Modelling of radiation-based thermal stress indicators for urban numerical weather prediction

Two widely used radiation-based thermal stress indices, the Wet-Bulb Globe Temperature (WBGT) and the Universal Thermal Climate Index (UTCI), are implemented into the Canadian urbanized version of the GEM Numerical Weather Prediction (NWP) model in order to improve heat-health meteorological products in urban areas. Predictions with 250-m grid spacing over the Greater Toronto Area (GTA), Canada, reveal that spatial distribution of both WBGT and UTCI are similarly sensitive to mesoscale features such as the lake-breeze flows. Accurate prediction of WBGT and its indicators is found as evaluated with measurements during clear-sky and cloudy condition cases. In particular in clear-sky conditions the scattering index of solar radiation from the atmospheric model is found to be more realistic than fixed values. Links between intermediate important variables representing thermal load on a body, the mean radiant temperature (TMR), and a synthetic variable equivalent to the globe temperature (TG) as measured with a globe thermometer sensor are closely analyzed. Results show that the use of distinct TMRWBGT and TMRUTCI leads to differences of up to 50% due to a different energy partitioning and that TMR and TG are linked through a hysteresis cycle.