Implementing regional downscaling capability in the STEAMER radionuclide dispersion prediction system based on multi-nested ROMS model

Kamidaira, Yuki   ; Kawamura, Hideyuki  ; Kobayashi, Takuya ; Uchiyama, Yusuke*

Oceanic regional downscaling capability was implemented into Short-Term Emergency Assessment system of Marine Environmental Radioactivity (STEAMER) developed by Japan Atomic Energy Agency to enable us to predict more realistically the oceanic dispersion of radionuclides at higher spatiotemporal resolutions for broader applications. The system consisted of a double-nested oceanic downscaling circulation model with tidal forcing and an oceanic radionuclide dispersion model. This system was used to comparatively examine downscaling and tidal effects on the dispersion of radionuclides hypothetically released from the Fukushima Daiichi Nuclear Power Plant in the colder season. The simulated dissolved Cs-137 distribution had seasonal variability and was different from that obtained using coarser-resolution models because downscaling enhanced both horizontal and vertical mixing. Temporal Cs-137 fluctuations were slightly amplified as a result of the suppression of horizontal mixing and enhancement of vertical mixing by tides. In addition, the submesoscale effects strengthened the three-dimensional Cs-137 fluctuations by 10 times, while the tidal effects promoted slightly increased the intensity of three-dimensional Cs-137 fluctuations by approximately 3%. This indicated that the submesoscale effects substantially surpassed tidal forcing in oceanic mixing in the coastal margin off Fukushima in the colder season.