WRF–City–LES Modelling of the Urban Thermal Environment in Sofia

Abstract. Urban areas are increasingly exposed to thermal stress owing to the combined effects of global warming and the Urban Heat Island (UHI). High-resolution numerical modelling is therefore needed to assess local thermal environments and support heat-mitigation planning. This study applies a high-resolution WRF–City-LES downscaling approach to simulate the urban thermal environment of Sofia, Bulgaria. The mesoscale Weather Research and Forecasting (WRF) model provides regional- to urban-scale meteorological forcing, whereas the microscale City-LES model resolves city-block-scale thermal and wind environments in the Lozenets district by explicitly representing buildings, trees, land-surface characteristics, and building-related anthropogenic heat (AH). Compared with the mesoscale WRF simulation, City-LES more accurately represents the near-surface air temperature at the observation sites, reducing the mean absolute error (MAE) from 1.4 °C to 0.3 °C and the root mean square error (RMSE) from 1.4 °C to 0.3 °C. The high-resolution micro-simulation also captures fine-scale spatial variability in near-surface air temperature and outdoor thermal comfort that WRF did not resolve. The spatial distributions of Mean Radiant Temperature (MRT) and Universal Thermal Climate Index (UTCI) reveal city-block-scale differences related to buildings, tree canopies, and radiative conditions. The AH sensitivity experiment showed that building-related AH had a limited effect on domain-averaged near-surface air temperature under the present implementation. These results demonstrate that the WRF–City-LES downscaling approach provides a useful basis for urban climate assessment and future urban digital twin applications, including scenario-based evaluation of heat-mitigation strategies.