Baltic Sea Ice Climate Workshop 2005 
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forcing used. The model resolves ice thickness distribution, i.e. ice concentrations of variable 
thickness categories, redistribution of ice categories due to deformations, thermodynamics of 
sea-ice, horizontal components of ice velocity and internal stress of the ice pack. 
The redistribution function is dependent on ice thickness, concentration and the strain rates 
(Thorndike et al., 1975, Hibler, 1986). Continuum scale sea ice models resolve an average 
behavior of the pack ice and the subgrid processes are neglected or taken into account in a 
simplified manner. The following assumptions of the deformation processes in the present 
model have been made i) deformed ice is generated only from undeformed ice categories i.e. 
rafted ice is not deformed further in the model ii) cross-over thickness determines whether the 
undeformed ice is rafted or ridged. This assumption is based on the Parmeter (1975) law and 
field observations (c.f. Rothrock, 1979). It is also assumed that the thinnest 15 % of the ice 
categories experience deformations (Thorndike et at. 1975). Further assumptions are that the 
shear deformations are not taken into account and the shape and porosity of the ridges is 
constant. These assumptions are based on the field observations (Timco and Burden, 1997; 
Kankaanpaa, 1997). 
Ice motion is determined by the time dependent momentum balance equation, which takes into 
account a Coriolis force, wind and water stresses, sea surface tilt term and an internal stress. 
The internal stress of pack ice is calculated according to the viscous-plastic rheology (Hibler, 
1979) but also relates consumption of the kinetic energy to the ice pack deformations 
(Rothrock, 1975). 
The sea-ice model employs curvi-linear co-ordinates. Variables are spatially discretized in a c- 
crid. The advective part of the ice thickness and concentration evolution equation is solved by 
an upwind method. Momentum balance is solved by the line successive relaxation procedure 
proposed by Zhang and Hibler (1997). 
Present set-up of the model predicts evolution of five undeformed and two deformed ice 
categories. Ice categories are "advected" in the thickness space without any limits, except that 
the thinnest category is not allowed to exceed 10 cm. Deformed ice is divided into separate 
categories of rafted and ridged ice types. 
Horizontal resolution of the model is 1 nm. The model was forced by the daily NCEP/NCAR 
reanalysis. Initial SST is obtained from the ice charts. 
Weather conditions 
The model was forced by the daily NCEP/NCAR reanalysis. The NCEP/NCAR data is from the 
global atmospheric model of resolution 2.55(2.5°. I n order to study utility of the NCEP/NCAR 
data in regional studies, the data were compared to the Tallinn meteorological observations. 
The figure 1 shows a good correlation between the reanalysed and measured wind data. The 
correlation coefficient is 0.91 and 0.88 for meridional, zonal components respectively.