A multi-scale approach combining MOLOCH and PALM-4U for simulating urban micro-meteorology in an italian neighborhood

City development has significantly transformed local climate conditions, modified wind patterns, and variations in altered air quality. These transformations have important consequences for human health, energy usage, and environmental sustainability. Improvements in Computational Fluid Dynamics (CFD) modeling have offered essential tools for examining the intricate nature of urban microscale processes. These allow researchers to replicate airflow around structures, winds at pedestrian levels, and the spread of pollutants with high spatial detail, considering turbulence movements. In the Large-eddy simulation (LES) model, the significant turbulent eddies are explicitly resolved and modeled. Since a proper definition of urban morphology has been recognized as vital to improve model efficiency, reliable numerical simulations at the local level necessitate a suitable fine-scale depiction of the urban fabric.

This study employs LES simulations to characterize the daily cycle of key micro-meteorological parameters at high spatio-temporal resolution over the Bolognina district during summertime under high-pressure conditions. Situated within the municipality of Bologna, Bolognina exemplifies many urbanisation features typical of Italian cities. For the first time, PALM-4U was implemented using off-line nesting within the GLOBO-BOLAM-MOLOCH modeling system. It is important to highlight that the MOLOCH meteorological model, specifically developed for the Italian peninsula, provides highly accurate mesoscale predictions compared to other state-of-the-art models such as COSMO and WRF. This accuracy is particularly critical when using MOLOCH as a meteorological driver for LES urban simulations, where precise mesoscale input as well as high-quality urban morphological data significantly influence the results.

To develop the static driver for Bolognina, diverse data sources—such as remote sensing, municipal datasets, and open-access data—were utilized. Additionally, a dedicated census was conducted for privately-owned trees. In total, over 5,000 trees, both public and private, were cataloged within a 1 km² area. 

The experiment was conducted over a three-day period, from August 23 to 25, 2023, under weather conditions characterized by clear skies, calm winds, and strong daytime insolation - ‘ideal’ for the development of the Urban Heat Island (UHI). This case study primarily serves to assess the numerical stability of the novel meteorological dynamic driver. Additionally, the impact of different pavement types on micrometeorological profiles and on the partitioning of available energy at the surface was analysed to investigate how materials with varying heat capacities and urban vegetation can enhance or mitigate the UHI effect and the thermal comfort.