How much could trees and other vegetation cool urban streets during an extreme heat wave?

Details

Urban heat island effects can intensify heat waves and air pollution episodes. Researchers examined how urban trees could dampen this effect during an extreme heat wave in the Washington, D.C., and Baltimore, Maryland, metropolitan area. They incorporated trees, soil and grass into the coupled Weather Research and Forecasting model and urban canopy model (WRF-UCM), parameterizing these natural surfaces alongside roads and buildings.

The modified model was run with 50% tree cover over urban roads and urban streets narrowed by 10% to make room for soil and grass alongside roads and buildings. Averaged across all urban areas, the added vegetation lowered surface air temperature in urban street canyons by 4.1 K, road-surface temperature by 15.4 K and building-wall temperature by 8.9 K, through tree shading and evapotranspiration. These temperature changes propagated downwind, changing the temperature gradient associated with the Chesapeake Bay breeze and consequently its strength.

The model also examined building height: reducing commercial building heights by 8 m and residential building heights by 2.5 m produced daytime surface and near-surface air temperatures up to 0.4 K higher because of reduced building shade, and nighttime temperatures up to 1.2 K lower because less longwave radiation was trapped in urban street canyons.