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

TLDR

Urban heat island effects can intensify heat waves and air pollution episodes. The study examines how urban trees dampen the UHI during an extreme heat wave in the Washington, D.C., and Baltimore metropolitan area by integrating trees, soil, and grass into the WRF‑UCM. The modified WRF‑UCM parameterizes trees, soil, grass, roads, and buildings, running scenarios with 50 % tree cover on roads and a 10 % street‑width reduction to accommodate soil and grass. Adding vegetation lowered street‑canyon air temperature by 4.1 K and reduced road‑surface and building‑wall temperatures by 15.4 K and 8.9 K, respectively, while also altering the Chesapeake Bay breeze; reducing building heights increased daytime temperatures by up to 0.4 K and decreased nighttime temperatures by up to 1.2 K.

Abstract

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.