Diurnal changes in urban boundary layer environment induced by urban greening

Urban green infrastructure has been widely used for mitigating adverse environmental problems as well as enhancing urban sustainability of cities worldwide. Here we develop an integrated urban-land-atmosphere modeling framework with the land surface processes parameterized by an advanced urban canopy model and the atmospheric processes parameterized by a single column model. The model is then applied to simulate a variety of forms of green infrastructure, including urban lawns, shade trees, green and cool roofs, and their impact on environmental changes in the total urban boundary layer (UBL) for a stereotypical desert city, viz. Phoenix, Arizona. It was found that green roofs have a relatively uniform cooling effect proportional to their areal coverage. In particular, a reduction of UBL temperature of 0.3 °C and 0.2 °C per 10% increase of green roof coverage was observed at daytime and nighttime, respectively. In contrast, the effect of greening of street canyons is constrained by the overall abundance of green infrastructure and the energy available for evapotranspiration. In addition, the increase in urban greening causes boundary-layer height to decrease during daytime but increase at nighttime, leading to different trends of changes in urban air quality throughout a diurnal cycle.