Roles of Urban Tree Canopy and Buildings in Urban Heat Island Effects: Parameterization and Preliminary Results

Abstract

Urban heat island (UHI) effects can strengthen heat waves and air pollution episodes. In this study, the dampening impact of urban trees on the UHI during an extreme heat wave in the Washington, D.C., and Baltimore, Maryland, metropolitan area is examined by incorporating trees, soil, and grass into the coupled Weather Research and Forecasting model and an urban canopy model (WRF-UCM). By parameterizing the effects of these natural surfaces alongside roadways and buildings, the modified WRF-UCM is used to investigate how urban trees, soil, and grass dampen the UHI. The modified model was run with 50% tree cover over urban roads and a 10% decrease in the width of urban streets to make space for soil and grass alongside the roads and buildings. Results show that, averaged over all urban areas, the added vegetation decreases surface air temperature in urban street canyons by 4.1 K and road-surface and building-wall temperatures by 15.4 and 8.9 K, respectively, as a result of tree shading and evapotranspiration. These temperature changes propagate downwind and alter the temperature gradient associated with the Chesapeake Bay breeze and, therefore, alter the strength of the bay breeze. The impact of building height on the UHI shows that decreasing commercial building heights by 8 m and residential building heights by 2.5 m results in up to 0.4-K higher daytime surface and near-surface air temperatures because of less building shading and up to 1.2-K lower nighttime temperatures because of less longwave radiative trapping in urban street canyons.

Urban warming reduces aboveground carbon storage

“…In our study's most conservative interpretation, urban warming was associated with reductions in ecosystem services that contribute to climate regulation [47]. We increasingly rely on urban forests to provide these services as deforestation of more natural forests continues [48] and as more people move to cities where they benefit directly from urban trees.…”

Remote Sensing Techniques for Urban Heating Analysis: A Case Study of Sustainable Construction at District Level

“…Finally, the construction of relatively high buildings with respect to the previous flat car park lot produced a shading effect and a probable improvement of the local air circulation. The impact of building design on the shadow effect was assessed in [43], showing how shorter urban buildings cause higher surface and near-surface temperatures during the daytime. Hu and Liao [44] suggest how a suitable space distance of buildings contributes to improve ventilation between them, and the new building position in Corso del Popolo seems to match the favourable conditions in [44].…”

The relationship between neighbourhood tree canopy cover and heat-related ambulance calls during extreme heat events in Toronto, Canada

“…Although the low sky view factor from buildings can result in radiative trapping at night and increase the nighttime UHI, the relatively cooler surfaces beneath trees result in less emitted long-wave radiation to be trapped and the evaporative cooling provided by the trees also decreases air temperature (Loughner et al, 2012). A modelling study in Washington, DC found that the additional tree shading and evapotranspiration provided by vegetation in an urban area decreased surface air temperatures in urban street canyons by 4.1 • C, road surface temperatures by 15.4 • C, and building wall temperatures 8.9 • C (Loughner et al, 2012). Coseo and Larsen (2014) also found that neighbourhoods with a higher percentage of impermeable surface cover, as compared to those with higher canopy cover, had higher nighttime surface temperatures.…”