Dynamically downscaled projection of urban outdoor thermal stress and indoor space cooling during future extreme heat

Abstract

While projected urban air temperatures under climate change and urbanization have received attention, projections of pedestrian thermal stress are scarce and usually produced by statistical downscaling. In this study, we present and evaluate a dynamical downscaling methodology to assess urban outdoor thermal exposure. We dynamically downscale Earth system model output with a mesoscale weather, urban canopy and building energy model (WRF-BEP-BEM), then use the downscaled output as boundary conditions for a microscale model (TUF-Pedestrian) to determine metre-scale mean radiant temperature. Using this methodology, we assess outdoor heat exposure and air conditioning loading during typical heatwaves at the start and end of century in Phoenix and Toronto.Results reveal that air conditioning loading would double in Phoenix by the end of century under an RCP 8.5 climate and urban development scenario; and it may double in Toronto even without urban development. This is compounded by up to 2 additional hours/day of extreme heat stress in Phoenix (5.5 additional hours/day of strong heat stress in Toronto). Using our dynamical downscaling methodology, we find that projected climate change-induced increases in outdoor heat stress largely result from higher air temperatures (Phoenix: 69–87%; Toronto: 60–64%), with smaller contributions from increased mean radiant temperature and humidity.