MyOcean Kd 490

Summary

Provider: MyOcean (CNR/ACRI)

Contact person: ???

Contact for production: MyOcean Service Desk Send mail

Algorithm development: ICESS

Kd490 is the diffuse attenuation coefficient of downwelling irradiance at 490 nm and is used as a measure of the turbidity of water. It measures values of the biological component of Kd490 and is used to produce time series analysis of water quality by international, national and regional research institutions, and environmental organizations.These are produced for the Mediterranean by the Group for Satellite Oceanography (GOS-ISAC) of the Italian National Research Council (CNR), in Rome, and globally by ACRI.

Units are: m-1

Data Portal: MyOcean 2

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Platform & processing

The diffuse attenuation coefficient (490 nm) is computed directly from the GSM merged chlorophyll product. Data for the Mediterranean are processed by CNR (using SeaDAS v6.1) and data for the Global Ocean a re processed by ACRI in association with MyOcean. (In theory, the software is available, see http://www.odesa-info.eu/distrib/). See the MyOcean Remote Sensing Reflectances page for details of CNR's processing.

Flags for land/cloud, flags for bad radiometric quality of spectral bands (i.e. saturated pixels, sun glint etc.) have already been applied to input data – ther are no specific flags at this level.

Algorithm

MODIS Level-0 raw data are routinely processed up to Level-3 with the SeaWiFS Data Analysis System (SeaDAS) software package version 6.1 available from NASA website.

The merged Kd(490) is computed directly from the GSM merged CHL1, using the following equation:

Kd (λ) = Kw (λ) + χ( λ) × chle( λ )

with: Kw(490) = 0.0166 m-1

χ(490) = 0.08349

e(490) = 0.63303

Product uncertainties

Cloud cover limits coverage of the sea-surface. The error of the merged product when compared with in situ values was estimated to be ~0.15 log 10 Kd for the merged case and, for SeaWiFS, MODIS, and MERIS taken individually, albeit with a small sample size, were 0.11, 0.07, and 0.09 respectively. Case 1 products may not work in case 2 waters depending on the nature of the water. Temporally the water may change through phytoplankton growth/decay, changes in nutrient inputs etc.

Product dissemination

These products are not distributed via GEONETCast, but are available through MyOcean via the internet.

Product validation

MyOcean state that this product is meant for use for educational purposes and for the managing of the marine safety, marine resources, marine and coastal environment and for climate and seasonal studies.

MyOcean provides validation documentation for both CNR and ACRI products.

The chlorophyll-a algorithm has been peer reviewed, see references on the Chlorophyll-a page

Chlorophyll-a was calibrated with in-situ data primarily to establish sensor uncertainties used in the GSM approach. (see http://www.globcolour.info/validation/report/GlobCOLOUR_FVR_v1.1.pdf).

Data characteristics

Mediterranean

Resolution: 1km

Map projection: cylindrical equirectangular

European Seas / North Africa

Resolution: 2km

Map projection: regular equirectangular

Global Ocean

Resolution: 4km, 25km or 100km resolution.

Map projection: All resolutions now use regular equirectangular.

Availability

The typical delay of product is less than 24 hours for Near Real Time (NRT) products. Delayed Time (DT) products are made available to the user with 5 days of acquisition. DT data are processed using precision orbital data and improved meteorological fields used for atmospheric correction when compared to NRT data. Reanalysis (RAN) products for the Mediterranean are the output of period but infrequent (after some years) reprocessings using the same configuration throughout. Global RAN products are available from 30 days after acquisition as daily, 8-day and monthly composites

Notes

Information about processing version changes are not widely disseminated. If processing changes the archive should usually be reprocessed for consistency with current NRT data although whether this mechanism is in place is not known. For some datasets the RAN products are continuously updated 30 days after satellite acquisition, but in this case there is no guarantee that the configuration is unchanged.

Useful references:

Garver, S. A., & Siegel, D. A. (1997). Inherent optical property inversion of ocean color spectra and its biogeochemical interpretation: I. Time series from the Sargasso Sea. Journal of Geophysical Research, 102, 18607 - 18625.

Gordon, H. R., Brown, O. B., Evans, R. H., Brown, J. W., Smith, R. C., Baker, K. S., et al. (1988). A semi-analytic radiance model of ocean color. Journal of Geophysical Research, 93, 10909 - 10924.

Maritorena, S., Siegel, D. A., & Peterson, A. (2002). Optimization of a semi-analytical ocean color model for global scale applications. Applied Optics, 41(15), 2705 - 2714.

Maritorena, S., Siegel, D. A. (2005). Consistent merging of satellite ocean color data sets using a bio-optical model. Remote Sensing of Environment 94:429-440. [pdf]

McClain, C. R., Arrigo, K. R., Esaias, W., Darzi, M., Patt, F. S., Evans, R. H., et al.(1995). SeaWiFS Algorithms, Part 1. NASA Tech. Memo. 104566, Vol. 28 Greenbelt, Maryland: NASA Goddard Space Flight Center.

Siegel, D. A. S. Maritorena, N. B. Nelson, M. J. Behrenfeld, and C. R. McClain. 2005. Colored dissolved organic matter and its influence on the satellite-based characterization of the ocean biosphere. Geophys. Res. Letters, 32, L20605, doi:10.1029/2005GL024310.

Werdell, P.J. (2005). Ocean color K490 algorithm evaluation. http://oceancolor.gsfc.nasa.gov/REPROCESSING/SeaWiFS/R5.1/k490_update.html