Due to melting ice during the summer months, passive microwave data is mostly unusable for determining ice motion; and because cloud cover obscures ice detection in AVHRR's infrared and visible wavelengths, far fewer vectors are generated. Only the relatively few buoy vectors are available all year, and these are mostly located on multi-year ice. Since there is much more first year ice covering the Arctic basin in the last decade, ice motion vectors derived from NCEP/NCAR wind reanalysis fields have been added. The NCEP/NCAR estimates are interpolated to the 25 km EASE-Grid projection. Thorndike and Colony (1982) found that, during summer conditions, the ice moves approximately one percent the speed of the wind and has a turning angle of about 20 degrees from the wind. This general rule was applied to the NCEP/NCAR winds. Adding these vectors deviates from only using vectors derived from remote sensing instruments, but they help to provide information to areas with little coverage.
Daily Averaged Ice Motion Vectors from NCEP/NCAR Winds
The accuracy of the ice vectors from wind fields will not be as good as from other sources. First, the wind fields are from atmospheric models, not directly from satellite or buoy observations. Second, the relationships between winds and ice motion are just general rules. Comparisons with buoys show mean differences of 1.2 cm/sec and Root Mean Square (RMS) error of 6.1 cm/sec.