Impact of Tree Leaf Area Density on Cooling and Ventilation of an Urban Neighborhood

Abstract

The impact of trees during heat waves can be diverse, as their interaction with their surroundings depends on several parameters that modify shading, ventilation potential and transpiration rate. A multiscale coupled model is presented that allows the detailed analysis of the local impact of vegetation as a mitigation measure for urban heat islands. A case study is performed on an urban neighborhood in Zurich, Switzerland, with an aim to improve the understanding of physical processes in urban microclimate subjected to a heat wave. A parametric study presents the impact of varying the leaf area density (LAD) of the existing trees in the neighborhood. Comparisons of surface temperatures and rate of transpiration with available measured data show a good agreement. The results show that urban trees can reduce heat storage during the day due to shadowing, especially when they are in groups. The reduction in air temperature due to transpiration largely depends on LAD, wind-flow patterns and urban morphology. The results also indicate locations with an increase in air temperature due to the presence of trees.