Daily ice motion vectors are computed from Advanced Very High Resolution Radiometer (AVHRR), Scanning Multichannel Microwave Radiometer (SMMR), Special Sensor Microwave/Imager (SSM/I), and International Arctic Buoy Programme (IABP) buoy data. Daily gridded fields combine data from all sensors, from November 1978 through December 2006. Grids extend from 48.4° N to 90° N, and from 53.2° S to 90° S. ... Mean gridded fields include yearly, monthly, and weekly means, and a mean for the entire time series. Both daily and mean gridded fields are in 2-byte integer format, reprojected to 25 km Northern and Southern Hemisphere EASE-Grids. Browse images of mean fields are available in Portable Network Graphics (PNG) and Enhanced Postscript (EPS) format. Raw ice motion vectors from each sensor are in tab-delimited ASCII text format. The complete data set is available via FTP. Registration for data is optional, but encouraged; registered users automatically receive email notification of data updates.
Direct comparisons between the more accurate buoy measurements and vectors derived from passive-microwave data are difficult because of time differences. The SMMR data are composites over two days, and the SSM/I images are composites over a 24-hour period. Each pixel in an image may contain information from more than one orbit. With this problem in mind, Fowler compared vectors derived from ... passive-microwave imagery with those from buoy data. For 85-GHz data, he found 74,381 pairs of buoy and SSM/I vectors that were less than 50 km apart. The mean difference in the u component was -0.05 cm/sec with a Root Mean Square (RMS) error of 4.16 cm/sec. The mean difference in the v component was 0.39 cm/sec with an RMS error of 4.23 cm/sec. For 37-GHz data, the mean difference in the u component was 0.04 cm/sec with a Root Mean Square (RMS) error of 5.05 cm/sec. The mean difference in the v component was 0.74 cm/sec with an RMS error of 5.24 cm/sec. With AVHRR 5-km ima gery, ice motion accuracy is about 2 cm/sec. Fowler compared vectors derived from AVHRR imagery with those from buoy data. He found 26,820 pairs of AVHRR and buoy vectors that were less than 50 km apart. The mean difference in the u component was -0.12 cm/sec with a Root Mean Square (RMS) error of 3.31 cm/sec. The mean difference in the v component was 0.07 cm/sec with an RMS error of 3.29 cm/sec. It is difficult to assign an error value to a buoy velocity. The position accuracy of the buoys is about 0.5 km. The locations are interpolated to the 6-hour interval times. The accuracy is then dependent upon the initial position error and the interpolation error. It could be assumed that the overall error would be less than 1 cm/sec for the average velocity over 24-hours. For the gridded data, Fowler found that the mean difference between the interpolated u components and the buoy vectors was 0.1 cm/sec with a Root Mean Square (RMS) error of 3.364 cm/sec. For the v component, t he mean was 0.4 cm/sec with an RMS error of 3.39722.
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