Comparisons of transport and dispersion model predictions of the joint urban 2003 field experiment

For a hazardous material release in a city or densely populated area, effective mitigation requires an understanding of the transport and dispersion of these hazards in the complex urban environment. Improved characterization and understanding of urban transport and dispersion will allow for more robust modeling. The Defense Threat Reduction Agency has developed a Hazard Prediction Assessment Capability (HPAC) that includes features to address hazardous releases within an urban environment. During the summer of 2003, a series of tracer gas releases were carried out in Oklahoma City, Oklahoma, and extensive meteorological and tracer concentration measurements were collected in a field experiment known as Joint Urban 2003 (JU03). This analysis uses the observations of JU03 to evaluate "Urban HPAC." Twenty sets of simulations, or "predictions," using four Urban HPAC modes and five meteorological input options, were created and compared using a variety of metrics. Strong consistency was found between the conclusions of this study and those of two previously reported Urban HPAC evaluations. For example, improved predictions were associated with the inclusion of a simple empirically based urban dispersion model within HPAC, whereas improvements associated with the inclusion of a more computationally intensive urban wind module were not found. In this study, two new results are reported. First, there was a substantial difference in the performance of Urban HPAC as a function of release time-day or night that was not discovered earlier because the previously examined urban field experiments focused on nighttime releases only. Daytime releases tended to be underpredicted and nighttime releases tended to be overpredicted. Second, and with respect to the under- and overpredictions described above, the inclusion of the new "Micro" Stationary Wind Fit and Turbulence (SWIFT) " SPRAY" (MSS) Urban HPAC mode typically led to less underprediction during the day and less overprediction at night than the other Urban HPAC modes. In addition, predictions that included MSS typically resulted in the least scatter between observations and predictions. These improvements warrant further investigation to determine whether this conclusion can be extended to other urban environments.