Observation of an Urban Plume during CAPITOUL experiment

An urban plume has been observed the 9th and 10th march 2004 over the city of Toulouse during the CAPITOUL experiment. The meteorological conditions, light synoptic wind and sunny cold (between 0 and 5 Celcius degrees) conditions, were favorable to the establishment of urban induced circulations in the boundary layer.

Observations were conducted both at the surface (surface energy balance in city center and rural area), UHI measurement network, profilers and completed by in-situ measurements in the boundary layer: radiosoundings (upwind rural and in the city center) and aircraft.

The contrast between the rural and urban surface energy budgets is clear. At night the city warms the atmosphere by more than 50 W/m2 continuously through the night. this creates an UHI and over the city an instable boundary layer of more than 100m deep takes place. At day, the larger warming from the city surface induces a boundary layer of 1100m deep, instead of only 900m in the countryside upwind. The urban dome is then approximately of 200m. Aircraft measurements measured a thermodynamical plume, with an increase of air temperature at 300m of height of approximately 1.5 degree when the airplane flew from upwind to above city. This warmer air is advected downward on approximately 50km.

Passive tracer release (SF6) were also conducted during this day, with surface measurements at sunrise, in a relatively suburban stable boundary layer, and on the afternoon in an unstable one. In the latter case, the SF6 concentrations were measured both at the surface and by the airplane. Validation of a dispersion model with these data give good results.

Some other cases of city induced circulations in the boundary layer will also be briefly presented.