Short-Term Emergency Assessment System of the Marine Environmental Radioactivity

The Japan Atomic Energy Agency (JAEA) developed a Short-Term Emergency Assessment system of the Marine Environmental Radioactivity (STEAMER) to predict the oceanic dispersion of radionuclides that are released into the North Pacific from nuclear facilities. STEAMER is in daily test operation at JAEA to evaluate the functioning of the system. STEAMER receives online oceanographic forecast data that is computed by eddy-permitting ocean general circulation models. Oceanic dispersion simulations are performed daily to predict the oceanic dispersion of a hypothetical release of Cs-137 from two nuclear facilities over 30 days. In this study, the predictability of oceanic dispersion simulations is validated using oceanographic forecast and reanalysis data that were saved over past several years. A lot of oceanic dispersion simulations are performed for a hypothetical release of Cs-137 from the Fukushima Daiichi Nuclear Power Plant. Oceanic dispersion simulations that are driven by oceanographic reanalysis data are assumed to be true solutions. The predictability is estimated quantitatively comparing forecast and reanalysis simulations. Ensemble forecast simulations are also performed applying the Lagged Average Forecast method that improves the predictability successfully. Moreover, a Regional Ocean Modeling System (ROMS) is applied to simulate submesoscale ocean circulations and tidal currents in the coastal and offshore areas. Submesoscale eddies enhance horizontal and vertical mixing of Cs-137. On the other hand, tidal currents suppress horizontal mixing of Cs-137 and enhance vertical mixing of Cs-137. A regional downscaling system using ROMS will be in operation in STEAMER in the future.