A city can strongly modify the structure of the Atmospheric Boundary Layer. The presence of the buildings alters the wind, temperature and turbulent fields. A good comprehension of the involved mechanisms is crucial to simulate the pollutant dispersion. Two scales are involved in this problematic: an urban scale of few kilometers, with particular "urban effects", and a meso-scale of few hundreds of kilometers representing the flow fields of the mesoscale circulation. The dynamical model, which produces the meteorological fields (wind, temperature, turbulent coefficients, etc.), must be able to represent the most important mesoscale phenomena, and, at the same time, to take into account the urban effects in the most accurate way. For computational reasons, the horizontal resolution of the domain, of the order of 100 km (mesoscale), cannot be fine enough to represent the impact of every building and at the same time have a domain size large enough to take into account mesoscale circulation. The integration of both scales into the dynamical model to provide a full three-dimensional picture of the urban flow and turbulence structure (in particular, the 'rural-urban transition') requires a parameterization for the urban effects.

The integration of a new urban surface exchange parameterization into a mesoscale model, both developed at the Laboratory for Air Pollution of the Swiss Federal Institute of Technology, is applied in the case of the BUBBLE experiment. The numerous measurement data will be used to validate and possibly improve all these parameterizations. The main result of the simulation may also serve as a 'model' or guidance for improving NWP models (i.e., their surface exchange description) once their resolution is sufficiently small to capture with several grid points the main urban areas (e.g. the Swiss 'local model').