Research Program Area: Climate Change
Cool roofs reflect sunlight and therefore can reduce cooling energy use in buildings. Further, since roofs cover about 20 - 25% of most cities, widespread deployment of cool roofs could mitigate the urban heat island effect and partially counter urban temperature increases associated with global scale climate change. The magnitude of these potential benefits for a given city depends on the increase in albedo that can be achieved using reflective roofs. Assessing this increase requires knowledge of roof albedo at the city-scale, which until now has been unknown due to a lack of reflectance data with sufficient spatial coverage, spatial resolution, and spectral information. In this work we use multiband aerial imagery to derive the albedos of individual roofs in seven California cities: Los Angeles, Long Beach, San Diego, Bakersfield, Sacramento, San Francisco, and San Jose. The radiometrically calibrated, remotely sensed imagery has high spatial resolution (1 m) and four narrow band reflectances: blue, green, red, and near-infrared. First, we locate roof pixels within GIS building outlines. Next, we use laboratory measurements of the solar spectral reflectances of 190 roofing products to empirically relate solar reflectance (albedo) to reflectances in the four narrow bands; the empirical relationship well predicts albedo as indicated by a low root-mean-square of the residuals of 0.016. Albedos computed from remotely sensed reflectances are calibrated to ground measurements of roof albedo in each city. The error (accuracy) at 90% confidence interval of the calibrated albedos is found to vary by city from 0.00 - 0.01 at low albedo and 0.06 - 0.14 at high albedo.
The fraction of urban area covered by roofs ranged by city from 10 to 25%. City-wide average roof albedo ranged from 0.17 ± 0.08 to 0.20 ± 0.11 (mean ± standard deviation) for five of the cities; values were higher in Sacramento (0.24 ± 0.11) and San Diego (0.29 ± 0.15). Buildings with small roofs were found to constitute a large fraction of city roof area and to have low mean albedos. This suggests that efforts to increase urban albedo through the use of reflective roofs should include small roofs, which are presumably mostly residential. Roof albedos derived for Bakersfield were used in a regional climate model (WRF) to estimate temperature changes attainable by converting the current stock of roofs to "cool" high albedo roofs. It was found that seasonal mean afternoon (15:00 LST) temperatures could be reduced by up to 0.2 °C during both the summer and winter. Changes in precipitation were not significant at the 95% confidence level.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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