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News Article

How urban trees can keep city dwellers comfortable

Urban trees moderate wind and reduce energy use

Even a single urban tree can help moderate wind speeds and keep pedestrians comfortable as they walk down the street, according to a recent University of British Columbia study that also found losing a single tree can increase wind pressure on nearby buildings and drive up heating costs.

The researchers used remote-sensing laser technology to create a highly detailed computer model of a Vancouver neighbourhood down to every tree, plant and building. They then used computer simulation to determine how different scenarios -- no trees, bare trees, and trees in full leaf -- affect airflow and heat patterns around individual streets and houses. The neighbourhood currently has a small fraction of trees that are taller than buildings, and eight years of continuous wind and turbulence measurements are available from a 30 m meteorological tower.

"We found that removing all trees can increase wind speed by a factor of two, which would make a noticeable difference to someone walking down the street. For example, a 15 km/h wind speed is pleasant, whereas walking in 30 km/h wind is more challenging," said lead author Marco Giometto, who wrote the paper as a postdoctoral fellow in civil engineering at UBC.

Trees also moderated the impact of wind pressure on buildings, particularly when it goes through small gaps in and between buildings.

"Wind pressure is responsible for as much as a third of a building's energy consumption. Using our model, we found that removing all the trees around buildings drove up the building's energy consumption by as much as 10 per cent in winter and 15 per cent in summer," said Giometto.

The researchers compared the simulated scenarios against a decade of measured wind data from a 30-metre-tall research tower operated by UBC in the same Vancouver neighbourhood. They discovered that even bare trees in the winter months can moderate airflow and wind pressure, contributing to a more comfortable environment.

"Even bare branches play a role. Deciduous trees, which shed their leaves every year, reduce pressure loading on buildings throughout the year-it's not only evergreens that are important in the city," said Marc Parlange, who supervised the work while a professor of civil engineering at UBC.

The model, piloted last year, is the first to simulate a real urban neighbourhood in extreme detail, added study co-author and UBC geography professor Andreas Christen.

"Information from such models can improve weather forecasts in order to predict the effects of a storm on a building and pedestrian level," said Christen. "It could also help city planners in designing buildings, streets, and city blocks to maximize people's comfort and limit wind speed to reduce energy loss."

The researchers found that the effects of trees in the urban environment were twofold. On one hand, they act as a direct momentum sink for the mean flow; on the other, they reduce downward turbulent transport of high-momentum fluid, significantly reducing the wind intensity at the heights where people live and buildings consume energy.

As well as reducing wind speeds, urban trees have many other environmental benefits. McDonald et al. (2016) report how trees cool the air by casting shade and releasing water vapour, and that leaves filter the fine particulate matter which is one of the most dangerous forms of air pollution. Rahman et al. (2017) report on the transpirational cooling effects of urban trees in contrasting sites in Munich, Germany. Graham et al. (2017) found that higher urban tree canopy cover reduced ambulance calls for heat stress during extreme heat events in Toronto, Canada.

Hong and Lin (2015) report on optimum building layout patterns and arrangement of trees for a
pleasant thermal comfort and wind environment at pedestrian level. Shahidan (2015) discuss how tree density and clusters of trees affect wind speed and urban microclimate.

Journal Reference

M.G. Giometto, A. Christen, P.E. Egli, M.F. Schmid, R.T. Tooke, N.C. Coops, M.B. Parlange. Effects of trees on mean wind, turbulence and momentum exchange within and above a real urban environment. Advances in Water Resources, 2017; 106: 154 DOI: 10.1016/j.advwatres.2017.06.018

Article details

  • Author(s)
  • David Simpson
  • Date
  • 17 August 2017
  • Source
  • Subject(s)
  • Arboriculture