Evidence of interaction between synoptic and local scales in the surface layer over the Paris area

Atmospheric boundary layer (ABL) turbulent processes in the Paris area have been documented in the framework of the Etude de la Couche Limite en Agglomeration Parisienne (ECLAP). Under anticyclonic conditions, simulations are made with a 'column' modelling approach, based on the three-dimensional version of the non-hydrostatic mesoscale model MERCURE restricted to a small domain. This 'column' model uses existing state-of-the-art surface-layer parameterizations (the addition of the convective velocity scale to the mean wind speed in near free convection periods, the prescription of the effective dynamical roughness length as well as a differentiation between dynamical and thermal roughness lengths). To ensure the representativeness of the comparison between measurements and simulations, the dynamical and thermal effective roughness lengths characterizing the experimental site are prescribed explicitly in the model, using sonic anemometer measurements. We show that the parameterizations implemented in MERCURE for this study enable a good description, by the three-dimensional model, of the observed complex ABL dynamics. We also show that in the region of Paris, the synoptic scale and mesoscale dynamics can have a dramatic impact on the ABL dynamics and turbulent processes at the local scale. This study is a first attempt at improving our ability to predict meteorological factors affecting urban air quality.