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Abstract Different heat mitigation technologies have been developed to improve the thermal environment in cities. However, the regional impacts of such technologies, especially in the context of a tropical city, remain unclear. The deployment of heat mitigation technologies at city‐scale can change the radiation balance, advective flow, and energy balance between urban areas and the overlying atmosphere. We used the mesoscale Weather Research and Forecasting model coupled with a physically based single‐layer urban canopy model to assess the impacts of five different heat mitigation technologies on surface energy balance, standard surface meteorological fields, and planetary boundary layer (PBL) dynamics for premonsoon typical hot summer days over a tropical coastal city in the month of April in 2018, 2019, and 2020. Results indicate that the regional impacts of cool materials (CMs), super‐cool broadband radiative coolers, green roofs (GRs), vegetation fraction change, and a combination of CMs and GRs (i.e., “Cool city (CC)”) on the lower atmosphere are different at diurnal scale. Results showed that super‐cool materials have the maximum potential of ambient temperature reduction of 1.6°C during peak hour (14:00 LT) compared to other technologies in the study. During the daytime hours, the PBL height was considerably lower than the reference scenario with no implementation of strategies by 700 m for super‐cool materials and 500 m for both CMs and CC cases; however, the green roofing system underwent nominal changes over the urban area. During the nighttime hours, the PBL height increased by CMs and the CC strategies compared to the reference scenario, but minimal changes were evident for super‐cool materials. The changes of temperature on the vertical profile of the heat mitigation implemented city reveal a stable PBL over the urban domain and a reduction of the vertical mixing associated with a pollution dome. This would lead to crossover phenomena above the PBL due to the decrease in vertical wind speed. Therefore, assessing the coupled regional impact of urban heat mitigation over the lower atmosphere at city‐scale is urgent for sustainable urban planning. 

Abstract This article investigates the effect of urban expansion and climate change impacts on heat stress (HS) for Arizona's (AZ; USA) two largest urban agglomerations, the Phoenix and Tucson metropolitan areas, under relatively dry and moist warm conditions with the Weather Research and Forecasting (WRF)‐urban modeling system. We dynamically downscale two contemporary summers, one dry and one moist, relatively to their respective seasonal‐mean specific humidity across AZ. Urban expansion impacts on HS are assessed by performing two identical simulations for each contemporary summer using different land use‐land cover representations: one simulation with the current urban landscape, and one simulation replaces the urban cover with the region's most representative MODIS vegetation type. Climate change impacts on HS are evaluated by performing four additional future simulations, two via dynamical downscaling of relatively dry conditions (one summer under the RCP8.5 and one summer under the RCP4.5 emissions pathways) and two of relatively moist conditions (one summer for each RCP pathway). The selection of future summers is based on their respective seasonal‐mean specific humidity across AZ from an end‐of‐century analysis of 2086–2100. We characterize impacts on HS by examining changes in near‐surface air temperature, Heat Index (HI), and the Universal Thermal Climate Index (UTCI) across urban areas under dry and moist warm conditions. Our results demonstrate that climate change impacts on HS are not well captured by examining only the projected changes in air temperature and are dependent on the bioclimate index considered. Additionally, we apply a new human heat balance (HHB) approach to evaluate the number of hours per day that an acclimatized and non‐acclimatized person would experience uncompensable HS and compare these results (with the number of hours per day) that we obtain when the HI and UTCI surpass commonly used thresholds considered “dangerous” and of “extreme heat stress”, respectively. The HI and UTCI overestimate the number of hours per day that a healthy, acclimatized person would experience uncompensable HS and underestimate dangerous HS for a non‐acclimatized person under both dry and moist conditions, emphasizing that standard metrics may not produce the most informative physiological estimates of HS.