Atmospheric Modeling of ¹³⁷Cs Plumes From the Fukushima Daiichi Nuclear Power Plant—Evaluation of the Model Intercomparison Data of the Science Council of Japan

Since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011, atmospheric simulation models have improved our understanding of the atmospheric behavior of radionuclides. Model intercomparisons provide valuable and useful information for evaluating the validity and variability of individual model results. In this study, we compared results of seven atmospheric transport models used to simulate ¹³⁷Cs released from the FDNPP to the atmosphere. All model results used in this analysis had been submitted for a model intercomparison project of the Science Council of Japan (2014, http://www.scj.go.jp/en/report/index.html). Here we assessed model performance by comparing model results with observed hourly atmospheric concentrations of ¹³⁷Cs, with a particular focus on nine plumes over the Tohoku and Kanto regions. The intercomparison results showed that model performance in reproducing ¹³⁷Cs concentrations was highly variable among different models and plumes. In general, models better reproduced plumes that passed over many observation stations. The performance among the models was consistent with the simulated wind fields and the source terms used. We also assessed model performance in relation to accumulated ¹³⁷Cs deposition. Simulated areas of high ¹³⁷Cs deposition were consistent with the simulated ¹³⁷Cs plume pathways, though the models that best simulated atmospheric ¹³⁷Cs concentrations were different from those that best simulated deposition. The ensemble mean of all models consistently reproduced atmospheric ¹³⁷Cs concentrations and deposition well, suggesting that use of a multimodel ensemble results in more effective and consistent model performance.