The UBL/CLU-Escompte experiment aimed at documenting the four-dimensional structure of the urban boundary layer in connection with the urban canopy thermodynamics during a summer period of low wind and breeze conditions, from June 5 to July 15, 2001. The objective was mainly to construct a data base allowing to test urban energy exchange schemes and high resolution meteorological and chemistry-transport models. The project took advantage of the large experimental set-up of the campaign ESCOMPTE (http://medias.obs-mip.fr/escompte) over the Berre-Marseilles area, especially as concerns remote sensing from ground, airborne measurements, and the intense documentation of the regional meteorology. UBL/CLU appeared thus as an “associated project” of ESCOMPTE, with an additional experimental set-up to document the fine scale dynamics and thermodynamics of the urban atmosphere over the Marseilles area, situated at Mediterranean coast, and involving more than one million of inhabitants.

The instrumentation was deployed at 5 main sites, along the North-South axis of the city, roughly parallel to the shoreline. Four urban sites were equipped with micrometeorological masts raising some 12 to 20 m above the urban canopy, where all the aerodynamic and radiative fluxes necessary to monitor the canopy surface energy budget were continuously measured. The central site, located in the rather uniform, 19th Century, dense part of the city, was also equipped with an array of IR radio-thermometers, either fixed to monitor the surface temperature of selected elementary surfaces, or hand-held to evaluate surface temperature distributions during some periods of intense observation. IR radio-thermometers were also operated at the North site located in a suburban area of mixed constructions. The two sub-urban sites were equipped with mini-SODARs sounding the atmospheric surface layer while a fourth site, close to the city center was equipped with a wind profiler UHF radar and a tethered balloon occasionally measuring thermodynamic and ozone profiles from 20 to 300 m. Two scintillometers were set to measure the heat flux over the city center, with 2.5 km optical paths oriented N-S and E-W. At the hilly northern borderline of the city, a “reference” site hosted a RASS-SODAR sounder, and two 3-D scanning LIDARS measuring O3 concentration, particle concentration, and wind, over a range of 5 to 10 km. The set-up also included an array of 20 T-RH continuous recorders at a 6 m height over the ground, while transect T-RH measurements were occasionally made from the “T-RH Clio” car.

Most measurements were recorded continuously. Two types of intensive observation periods were more densely documented : the ESCOMPTE IOPs, generally during breeze situations, with a few airplane flights documenting the turbulent fields within or at the top of the boundary layer ; the TIR IOPs, when an airplane equipped with a thermal infrared mapping camera scanned the urban canopy from different directions with respect to the sun, and at different times in the day.

While 11 groups participated in the field experiment, more than 17 groups participate in the data analyses, from at least 3 points of view : surface temperature and heat flux remote sensing from satellite visible and IR channels ; structure of the urban boundary-layer ; numerical simulation of urban air quality with high resolution chemistry-transport models.