Wind tunnel modelling of urban turbulence and dispersion over the City of Basel (Switzerland) within the BUBBLE project

Urban turbulence and dispersion of a passive, chemically inert scalar in an urban environment are studied in an extensive wind tunnel experiment in the large boundary layer wind tunnel of the Meteorological Institute at the University of Hamburg, Germany. The wind tunnel study is conducted within the BUBBLE project which also includes extensive field measurements and numerical modelling (http://www.unibas.ch/geo/mcr/Projects/BUBBLE).

The wind tunnel model of Basel (scale 1:300) is based on a digital model of Basel and covers full scale roughly 2.4km x 1.2km of urban roughness including more than 3.000 building structures (mean building height approx. h=14.6m). All experiments are conducted under neutral stratification. Focus of the turbulence study is the surface layer above roof level covering the height range from approx. z=21m to z=100m. The dispersion experiment is designed to model a tracer release field experiment in Basel carried out in 2002 during the BUBBLE field campaign.

Deep within the urban model (downstream of an urban fetch of approx. 1.3km) vertical and horizontal turbulence profiles are measured with Laser Doppler Anemometry. A layered turbulence structure is observed with a roughness sublayer (RSL) reaching up to approx. 51m (3.5h) and an inertial sublayer (ISL) covering the height range from approx. z=51m to z=81m. The ISL is characterised by the logarithmic mean wind profile and a constant flux profile (u’w’). Based on horizontal averages these profiles can be extended downwards well within the RSL (down to at least 27m (1.8h)) which is otherwise characterised by clearly 3-dimensional turbulence. The Reynolds flux u’w’ is further studied by a quadrant analysis and horizontally averaged profiles of ejection-sweep differences S_0 are calculated showing a slight dominance of ejections within the ISL which is in contrast to findings in wind tunnel studies with non-urban, idealized roughness.

Comparisons to local field measurements in Basel give good agreement of the turbulence characteristics up to z=75m, thereby supporting the applicability of the wind tunnel findings to full scale urban scenarios.

For the wind tunnel dispersion experiment ethane is released from a point source above roof level (z~18m). The concentration field above roof level (from z=27m to z=69m) is determined systematically on a 3-dimensional grid of measurement points covering a downstream distance from the point source of approx. 400m to 1.000m. Additionally, concentration time series are recorded at the field sampling locations. The concentration measurements are compared to the field measurements and to the predictions of urban dispersion models. Especially the simple Gaussian plume model seems to give a good parameterisation of the measured concentration fields. All concentration measurements are done with FFID instrumentation.