The Regional Impact of Urban Heat Mitigation Strategies on Planetary Boundary Layer Dynamics Over a Semiarid City

The rapid urbanization and associated landscape changes strongly modulate heat and moisture transfer processes on the urban surface as well as in the planetary boundary layer (PBL) via urban land-atmosphere interactions. In this study, we employed the mesoscale Weather Research and Forecasting (WRF) model with realistic urban dynamics to assess the effects of two urban landscaping strategies for urban heat mitigation (viz., green roofs and white roofs) on the PBL dynamics for hot summer periods over a semiarid city, Phoenix of Arizona. Our results show that the effects of green roofs and white roofs on PBL dynamics are markedly different at daytime and nighttime. At daytime, both roofing systems reduce the sensible heat flux significantly by ~150 W/m², lower the PBL height by ~700 m, and decrease the maximum convective available potential energy (CAPE) by ~40 J/kg over the built terrain. At nighttime, both sensible and latent heat fluxes increase with green roofs by ~4 and ~6 W/m², respectively. In contrast, with white roofs, a marginal reduction of sensible heat flux by ~4 W/m² was observed, owing to the remnant effect of daytime cooling. In addition, both roofing systems reduce the CAPE over the urban core but increase the CAPE over the rural surrounding, implying that the use of green or white roofs may potentially enhance the probability of precipitation toward the outskirt of the city.