Establishing a baseline for thermal stress conditions-A high-resolution radiative perspective

As Earth's climate warms, outdoor thermal conditions that threaten human life are becoming more frequent and severe. To prepare and adapt effectively to this challenge, an understanding of both current baseline thermal conditions and how thermal conditions will change throughout the 21st century is vital. However, current efforts to measure and model baseline thermal conditions have generally ignored the contributions of radiation, and have been performed at coarse temporal and spatial resolutions. In this study, we present the first hourly multidecadal bioclimatology of mean radiant temperature and the Universal Thermal Climate Index, driven by meteorological reanalysis data and processed using the Solar and LongWave Environmental Irradiance Geometry model. This process allows for the calculation of the influence of urban geometry on radiation fluxes at 1 m spatial scale. The results demonstrated that this technique provides significant insight into thermal stress conditions, while also presenting some inherent challenges. The presence of bimodal thermal stress conditions driven by shade, significant variability of heat stress conditions on multiple time scales, and differences in the factors that drive extreme heat and cold stress were revealed. Our study showed that such high-resolution modelling of thermal stress is a feasible technique with the potential to provide significant value in understanding outdoor thermal stress in complex urban environments.