Quantifying the impacts of large-scale implementation of white roofs on the climate of Montreal Island through integrated super-resolution modeling

• Large-scale implementation of white roofs indicates different responses for urban and non-urban fractions over Montreal island • Significant cooling noted for urban fractions, while warming is noted for some non-urban fractions • Warming of non-urban fraction is linked primarily with local winds such as land and sea breezes • Integrated high-resolution modelling is critical for better quantification of urban heat mitigation strategies The net impact of large-scale implementation of white roofs, one of the many urban heat mitigation strategies widely being adopted, on the urban climate is investigated. This is achieved through super-resolution (250 m) regional climate model simulations, with and without white roofs, for summer, for the island of Montreal, Canada. Being surrounded by water bodies, local winds such as land and sea breezes are important for the island, which are captured by the super-resolution climate simulation along with other climate characteristics. Grid-averaged surface and 2 m temperature results suggest general cooling with white roof implementation. However, analysis shows warming for the non-urban fractions in different parts of the island as a result of the complex interactions between various aspects of the regional climate system. During daytime, this warming, particularly noted for the west and central parts of the island, is associated with reduced latent heat flux in the white roof simulation, caused by reduced surface energy, weakening of sea breeze circulation, and decreased soil moisture from reduced convective precipitation. During nighttime, the white roof enhanced land breeze also warms the non-urban fractions of low-density urban zones in the eastern and western parts of the island, through advection of relatively warmer air from nearby urban areas. The new insights on the adverse effect of large-scale white roof implementation, primarily triggered by changes in local/non-synoptic winds, highlight the need for integrated modelling in quantifying the net impact of urban heat mitigation strategies, which is crucial for providing equitable benefits to all regions.