Extreme heat events such as heatwaves or urban heat island effect are some of the noticeable outcomes of climate change within cities that can affect citizens' quality of life significantly. Effects range from slight thermal discomfort to extreme heat stress and even heat-related mortalities. Although there is a need for an immediate response to the problem of extreme heat in cities, there is still uncertainty in developing effective and place-based heat mitigation strategies. Previous studies, mainly by doing parametric analysis and changing one factor at a time, tried to evaluate a single point in the parameters space in just a one-time slice, resulting in narrow conclusions with limited applicability. However, in this study, by employing a three-dimensional model and conducting sensitivity analysis, we tried to assess the relative impacts of morphological and vegetation parameters in reducing thermal stress during an extreme heat event and evaluate how the magnitude of their effects might change in various contexts and different hours a day. We implemented the weather-related data of the Chicago heatwave in July 1995 for our simulations, in which 514 heat-related deaths, mostly among racial and ethnic minorities, had happened. Our findings demonstrate physical parameters such as urban morphology and surface material have the highest cooling impact during the hottest hours of the day, while vegetation parameters exhibit almost constant effects during a day. However, when analyzing specific time steps, the results revealed that the effect of vegetation parameters on modifying thermal stress largely depends on the other physical and morphological parameters. This study hopes its findings by better understanding the impacts of influential parameters under a variety of contexts help planners craft place-based and effective heat mitigation strategies. Providing safe and equitable urban environments for all citizens and reducing heat-related casualties are the overreaching goals of this study.

Extreme heat exposure as a result of the interactions between heat waves and urban heat islands can lead to resident fatalities. Heat exposure is already the leading environmental cause of death, and with global climate change and growing urban populations, risk of heat exposure fatality is expected to become more severe, especially among marginalized groups. These conditions make the spatial heterogeneity of land surface temperatures particularly relevant to urban planners, who need to consider equitable, safe environmental conditions for all residents. Within cities however, there can be much variation in surface air temperatures, following physical heterogeneity of development and vegetation, which have interactive effects on urban canyon-scale microclimate. Yet data gaps in key parameters of physical heterogeneity result in uncertainty in urban canyon scale air temperature variability. In this study, we use a three-dimensional computational fluid dynamics model, ENVI-met, to conduct a global sensitivity analysis of physical morphology and vegetation parameters (e.g. aspect ratio, orientation toward prevailing winds and green coverage) in idealized urban street canyons to quantify both parameter uncertainty and scenario variability. Our findings demonstrate although vegetation parameters have a great influence on modifying canyon’s surface temperature, their cooling effect strongly depend on canyon’s physical morphology conditions. The results of those sensitivity analysis can be used to develop a policy-relevant typology of street canyons that can be applied in an adaptive planning framework as one component of population risk quantification.