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Ice Motion from SMMR, SSM/I-SSMIS, and AMSR-E

Methods

SMMR data are available from 1978 to 1987, SSM/I-SSMIS data are available from 1987 through the present, and AMSR-E data are available from 2002 to 2011. The SMMR 37 GHz channel is used for tracking ice, while SSM/I 37 GHz and 85 GHz channels are used. Both channels have dual polarizations that are treated independently for ice tracking. The 85 GHz channels are at a 12.5 km resolution, and the 37 GHz channels are at 25 km resolution. The data from AMSR-E are similar to the SSMI-SSMIS data except that the resolutions are twice as fine and the higher resolution AMSR-E channel is 89 GHz.

The daily SMMR and SSM/I-SSMIS images are reprojected from a polar stereographic projection to a corresponding EASE-Grid projection. The images are 24-hour composites of all available passes, and they cover sea ice areas of both polar regions completely. The same MCC methods used for AVHRR were applied to the two daily SMMR images (37H GHz and 37V GHz channels) and four SSM/I images (37H GHz, 37V GHz, 85H GHz, and 85V GHz channels).

Vectors from the SSM/I channels were merged together, with vectors available for both frequencies in most cases. The precision of the vectors from the 85 GHz channel is better than that from the 37 Ghz channel; however, periods of high water vapor affect 85 GHz more than 37 GHz, and the ability to track ice is reduced. Vectors from 85 GHz are preferable. In cases where 85 GHz and 37 GHz vectors overlap, the 37 GHz vector is removed.

Ice motion calculations from AMSR-E were completed using only the 89V GHz channel.

The following image shows ice motion from SSM/I on 20 April 2000.

SSM/I ice motion
Daily-averaged ice motion vectors from SSM/I

Accuracy

Direct comparisons between the more accurate buoy measurements and vectors derived from passive microwave data are difficult due to 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, and each pixel represents the composite motion of ice within the pixel area, while buoy motion tracks the drift of a specific ice floe or patch of ice. With these problems in mind, vectors derived from passive microwave imagery were compared with those from buoy data.

For 85 GHz data, there were 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.

The pixel resolution of the AMSR-E data is similar to the resolution of the AVHRR data, and the comparison with buoys showed much the same error, with a mean error of 0.19 cm/sec and root-mean-sqaure error of 3.52 cm/sec.