EAMNet Kd490 - 1km

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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

Kd490 is diffuse attenuation coefficients of downwelling irradiance at 490 nm and is used as a measure of the turbidity of water. Values of the biological component of Kd490 range from ~0.001 to 3 (see http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/kdv4/ocfit_kd2mr.png). It is used for monitoring water turbidity, time series analysis and water quality by international, national and regional research institutions, and environmental organizations.

Units: m-1

Data Portal: EAMNet

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Platform & processing

The NASA Aqua MODIS chlorophyll product is ~1.1km resolution from 2002-date. NASA data are processed to level 2 using NASA specific software. The user version of the processing software is called SEADAS and is available at http://oceancolor.gsfc.nasa.gov/seadas/.


MODIS: uses a polynomial algorithm based on the 488 nm to 547 nm band ratio (Mueller, 2000; Werdell, 2005; see http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/kdv4/).

Product uncertainties

Cloud cover limits coverage of the sea-surface. The error in the latest algorithm using the NASA NOMAD dataset is ~0.19 log 10 Kd . The error by comparing MODIS retrievals with in situ values is likely to be larger.

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. The basic data are distributed by Internet by NASA. The data produced for DevCoCast are also distributed via the internet. The ty ypical delay of product is a minimum of 6 hrs from acquisition of data by NASA or ESA, varying on the region covered. Products from are delivered on best effort basis only.

Individual overpasses give an instantaneous "snapshot" typically once per day over equatorial regions (for MODIS).

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.

Product validation

The algorithms to produce Kd490 are peer-reviewed (see references).

MODIS algorithms are produced by comparison with in situ data with strict time and space matchup criteria (see http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/kdv4/). SeaWiFS and MODIS data use a common processing system.

Data characteristics

Data input MODIS Rrs data at ~1.1 km resolution output from the level 1 to 2 atmospheric correction. The area covered at 1.1 km depends on the region.

Data is 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.


The algorithm changes depending on the processing version - need to see latest information from NASA. When the MODIS processing changes the archive is also usually reprocessed so archives should be consistent with current NRT data.

Useful references:

Mueller, J.L. (2000) SeaWiFS algorithm for the diffuse attenuation coefficient, K(490), using water-leaving radiances at 490 and 555 nm. In O'Reilly, J.E. and co-authors: SeaWiFS Post launch Calibration and Validation Analyses, Part 3. S.B. Hooker and E.R. Firestone, Eds., NASA/TM-2000-206892, Vol. 11, NASA Goddard Space Flight Centre, Greenbelt, Maryland, 24-27.

Werdell, (2005) http://oceancolor.gsfc.nasa.gov/REPROCESSING/SeaWiFS/R5.1/k490_update.html