  Nowadays, cities are frequently exposed to heatwaves, worsening the outdoor
thermal comfort and increasing cooling energy demand in summer. Urban forestry
is seen as one of the viable and preferable solutions to combating extreme heat
events and urban heat island (UHI) in times of climate change. While many
cities have initiated tree-planting programmes in recent years, the evolving
impact of trees on street microclimate, in a time span of up to several
decades, remains unclear. We investigate the cooling effects of linden trees in
five groups, i.e., 10-20, 20-30, 30-40, 40-60, and 60-100 years old. The leaf
area index (LAI) and leaf area density (LAD) vary nonlinearly as the trees
grow, peaking at different ages. Computational fluid dynamics (CFD) simulations
solving microclimate are performed for an idealized street canyon with trees of
varied age groups. Turbulent airflow, heat and moisture transport, shortwave
and longwave radiation, shading and transpiration are fully coupled and solved
in OpenFOAM. The meteorological data, including air temperature, wind speed,
moisture, and shortwave radiation of the heatwave in Zurich (June 2019), are
applied as boundary conditions. The results show that young trees in the age
group of 10-20 years old provide little heat mitigation at the pedestrian level
in an extreme heat event. Optimal heat mitigation by trees is observed for the
group of 30-60 years old trees. Finally, the potential impact of growing trees
as a heat mitigation measure on air ventilation is evaluated.
