The climate effects of increasing the albedo of roofs in a cold region†

Urban heat island (UHI) phenomenon has been observed in many large cities located in cold regions (e.g. Montreal in Canada) during summer. One of the well-known strategies to mitigate the temperature rise of urban areas is increasing their albedo. Roofs cover about 25% of urban areas and increasing their reflectivity would have a significant effect on the total energy budget of a city. We have studied the effect of increasing the albedo of roofs on the air and skin temperature distributions of the Greater Montreal area. We performed simulations for one-day summer episode (12 July 2005) using the Weather Research and Forecasting (WRF) mesoscale model. The WRF solver (version 3.4.1) is coupled with three different urban canopy models (UCMs): slab, single-layer, and multi-layer. We used all three UCMs by increasing the roof albedo from 0.2 to 0.8 and compared the results. All models simulated a well-defined UHI over areas with high concentration of roofs. They predicted a maximum air temperature decrease of about 1 K by implementing cool roofs. The difference between the skin (surface) temperature of urban area and its surrounding was about 9 K. The maximum air temperature difference between the urban and suburban areas was about 4 K.

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