Assessment of the amount of Cesium-137 released into the Pacific Ocean after the Fukushima accident and analysis of its dispersion in Japanese coastal waters

International audienceNumerical modeling was used to provide a new estimate of the amount of 137Cs released directly into the ocean from the Fukushima Daiichi nuclear power plant (NPP) after the accident in March 2011 and to gain insights into the physical processes that led to its dispersion in the marine environment during the months following the accident. An inverse method was used to determine the time-dependent 137Cs input responsible for the concentrations observed at the NPP's two liquid discharge outlets. The method was then validated through comparisons of the simulated concentrations with concentrations measured in seawater at different points in the neighborhood of the plant. An underestimation was noticed for stations located 30 km offshore. The resulting bias in the release inventory was estimated. Finally, the maximum 137Cs activity released directly to the ocean was estimated to lie between 5.1 and 5.5 PBq (Peta Becquerel = 1015 Bq) but uncertainties remain on the amount of radionuclides released during the first few days after the accident. This estimate was compared to previous ones and differences were analyzed further. The temporal and spatial variations of the 137Cs concentration present in the coastal waters were shown to be strongly related to the wind intensity and direction. During the first month after the accident, winds blowing toward the south confined the radionuclides directly released into the ocean to a narrow coastal band. Afterwards, frequent northward wind events increased the dispersion over the whole continental shelf, leading to strongly reduced concentrations

Low-order pressure gradient schemes in sigma coordinate models: The seamount test revisited

International audienceThis paper revisits the classic seamount test used in numerous previous studies to evidence the sigma errors of the pressure gradient force (PGF) and their long-term effects on circulation. Two kinds of analysis are developed. We first consider the initial PGF errors. Then, the global level of erroneous kinetic energy is computed along a 180-day simulation. The long-term circulation appears to be better correlated to the initial vorticity errors than to the initial error diagnostics. The original feature of this study is to reconsider the currently admitted idea that Density-Jacobian type PGFs perform better than the primitive sigma formulation discretized in a straightforward way (hereafter Straightforward-Primitive PGF). Errors on the discrete hydrostatic pressure are actually closely related to the way the density field is initialized. If a mass conserving method is preferred to a straightforward initialization, the rectangular integral of the Straightforward-Primitive PGF is likely to be more accurate than the trapezoidal rule usually involved in Density-Jacobian PGFs. Errors on the vorticity field of the Straightforward-Primitive PGF depend on the discretization of the hydrostatic correction term. A modified version of the Straightforward-Primitive PGF is shown to be in better agreement with the concept of bottom torque consistency. The seamount tests show that this so-called Modified-Primitive PGF performs globally better than the current low-order Density-Jacobian PGFs

Considerations on Open Boundary Conditions for Regional and Coastal Ocean Models

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3D phase-resolved wave modelling with a non-hydrostatic ocean circulation model

International audienceA phase-resolved wave model is derived from an ocean circulation model for the purpose of studying wave-current effects in nearshore zones. One challenge is to adapt the circulation model to the specificities of wave physics. This mainly concerns the consideration of nonhydrostatic effects and the parametrization of wave breaking. The non-hydrostatic pressure is calculated using the artificial compressibility method (ACM). The ACM-induced errors on wave dispersion properties are examined in detail in the context of the linear theory using idealized test cases. The possible compromise between the precision achieved on nonhydrostatic physics and the adjustable CPU cost of the ACM method is looked at in detail. The modification of the wave characteristics by the bathymetric slope and the breaking of waves are then examined from a linear slope beach laboratory experiment. Finally the model is evaluated on the issue of rip currents and their feedback on the wave field using a laboratory experiment of a beach with a bar intersected by channels

Considerations on Open Boundary Conditions for Regional and Coastal Ocean Models

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Secondary flows induced by wind forcing in the Rhône region of freshwater influence

Secondary flows induced by the blocking effect of a river plume on a transverse upwelling are investigated in a microtidal region of freshwater influence (ROFI). A nested version of the SYMPHONIE primitive-equation free-surface model for 3-D baroclinic coastal flows has been developed for the Rhône ROFI. The main characteristics of the model are a generalized sigma coordinate system in finite differences, using a time splitting for external and internal modes and high-order numerical advection schemes for density fields in combination with an modified turbulence closure scheme. The nesting system consists of two grids forced by the high-resolution ALADIN model atmospheric data. The coarse grid of 3 km resolution for the whole Gulf of Lions allows the forcing of the Liguro-Provençal large-scale current when the fine mesh of 1-km resolution is centred on the river mouth of the Grand Rhône. Documented field experiments from the Biodypar 3 field campaign performed during March 1999 are used for validation. Numerical results, CTD profiles and a SPOT TSM visible image are in good agreement concerning the shape and structure of the river plume. Other coastal flow features can be observed from satellite imagery. Computations of realistic situations recover these main secondary structures. Complementary process-oriented runs give an explanation of how the coastal upwelling induced by an inhomogeneous offshore wind is destabilized by the combination of the river plume and along-shelf current-blocking effects. In the end, a factor-separation analysis provides evidence that the locally non-linear effects in momentum contribute to the occurrence of secondary vortices

Circulation in a stratified and wind-forced Gulf of Lions, NW Mediterranean Sea: in situ and modeling data

International audienceADCP, thermosalinograph, and meteorological data were collected during a short cruise (SARHYGOL 3; June 13–15, 2000) throughout the Gulf of Lions, NW Mediterranean Sea. The 3-D hydrodynamical model Symphonie reproduces well the circulation features observed in situ. The Northern Current (NC)—geostrophic current bordering the gulf along its continental slope—is well modeled with slight differences in its position. W hen the modeled stratification of the gulf is shallower than the measured stratification, the modeled NC is further offshore than the real NC (and reciprocally). The model allows the determination of the life-time and generating processes of the intrusions of the NC on the shelf. During the cruise, two different types of intrusions of the NC are found: (1) an intrusion at the eastern side of the gulf, that lasted 5 days and was due to a combination of the water column stratification and wind-forcing effects at the start of the continental shelf; (2) an intrusion at the center of the gulf, that lasted less than 12 h and was generated by the proximity of a positive and a negative wind stress curls. The coupled in situ data/model analysis also provides a better understanding on inertial oscillations. A strong inertial oscillation, with maximum amplitude of 60 cm/s, is clearly observed at the western side of the gulf due to the absence of the NC there. The model also exhibits this oscillation, and provides its temporal variations. The analysis, closely coupling in situ measurements and model results, provides information that would not have been obtained using either data separately

Application of an inverse method to coastal modelling

Free surface coastal models currently suffer from the difficulty of having to specify the global circulation during the initialization process and along the open boundaries. As an alternative to the long spinup periods, an original explicit approach based on inverse techniques has been developed. Data originating from in situ observations and/or ocean general circulation models are optimally interpolated over the small-scale grid in such a way that the tendency terms are reduced to physically consistent values. The errors on the “true” tendencies and the truncated nonlinear term are evaluated to compute the model covariance. The observation covariance matrix is divided into two parts: the homogeneous, isotropic matrix calculated with a global energy spectrum; and a parameterized nonhomogeneous, nonisotropic matrix. The inverse method is applied to the study of the interaction of a barotropic alongshore current over a narrow canyon. The transient processes following the initialization are drastically reduced and the analysis field can efficiently be used in a flow relaxation scheme along the open boundaries

Origin and dynamics of mesoscale eddies in the Catalan Sea (NW Mediterranean): Insight from a numerical model study

Past observations and satellite sea surface temperature imagery indicate the presence of mesoscale anticyclonic eddies drifting along the Catalan coast. In September 2001 one of these anticyclonic eddies was surveyed over the shelf break during an océanographie cruise which permitted the 3-D description of its structure. In this work we investigate the origin and dynamics of such >Catalan eddies> using a numerical circulation model of the northwest Mediterranean at 3 km resolution driven by high-resolution atmospheric analyses and compare model eddies with the observations in the Catalan Sea. We identify two zones of eddy formation in the Gulf of Lions, in front of the city of Marseille and at the southeast of coast of Roussillon, from which anticyclonic eddies are observed to drift toward the Catalan Sea. The hydrology and dynamics of the structures observed in the simulations are characterized. Sensitivity experiments and energy analysis are performed which allow us to identify the mechanisms associated with their generation. Properties of the eddy observed during the 2001 cruise at the Catalan shelf break are found to compare well with model eddies generated at the southeast of the Roussillon coast. The model relates the origin of these eddies to the separation of the coastal current downstream from Creus Cape: flow separation is linked to intense downwelling taking place in front of the Roussillon coast when strong northwesterly winds events occur. Copyright 2009 by the American Geophysical Union.This work has been partially performed in the framework of the project "Génesis, evolución temporal, translación y efectos de remolinos de mesoescala sobre la plataforma continental Catalana (MAR1999-1010)" which has been funded by the Spanish Comission Interministerial de Ciencia y Tecnología (CYCIT) and the project MFSTEP funded by the European CommissionPeer Reviewe