13 
4.2 The circulation model 
At the BSH ocean models for the North East Atlantic, the North Sea and Baltic are 
routinely run during the night (Fig. 3). A 40 km grid covering the north-east Atlantic is 
used to provide external surges at the open boundaries in the northern North Sea and 
the entrance to the English Channel. The NE Atlantic model is two-dimensional and is 
driven only by meteorological forecasts. Hydrodynamics in the North Sea and Baltic are 
simulated by three-dimensional models which are interactively coupled. Grid spacing in 
the German Bight and western Baltic Sea is 1.8 km (AX= 100"; Arp = 60"), and 10 km 
(Ak= 10 Arp = 6') in the other parts of the North Sea and Baltic. 
340” 345’ 350" 355" 0" 5" 10" 15" 20" 25" 30" 
Fig.2: Grid nets of the BSH's circulation models 
(NE Atlantic: 24 nm, North Sea and Baltic: 6 nm with nested 1 nm grid) 
Concerning vertical discretization, z-level models are used, with a maximum of 14 layers 
in the deepest area of the Norwegian trench. In the model, the thickness of the surface 
layer (for zero water level) must exceed the maximum negative water level in order to 
avoid emptying of grid cells. Due to high tidal amplitudes in the English Channel, the 
thickness of the first layer is set to 8 m. Each of the next 4 layers is 4 m thick, followed 
65 
60 
55 
50