Ice Motion from SMMR and SSM/I

Methods:

SMMR data are available from 1978 to 1987, and SSM/I data are available from 1987 through present. 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.

Fowler reprojected daily SMMR and SSM/I images in a polar stereographic projection to corresponding EASE-Grids. The images are 24-hour composites of all available passes, and they cover both sea ice areas of 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.

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

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 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, 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, 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.