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Ocean Colour CCI chlorophyll product compared to CMEMS and GlobColour

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The OC-CCI chlorophyll product (units of mg/m^3) is derived from merged MODIS-Aqua, SeaWiFS, VIIRS and MERIS data. The SeaWiFS and VIIRS data are processed from Level 1 to 2 within the project using the NASA standard processing (SeaDAS 7.0), and POLYMER is used for the MERIS atmospheric correction. In v2 MODIS was processed using SeaDAS while in V3.0 POLYMER is used. Then the MODIS, VIIRS and MERIS data are band shifted to the six main SeaWiFS bands and a bias correction applied before merging, after which the chlorophyll algorithm is applied. The OC-CCI products have used different chlorophyll algorithms for each version:

  • v2.0: OC4v6; generated in 2015 with the current input dataset versions, VIIRS not included at this stage.
  • v3.0: an Ocean Colour Algorithm Blending (OCAB) approach has been used to combine two chlorophyll-a algorithms to obtain optimal overall results: OC5 algorithm (Gohin et al., 2002 IJRS, 2005 RSE) and OC4v6-OCI algorithm (Hu et al., 2012 JGR) as it performed better in the clearest oceanic waters (optical water classes 1 to 10). See the OCAB ATBD for more details on the blending algorithm http://www.esa-oceancolour-cci.org/?q=webfm_send/587

The Copernicus Marine Environment Monitoring Service (CMEMS) products use the OC-CCI-v2.0 4km bias-corrected/merged remote-sensing reflectance (Rrs, surface ratio of upwelling radiance emerging from seawater to downwelling radiative flux in air) for the global products, with 1km data processed for the regional datasets. Then, either the OC4 or OC5CI chlorophyll algorithms are applied alongside Rrs being made available. See MyOcean Product User Manual http://marine.copernicus.eu/documents/PUM/CMEMS-OC-PUM-009-ALL.pdf

The GlobColour RAN (reanalysis) products are produced by ACRI-ST using the GlobColour processor that merges MODIS-Aqua, SeaWiFS and MERIS; the dataset was originally generated in 2008 and has since been reprocessed (2nd reprocessing). There are several chlorophyll products available:

  • CHL1 merged products that include:
    • GSM derived product based on merged Level 2 data, following the approach proposed by Maritorena and Siegel (2005, RSE);
    • OC4Me algorithm (an inter-calibration is applied to the MODIS-Aqua and SeaWiFS Level 2 chlorophyll products to get MERIS algorithm compatible inputs) with a weighted averaging merging (AVW) method then applied to merge the adjusted MODIS and SeaWiFS products with MERIS data – there is also a simple average (AV) single sensor OC4Me product;
  • CHL2 that’s the MERIS C2R Neural Network algorithm (only available for input MERIS data).

See GlobColour Product User Guide http://www.globcolour.info/CDR_Docs/GlobCOLOUR_PUG.pdf