Advancing urban atmospheric modeling with coupled mesoscale–LES: A Bologna heatwave case study

This study presents the first application of a coupled PALM and MOLOCH modeling system over the city of Bologna, Italy, aimed at advancing the understanding of urban-scale meteorology through high-resolution numerical simulations. Focusing on a specific heatwave event in August 2023, which serves as a proxy for increasingly frequent extreme conditions in the Mediterranean basin, we investigate the intricacies of atmospheric dynamics at the micro-scale. A novel software, MOLOCH4PALM, is introduced for preparing initial and time-dependent boundary conditions for PALM by ingesting 3D meteorological fields from the mesoscale MOLOCH model in an offline nesting configuration. This integration significantly improves the representation of urban atmospheric dynamics, enabling more realistic simulations of temperature patterns and energy exchanges across scales. Validation against observational data collected during the heatwave confirms the reliability of MOLOCH in capturing large-scale temporal variability, despite a consistent cold bias. In contrast, the coarse PALM configuration (50 m resolution) showed poor performance, with low correlation and a pronounced warm bias. Increasing PALM’s resolution to 10 m and 2 m corrected these biases, with the 10 m configuration offering the best balance between accuracy and computational efficiency. Additionally, wind speed simulations demonstrated the robustness of the coupled system, particularly under calm wind conditions, highlighting its suitability for urban microclimate studies and heat mitigation planning. Further analysis using multiple linear regression quantified how urban morphology, represented by the Sky View Factor (SVF), and wind speed independently influence 2 m air temperatures across various surface materials. The results revealed distinct diurnal patterns. Higher SVF values increased daytime temperatures due to greater solar exposure but accelerated nighttime cooling through enhanced longwave radiation loss. Conversely, while higher wind speeds promoted cooling during peak insolation, they were linked to warmer nighttime temperatures due to atmospheric mixing. These findings underscore the critical role of urban design in modulating thermal comfort.