Modeling outdoor heat stress at meter-scale resolution: Validation of the UrbClim-HiREx framework

Climate change and ongoing urbanization are increasing human exposure to heat stress in cities, where strong spatial contrasts in shading and imperviousness generate large variations at the scale of streets and public spaces. Urban adaptation planning therefore requires high-resolution information on outdoor heat stress. Microscale climate models provide such information, but their reliability must be demonstrated at meter-scale spatial resolution. This study presents a validation approach for such models through a detailed evaluation of the coupled UrbClim-HiREx modeling framework. UrbClim is a fast urban boundary layer model operating at 100 m resolution, while HiREx is a radiation-based model that simulates wet-bulb globe temperature (WBGT) at 1 m resolution. Model performance is assessed against a dense observational dataset collected during 7 consecutive heat wave days in June 2023 in Ghent, Belgium, spanning multiple urban microenvironments within a 300 m × 300 m area. UrbClim reproduces air temperature and humidity with high accuracy and provides realistic pedestrian-level wind speeds after in-canopy extrapolation, yielding a reliable forcing for HiREx. HiREx reproduces the diurnal evolution of WBGT with root mean squared errors of 0.95 °C at an open site and 0.51 °C beneath tree canopy. It resolves transient shading effects and meter-scale spatial variability, with hourly daytime root mean squared errors across the microenvironments remaining below 1.59 °C. The model reproduces the observed reductions in peak daytime heat stress under tree shade of approximately 4 °C WBGT. These findings provide the most detailed validation of the UrbClim-HiREx modeling framework to date and establish a transferable approach for evaluating microscale heat stress models. The results confirm the framework’s suitability for identifying urban heat stress hotspots and assessing adaptation strategies at meter-scale resolution.