128 finite di?erence method for the C grid and the principal equations of the physical engine are detailed in Ref. [XI-19–20]. The model uses a stretched horizontal grid with a variable horizontal resolution, from 600 m × 600 m at the nearest grid point from Fukushima, to 5 km × 5 km o?shore. The vertical grid is based on a generalized s-coordinate system. The 30 vertical levels are irregularly distributed, with increased resolution near the sea surface. The model was initialized and forced at its lateral boundaries with the global NCOM real time operational ocean model of the United States of America Navy [XI-12] operated by the National Oceanic and Atmospheric Administration (NOAA). At the sea surface, the ocean model is forced by the meteorological ?uxes delivered every 3 hours by the European Centre for Medium-Range Weather Forecasts (ECMWF). The tidal forcing at the lateral open boundaries is provided by the T-UGO model, implemented for this purpose by the SIROCCO team on coast of the Japanese Pacific Ocean. The main rivers of the region (between 35.5°N and 38.5°N), the Tone to the south, and the Natori and Abukuma to the north, were introduced into the model on the basis of climatological freshwater discharges (190, 17 and 67 m3/s respectively). The model was initialized on 21 February 2011. Currents and vertical diffusivities computed by the model were averaged over 3 hour periods and stored to compute the advection and di?usion of tracers in o?-line mode, using an Eulerian representation (Figure XI-2). An inverse- method calculation was carried out to estimate the source term. FIG.XI-2. Calculated surface currents on 15 March 2011.