A semi-empirical model for the effect of trees on the urban wind environment

High-density urban areas are often associated with limited outdoor natural ventilation. Given the growing call for more vegetation in cities, it is important to study the wind resistant of urban trees in order to address outdoor natural ventilation problem in the landscape planning. Currently, Computational Fluid Dynamics (CFD) simulation and wind tunnel experiment can only model the simplified street canyon with roadside trees, at the expense of intensive technical support and high computational cost. Thus, they are mostly for the research purpose only, and the impact of research outputs on the landscape planning remains low. In this study, we developed a practical semi-empirical model to provide scientific understandings for the landscape planning practice. The new model was developed based on the balance between momentum flux and the drag force of both buildings and trees on air flow. Friction velocity (u*) was modeled and validated by existing CFD and wind tunnel data, and effective frontal area density (λf_tree) was estimated by the measured leaf area index. Effects of urban context density and trees (i.e. plant canopy density and typology) on wind environment were clarified. This research correlated the urban density and tree geometry indices with wind speed, thereby enabling planners to calculate trees' effects on airflow using their in-house data. With such new practical tool and understandings, the knowledge-based landscape planning can be conducted to introduce more trees into urban areas, while avoiding negative effects of trees on the outdoor wind environment at cities at the same time.