Wind tunnel studies offer an excellent opportunity for systematic and extensive turbulence measurements with boundary conditions which are better known and controlled than in full scale experiments. 

This is the motivation for the realization of an extensive wind tunnel model of the city part "Kleinbasel" at the scale 1:300. The model covers an area of a bit less than 3 kmē of urban roughness densely covered by over 3 200 houses as it is typical for Basel and other european urban environments of comparable size. The reference is given by a 3d digital urban model provided by the local authorities supplemented by over 200 photos documenting street canyons, buildings arrangements and measures as well as details of the respective roof shapes.

Location of the wind tunnel study is 'WOTAN', the large boundary layer wind tunnel of the meteorological institute at the University of Hamburg, Germany. 'WOTAN' was put into operation in 2001 and since then various studies concerning urban turbulence and pollutant dispersion have been delivered successfully. The test section is 18m long, 4m wide with an ajustable ceiling height of 2.75-3.25m to compensate pressure gradients along the wind tunnel axis. It is equipped with LDA measurement facilities which allow simultaneous measurements of two velocity components to determine, e.g., mean wind speeds, turbulence intensities and turbulent fluxes as well as turbulence spectra and integral length scales. Tracer gas concentrations can be measured with FID instrumentation.

The wind tunnel study consists of two phases: turbulence measurements and pollutant dispersion experiments. The first phase, conducted in summer 2003, focuses on the set up of propper boundary conditions for the urban flow followed by systematic measurements of vertical and horizontal turbulence profiles. Special attention is given to the approach flow to match as well as possible the atmospheric boundary layer (ABL) turbulence characteristics appropriate for Kleinbasel. Goal of the turbulence measurements are, firstly, the elucidation of the layered turbulence structure above roof level in terms of a roughness sublayer (RSL) and an intertial sublayer (ISL) and, secondly, a quantification of the horizontal variability of turbulence characteristics within the RSL. This is expected to complement and help the interpretation of the full scale measurements which have been made within the model area at Basel-Sperrstrasse (BSPR). 

The second phase, due in 2004, encompasses pollutant dispersion experiments to model the full scale tracer gas release experiment which has been undertaken during the IOP in Kleinbasel (2002). A comprehensive data set of tracer gas concentrations at various locations, including the full scale sampling locations, is expected to provide further insights in the turbulent dispersion dynamics.

Naturally, a succesful delivery of this wind tunnel study will provide an extensive reference data set for urban turbulence and pollutant dispersion particularly suited for the comparison with numerical model predictions.