Ensemble Dispersion Modeling: “All for One, One for All!”

Almost every country has adopted modeling systems to forecast the consequences of atmospheric dispersion at various scales. In particular long range transport and dispersion (LRTD) models are used to forecast the dispersion of large emissions of harmful pollutants from point sources such as, for example, atmospheric dispersion of radioactive gasses from NPP or other sources. The comparison of state-of-the-art model results with observations (e.g. Draxler, 1983; Klug et al., 1992; Girardi et al., 1998) has shown unequivocally that among the various approaches to atmospheric dispersion modeling, none is systematically performing better than others. Two are the main sources of uncertainty in the model results: one connected to the atmospheric circulation forecast and one dependent on the way in which atmospheric dispersion has been modeled. Therefore none of the models evaluated can be identified as “the model” to be used. On the other hand, model results are used for the assessment or forecast of conditions that may involve adoption of countermeasures for the protection of the population, for which a high level of accuracy is required.