Simulation of a summer urban breeze over Paris

Numerical simulations for an anticyclonic summer episode in the Paris area have been performed at the meso-gamma scale for a 48-hour period, and compared to observations from a dense operational observational network. The meteorological stations have been classified, according to the extent of urbanization of their surroundings, into four classes (central Paris, urban, suburban, and rural). The atmospheric model, coupled with an urban surface scheme, correctly reproduces the temperature (within 1 K from the observations) and humidity. The intense urban heat island during the night is also well represented. Following the validation, the model is used to quantify atmospheric effects of Paris on the boundary layer, through a comparison with a purely rural simulation. At night, the model simulates a neutral or even slightly unstable boundary layer to a depth of 200 m over the city. In contrast, a very stable layer formed in the countryside. During the day, the boundary layer was more turbulent and 500 m deeper over Paris; vertical velocities of up to 1 m s(-1) were created over the city. This leads to an urban breeze with convergence at low levels (with winds around 5 to 7 m s(-1)), and divergence at the boundary-layer top (with similar wind speeds). The horizontal extent of the breeze reaches for more than 50 km from the city centre, and could have an important impact on pollutant diffusion in the area for calm days. Finally, three other spring cases are presented briefly. These show that an urban breeze develops if the synoptic wind is weak enough or disorganized; an urban plume develops otherwise.