Changes to Level-2 flags
Four flags were removed as they were not being used for any masking or quality assessment. This provides additional spare bits for future development. A product warning flag was added. This can be set by any derived product to indicate reduced quality. The flages coded in RED indicate those that are applied as standard masks in the RRS Level-3 suite (though other Level-3 suites may use alternate masking). BLUE indicates a change.
Stray light from adjacent, bright sources such as cloud edges is known to contribute to error in SeaWiFS and MODIS ocean color retrievals. In MODIS/Aqua Reprocessing 1.1 (2005), pixels within a 7x5-pixel region around bright pixels were flagged as straylight and masked in Level-3. This decision was based on the point-spread function as described in Meister et al. 2008, coupled with the lack of an adequate straylight correction capability for MODIS. In contrast, the SeaWiFS straylight masking was reduced in SeaWiFS Reprocessing 4.0 (2004), as it was believed that the straylight correction algorithm was sufficient for accurate ocean color retrieval. However, recent analysis has shown that SeaWiFS straylight artifacts are a significant source of discrepancy between SeaWiFS and MODIS ocean color retrievals in oligotrophic water, where the signal in the green wavelengths is very small and highly sensitive to radiometric contamination. For Reprocessing 2009, therefore, the SeaWiFS straylight masking has been restored to pre-2004 levels. Specifically, a 5x3 diamond-shaped dilation filter is now applied around bright pixels, where pixels that fall into the filter kernel are flagged for straylight and masked in Level-3 binning.
As shown in the Figure above, one impact of this additional flagging & masking is a significant loss in SeaWiFS global Level-3 coverage on short time-scales. It should be emphasized that the straylight-contaminated pixels are flagged at Level-2 and masked at Level-3, so there is no impact to Level-2 coverage. And again, this masking scenario is very similar to what existed in global SeaWiFS products prior to 2004. The positive impact of this change is a 50% reduction in the discrepancy between SeaWiFS and MODIS oligotrophic chlorophyll retrievals, and significantly improved radiometric agreement in the green to red spectral regime.
The sea ice flag is used to mask retrievals of photosynthetically active radiation (PAR), as ice contamination would confuse the algorithm. In previous processings, this flag was set via a fixed monthly climatology. It is now more realistically set using NSIDC near real time ice fraction data and a threshold 10%. Pixels with an ice fraction above the threshold get flagged as sea ice. For more information see the ancillary data documentation.