Urban-breeze circulation during the CAPITOUL experiment: observational data analysis approach
Résumé
An experimental study focused on 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 to Feb. 2005) is presented. The influence of the urban breeze circulation on the structure and the dynamics of the Atmospheric Boundary Layer is analysed by using ground stations, wind profilers (UHF radar), radio soundings and aircraft data. The IOP5 is a summertime anticyclonic situation with low wind, strong insulation and a deep boundary layer with a sheared wind profile. The temporal and spatial relationship between the Surface Energy Balance (SEB), the diurnal evolution of the surface temperature and the dynamics of the airflow within the mixed layer are studied for urban and rural sites. The vertical and horizontal structure of the urban heat island are analysed from both surface stations and aircraft data. The nocturnal urban heat island reaches 5 C. In the early morning, the city air becomes 1 C cooler than in the countryside (due to efficient heat storage and building shade on the city urban canopy), then warmer again (1 C) in the afternoon, due to negligible evaporation and strong sensible heat flux in the city. The daytime heat island is also present in altitude, being localized above the city centre at 350m of height (1 C) and advected to leeward at 1100m of height (0.5 C). This diurnal heat island is associated to an urban-breeze circulation characterized by a surface convergence towards the city at low levels and a divergence in upper boundary layer. The urban breeze grows in intensity from 2ms1 (at 12.00 UTC) to 5-6ms1 (at 18.00 UTC). Aircraft measurements show that the urban breeze horizontal extension is 2 to 3 times larger than the city size.