Tree effects on urban microclimate: Diurnal, seasonal, and climatic temperature differences explained by separating radiation, evapotranspiration, and roughness effects

Naika Meili, Gabriele Manoli, Paolo Burlando, Jan Carmeliet, Winston T.L. Chow, Andrew M. Coutts, Matthias Roth, Erik Velasco, Enrique R. Vivoni, Simone Fatichi

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

Abstract

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.

Original languageEnglish (US)
Article number126970
JournalUrban Forestry and Urban Greening
Volume58
DOIs
StatePublished - Mar 2021

Keywords

  • Ecohydrology
  • Evapotranspirative cooling
  • Land-Atmosphere interactions
  • Nature based solutions
  • Urban climate
  • Urban greenery

ASJC Scopus subject areas

  • Forestry
  • Ecology
  • Soil Science

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