Relative influence of urban and orographic effects for low wind conditions in the Paris area

The relative roles of land-use and orography on the meteorological fields in the Paris area are studied by means of numerical simulations of two Special Observing Periods (SOP), of the ECLAP experiment. Sensitivity experiments have been performed with flat orography to investigate the strength of the urban effects, and with a uniform land-use surface to replace the surface heterogeneities, to isolate the orographic forcing. Comparisons of the simulated fields with a reference simulation including all forcing are analysed. It is found that during these two SOP of spring-time conditions, despite the low terrain elevation, the orographic effect is the main forcing of the dynamic field but that although the land use is the dominant factor reproducing the thermal evolution of the boundary layer, the orography alone also plays a role in the temperature pattern with slope flows in and out of the Paris basin.