High-resolution Assessment of Heat Mitigation Strategies in the City of Toronto

Toronto is vulnerable to the negative impacts of extreme heat episodes, due to its highdensity of buildings, regular summertime heat episodes, and complex weather patternsinfluenced by Lake Ontario. As warming trends are anticipated to continue, the number ofextreme heat days (warmer than 30 OC) will also increase. A high frequency of extreme heat daysis associated with increased heat related mortality. Despite this, studies assessing strategies ofheat mitigation in Toronto, which involve implementing infrastructure to reduce heat hazards,are currently limited. Using the mesoscale (~ 1 km resolution) climate model, Weather Research and Forecasting (WRF), various heat mitigation strategies, including addition of vegetation and high albedo surfaces, are explored. To quantify the impacts of urbanization, we modelled climatic conditions in the Toronto region with and without urban development. This step established that under the same boundary conditions, the presence of urban features significantly increases diurnally averaged temperatures in Toronto by as much as 4 oC. Having established that urbanization has led to higher temperatures, we then modelled the impacts of adding cool roofs and vegetation to urbanized Toronto simulations, finding that having 80% vegetation in all grid cells significantly reduces day-time diurnal temperatures. Lastly, our project will include a novel addition of street trees into WRF, allowing users to calculate tree-building interactions, including evapotranspiration and radiation trapping effects. The results of this study willbe used in real time by building simulation teams to determine which mitigation strategies will best prevent overheating.