24 3.1.2. Present modelling exercises and applied models Generally speaking, a marine dispersion model consists of two submodels, i.e. a hydrodynamic model will provide the water circulation required to solve the advective transport of radionuclides and the dispersion model will use such currents to calculate transport including advection, mixing produced by turbulence and other processes like radioactive decay or interactions of dissolved radionuclides with suspended matter and bottom sediments. A sequential chain of dispersion exercises was carried out within the MODARIA Programme in such a way that the reasons of the discrepancies between models could be assessed, i.e., if they are due to the hydrodynamic part, the dispersion part, and the ultimate reasons. The four modelling exercises are summarized in Table 9 below. The overall idea was to harmonize models, making them run with the same forcing in a step-by-step procedure, in such a way that the main agent in producing discrepancies between models could be found. This type of model intercomparison exercise has never been carried out before. Initially, dispersion exercises were carried out with a “tracer” (which is taken to mean an entirely conservative radionuclide, i.e. no significant decay, no interaction with sediments), thereby ensuring that all parameters describing water–sediment interactions are avoided. In addition, a constant hypothetical source term was used by all models. Four exercises were carried out, although the final exercise was split into two parts. In Exercise 1 each model used its own water circulation and set of parameters, e.g. horizontal and vertical di?usion coe?cients, after which all models used the same hydrodynamic description given in Exercise 2. All parameters were homogenized in Exercise 3 and, finally, a realistic source term both for direct releases into the ocean and atmospheric deposition on the sea surface was used in Exercise 4. This method allows a comparison to be made between model results and measurements of 137Cs in water and sediments. 137Cs was introduced from Exercise 2 onwards. Exercise 3 was carried out with and without suspended matter particles in the water column. Exercise 4, was split into two exercises, 4a and 4b. A common model configuration was used in Exercise 4a. Each model was run with its own set-up and parameterization during Exercise 4b and a range of 137Cs concentrations in water and sediments were produced which may be regarded as some kind of model uncertainty assessment. The dispersion models used in the Pacific Ocean radiological scenario are summarized in Table 10. More detailed descriptions are given in the corresponding annexes and respective references. Some of these models make their own calculation of hydrodynamics (e.g. SELFE/IMMSP/KIOST, NTUA, Sisbahia (Sistema Base de Hidrodinamica Ambientâl) models), while others import water circulation from operative ocean forecasting models (e.g. KAERI, JAEA and USEV models). The characteristics of these ocean circulation models are brie?y described in Annex XI except for SELFE which is provided in Annex IV. A comparison of hydrodynamic model outputs was necessary to carry out Exercise 2, since a common hydrodynamic description to be used by all models had to be selected. Such a quantitative comparison of hydrodynamic model outputs is described in Section 3.3.2. The results of Exercises 1–4 described above are discussed in detail in sections 3.2 to 3.5