Evaluation of Transport and Dispersion Models: A Controlled Comparison of HPAC and NARAC Predictions

: The goal of this task is to improve the verification, validation, and accreditation (VV&A) of hazard prediction and assessment models. These studies are part of a larger joint VV&A effort that DTRA and the Department of Energy, via the Lawrence Livermore National Laboratory (LLNL), are conducting. This joint effort includes comparisons of the LLNL and DTRA transport and dispersion modeling systems National Atmospheric Release Advisory Center (NARAC) and Hazard Prediction and Assessment Capability (HPAC), respectively. The Institute for Defense Analyses role includes conducting comparisons between the models and providing analysis and discussions associated with these examinations. This paper provides detailed comparisons of HPAC and NARAC predictions for 17 releases. The 17 trials were chosen with the desire for simplicity, with the expectation that simple releases would test only the core transport and dispersion algorithms within each model. Consequently, all releases were chosen to have idealized weather a single unidirectional wind observation of 5 meters per second at 10 meters altitude and only flat terrain was considered. The 17 trials did allow for the comparison of model predictions as a function of atmospheric stability condition, source type (gas or particle), release duration, release altitude, and particle size.