Data regions:

The regional images' extents are as follows:

Mauritania	(18N, 24W)	(10N, 14W)
Senegal	(10N, 24W)	(0, 14W)
Guinea	(10N, 14W)	(0, 6W)
Cote d'Ivoire	(8N, 6W)	(0, 4E)
Nigeria	(8N, 4E)	(5S, 13E)
Angola	(4S, 4E)	(15S, 16E)
Namibia	(14S, 4E)	(28S, 16E)
South Africa (west)	(26S, 12E)	(37S, 22E)
South Africa (east)	(31S, 18E)	(41S, 28E)
South Africa (south)	(27S, 27E)	(37S, 40E)
Tanzania	(5S, 37E)	(14S,50E)
Mozambique Channel	(14S, 32E)	(28S, 50E)
Somalia (south)	(5N, 38E)	(5S, 50E)
Somalia (north)	(18N, 43E)	(5N, 60E)

Data products:

MERIS data:

Data availability

As from July 2007, MERIS data is received in near-real time at the University of Cape Town. So new MERIS data will be available as soon as it is processed. Archived data (older than July 2007) will be made available in due course. Note the image coordinates for older data may not be identical to newer images, this is due to differences in the processing chain.

Algal 1 index

The MERIS Algal 1 pigment index is a measurement of the concentration in Log10(mg/m3) of phytoplankton (algae) in the water. The concentration is derived by the direct relationship between the ratio of the blue and green signal leaving the water surface and the concentration of algal pigments. The relationship, based on published data, is valid over clear (Case 1) waters and spans a concentration range from mg/m3 to tens of g/m3.

Case 1 waters are those whose optical properties are dominated by phytoplankton and their associated materials (debris, heterotrophic organisms and bacteria, excreted organic matter). These conditions are generally applicable in the open ocean. The MERIS Algal 1 index is a semi empirical algorithm which calculates chlorophyll a content based on the ratios of 4 reflectance bands.

Inputs are the normalised water-leaving reflectances from the 4 bands, as well as auxiliary optical data (full details can be reviewed in section 2.1.2, ATBD 2.9). The ratios are calculated from the bands centred on 560nm (green), paired with each of 443, 490 and 510nm (all blue-green). A "best fit" of these 3 ratios compared with empirical data is then calculated as a function of chlorophyll and the chlorophyll estimate is retrieved. A further processing step removes the influence of bi-directional reflectance effects.

This algorithm is only appropriate for Case 1 waters and becomes unreliable in very high chlorophyll conditions.

Algal 2 index

The MERIS Algal 2 pigment index is a measurement of the concentration in Log10(mg/m3) of phytoplankton (algae) in the water, but is part of a suite of oceanic products derived by inverting a model of the optical properties of the ocean through the use of a neural network. The other oceanic products retrieved are suspended matter and yellow substance. This algorithm can be used for both clear (case 1) and coastal (case2) waters.

The algorithm is based on inverse modelling of the radiative transfer using a Monte Carlo photon tracing code. The parameterisation is done by a neural network, whose coefficients are determined from a table of reflectances, angles and the corresponding concentrations by the feed-forward back-propagation optimisation technique.

Input to the algorithm are the directional water leaving radiance reflectances, Uw, of 8 MERIS bands, which are defined as Uw(Tv_ 'Iv_=Lw(Tv_ 'Iv_/Ed(Ts), with Lw, the water leaving radiance, and Ed, the down-welling irradiance just above the sea surface, and the three angles, Tv_ the zenith angle of the observation angle, 'Iv_ the observation azimuth relative to the sun azimuth, and Ts, the solar zenith angle. (All adjusted for atmospheric effects.) Further details can be found in ATBD 2.12.

Case 2 waters are those whose optical properties are further complicated with the presence, in addition to phytoplankton, of significant amounts of coloured dissolved organic matter (or yellow substance) and suspended particulate matter. These waters are generally found in coastal regions.

The Algal 2 algorithm can be applied to both Case 1 and Case 2 waters, and is suitable for varying concentrations of all the constituents.

Yellow substance (Gelbstoff)

The MERIS yellow substance product is a measurement of the Gelbstoff absorption at 440nm in m-1. It is derived by inverting a model of the optical properties of the ocean by the use of a neural network. The model describes yellow substance as non-scattering absorbing matter. Yellow substance (also referred to as "Gelbstoff" or CDOM) is decayed organic material that has been dissolved in the marine waters and is usually transported into the sea by rivers.

For more information read the notes on Algal 2 or refer to ATBD 2.12.

Suspended sediments

The MERIS suspended matter product is a measurement of the suspended sediments concentration in Log10(g/m3). It is derived by inverting a model of the optical properties of the ocean by the use of a neural network. The model describes suspended matter as a scattering particle with very little absorption for which a more appropriate name would be "total suspended mineral matter" concentration.

For more information read the notes on Algal 2 or refer to ATBD 2.12.

KD490 - diffuse attenuation coefficient

The diffuse attenuation coefficient at 490nm describes the vertical attenuation of downward irradiance at this wavelength, and is given as a coefficient per metre. This quantity is thus the rate of change of Ed (downward irradiance) with depth.

No idea on how we make it or where the algorithm comes from!!!!!