Greenery Systems for the Mitigation of the Urban Heat Island: A Simulation Experience for Southern Italy

This paper discusses the potential effectiveness of different green-based strategies for the mitigation of the Urban Heat Island effect and the consequent thermal discomfort in a crowded urban area located close to Catania, Southern Italy. The proposed mitigation solutions rely on the introduction of greenery systems either at a building scale or at an urban scale, namely: (i) green walls, (ii) green roofs and (iii) introduction of trees and urban vegetation at street level. The study relies on numerical simulations performed through ENVI-met, and aims to quantify the reduction in the outdoor air temperature and the outdoor Mean Radiant Temperature, as well as the variation in the wind speed and the relative humidity. The simulations refer to a 200 m square urban area, but the selected strategies just apply to a series of buildings in a smaller area located in the middle of the entire simulation domain. The results of the simulations suggest that the two solutions applied at the building scale do not introduce visible mitigation effects at the street level (that is to say at a height of 1.5 m above the street surface). On the contrary, significant improvements are observed with the extensive use of different species of trees along the streets and in the squares: indeed, the outdoor air temperature decreases by around 0.5 ℃ or even 1 ℃ at street level compared with the current state, especially in the hottest hours of the day. However, further simulations with ENVI-met suggest that green roofs and green walls are very effective to mitigate the indoor climate in summer. Hence, a suitable combination of the three proposed solutions is the most interesting approach for addressing both indoor and outdoor comfort issues.