Concentration fluctuations in a downtown urban area. Part II: analysis of Joint Urban 2003 wind-tunnel measurements

Abstract

An analysis of concentration time series measured in a boundary-layer wind tunnel at the University of Hamburg is presented. The measurements were conducted with a detailed aerodynamic model of the Oklahoma City (OKC) central business district (CBD) at the scale of 1:300 and were part of the Joint Urban 2003 (JU2003) project. Concentration statistics, as well as concentration probability density (PDF) and exceedance probability (EDF) functions were computed for street- and roof-level sites for three different wind directions. Taking into account the different length scales and wind speeds in the wind-tunnel (WT) and full-scale experiments, dimensionless concentrations and a dimensionless time scale are computed for the comparison with data from the JU2003 full-scale tracer experiments, conducted in OKC in 2003. Using such dimensionless time, the WT time series cover a ~20 times longer time span than the JU2003 full-scale time series, which are analysed in detail in an accompanying, first part of this paper. The WT time series are thus divided into 20 consecutive blocks of equal length and the statistical significance of parameters based on relatively short records is assessed by studying the variability of the concentration statistics and probability functions for the different blocks. In particular at sites closer to the plume edge, the results for the individual blocks vary significantly and at such sites statistics from short records are not very representative. While the location of three sampling sites in the WT closely matched the sites during the full-scale experiments, the prevailing wind directions during the JU2003 releases were not exactly matched. The comparison between full-scale and WT concentration parameters should thus primarily be interpreted in a qualitative rather than direct quantitative sense. Given the differences in mean wind directions and concerns about the representativeness of full-scale concentration statistics, the WT and full-scale results compared well. The 98 percentile concentrations for almost all full-scale releases analyzed are within the scatter of the percentiles observed in the block analysis of the WT time series. Furthermore, the concentration percentiles appear linearly correlated with the fluctuation intensities and the linear relationships determined in the wind tunnel agree well with full-scale results.