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Urban heat island (UHI) phenomenon has been observed in many populated 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 significant effect on the total energy budget of a city. Changing the surface energy budget can directly affect the air temperature near ground and the vertical wind speed. 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 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. The slab UCM is a one-dimensional model where the surface properties of urban areas (e.g. albedo, thermal storage, etc.) are kept constant. The single-layer UCM considers the albedo of different urban surfaces (i.e. roof, road, and wall), the wind effect in the canopy, and radiation trapping between buildings. Multi-layer UCM has the ability of simulating the effect of turbulence and momentum sink, to estimate the vertical heat exchange more accurately.
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 ºC 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.