Description and assessment of the OSTIA reanalysis. - Met Office

2.5 OSI-SAF sea-ice concentration reprocessing
A daily sea ice concentration field is provided with the OSTIA reanalysis. This is used in to update
the background SST field (see section 3.3) with the aim of improving the consistency between the
SST and the sea ice. The ice concentration field is also used in the quality control procedure to
mask out observations in all of the datasets for regions which appear to be under ice (section 3.2).
This is particularly important for the Pathfinder data owing to an issue of erroneous data in the Arctic
in the boreal summer, discussed in section 2.2.
The OSTIA ice field is derived from the OSI-SAF sea ice concentration reprocessing dataset
between 1985 and 2007 (Eastwood et al., 2010) by regridding from a 10 km resolution polar stereographic
grid on to the OSTIA 1/20 o equal-area grid. A QC procedure was performed by eye on the
ice concentration files before inclusion in the reanalysis to remove files with regions of missing or
poor data. For days when the ice concentration field was not available due to satellite malfunction
or planned maintenance (Tonboe and Nielsen, 2010), or did not pass our QC checks, the previous
day’s ice concentration field was persisted. Appendix A gives a complete list of missing days.
For large gaps in the data (7 days or more), the first file available after the outage has been
copied to the date in the middle of the outage. This is because (making the assumption that the
growth/retreat of ice extent can be approximated as linear) after the halfway point the ice concentration
field will be closer to that in the later file, and less close to that persisted from the file at the start
of the outage. This aims to reduce large jumps in the SST and sea ice consistency. This method
could be improved, for example by using an interpolation method for the sea ice concentration in
the data gap to produce a daily file. Analysis of the effectiveness of this procedure is included in
section 4.4.2.
Areas of missing data in the ice concentration field due to missing scan lines, missing orbits
and the polar observation hole were filled by OSI-SAF. This was accomplished using an algorithm
employing both spatial and temporal interpolation (Eastwood et al., 2010). However, for this version
of the reprocessed ice concentration data there still remained missing data within some sea ice files.
For missing regions enclosed within the ice concentration field a spatial 2-D bilinear interpolation
was used during the OSTIA regridding process. For missing regions which extended beyond the
ice edge the region could not be filled using this method and the file was rejected. Over open water
the effect of atmospheric noise on the passive microwave sensor increases the ice concentration
above 0% (Tonboe and Nielsen, 2010). Therefore data with a concentration of less than 15% was
set to 0% for the OSTIA reanalysis.
The OSI-SAF sea ice concentration dataset is comprised of data from the SMMR (Scanning
Multichannel Microwave Radiometer) instrument between January 1985 and July 1987 and the
various SSM/I (Special Sensor Microwave/Imager) instruments between July 1987 and December
2007. Lists of the satellites carrying the instruments and overlap periods between instruments are
given by Eastwood et al. (2010). The SMMR instrument, aboard the Nimbus 7 satellite, operated
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