The effects of uncertainties in boundary layer heights on urban dispersion during the daytime in Oklahoma City

Analyses of the tracer concentration measurements made over Oklahoma City during the Joint Urban 2003 urban dispersion experiment show that the concentrations measured in the morning during Intensive Operating Period (IOP) 5 were substantially higher than the concentrations measured under apparently similar meteorological conditions during the other daytime IOPs. This difference is seen even if the measured concentrations C are normalized by the release masses Q. An urban dispersion model prediction using model default boundary layer inputs also cannot explain the differences between concentrations measured during IOP 5 and the other daytime IOPs. This paper summarizes the results of an investigation to determine the cause of these anomalously high tracer concentrations during IOP 5. We investigated radiosonde soundings taken within Oklahoma City, measurements of vertical optical turbulence profiles by a frequency-modulated/continuous-wave (FM/CW) boundary layer radar, and synoptic weather maps. We found that the observed boundary layer heights were only about 100 to 200 m while the model-assumed boundary layer heights were three to six times larger. Urban dispersion model predictions for IOP 5 are in much better agreement with the measured concentrations when the lower boundary layer heights derived from the meteorological measurements are substituted for the default boundary layer heights.