Radiation, evapotranspiration, and roughness effects of urban trees on local microclimate: A modelling study

Abstract

The increase in urban air temperature caused by urban heat and climate change can have negative effects on the outdoor thermal comfort (OTC) as well as on the energy demand for air-conditioning. Nature-based solutions, such as the increase in urban biomass, are often proposed to mitigate excessive urban heat. Trees are expected to decrease temperatures due to shade provision on surfaces and evapotranspiration but their canopy blocks wind flow, thus potentially induce warming by reduction of heat removal. Several studies have shown that trees have a varying potential for air temperature reduction throughout the diurnal cycle as well as in different climates. Studies that partition and attribute the temperature reduction to the aforementioned effects are still lacking though, thus making the explanation of the observed differences difficult. To address this knowledge gap, we use the mechanistic urban ecohydrological model, Urban Tethys-Chloris (UT&C, Meili et al. 2019), which accounts for radiation, evapotranspiration and roughness effects of trees in the urban canyon. Turning these components on and off by means of virtual experiments allows us to quantify their contribution to the air and surface temperature modification caused by the tree cover. The results are analysed for compact low-rise residential areas (LCZ3) in four different climates (Phoenix, Singapore, Melbourne, Zurich). We find that tree evapotranspiration is able to lower 2 m air temperature at maximum by 3-4°C in all four climates as stomatal closure due to high vapour pressure deficits in dry and hot cities limit the transpirative cooling effect during mid-day. Counterintuitively, tree-radiation interaction increases the 2 m air temperature up to 2°C at noon time even though a decrease in surface temperatures is observed. While the surfaces underneath the tree canopy receive less radiation due to shading, the overall absorbed solar radiation within the canyon increases due to radiation trapping. In the analysed scenarios, the presence of trees leads to a decrease in the city roughness hindering turbulent energy exchange and thus, increasing the 2 m air temperature in all climates during daytime. The tree-radiation and tree-roughness effects on 2 m air temperature during night vary in different climates due to atmospheric stability effects. Combining the different tree effects as in the real world, leads to a distinct diurnal pattern of air temperature reduction which is consistent with the observations in the literature. The numerical experiment allows reconciling differences in temperature changes induced by trees across the diurnal cycle and in various climates. The results could be used to guide green cover and tree type selection in cities and inform future studies aimed at optimizing the role of urban greening for improving local microclimatic conditions.