EAMNet Remote Sensing Reflectances - 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

Water-leaving radiances (Lw) are the primary retrieved level-2 quantity after atmospheric correction of the level-1 data. The data are mainly converted into normalised remote sensing reflectance (Rrs) and used as inputs to algorithms to compute Chl-a and other in-water parameters (e.g. Kd490) but may be used individually or as a three-band false-colour composite to show area of enhanced brightness, for example, due to coccolithophore blooms.

The Rrs products are avaliable at various wavelengths. They are used for monitoring ocean colour and visualising some phytoplankton types or suspended particulates by, international, national and regional research institutions, and environmental organizations.

Units: nLw (mW cm-2 um-1 sr-1), Rrs (sr-1).

Data Portal: EAMNet

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Platform & processing

The ESA Envisat MERIS radiance products provided are the outputs of the case 1 / open ocean or case 2 / coastal water algorithms at 1.2 km resolution from 2002 to date. The NASA Aqua MODIS radiance is case 1 at ~1.1 km resolution 2002-date.

nLw are mainly used as input to other algorithms e.g. for regional chl-a estimation; but are also used to look at the colour of the ocean e.g. to look at scattering in coccolithophore blooms

Main data input are level 2 MERIS or MODIS data at ~1.1 km resolution output from the level 1 to 2 atmospheric correction.Algorithm

ESA data are processed by ESA to level 2 using specific software; 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/. SeaDAS can also process MERIS data.

MODIS: uses an atmospheric correction procedure e.g. Gordon and Wang 1994; MERIS uses the atmospheric correction procedure of Antoine and Morel, 2005 and in bright water uses Aiken and Moore (2000).

Product uncertainties

Cloud cover limits coverage of the sea-surface; similarly sun-glint or high levels of aerosol mask the surface signal. The limitation is the accuracy of the atmospheric correction procedure together with the calibration and signal to noise of the instrument.

Case 1 retrievals will have an error 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 currently. The basic data are distributed over the internet by ESA and NASA respectively. Data produced for DevCoCast are also distributed via the internet. Typical delay of product is a minimum of 6 hrs from acquisition of data by NASA or ESA. Products from are delivered on best effort basis only.

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 chl-a and the atmospheric correction are peer-reviewed (See references)

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/). 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.

NASA undertake vicarious calibration and comparison of nLw with in situ data (see http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/validation/ for SeaWiFS 2009 reprocessing).

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 (see master product list).

Notes

The algorithm changes depending on the processing version - need to see latest information from NASA (e.g. http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/) and ESA. When the MODIS or MERIS processing changes the archive is also usually reprocessed so archives should be consistent with current NRT 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.

Gordon, H. R., and Wang, M. (1994), Retrieval of water-leaving radiance and aerosol optical thickness over the oceans with SeaWiFS: a preliminary algorithm. Appl. Opt. 33:443-452.

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.