Atmospheric transport models for hazardous releases: Variability and uncertainties-findings of COST Action ES1006 and operational experiences

Atmospheric transport models are in use to model the airborne transport of toxic or radioactive gases or aerosols in large, regional or very local scale events. Different approaches are available, ranging from simple parametric models and Gaussian methods to Lagrangian dispersion models and advanced CFD-based modelling suites. The variety of the models implemented in today's emergency response systems, used in crisis management and emergency response planning, therefore ranges from very simple, robust and fast approaches to highly sophisticated model systems taking into account changes in the meteorological conditions in time and space, terrain and building effects etc. The various methodologies have advantages and disadvantages with respect to their computational efficiency, accuracy, reliability of the results and many more. For any accidental release scenario, authorities may come to different decisions and a variety of instructions may be given to emergency responders, depending on the simulation tools applied. Which variability in model results can be expected in emergency cases depending on the atmospheric transport model used? How can these uncertainties be explained? How may these be handled in real time application? These questions are addressed in the paper. The performance of different models for local hazardous releases in built-up areas is illustrated by selected results from model evaluation exercises undertaken in the frame of COST Action ES 1006: non-blind and blind test cases, for continuous as well as for puff releases, sensitivity studies and applications of the same model by different users. A comprehensive overview of all results of these model evaluation exercises is given by Baumann-Stanzer et al. (2015). Regional to large scale atmospheric dispersion calculations from different emergency response tools, e.g. for radiological hazards, may also render significant differences in the simulated affected areas-due to different meteorological input but even in case of identical meteorological input and source term due to differences in the applied model physics. This is exemplified and discussed based on a model comparison study using the model systems TAMOS, RODOS and ESTE: comparisons of model results based on the same meteorological forecasts under various weather conditions and for different release scenarios.