Sea Ice Index Processing Resumed with DMSP-F18 Satellite Data

On 05 April 2016, the vertically polarized 37 GHz (37V) channel of the SSMIS instrument on the DMSP-F17 satellite began yielding compromised brightness temperature data. This channel is one of those used to estimate sea ice concentration shown in the Sea Ice Index, so data processing was temporarily suspended.

NOAA@NSIDC is pleased to announce that, today, Sea Ice Index processing has resumed and the time series now uses the SSMIS instrument on the DMSP-F18 satellite beginning 01 April 2016. These sea ice concentration values come from the NSIDC Near-Real-Time DMSP SSMIS Daily Polar Gridded Sea Ice Concentrations product. Prior to transitioning this near-real-time data set to the F18 SSMIS instrument, NSIDC investigated whether or not algorithm parameters would need to be calibrated to minimize the difference between sea ice extents derived from F17 and F18.

Because each sensor and spacecraft orbit is slightly different, algorithm parameters called tie points sometimes need adjustments so that the time series is consistent.  Inconsistencies would be marked by sudden false jumps or drops in derived ice extent at the point when the algorithm started using data from the new instrument. These tie-point adjustments are derived from regressions of brightness temperatures during satellite overlap periods. To determine if an adjustment to algorithm tie points would be needed for F18 data, NSIDC ran the algorithm both with and without adjustment to produce ice concentration data from both F17 and F18 data.  Ice extents derived from the ice concentration data were then compared over a 12-month period from 01 March 2015 through 29 February 2016 when the SSMIS instruments on both satellites were operational.

After investigating the difference in ice extent as the algorithm tie points were varied, it was found that the current F17 tie points provided the best match in sea ice extent for the overlap period, so no adjustment in algorithm parameters was needed for F18. Results are shown in the figures at the bottom. The average difference between F18- and F17-derived sea ice extents for the overlap time period is approximately -20,000 sq km. See the Error Sources section of the Near-Real-Time DMSP SSMIS Daily Polar Gridded Sea Ice Concentrations documentation for more information on the input data.  For more information on algorithm tie points see the Consistency of the Data Record section in the Sea Ice Index documentation.

The figures below show the relative agreement of sea ice extent from F17 (green line) and F18 (orange line) for the Northern Hemisphere (top) and Southern Hemisphere (bottom) for March 2015 through February 2016.

N_extent_ts_f18-f17

S_extent_ts_f18-f17

Data Set DOI: http://dx.doi.org/10.7265/N5QJ7F7W

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