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Journal Articles

Development of regional downscaling capability in STEAMER ocean prediction system based on multi-nested ROMS model

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

Journal of Nuclear Science and Technology, 56(8), p.752 - 763, 2019/08

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 $$^{137}$$Cs distribution was different from that obtained using coarser-resolution models because downscaling enhanced both horizontal and vertical mixing. The suppression of horizontal mixing and the promotion of vertical mixing by tidal forcing synergistically reduced offshore $$^{137}$$Cs transport. In addition, the submesoscale effects strengthened the three-dimensional $$^{137}$$Cs fluctuations by $$<$$10 times, while the tidal effects promoted slightly increased the intensity of three-dimensional $$^{137}$$Cs 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.

Journal Articles

Influences of the Kuroshio on interisland remote connectivity of corals across the Nansei Archipelago in the East China Sea

Uchiyama, Yusuke*; Odani, Sachika*; Kashima, Motohiko*; Kamidaira, Yuki; Mitarai, Satoshi*

Journal of Geophysical Research; Oceans (Internet), 123(12), p.9245 - 9265, 2018/12

no abstracts in English

Journal Articles

A Biological network around Okinawa Island analyzed with a coupled high-resolution ocean circulation and lagrangian particle tracking model

Uchiyama, Yusuke*; Miyagawa, Tsubasa*; Odani, Sachika*; Kamidaira, Yuki; Odani, Sachika*

Doboku Gakkai Rombunshu, B2 (Kaigan Kogaku) (Internet), 74(2), p.I_1291 - I_1296, 2018/11

no abstracts in English

Journal Articles

Submesoscale mixing on initial dilution of radionuclides released from the Fukushima Daiichi Nuclear Power Plant

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

Journal of Geophysical Research; Oceans (Internet), 123(4), p.2808 - 2828, 2018/04

We developed a submesoscale eddy-resolving oceanic dispersal modeling system consisting of a double nested oceanic downscaling model and an offline oceanic radionuclides dispersion model to investigate influences of submesoscale coherent structures (SCSs) and associated ageostrophic secondary circulations (ASCs) on the three-dimensional (3D) dispersal and initial dilution of the dissolved radioactive $$^{137}$$Cs accidentally released from the Fukushima Daiichi Nuclear Power Plant (FNPP1) occurred since March 2011. The extensive model-data comparison demonstrates that the elaborated innermost high-resolution model at a lateral grid resolution of 1 km successfully reproduces transient mesoscale oceanic structures, the Kuroshio path and stratification, and spatiotemporal variations of 3D $$^{137}$$Cs concentrations. These SCSs and ASCs occurred primarily due to shear instability with baroclinic instability as the secondary mechanism, according to energy conversion and spectral analyses. The vertical $$^{137}$$Cs flux analysis was performed with decomposition of the variables into the mean, mesoscale, and submesoscale components using frequency and wavenumber filters. The vertical $$^{137}$$Cs flux analysis explained that 84% of the FNPP1-derived $$^{137}$$Cs was transported downward below the mixed layer by eddies, with the major contributions from ASCs induced by submesoscale eddies.

Journal Articles

Inventory analysis of suspended radiocesium derived from Niida River during a typhoon-induced flood event

Uchiyama, Yusuke*; Azuma, Kohei*; Odani, Sachika*; Iwasaki, Toshiki*; Tsumune, Daisuke*; Kamidaira, Yuki; Shimizu, Yasuyuki*; Onda, Yuichi*

Doboku Gakkai Rombunshu, B2 (Kaigan Kogaku) (Internet), 73(2), p.I_685 - I_690, 2017/10

no abstracts in English

Journal Articles

Influence of the Kuroshio on dispersal of coral spawn and larvae around Ryukyu Islands

Odani, Sachika*; Uchiyama, Yusuke*; Kashima, Motohiko*; Kamidaira, Yuki; Mitarai, Satoshi*

Doboku Gakkai Rombunshu, B2 (Kaigan Kogaku) (Internet), 73(2), p.I_1315 - I_1320, 2017/10

no abstracts in English

Journal Articles

Eddy-induced transport of the Kuroshio warm water around the Ryukyu Islands in the East China Sea

Kamidaira, Yuki; Uchiyama, Yusuke*; Mitarai, Satoshi*

Continental Shelf Research, 143, p.206 - 218, 2017/07

 Times Cited Count:7 Percentile:13.38(Oceanography)

In this study, an oceanic downscaling model in a double-nested configuration was used to investigate the role played by the Kuroshio warm current in preserving and maintaining biological diversity in the coral coasts around the Ryukyu Islands (Japan). A comparison of the modeled data demonstrated that the innermost submesoscale eddy-resolving model successfully reproduced the synoptic and mesoscale oceanic structures even without data assimilation. The Kuroshio flows on the shelf break of the East China Sea approximately 150-200 km from the islands; therefore, eddy-induced transient processes are essential to the lateral transport of material within the strip between the Kuroshio and the islands. The model indicated an evident predominance of submesoscale anticyclonic eddies over cyclonic eddies near the surface of this strip. An energy conversion analysis relevant to the eddy-generation mechanisms revealed that a combination of both the shear instability due to the Kuroshio and the topography and baroclinic instability around the Kuroshio front jointly provoke these near-surface anticyclonic eddies, as well as the subsurface cyclonic eddies that are shed around the shelf break. Both surface and subsurface eddies fit within the submesoscale, and they are energized more as the grid resolution of the model is increased. An eddy heat flux (EHF) analysis was performed with decomposition into the divergent (dEHF) and rotational (rEHF) components. The rEHF vectors appeared along the temperature variance contours by following the Kuroshio, whereas the dEHF properly measured the transverse transport normal to the Kuroshio's path. The diagnostic EHF analysis demonstrated that an asymmetric dEHF occurs within the surface mixed layer, which promotes eastward transport toward the islands. Conversely, below the mixed layer, a negative dEHF tongue is formed that promotes the subsurface westward warm water transport.

Journal Articles

Development of a prediction system for radionuclide dispersion in Fukushima coast

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

Doboku Gakkai Rombunshu, B2 (Kaigan Kogaku) (Internet), 72(2), p.I_451 - I_456, 2016/11

no abstracts in English

Journal Articles

Impact of mesoscale recirculation of the Kuroshio on asymmetric oceanic structure around Okinawa Island

Uchiyama, Yusuke*; Odani, Sachika*; Yamanishi, Takafumi*; Kamidaira, Yuki; Mitarai, Satoshi*

Doboku Gakkai Rombunshu, B2 (Kaigan Kogaku) (Internet), 72(2), p.I_481 - I_486, 2016/11

no abstracts in English

Journal Articles

An Eddy heat flux analysis on enhancement of submesoscale mixing around the Ryukyu Islands

Kamidaira, Yuki; Uchiyama, Yusuke*; Mitarai, Satoshi*; Nakada, Shohei*

Doboku Gakkai Rombunshu, B2 (Kaigan Kogaku) (Internet), 71(2), p.I_1219 - I_1224, 2015/11

BB2015-0179.pdf:4.2MB

no abstracts in English

Journal Articles

Removal of cesium using cobalt-ferrocyanide-impregnated polymer-chain-grafted fibers

Ishihara, Ryo*; Fujiwara, Kunio*; Harayama, Takato*; Okamura, Yusuke*; Uchiyama, Shoichiro*; Sugiyama, Mai*; Someya, Takaaki*; Amakai, Wataru*; Umino, Satoshi*; Ono, Tsubasa*; et al.

Journal of Nuclear Science and Technology, 48(10), p.1281 - 1284, 2011/10

AA2011-0190.pdf:0.45MB

Journal Articles

Characterization of hydrogen embrittlement of Ti and Ti-5Ta alloys using constant load test

Yokoyama, Kenichi*; Murata, Yusuke*; Shiimori, Fusae*; Sakai, Junichi*; Yamamoto, Masahiro; Uchiyama, Gunzo; Nojima, Yasuo*; Fujine, Sachio*

Fushoku Boshoku Kyokai Dai-58-Kai Zairyo To Kankyo Toronkai Koenshu, p.11 - 12, 2011/09

Characteristics of hydrogen embrittlement of pure Ti and Ti-5Ta alloys were evaluated by stress loading and constant load tests. Mechanical properties of these materials were affected by hydride formation of surface layers. Although elongation was reduced by hydrogen absorption, embrittlement was observed only in the surface hydride area. It is also suggested that Ti-5Ta alloy was better susceptibility of hydrogen embrittlement than pure Ti.

Oral presentation

Surface mixing and dispersion of radioactive tracer due to submesoscale eddies off the northeastern Pacific coast of Japan

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

no journal, , 

A multiple nesting technique enables us to examine submesoscale eddies and their effects on surface mixing and associated material transport (e.g., Kamidaira et al., 2016). In this study, downscaling simulations for the coastal margin off Fukushima are conducted by using ROMS with a horizontal resolution of 1 km. The oceanic initial and boundary conditions are given by an oceanic data assimilation system MOVE developed at Meteorological Research Institute (MRI), Japan Meteorological Agency (JMA). Energy conversion analysis exhibits that surface eddy potential energy to eddy kinetic energy (EKE) conversion through baroclinic instability is substantial in vorticity generation in this area. A further simulation is performed on oceanic dispersion of a radioactive tracer released from the Fukushima Dai-ichi Nuclear Power Plant to investigate eddy-induced mixing using an offline, oceanic tracer dispersion model SEA-GEARN developed at Japan Atomic Energy Agency (JAEA) (Kobayashi et al., 2007). A spatial moment analysis with respect to the center of gravity of the concentration reveals that tracer dispersion occurs with two distinct regimes leading to anisotropic transport in the zonal and meridional directions. The first regime lasting for about 5-8 days after the tracer release are mainly caused by coastally-trapped alongshore jet and submesoscale eddy-mixing. The subsequent regime occurs after the tracer is sufficiently dispersed offshore where mesoscale eddies play more dominant roles. Moreover, time series of zonal and meridional tracer variances fluctuate with enstrophy, area averaged density anomaly and EKE. These results clearly illustrate that fluctuating submesoscale and mesoscale eddy field induced by baroclinic insatiability evidently affects the tracer dispersion off Fukushima.

Oral presentation

Impact of the Kuroshio on dispersal of coral spawn and larvae around Ryukyu Islands in the East China Sea

Odani, Sachika*; Uchiyama, Yusuke*; Kashima, Motohiko*; Kamidaira, Yuki; Mitarai, Satoshi*

no journal, , 

We conduct a double nested high-resolution synoptic ocean modeling using ROMS with grid spacing down to 1 km coupled with an offline Lagrangian particle tracking model. The modeled dispersal of coral spawn and larvae released from about 20 major islands and lagoons are examined to quantify connectivity using Lagrangian probability density functions (PDFs) of the Lagrangian particles among Ryukyu Islands. We introduce 145 patches with a diameter of 1km around Sekisei Lagoon between Yaeyama and Ishigaki Islands close to Taiwan to compare with an in-situ surface drifter measurement. Lagragian particles are released from the patches in spring for 2012-2015 and are tracked for the advection time of 3 weeks suitable to representing active spawning and lifespan of coral spawn and larvae. The PDF analyses suggest that the particles mostly remain near the released areas, while about 10% of the particles are entrained by the Kuroshio to travel long distance, leading to inter-island connectivity. Some of these particles are further transported northeastward, and then partially trapped by the Kuroshio Counter Current formed between the Kuroshio and Main Island to approach Okinawa Main Island. We reveal that eastward current in the south the lagoon occurred shortly after the releases suppresses the entrainment and the resultant long-distance transport.

Oral presentation

Submesoscale eddy-induced mixing and dispersion of radioactive tracer off the northeastern Pacific coast of Japan

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

no journal, , 

no abstracts in English

Oral presentation

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*

no journal, , 

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.

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