Model Intercomparison of Atmospheric Cs-137 From the Fukushima Daiichi Nuclear Power Plant Accident: Simulations Based on Identical Input Data

A model intercomparison of the atmospheric dispersion of cesium-137 (Cs-137) emitted after the Fukushima Daiichi Nuclear Power Plant accident in Japan was conducted to understand the behavior of atmospheric Cs-137 in greater detail. The same meteorological data with a fine spatiotemporal resolution and an emission inventory were applied to all models to exclude the differences among the models originating from differences in meteorological and emission data. The meteorological data were used for initial, boundary, and nudging data or offline meteorological field. Furthermore, a horizontal grid with the same resolution as that of the meteorological data was adopted for all models. This setup enabled us to focus on model variability originating from the processes included in each model, for example, physical processes. The multimodel ensemble captured 40% of the atmospheric Cs-137 events observed by measurements, and the figure of merit in space for the total deposition of Cs-137 exceeded 80. The lower score of the atmospheric Cs-137 than that of the deposition originated from the difference in timing between observed and simulated atmospheric Cs-137. Our analyses indicated that meteorological data were most critical for reproducing the atmospheric Cs-137 events. The results further revealed that differences in Cs-137 concentrations among the models originated from deposition and diffusion processes when the meteorological field was simulated reasonably well. The models with small deposition fluxes produced higher scores for atmospheric Cs-137, and those with strong diffusion succeeded in capturing the high Cs-137 concentrations observed; however, they also tended to overestimate the concentrations.