EAMNet Surface Chlorophyll - 1km

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Summary

Providers: Plymouth Marine Laboratory Prospect Place Plymouth PL1 3DH UK &
Department of Oceanography University of Cape Town, Rondebosch, Cape Town, South Africa, 7701

Contact person: Steve Groom, Send email

Contact for production: NEODAAS Helpdesk Send mail

Algorithm development: NASA (MODIS); ESA (MERIS), PML/UCT for mapping

Chl-a is used as an estimate of phytoplankton abundance or biomass although as chl-a is a pigment the ratio of chl-a to carbon in a phytoplankton assemblage is quite variable. Chl-a can be used to compute primary production and in high concentrations can indicate eutrophication. Remotely sensed Chl-a is used by international, national and regional research institutions, and environmental organizations for change detection, time series analysis, early warning of harmful algal blooms, and productivity

Units: Milligrams of chl-a per cubic meter (mg m-3); values range from ~0.01 to 64

Data Portal: EAMNet

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Platform & processing

The ESA Envisat MERIS chlorophyll products provided are "algal_1"(case 1 / open ocean surface chlorophyll) and "algal_2" (case 2 / coastal waters surface chlorophyll) at 1.2 km resolution from 2002 to date. The NASA Aqua MODIS chlorophyll product is case 1 / open ocean surface chlorophyll at ~1.1km resolution for 2002 to date.

MERIS data are processed by ESA to level 2 using specific software; NASA data are processed to level 1 and level 2 using NASA specific software. The user version of the NASA processing software called SeaDAS is available at http://oceancolor.gsfc.nasa.gov/seadas/. SeaDAS can also process MERIS data.

Algorithm

MODIS: uses OC5, a maximum band ratio switching algorithm using bands around 443, 490 and 555 nm (e.g. see Gohin et. al (2008)). MERIS: algal_1 uses a blue to green band ratio algorithm (see http://envisat.esa.int/instruments/meris/pdf/atbd_2_09.pdf); algal_2 uses a neural network approach (see http://envisat.esa.int/instruments/meris/pdf/atbd_2_12.pdf)

Main data inputs are level 2 MERIS or MODIS data at ~1.1 km resolution output from the level 1 to 2 atmospheric correction. The algorithm changes with the processing version - please refer to the latest information from NASA and ESA.

For MODIS see O'Reilly et al, 2000; for MERIS algal_1 see Morel and Antoine (2007); for algal_2 see Doerffer and Schiller (2007) and Schiller and Doerffer (2005).

Product uncertainties

Cloud cover limits coverage of the sea-surface. Chl-a is used as an estimate of phytoplankton abundance or biomass although chl-a is a pigment and the ratio of chl-a to carbon in a phytoplankton assemblage is quite variable. Case 1 chl-a estimates are computed in case 2 areas but may be incorrect depending on the nature on the water. In case 1 waters chl-a retrievals are generally quoted with an accuracy of ~0.23 log10C, that is for a value of Chl-a of 1mgm-3 the error is 10-0.23 to 10+0.23 i.e. 0.59 - 1.7 mg m-3. NASA publish intercomparisons during reprocessing (e.g. http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/ocv6/ocval_swf.png). The error is generally greater for case 2 waters

Case 1 products may not work in case 2 waters depending on the nature of the water. The water contituents may change with time through phytoplankton growth/decay, changes in nutrient inputs etc.

Product dissemination

Only MODIS chlorophyll is distributed via GEONETCast (a global lower resolution map is distributed by JRC). The basic data are distributed over the internet by ESA and NASA respectively, at various levels of processing. The DevCoCast produced data are also distributed via the internet.

The typical delay of product is a minimum of 6hrs from acquisition of data by NASA or ESA, depending on the region covered. Composites are produced on day i for the period i-8 to i-1. MODIS are reprocessed once actual meteorological data are available - these are termed "refined" products. MODIS and MERIS processing systems are currently fully operational. The SeaWiFS instrument failed in 2010 consequently only archive data is available

Product validation

The algorithms used to produce chl-a and the atmospheric correction are peer-reviewed

MODIS processing chains are validated by comparison with in situ data with strict time and space matchup criteria (see http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/ocv6/ocval_mod.png). Operational ESA products are not currently validated against field measurements though comparisons are made (e.g. http://www.obs-vlfr.fr/Boussole/html/calibration/matchup_r.php#meris).

SeaWiFS and MODIS data use a common processing system and NASA undertake intercomparisons

Data characteristics

The area covered at 1.1 km depends on the region.

Individual overpasses give an instantaneous "snapshot" typically once per day over equatorial regions (for MODIS) and every two days (for MERIS).

Data are presented using the Mercator projection.

Flags for land/cloud, flags for bad radiometric quality of spectral bands i.e. saturated pixels, sun glint etc are used.

Notes

When the MODIS or MERIS processing changes, the archive is usually reprocessed as well, so archives should be consistent with current near-real-time data.

Useful references:

Aiken J. and G. Moore (2000) Case 2 (S) Bright Pixel Atmospheric Correction, MERIS ATBD 2.6 [pdf]

Antoine D. and A. Morel (2005) Atmospheric Correction of the MERIS observations Over Ocean Case 1 waters, MERIS ATBD 2.7, [pdf]

Doerffer, R., and H. Schiller (2007), The MERIS case 2 water algorithm, International Journal of Remote Sensing, 28, 517-535.

Gohin, F., Saulquin, B., Oger-Jeanneret, H., Lozac'h, L., Lampert, L., Lefebvre, A., Riou, P., and Bruchon, F. (2008): Towards a better assessment of the ecological status of coastal waters using satellite-derived chlorophyll-a concentrations, Remote Sensing of Environment, 112, 3329-3340, 10.1016/j.rse.2008.02.014

Morel A. and D. Antoine (2007) ATBD 2.9 - Pigment Index Retrieval In Case 1 Waters (http://envisat.esa.int/instruments/meris/pdf/atbd_2_09.pdf)

Schiller, H., and R. Doerffer (2005), Improved determination of coastal water constituent concentrations from MERIS data, IEEE Transactions on Geoscience and Remote Sensing, 43, 1585-1591.

O'Reilly, J.E., and 24 co-authors, 2000: SeaWiFS Post launch Calibration and Validation Analyses, Part 3. NASA Tech. Memo. 2000-206892, Vol. 11, S.B. Hooker and E.R. Firestone, Eds., NASA Goddard Space Flight Centre, 49 pp.