Urban-breeze circulation during the CAPITOUL experiment: numerical simulations

In this study we present a numerical simulation of the urban-breeze circulation observed in Toulouse, South-West of France, during the Intensive Observation Period number 5 (IOP5, 3rd and 4th July 2004) of the CAPITOUL experiment (Feb. 2004-2005). The numerical simulation is performed with the non-hydrostatic atmospheric model MesoNH (Lafore et al. 1998) coupled with the urban surface scheme TEB (Masson 2000). Four two-way, grid-nested models with horizontal grid resolution of 12 km, 3 km, 1 km and 0.25 km are used. The diurnal cycle of temperature, the nocturnal heat island and the early morning cool island are reproduced by the model. For the urban-breeze period, between 12.00 UTC to 18.00 UTC, the heat island structure and the simulated turbulent fluxes are discussed based on the observed surface energy balance and urban canopy temperature. The numerical simulations confirm the presence of a convergent circulation at the surface towards the city centre and a divergent counter-current 1500 m above the ground. The intensity of the urban-breeze circulation is of the order of 1.5 m s⁻¹ and its extension, in the mean wind axis, is two times the diameter of the city. The dynamical perturbation on the ABL due to the roughness of the city is only significant up to 50 m of height, the urban breeze circulation being caused by the pressure gradient due to the UHI-induced thermal effects. An evaluation of the improvement on the ABL thermodynamics representation when going down to 250 m of horizontal resolution instead of 1 km is also presented.