Comparisons of sea-ice velocity fields from ERS-1 SAR and a dynamic model

Numerical models for sea-ice thickness distribution and velocity are used f or ice-dynamics research and ice forecasting. In the modeling work, ERS-1 S AR is an excellent tool, in particular by providing spatial ice-velocity fi elds as described in the present Baltic Sea study. Ice velocities were extr acted from SAR data with 3 and 6 day time intervals using the optical-flow method. A considerable stiffening of the ice pack was observed due to the c hange in the character of ice deformation under compression from rafting to ridging as the minimum ice thickness increased from 10 to 30 cm. The coast al alignment was strong in the ice motion and the character of ice deformat ion under compression from rafting to ridging a the minimum ice thickness i ncreased from 10 to 30 cm. The coastal alignment was strong in the ice moti on and the coastal boundary layer width was 20 30 km. An analysis of the SA R data with an ice-dynamics model showed that the observed overall ice-velo city field could be produced using the Hibler viscous-plastic ice rheology. The compressive strength of the ice (over 10 km scales) was 2.5 x 10(4) N m(-2) +/- 50% for ridging and negligible for rafting of very thin ice. The shear strength was significant and the normal yield ellipse aspect ratio of 2 was valid. The 3 day time interval is valid for updating an ice model bu t for detailed ice-dynamics investigations a data frequency of 1 d(-1) or h igher would be preferable.