High-resolution simulations of the urban atmosphere in sea-breeze conditions

Large-eddy simulations of the Intensive Observation Period (IOP 2b) of the UBL-Escompte experiment have been performed with the SUBMESO atmospheric model (based on ARPS) on three nested domains whose horizontal resolution ranges from 990m to 110m. The SM2U soil model (Dupont & Mestayer 2006) is used to prescribe in the atmospheric model the heat and humidity fluxes between the canopy and the atmosphere.

The simulation results are compared with measurements at the ground stations that have been deployed over Marseille to provide temperature, humidity and for some of them high frequency turbulent fluxes and wind components. This large amount of data is very useful to assess the accuracy of the simulation results and our choices of large-scale forcing and of surface/canopy description. The agreement between simulated and measured energy budget components is very sensitive to the surface description (water content of deep soil and humidity of natural surfaces, albedo and emissivity of artificial surfaces…). The time accuracy of the sea-breeze arrival above the studied site is very important too, particularly at high resolution to capture the instantaneous changes in flux behavior and their differences with the diurnal cycle of urban areas located far from the coast. The competing influences of the sea-breeze and of the urban physical processes are further analysed by considering (1) the atmospheric stratification, (2) the heat and momentum turbulent fluxes and (3) the turbulent kinetic energy in the lower atmosphere, as provided by the large eddy simulation.