Jun 25, 2014
Green roofs can reduce urban heat island effect
Several cities in the US are encouraging the use of green or cool roofs on buildings in an attempt to reduce the urban heat island effect. But assessing the effectiveness of this policy has been challenging. Most models look at the effect of a green or cool roof on one building but up-scaling this to a city or metropolitan area is difficult due to different urban landscapes and their effect on atmosphere exchanges.
Now researchers from Princeton University in the US have developed a new urban canopy model (UCM) that they claim is ideal for city-scale investigations into the mitigation of urban heat island effects. Dan Li and his colleagues tested their model on the Baltimore-Washington metropolitan area during a four-day heatwave and found that a reduction of surface temperature of 1 °C could be achieved if 30% of all roofs in the area were green or cool roofs.
“This is a relatively large temperature change and can easily be achieved because green and cool roofs are so straightforward to implement,” Li told environmentalresearchweb. “Our model is capable of simulating fractional coverage and we found that as the green and cool roof fractions increase, the surface and near-surface urban heat islands are reduced almost linearly.” Princeton’s UCM was used in conjunction with the Weather Research and Forecasting (WRF) model on the Baltimore-Washington metropolitan area during a heatwave on 7–10 June 2008.
While simulations showed a significant reduction of the surface heat island effect with only 30% take-up of green or cool roofs, they also showed that reducing the air temperature is more challenging. “Reducing the near-surface urban heat island by 0.5 °C requires the green roof fraction to be close to 90% and the cool roof fraction has to be close to 95%,” explained Li. “While this target is perhaps not achievable, our results still show that a significant reduction in surface temperature using green or cool roofs is possible.”
That said, these simulations assume that green roofs are maintained properly and that the albedo of the cool roofs is 0.7. “The moisture of green roofs must be maintained either by rain or irrigation and cool roofs must be kept relatively clean in order to be effective,” said Li.
Li and his colleagues are currently using their model to simulate the effect of green and cool roofs on New York City and hope that it could also be incorporated into global climate models.
- The effectiveness of cool and green roofs as urban heat island mitigation strategies Dan Li et al. 2014 Environ. Res. Lett. 9 055002
- Quality and sensitivity of high-resolution numerical simulation of urban heat islands Dan Li and Elie Bou-Zeid 2014 Environ. Res. Lett. 9 055001
- ERL Focus on Environmental Assessments in the Built Environment
- Dan Li, Princeton University
About the author
Nadya Anscombe is a contributing editor to environmentalresearchweb.