Fo 
interaction (passive) with the current field and which do not change 
(conservative) their properties. 
Methodics of the Numerical Simulation 
(description of the models of the IfMH and the DHI) 
The. dispersion of substances soluble in water can be simulated by a 
model system consisting of a current model and a dispersion model. 
Owing to the fact that the substances under consideration are pas- 
sive substances which do not influence the circulation, these models 
can be operated separate from one another. Hereby, the current model 
supplies the input data for the dispersion model. 
Such a model system was operated not only in the Deutsches Hydro- 
graphisches Institut but also in the Institut für Meereskunde der 
Universität Hamburg. 
Two Tables (see Annex) show the characteristics of these systems in 
a comparison. 
The significance of the differences of the two model systems is gone 
into in the following Chapters. 
Hitherto, the two current models have found widely different appli- 
cation, not only in Institut für Merreskunde but also in the Deut- 
sches Hydrographisches Institut. As far as possible, the model re- 
gults were verified with the aid of observational data (Backhaus, 
1985; Müller-Navarra and Mittelstaedt, 1985; Duwe, 1983; Pohlmann, 
1985). 
Deseriptions of the dispersion models are to be found in Maier- 
Raimer, 1973; Müller-Navarra and Mittelstaedt, 1985; as well as 
Hainbucher et al., 1986, 
In the following spreading simulations of DHI resp. IfMH described, 
two different current fielda are taken as a basis, In order to com- 
pare the model results of the two Institutes with one another, one 
must judge which physical processes essentially influence in what 
temporal and spatial scales the pollutant spreading in the North 
Sea. 
In the dispersion calculations in both models & time dependent cur- 
rent in used, not a mean one. In the DHI model, the current. varies 
periodically with the semidiurnal My tide - the current is eyelic 
stationary - and is three-dimensional. The IfMH spreading model, for 
the period 1969 to 1982, uses every day a wean wind and 
tidaldependent two-dimensional currentfield. 
The three-dimensional currents actually calculated from the IfMEH 
current mödel were vertically integrated for the spreading simula- 
tions in order to reduce the considerabloe amount of data and thus 
to apply an emphasis upon the time dependency of the dispersion pro- 
blems.