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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.

The potential effects of a terrorist attack involving the atmospheric release of chemical, biological, radiological, nuclear, or other hazardous materials continue to be of concern to the United States. The Defense Threat Reduction Agency has developed a Hazard Prediction Assessment Capability (HPAC) that includes initial features to address hazardous releases within an urban environment. Improved characterization and understanding of urban transport and dispersion are required to allow for more robust modeling. In 2001, a scaled urban setting was created in the desert of Utah using shipping containers, and tracer gases were released. This atmospheric tracer and meteorological study is known as the Mock Urban Setting Test (MUST). This paper describes the creation of sets of HPAC predictions and comparisons with the MUST field experiment. Strong consistency between the conclusions of this study and a previously reported HPAC evaluation that relied on urban tracer observations within the downtown area of Salt Lake City was found. For example, in both cases, 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 wind field module were not found. The use of meteorological observations closest to the array and well above the obstacle array--the sonic anemometer measurements 16 m above ground level--resulted in predictions with the best fit to the observed tracer concentrations. The authors speculate that including meteorological observations or vertical wind profiles above or upwind of an urban region might be a sufficient input to create reasonable HPAC hazard-area predictions.

The tracer releases of the 'URBAN 2000' urban tracer and meteorological field experiment conducted in Salt Lake City, Utah, in October 2000 provided a wealth of data for comparison with the predictions of transport and dispersion models. Comparisons of several sets of predictions created with the U.S. Defense Threat Reduction Agency's urban Hazard Prediction and Assessment Capability (HPAC) model are compared with the observations of the URBAN 2000 field study. This analysis focuses on paired-in-space-and-time comparisons of predictions and observations and uses conventional statistics as well as a previously described transport and dispersion model user-oriented measure of effectiveness. Four urban HPAC model configurations and five weather input options were examined, and, thus, 20 sets of URBAN 2000 predictions were created. Novel features of this analysis include 1) an inclusive data protocol that allowed for the use of 94% of the observed URBAN 2000 sulfur hexafluoride (S[F.sub.6]) 30-min average concentrations, 2) the application of the previously developed user-oriented measure of effectiveness (MOE) to urban predictions and observations, and 3) the use of nonparametric hypothesis test procedures to guide objectively the search for significant performance differences between urban HPAC model configurations through over 25 000 comparisons of metrics. Robust findings are reported that include the statistical discernment of general trends associated with urban HPAC predictive performance as well as the identification of differences in performance among various model configurations.

A two-dimensional measure of effectiveness for comparing hazardous material transport and dispersion model predictions and field observations has been developed. This measure is used for comparing predictions and observations paired in space and time, and the components of this measure--overprediction, or false-positive fraction, and underprediction, or false-negative fraction--can illuminate strengths and weaknesses of a model in ways that many one-dimensional measures cannot. Comparisons of predictions of short-range field observations are used to illustrate features of this measure of effectiveness, including its computation based on dosages or based on a dosage threshold of interest. With this user-oriented measure of effectiveness, statistically significant resolution of transport and dispersion model performance differences as a function of downwind range and meteorological stability category grouping, as well as between different models, is described. Evaluation of probabilistic prediction outputs is also demonstrated with this measure of effectiveness. The user-oriented measure of effectiveness can be evaluated for different assumed hazardous materials and human effects of interest and, as such can relate the quality of a prediction for agents of greatly varying toxicity. A quantitative method that can aid in the communication of a user's risk tolerance is identified and allows one to divide the two-dimensional measure of effectiveness space into regions of relative acceptability for the particular user and application.