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

多段ネストROMSモデルに基づいたSTEAMERにおけるダウンスケーリング機能の実装

上平 雄基   ; 川村 英之  ; 小林 卓也 ; 内山 雄介*

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

STEAMERにROMSによるダウンスケーリングシステムを導入し、海況場及び原子力施設から放出された放射性物質の濃度分布の詳細な予報値が計算可能となるシステムを構築した。構築したシステムを用いて福島県沖のサブメソスケール現象及び潮汐が流動構造、濃度分布に与える影響を評価した。その結果、解析領域での濃度分散には季節性が見られ、三次元的な物質混合にはサブメソスケール現象による寄与が大きいことが示唆された。潮汐は水平の混合を抑制する一方で鉛直混合を強化し、結果として潮汐非考慮では見られなかった放出口近傍沿岸に張り付いた濃度分布を形成していた。潮汐による寄与はサブメソスケール現象に比べて小さいが、濃度希釈過程に影響を及ぼすことが示唆された。

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.