Reply on RC1

Abstract

Trees provide many ecosystem services in cities such as urban heat island reduction, water runoff limitation and carbon storage. However, the presence of trees in street canyons reduces the wind velocity in the street and limits pollutant dispersion. Thus, to get accurate simulations of pollutant concentrations, the aerodynamic effect of trees should be taken into account in air-quality models at the street level. The Model of Urban Network of Intersecting Canyons and Highways (MUNICH) simulates the pollutant concentrations in a street-network, considering dispersion and physico-chemical processes. It can be coupled to a regional-scale chemical-transport model to simulate air quality over districts or cities. The aerodynamic effect of tree crown is parameterized here, through their impacts on the average wind velocity in the street direction and the vertical transfer coefficient associated with the dispersion of a tracer. The parameterization is built using local-scale simulations performed with the Computational Fluid Dynamics (CFD) code Code_Saturne. Two-dimensional CFD simulations in an infinite street canyon are used to quantify the effect of trees depending on the tree characteristics (Leaf Area Index, crown volume fraction and tree height to street height ratio) using a drag-porosity approach. The tree crown slows down the flow and produces turbulent kinetic energy in the street, thus impacting the tracer dispersion. This effect increases with the Leaf Area Index and the crown volume fraction of the trees: the average horizontal velocity in the street is reduced up to 68 % and the vertical transfer coefficient up to 23 % in the simulations performed here. A parameterization of these effects on horizontal and vertical transfers for the street model MUNICH is proposed. Existing parameterizations in MUNICH are modified based on Code_Saturne simulations to account for both building and tree effects on vertical and horizontal transfers. The parameterization is built to get similar tree effects (quantified by a relative deviation between the cases without and with trees) between Code_Saturne and MUNICH. The vertical wind profile and mixing length depend on Leaf Area Index, crown radius and tree height to street height ratio. The interaction between the trees and the street aspect ratio is also considered.