Green roofs as a strategy for urban heat island mitigation in Bologna (Italy)

Green roofs have received much attention in recent years due to their ability to lower urban temperatures and reduce heat fluxes to the atmosphere. This study shows the results of an experimental investigation and a numerical simulation of the Engineering Campus of the University of Bologna, which have been done to estimate the potential of green roofs in mitigating the UHI effect. The profiles of the air temperature above an extensive green roof have been measured and compared to those measured above a traditional roof. The green roof showed a good ability to lower diurnal surface temperature as demonstrated by the fact that the difference between maximum temperatures for green surface and air was 5.1 °C at 03:00 pm, and consequently 18.9 °C below the reference surface temperature. Moreover, the experimental data have been used to validate a CFD model, realized with the software ENVI-met. The simulation model allows the estimation of the UHI phenomenon across the campus area, and to evaluate the effect of a hypothetical mitigation strategy, which consist in covering all the roofs with intensive green roofs. The simulation results are presented in terms of differences in air temperature at the pedestrian level. The mitigation strategy scenario allows up to 0.5 and 3 °C decrease in air temperature, during the night and the day respectively. The comparison between the two scenarios demonstrates the validity of green roofs in reducing the UHI and in improving human comfort