Review on urban tree modelling in CFD simulations: Aerodynamic, deposition and thermal effects

This paper reviews current parameterizations developed and implemented within Computational Fluid Dynamics models for the study of the effects linking vegetation, mainly trees, to urban air quality and thermal conditions. In the literature, passive mitigation via deposition is parametrized as a volumetric sink term in the transport equation of pollutants, while a volumetric source term is used for particle resuspension. The aerodynamics effects are modelled via source and sink terms of momentum, turbulent kinetic energy and turbulent dissipation rate. A volumetric cooling power is finally considered to account for the thermal (transpirational cooling) effects of vegetation. The most recent applications are also summarized with a focus on the relative importance of both aerodynamic and deposition effects, together with recent studies evaluating thermal effects. Those studies have shown that the aerodynamic effects of trees are stronger than the positive effects of deposition, however locally the pollutant concentration increases or decreases depending on the complex inter-relation between local factors such as vegetation type and density, meteorological conditions, street geometry, pollutant characteristics and emission rates. Unlike aerodynamic and deposition effects on pollutant dispersion which were also found in street far from trees, the thermal effects were in general locally restricted to the close vicinity of the vegetation and to the street canyon itself. Future requirements in CFD modelling include more in depth investigation of resuspension and thermal effects, as well as of the VOCs emissions and chemical reactions. The overall objective of this review is to provide the scientific community with a comprehensive summary on the current parameterizations of urban vegetation in CFD modelling and constitutes the starting point for the development of new parametrizations in CFD as well as in mesoscale models.