A computational approach for urban heat island mitigation through nature-based solutions modelling.

Worldwide, increasing urbanization and anthropic activities are causing environmental degradation, resulting in biodiversity loss, lack in ecosystem services provision vital for humans and other living organisms, and climate change. Heatwaves, exacerbated by the urban heat island phenomena (UHI), which consists in the flux of higher temperatures in urban areas compared to surrounding natural areas due to the high concentration of artificial materials with low level of albedo, are among the most critical climate change effects. In addition to the environmental risks associated with heatwaves, these extreme events can also have a strong impact on human health, especially for some social categories such as children or the elderly, as well as fragile subjects. Nature-based Solutions (NbS), defined as solutions inspired by nature or which consist in the integration of natural elements in the urban environment, can reduce the impact of heatwaves, improving local environmental quality. In particular, the positive contribution of NbS in improving outdoor thermal comfort is mainly related to the capacity of shading provided by vegetated elements, as well as the evapotranspiration capacity and the high levels of albedo of leaves. Although the introduction of NbS proved to be a promising strategy to reduce climate change-related risks, one of the most critical aspects is related to the design, planning, and prevision of their performances. Regarding this last aspect, computational modelling constitutes a promising approach due to its pre-design application and their cost-effectiveness. Several computational approaches have been developed during the last years to quantify the potential of group of trees and other NbS to locally reduce the effects of heatwaves and improve outdoor thermal comfort. While most of the models consider mainly urban morphology and solar radiation as meaningful parameters to run real time analysis, other more accurate approaches have been developed. Envi-met is among the computational-fluid dynamics software which allow designers to run accurate microclimate simulations also considering plants contribution. However, this approach presents some critical issues, related to its graphical interface for 3d modeling and the limited amount of information on plant performance. This paper proposes a methodological approach to improve the effectiveness of the simulations to quantify NBS performance in relation to outdoor thermal conditions, by working on the plant modelling. In particular, the parameters considered are: foliage albedo, transmittance, emissivity of leaves, leaf weight and the height and the width of plants. In this research we model for one urban case study selected plant species, considering the parameters listed above, and to compare the results of three simulation approaches for the analysis on outdoor comfort: 1) a simplified model in stationary conditions which considers geometric parameters, 2) a CFD model to consider thermodynamic parameters with default values from the ENVI-met database, 3) a CFD model which consider more accurate thermodynamic parameters coming from literature and other existing databases. The analysis and comparison of the results will allow understanding how the thermodynamic parameters of plants affect their performances and if the consideration of geometries only allows obtaining significant results (i.e., comparing thermodynamic and stationary conditions modeling).