A parameterization of the urban effects for mesoscale models

The dispersion of local emissions in urban areas and in proximity of them, is strongly dependent on the structure and characteristics of the boundary layer developping above the city. In this context a very important role is played by the interactions with the rural boundary layer and with the meteorological circulations induced by the geophysical chacteristics of the surroundings (ex. sea/land breeze, valley winds etc.). The numerical tool employed to study this phenomena should be able to run on a domain large enough to take into account the mesoscale circulations and, at the same time, have a good representation of the heat and momentum fluxes in the urban area (which is a key factor in the development of the urban boundary layer). For these reasons, in this work, the mesoscale model FVM (Finite Volume Model) was modified in order to parameterize the impact of the three active surfaces of the buildings (roofs, walls and street) on the momentum, energy and TKE budget equations. For the momentum, two different roughness lengths are defined for roof and street, respectively while the contribution of the walls is parameterized with a drag force approach. The sensible heat fluxes are determined as a function of the difference between the air temperature and the corresponding surface temperatures. A complete energy budget equation is solved for each of the three surfaces. The short and long wave radiative fluxes are computed by taking into account the shadows and multiple reflection effects of the street canyon element. The model and the parameterization are tested for 2D idealized cases of an urban boundary layer. A qualitative (and quantitative) comparison with measurements (Reynolds stress, tke profiles, heat storage, etc) is performed and an evaluation of the relative impact of the three surfaces is analysed. The impact of different synoptic forcings and different 'rural' characteristics on the structure of the urban boundary layer will be also presented.