Coupling Between Urban Sublayers: High-Resolution LES Modeling of Microclimate and Energy Dynamics in Bolognina

The rapid growth of the global urban population in recent years emphasises the need to understand urban environments in order to build sustainable, resilient cities and enhance residents' quality of life. Despite its significance, the urban microclimate remains one of the most complex and least understood phenomena, largely due to the heterogeneity of urban environments. The physical structure of cities alters the exchange of momentum, energy, and pollutants between the surface and the atmosphere, creating an urban surface layer where classical turbulence laws no longer apply.

This study uses LES (PALM-4U) simulations to analyse key micrometeorological parameters with high spatio-temporal resolution in the Bolognina district of Bologna. The area is a typical example of Italian urbanisation. PALM-4U is coupled with the GLOBO-BOLAM-MOLOCH system. The MOLOCH model, developed specifically for Italy, provides more accurate mesoscale predictions than models like COSMO and WRF, which is crucial for LES simulations. Data from remote sensing, municipal datasets, and a census of over 5,000 trees within a 1 km² area were used as static drivers for the Bolognina study.

The case study spans three days, from 23 to 25 August 2023, characterized by clear skies, intense daytime solar radiation, and light winds—optimal conditions for turbulent flow detachment from the surface and the development of the UHI effect.

The study aims to investigate i) the role played by the type of pavement and urban vegetation in mitigating or amplifying the UHI effect and influencing thermal comfort; ii) how different pavements and vegetation affect the micrometeorological processes in the roughness sublayer (RSL), thereby influencing the flow in the inertial sublayer (ISL); and iii) the intensity of the coupling between the RSL and ISL and how it modifies the closure of the energy balance in the urban boundary layer.