The modifications induced by an urban area on the planetary boundary layer structure play a very important role in the pollutant dispersion. A comprehension of the mechanisms involved is crucial for a correct understanding of the phenomena. In this work, a mesoscale numerical model, with a detailed urban surface exchange parameterization, is used to study the impact of a city on the boundary layer structure. The formulation takes into account the mechanical effects (sink of momentum induced by building’s drag, increase of TKE) as well as the thermal effects (shadowing and trapping of radiation in the street canyon which alter the surface energy budget). This parameterization is able to reproduce the most important features usually observed in urban areas (Reynolds stress profiles, storage, urban heat island, etc.).

Here the model is used as a tool to investigate the interactions between the thermal and mechanical effects. For a simple idealized domain, but for different urban morphologies and different meteorological conditions, a factor separation analysis technique is used in order to appreciate the importance of the thermal and mechanical urban factors in the modifications induced on the boundary layer structure. Results show that during the night, mechanical factors play a very important role in determining the depth of the neutral layer developing above the city. On the other hand, during day, thermal factors are more important in modulating the Planetary Boundary Layer height evolution either above the city, either downwind the city. The impact of both factors on the pollutants dispersion is also analyzed.