Die Kuste, 81 (2014), 255-271 
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surface boundary conditions; therefore the Stokes drift velocities are calculated within 
SeatrackWeb. Stokes drift velocities are computed from the two-dimensional wave energy 
spectrum. The wave spectrum is not yet imported from an operational wave forecast 
model, but is instead based on the parameterized spectrum presented in DONELAN et al. 
(1985) for fetch-limited growth. 
The appearance of sea ice is taken into account and influences almost all processes 
mentioned above. For instance the hydrodynamic model velocities are replaced by the ice 
drift velocity if ice concentrations are higher than 70 % and the particle is at the surface. 
Also die Stokes drift linearly decreases from 100 %, when the ice concentration is zero, 
to 0 %, when the ice concentration is 70 % or higher. Also the gravitational spreading of 
an oil slick linearly decreases to zero with increasing ice concentration (DlCKlNS 1992; 
VENKATESH et al. 1990). Oil dispersion by breaking waves from the surface down into 
the water column is also reduced for high ice concentrations higher than 30 %. Ice 
strongly damps the waves and limits the dispersion. 
Although many processes are included, still a high level of uncertainty originates from 
the ocean and wind model applied. To mimic some of the uncertainty it is possible to add 
extra uncertainty spreading randomly, whose magnitude is a function of the wind forecast 
uncertainty, to the movement of particles at the surface. 
2.3 Graphical User Interface 
SeatrackWeb users typically configure drift simulations and display the results via the 
graphical user interface (GUI). SMHI develops and continuously updates, respectively 
renews the SeatrackWeb GUI. At present two versions of the GUI exist: one is a Java 
Client/Server application and the other one is a JavaScript web application tested in 
common internet browsers. The web application version is the latest one, but the Java 
Client version is still commonly used. At the BSH the current operational setup uses Java 
Client and therefore, only this version of the GUI will be presented. 
Java Web Start starts the Java Client application on the user computer. Since the drift 
simulations itself are performed on the server site there are no specific requirements for 
the personal desktop computer performance. For more details about the SeatrackWebs 
Client/Server Java Application we refer to AMBJORN et al. (2011). 
After successful login a coastline map opens and more layers with additional infor 
mation may optionally be added. Fig. 1 shows the SeatrackWeb GUI of the BSH installa 
tion. For example, as it can be seen in Fig. 1, it is possible to display the location of oil 
and gas platforms, borders of the exclusive economic zone, marine traffic routes and bio 
logical sensitive areas. Furthermore, the bathymetric depths showing the resolution of the 
BSHcmod circulation model can be visualized. This information helps to identify how 
well ocean current fields are resolved in the drift simulation. 
To set up a drift simulation the user has to provide some information guided by the 
GUI through different menus, e.g. kind of substance/object, kind of outlet (continuous, 
amount, rate,...), position, start and end time of the simulation must be defined. 
SeatrackWeb provides strong support for expert users, which can choose the forcing 
wind/current fields, give additional wind drag for floating objects, choose the kind of oil 
or add uncertainty due to wind.