An Urban Street Canyon Heat Stress Calculation Method for Weather and Climate Models

Human thermal comfort in urban areas, where a growing percentage of the global population live, has been a topic of research for many years. Impacts of heatwaves are often greater in urban areas via the influence of urban heat islands, so the ability to determine indicators of urban heat stress on both weather forecast and climate projection timescales is crucial.

To address this, it is necessary to develop thermal comfort models that account for urban processes and utilise standard weather and climate model output data. In this work, we present a computationally efficient method to calculate wet-bulb globe temperature (WBGT; a standard heat stress metric) in an infinitely long street canyon for this purpose. The radiation calculation is analytical and accounts for longwave, shortwave direct and shortwave diffuse radiation transfer (allowing up to two reflections) to calculate mean radiant temperature of a black globe located anywhere in the canyon. This is combined with model estimates of near-surface temperature, humidity and wind speed to determine WBGT via standard empirical equations.

We calculate WBGT from hectometric weather model simulations over Paris and compare with WBGT values derived from black-globe thermometer measurements from street canyons during the PANAME field campaign. Additionally, we investigate the sensitivity of our WBGT calculations to geometric and optical properties of the canyon, and the background meteorological fields. We find that the model reproduces observed WBGT well in an urban canyon environment, and that values are most sensitive to shading, which is modulated by building geometry. Finally, we demonstrate a potential application of our method by calculating WBGT along the Paris 2024 Olympics marathon route and, comparing with published WBGT safety thresholds for long-distance running, demonstrate how our method could be used to advise event organisers on safety measures, such as event timing and route planning.