Reduction of the urban cooling effects of an intensive green roof due to vegetation damage

Abstract

Green roofs are increasingly being utilised as urban climate adaptation measures given good evidence of their passive cooling effects. Shading of conventional building surfaces, higher albedo and latent cooling from evapotranspiration mean that vegetated surfaces can be used to significantly lower surface and air temperatures in cities. The extent to which cooling properties may be affected by poor maintenance, green roof damage and the general health of the vegetation is not as well known. This study investigates the ability of an intensive green roof in Manchester, UK, to cool the air above it relative to an adjacent conventional, concrete roof. The monthly median air temperature was found to be 1.06°C lower at 300 mm over the green roof. Diurnal trends were apparent, with strongest cooling of 1.58°C, occurring at night when Urban Heat Islands are most common. A section of green roof was damaged by mismanagement in a drought period, which allowed an investigation into the impairment to the cooling effect when large areas of bare substrate are present. Daytime temperatures were higher over this section of the roof than over an adjacent healthy green roof, being a maximum 0.63°C warmer than the bare roof on average, in the late morning. Cooling still occurred at night, albeit with a lower maximum average relative cooling of 0.78°C. Implications of the spatial influence of green roof cooling are discussed along with the need to undertake careful maintenance and irrigation of the roofs for the benefits to be maintained.

Seasonal and Diurnal Thermal Performance of a Subtropical Extensive Green Roof: The Impacts of Background Weather Parameters

“…The cooling effects were more pronounced in summer than spring and fall, on sunny days than rainy and cloudy days, and in nighttime than daytime. The result of higher nocturnal cooling effects is consistent with Speak et al's study, which also found that across a day strongest cooling occurs at night [38]. However, the effects were mostly restricted to limited distance.…”

“…Evaluation of the temporal variations of green-roof thermal performance with respect to weather effects enables a deep and comprehensive understanding of its role as a UHI mitigation measure. Speak et al [38] compared the seasonal and diurnal cooling effects between a healthy green roof and another damaged green roof with exposed bare substrate. Results found that both green roofs display remarkable temporal variations in cooling effects.…”

Impacts on cooling energy consumption due to the UHI and vegetation changes in Manchester, UK

“…The simulation in IES-VE is limited to conductive heat exchange through the roof as well as the available construction materials in IES-VE. It does not account for air temperature cooling due to evapotranspiration that can take place within a height of 1-2 m above the green roof [65].…”

Impacts of Climate Change on Urban Areas and Nature-Based Solutions for Adaptation

“…Stressed, unhealthy or declining vegetation cover will also cause reduced ecosystem function. Speak et al (2013) showed that green roofs can lower the air temperature above the system with approximately 1 °C. The effect was increased at night by 50% coinciding with the time when UHI is the strongest.…”