The role of vegetation in mitigating Urban Heat Island effect: Development of a model for assessing the energy equilibrium of interventions in dense urban environments

The presence of vegetation in open spaces within urban areas has a relevant impact on the urban heat island phenomenon with a bulk of effects on the different terms of the surface energy balance (net radiation, sensible, latent and ground heat fluxes, respectively Rn, LE, H, G). These effects are the result of a combination of the action of factors like type of plants, weather conditions, water availability, time of the day, etc. Nowadays, many community authorities are adopting special planning tools to evaluate the environmental impact of urban interventions through significant indicators, with the aim of promoting environmental compensation strategies and, therefore, to increase the presence of vegetation in urban areas. This work investigates the impact of vegetation on urban microclimate referring directly to the quantitative effects on the terms of the surface energy balance and in particular to sensible and latent heat fluxes and their ratio (the Bowen ratio H/LE). Decreasing the value of the ratio H/LE, decreases the power of a surface to heat the area above it. The evaluation model has been applied to several case studies in Lyon (France) and Barcelona (Spain). These two cities are famous for enhancing “urban renaturalization” through open space interventions. The application shows how Lyon and Barcelona micro-meteorological conditions are influenced by urban morphology, materials, state of vegetation.