Impact of Trees on Urban Canyon Microclimate

Cities worldwide, including already hot locations such as Tempe, Arizona, are experiencing increases in temperature due to the combined effects of the built environment and climate change. Trees are a commonly cited strategy by cities that can be employed to mitigate urban heat, and therefore reduce the impact of the urban heat island; however, the extent to which the combination of trees, artificial surfaces, and building design impact thermal conditions in the urban environment through a complete diurnal cycle is unclear. While considerable research exists regarding the effects of tree deployment on daytime temperatures, there is little observational evidence of their impact on thermal conditions at night. We conducted an observational campaign in an urban canyon on Arizona State University’s Tempe campus over one complete diurnal cycle. Volunteers used a mobile cart containing a variety of meteorological sensors to measure a range of variables at pre-determined points within the canyon. Results show that trees raise overnight air temperature and mean radiant temperature while lowering daytime air temperature and mean radiant temperature, compared to treeless locations at the same time. They also show that trees are most effective at lowering daytime mean radiant temperature and air temperature when they are the primary source of shade. This research can be used to improve our understanding of urban climate dynamics as it is altered by urban features and canyons in areas with similar climates. It can also improve climate models (e.g., Urban WRF) through model evaluation, and be examined as a base for further urban climate research for urban heat mitigation in other cities.