Green roof effects on urban building surface processes and energy budgets

Green roofs, as a nature-based solution, can be an effective tool for cities to improve the thermal environment, save energy, and combat climate change. However, it remains unclear how green roofs change the energy budget of urban building surfaces. Therefore, we modeled the surface energy balance of bare and green roofs based on experimental observations over a full calendar year to compare the physical transport processes of energy flows on bare and green roofs. Observational data were used to quantify the seasonal influence of green and bare roof surfaces on the energy flows and to document how roof vegetation alteres surface energy flow via physical and biochemical processes. On the bare roof, the incident solar radiation was partitioned largely into heating the building structure, with approximately 32.3% of the solar radiation absorbed. The green roof intercepted and consumed nearly 89% of incident solar radiation. The green roof effectively reduced convective energy by 6.7% compared to the bare roof, while longwave radiation emitted by green roof surfaces was reduced by 24.89 Wm⁻². Evapotranspiration was the primary form of energy consumption on the green roof, accounting for 23.7% of the energy output. Results show that seasonal green roof evapotranspiration ranks as follows: winter > summer > spring > autumn. Reflected solar radiation, upward longwave radiation, and latent heat flux accounted for almost 95% of the total energy output of the green roof. Quantifying the energy balance impacts of green roofs on urban building surfaces improves the accuracy of urban climate models and supports adopting green roofs to foster sustainable urban development.