 4, 2022

Semi-arid urban environments are undergoing an increase in air temperatures, both in average temperatures and in the frequency and intensity of extreme heat events. Within cities, different varieties of urban landcovers (ULC) and their densities influence local air temperatures, either mitigating or increasing heat. Currently, understanding how various combinations of ULCs influence air temperature at the block to neighborhood scale is limited due to the complexities of urban energy balances at small scales. We quantified how ULC influences air temperature at 60 m resolution for day and nighttime climate normals and heatwaves, by integrating data from microclimate temperature sensor networks and high-resolution (1 m2) ULC for Denver Colorado’s urban core. We derived ULC drivers of air temperature using a structural equation model, and projected urban heat scenarios of climate normals and heatwaves throughout the extent of urban Denver. We found that, in conjunction with other ULCs, urban tree canopy reduced daytime air temperatures (-0.026° C per % cover), and the combination of impervious surfaces and buildings increased daytime air temperature (0.021° C per % cover). During night hours, irrigated turf and tree canopy reduced temperatures more than the daytime (-0.038° C per % cover). Overall, ULC drove ~17% and 25% of local air temperature during the day and night respectively, with regional factors making up the rest. During heatwaves, ULC influence on daytime air temperatures was strengthened. The results provided evidence showing daytime urban heat mitigation is due to increases in surface shading and transpiration from tree canopies and nighttime heat mitigation in a semi-arid environment is primarily due to transpiration-derived cooling. Our findings inform urban planners seeking to identify potential hot and cool spots within a semi-arid city and mitigate high urban air temperatures through using ULC withing larger urban climate mitigation strategies.