During July 2003, the extensive field campaign JOINT Urban 2003 (JU2003) took place in Oklahoma City, USA. In total, ten intensive observation periods (IOPs) were performed with typically three half-hour tracer releases during each IOP. One of the main objectives of this field campaign is the generation of reliable datasets for the validation of microscale dispersion models, which can be used as fast-response tools for accidental releases of toxic material in an urban environment. Although, the JU2003 campaign was the largest field study ever performed for such purposes, the field data represent only a limited set of atmospheric conditions that varied to a certain extent during each IOP. It was therefore felt, that laboratory datasets from wind-tunnel simulations of the flow and dispersion characteristics in downtown Oklahoma City will significantly enhance the validation capabilities of the JU2003 database. Such laboratory data have the advantage that the flow conditions can be precisely controlled and kept constant over a long period of time. Thus, it is possible to simulate the same steady-state situations as calculated by most of the micro-scale flow and dispersion models.

Using a scaled wind-tunnel model (model scale 1:300) of the central business district (CBD) of Oklahoma City, systematic wind tunnel tests are carried out at the University of Hamburg, Germany. The first phase of experiments was in Spring 2003 (i.e. before the field campaign started) and the main objective was to provide information for planning of the field studies. Laser light-sheet flow visualizations and measurements of concentration timeseries were performed for wind directions typically observed during July in Oklahoma City and for several possible source locations.

The wind-tunnel simulations revealed a very instantaneous/non-stationary behavior for certain release locations. The plume is erratically shifted by more than ± 45° around the mean wind direction. Consequently, long periods with or without measurable tracer gas concentrations must be expected for downwind sampling positions across the entire central business district. For all release locations short periods with a significant upwind transport of the tracer could be observed. Furthermore, an almost instant vertical mixing of the tracer up to the height of the tallest buildings was observed for all release locations. The almost immediate vertical transport of tracer was expected for sources within the CBD, but was also visible for release locations south of the CBD. Due to the latter results the number of rooftop tracer samplers was increased during the field campaign and first field data confirm the somewhat unexpected strong vertical mixing.