TY - JOUR
T1 - Tree effects on urban microclimate
T2 - Diurnal, seasonal, and climatic temperature differences explained by separating radiation, evapotranspiration, and roughness effects
AU - Meili, Naika
AU - Manoli, Gabriele
AU - Burlando, Paolo
AU - Carmeliet, Jan
AU - Chow, Winston T.L.
AU - Coutts, Andrew M.
AU - Roth, Matthias
AU - Velasco, Erik
AU - Vivoni, Enrique R.
AU - Fatichi, Simone
N1 - Funding Information:
The research was conducted at the Future Cities Laboratory at the Singapore-ETH Centre, which was established collaboratively between ETH Zurich and Singapore’s National Research Foundation ( FI370074016 ) under its Campus for Research Excellence and Technological Enterprise programme. G.M. acknowledges support by The Branco Weiss Fellowship – Society in Science administered by ETH Zurich. E.V. acknowledges a research fellowship granted by the Centre for Urban Greenery and Ecology of Singapore’s National Park Board . MeteoSwiss, the Federal Office of Meteorology and Climatology, is acknowledged for providing the station meteorological data for Switzerland.
Publisher Copyright:
© 2020 The Author(s)
PY - 2021/3
Y1 - 2021/3
N2 - Increasing urban tree cover is an often proposed mitigation strategy against urban heat as trees are expected to cool cities through evapotranspiration and shade provision. However, trees also modify wind flow and urban aerodynamic roughness, which can potentially limit heat dissipation. Existing studies show a varying cooling potential of urban trees in different climates and times of the day. These differences are so far not systematically explained as partitioning the individual tree effects is challenging and impossible through observations alone. Here, we conduct numerical experiments removing and adding radiation, evapotranspiration, and aerodynamic roughness effects caused by urban trees using a mechanistic urban ecohydrological model. Simulations are presented for four cities in different climates (Phoenix, Singapore, Melbourne, Zurich) considering the seasonal and diurnal cycles of air and surface temperatures. Results show that evapotranspiration of well-watered trees alone can decrease local 2 m air temperature at maximum by 3.1– 5.8 °C in the four climates during summer. Further cooling is prevented by stomatal closure at peak temperatures as high vapour pressure deficits limit transpiration. While shading reduces surface temperatures, the interaction of a non-transpiring tree with radiation can increase 2 m air temperature by up to 1.6 – 2.1 °C in certain hours of the day at local scale, thus partially counteracting the evapotranspirative cooling effect. Furthermore, in the analysed scenarios, which do not account for tree wind blockage effects, trees lead to a decrease in urban roughness, which inhibits turbulent energy exchange and increases air temperature during daytime. At night, single tree effects are variable likely due to differences in atmospheric stability within the urban canyon. These results explain reported diurnal, seasonal and climatic differences in the cooling effects of urban trees, and can guide future field campaigns, planning strategies, and species selection aimed at improving local microclimate using urban greenery.
AB - Increasing urban tree cover is an often proposed mitigation strategy against urban heat as trees are expected to cool cities through evapotranspiration and shade provision. However, trees also modify wind flow and urban aerodynamic roughness, which can potentially limit heat dissipation. Existing studies show a varying cooling potential of urban trees in different climates and times of the day. These differences are so far not systematically explained as partitioning the individual tree effects is challenging and impossible through observations alone. Here, we conduct numerical experiments removing and adding radiation, evapotranspiration, and aerodynamic roughness effects caused by urban trees using a mechanistic urban ecohydrological model. Simulations are presented for four cities in different climates (Phoenix, Singapore, Melbourne, Zurich) considering the seasonal and diurnal cycles of air and surface temperatures. Results show that evapotranspiration of well-watered trees alone can decrease local 2 m air temperature at maximum by 3.1– 5.8 °C in the four climates during summer. Further cooling is prevented by stomatal closure at peak temperatures as high vapour pressure deficits limit transpiration. While shading reduces surface temperatures, the interaction of a non-transpiring tree with radiation can increase 2 m air temperature by up to 1.6 – 2.1 °C in certain hours of the day at local scale, thus partially counteracting the evapotranspirative cooling effect. Furthermore, in the analysed scenarios, which do not account for tree wind blockage effects, trees lead to a decrease in urban roughness, which inhibits turbulent energy exchange and increases air temperature during daytime. At night, single tree effects are variable likely due to differences in atmospheric stability within the urban canyon. These results explain reported diurnal, seasonal and climatic differences in the cooling effects of urban trees, and can guide future field campaigns, planning strategies, and species selection aimed at improving local microclimate using urban greenery.
KW - Ecohydrology
KW - Evapotranspirative cooling
KW - Land-Atmosphere interactions
KW - Nature based solutions
KW - Urban climate
KW - Urban greenery
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U2 - 10.1016/j.ufug.2020.126970
DO - 10.1016/j.ufug.2020.126970
M3 - Article
AN - SCOPUS:85099794982
SN - 1618-8667
VL - 58
JO - Urban Forestry and Urban Greening
JF - Urban Forestry and Urban Greening
M1 - 126970
ER -