The Effect of Urban Surface Modification on the Near Surface Air Temperature and Urban Boundary Layer Evolution during Extreme Heat Waves in Montreal

Modification of urban surfaces through increasing the reflectivity of the surfaces or increasing the vegetation in urban areas are the main proposed scenarios for heat island mitigations in large cities. At Environment and Climate Change Canada, a sub-kilometer high resolution 3D urban Numerical Weather Prediction system has been employed to evaluate the sensitivity of near surface air temperature, wind speed and boundary layer evolution to the modification of the urban surfaces in Montreal. The modeling system was run with 250-m grid spacing for two intensive heat wave periods during summer 2010 which had a large impact on dwellers. Evaluation of the control forecasts and results from several mitigation scenarios will be presented. For example, increasing the albedo of the different urban surfaces (wall, roofs, roads) results in the reduction in surface air temperature (2-m) for all the hours during the heat waves. The average reduction of air temperature during the heat wave periods is 0.35 ^oC in urban areas with the maximum temperature reduced up to 1^oC . A smaller and more localized effect of increasing the vegetation fraction in the urban area on air temperature is found. The effect of each urban mitigation scenario on the wind speed and boundary layer structure is also investigated.