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sub-area. Its outer edges are not even required to be straight. Nonetheless, it is desirable 
that the grid transition across the outer boundary of the refined grid should be smooth 
with respect to both coastline and bathymetry. 
The transfer of grid data from fine to coarse grid and vice versa could be compared to a 
tracing on an overlapping strip, where data are transferred from one grid to the other by 
means of averaging and interpolation. It should be obvious that the BSH's joining method 
works with a minimum strip width. Details are a matter of taste and may depend on the 
property conservation conditions during transfer. In any special case, the choice of the 
data transfer formula does not hinge on shallow water hydrodynamics. It is something 
static, and the BSH model uses the simplest option. 
4 The operational model version 
As has been pointed out above, a model version has been in operational use since the 
beginning of 1999 which differs from the preceding model mostly in its grid resolution, 
simulation periods, meteorological forcing data, and output formats. 
4.1 Meteorological forcing 
The hydro- and thermodynamics of shallow seas are strongly influenced by momentum 
and vertical heat fluxes in the boundary layer between atmosphere and ocean. To 
provide the operational model of the Baltic Sea and the North Sea with atmospheric 
boundary values, forecast data from the GME/LM model system of the German Weather 
Service (DWD, Dorns and Schattier, 1999) are transferred on-line to the BSH’s computer 
system. Twice daily an 84-hour data set is transferred from DWD to BSH which contains 
hourly values of mean sea level pressure, total cloud cover, wind velocity, air 
temperature and humidity (Table 1) . The two latter parameters are taken from the 
bottom layer (LM: k=35, GME: k=31). The 10-m wind components are calculated from 
the pressure level height (~ 35 m) of the bottom layer by means of special extrapolation 
taking into account the stability conditions within the Prandtl layer.