2.7.2 Level 2 Products

2.7.2.1 Level 2 High-Level Organisation of Products

2.7.2.1.1 Reduced Resolution Geophysical Product

The high-level structure of the product is shown below in the table 2.25.

Table 2.12 - Reduced Resolution Geophysical Product high-level structure.

2.7.2.1.2 Extracted Cloud Thickness and Water Vapour

The high-level structure of the product is shown below in the table 2.26.

Table 2.13 - Extracted Cloud Thickness and Water Vapour high-level structure.

2.7.2.1.3 Extracted Cloud Thickness and Water Vapour for Meteo Users

The high-level structure of the product is shown below in the table 2.27.

Table 2.14 - Extracted Cloud Thickness and Water Vapour
for Meteo Users high-level structure.

2.7.2.1.4 Extracted Vegetation Indices

The high-level structure of the product is shown below in the table 2.28.

Table 2.15 - Extracted Vegetation Indices high-level structure.

2.7.2.1.5 Full Resolution Geophysical Product

The high-level structure of the product is shown below in the table 2.29.

Table 2.16 - Full Resolution Geophysical Product high-level structure.

2.7.2.2 Level 2 Geophysical Products

The MERIS Level 2 product are a distributed mixture of geophysical products and reflectances.

The product format is called distributed because the geophysical quantity found in the different data sets changes according to the different surfaces measured. The different product groups are: ocean colour products, land and cloud products.

The classification between the different product groups, ocean, land and cloud is made on a pixel by pixel basis by using the a priori determination of Level 1b further consolidated by the use of radiometric tests at level 2. The Level 2 classification information is provided on a pixel by pixel basis in the flag measurement data set.

The scientific background to the MERIS products can be found in a MERIS special issue of the International Journal of Remote Sensing Volume 20, Number 9, 15th June 1999.

2.7.2.2.1 Product description

For each measurement data set, the products will be distributed according to the table below.

Table 2.17 - Distributed product Table.

SM - YS: Suspended Matter & Yellow Substance
MGVI: MERIS global vegetation index
PAR: Photosynthetically Active Radiation for chlorophyll fluorescence applications
Aerosol t & e: Aerosol optical Thickness & Aerosol Angström factor
RS: Normalised water leaving radiance/reflectance

Figure 2.9 - Chlorophyll Map of Western coast of South Africa.

The open ocean data products consists of chlorophyll concentrations and several atmospheric parameters. In coastal zones more products are produced such as Total Suspended Matter and Yellow Substance Concentration. The land surface product contains MERIS global Vegetation Index (MGVI) and atmospheric parameters. In addition, surface reflectances will be made available, allowing the user to derive further products. Three cloud products are foreseen: cloud top height, cloud optical thickness and cloud albedo.

The next sections briefly present the MERIS Level 2 products.

2.7.2.2.2 Ocean products

2.7.2.2.2.1 Normalized water leaving radiance / reflectance

The MERIS Level 2 radiometric unit is atmospherically corrected water leaving reflectance. The atmospheric correction applied assumes that the water is absorbing in the NIR, and include a correction for those sediment loaded waters where this assumption fails.

For more information, see ATBD 2.7, and ATBD 2.6 for the bright water correction.

2.7.2.2.2.2 Algal Pigment Index I

The MERIS algal 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 waters and spans a concentration range from mg/m³ to tens of g/m³.

For more information, see ATBD 2.9.

2.7.2.2.2.3 Algal Pigment Index II

The second MERIS algal pigment index is also a measurement of the concentation 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 by the use of a neural network. The other oceanic products are suspended matter and yellow substance.

For more information, see ATBD 2.12.

2.7.2.2.2.4 Suspended matter

The MERIS suspended matter products is a measurement of the suspended sediments concentration in Log₁₀(g/m³). As described above, it is derived by inverting a model of the optical properties of the ocean by the use of a neural network. The model describe suspended matter as a scattering particle with very little absorption for which a more appropriate name would be "total suspended mineralic matter concentration.

For more information, see ATBD 2.12.

2.7.2.2.2.5 Yellow substance

The MERIS yellow substance product is a measurement of the Gelbstoff absorption in m^-1. As described above, 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 or "Gelbstoff" is decayed organic material that has been dissolved in the marine waters and is usually transported into the sea by rivers.

For more information, see ATBD 2.12.

2.7.2.2.2.6 Photosynthetically Active Radiation (PAR)

The MERIS PAR product is a measurement the amount of radiation in mEinstein.m² available to the Photosynthetically Active oceanic flora. It will be computed from atmospherically corrected irradiances using the method proposed by Calder et al. This product is intended for the study of chlorophyll fluorescence and should be used with the 681.25 nm band selected for this purpose.

For more information, see ATBD 2.18.

2.7.2.2.2.7 Aerosol optical thickness

The MERIS aerosol optical thickness is a measurement the opacity of the aerosol layers at 865 nm. It is measured by assuming that the ocean is a black surface in the NIR (see Normalized water leaving radiance / reflectance).

For more information, see ATBD 2.7.

2.7.2.2.2.8 The MERIS Aerosol Angström Coefficient

The MERIS aerosol Epsilon factor is a description of the aerosol assemblage detected over water bodies. It is expressed as the spectral slope of the Aerosol Reflectance in the near Infra Red, complemented by two flags which identifies whether they are absorbing or not.

To be consistent with the land aerosol product, the epsilon factor is converted into an Angström coefficient.

For more information, see ATBD 2.7.

2.7.2.2.3 Cloud products

2.7.2.2.3.1 Cloud optical thickness

The MERIS cloud optical thickness is a measurement the opacity of the cloud.

For more information, see ATBD 2.1.

2.7.2.2.3.2 Cloud albedo

The MERIS cloud albedo is a measurement the amount of radiation reflected by clouds.

For more information, see ATBD 2.2.

2.7.2.2.3.3 Cloud top pressure

The MERIS cloud top pressure is a measurement of pressure in hPa based on the absorption of solar radiation by oxygen. It is derived by inverting a model of the optical properties of the atmosphere and clouds by the use of a neural network.

One example of Cloud Top Pressure image is shown in Figure 2.2 - .

Figure 2.10 -

For more information, see ATBD 2.3.

2.7.2.2.3.4 Cloud Type

The MERIS cloud type product is determined by using a simple relationship between the cloud properties - optical thickness and height - and its type, as established by the International Satellite Cloud Climatology Project (ISCCP).

2.7.2.2.3.5 Cloud reflectance

The MERIS Level 2 cloud radiometric unit will consist of top of the atmosphere reflectance only.

For more information, see ATBD 2.16 .

2.7.2.2.4 Land products

2.7.2.2.4.1 Reflectance

The MERIS Level 2 land radiometric unit will consist of Rayleigh corrected reflectance. The atmospheric corrections over land does not include a correction for the aerosols contribution.

For more information, see ATBD 2.15.

2.7.2.2.4.2 Aerosol optical thickness

The MERIS cloud optical thickness is a measurement the opacity of the aerosol layers at 865 nm. It is measured by assuming that Dense Dark Vegetation has a known dependent standard reflectance value in the visible.

For more information, see ATBD 2.15.

2.7.2.2.4.3 Aerosol Angström Coefficient

The Angström coefficient describes the dependance of the aerosol optical thickness between the blue and the red spectral bands.

For more information, see ATBD 2.15.

2.7.2.2.4.4 Meris Global Vegetation Index

The MERIS Global vegetation index is a measurement of the presence of healthy live green vegetation. It has been optimised to be robust to atmospheric conditions and surface reflectance and constrained to provided an estimate of the fraction of absorbed photosynthetically active radiation.

For more information, see ATBD 2.10.

2.7.2.2.4.5 Meris Terrestrial Chlorophyll Index

The MERIS Global vegetation index is a measurement of the presence of healthy live green vegetation. It has been optimised to be robust to atmospheric conditions and surface reflectance and constrained to provided an estimate of the fraction of absorbed photosynthetically active radiation.

For more information, see ATBD 2.10.

2.7.2.2.5 Water Vapour products

The MERIS water vapour product is a measurement of the concentration in g.m^-2 of water vapour found in the total atmospheric column. This product is of particular interest over land where the signal is high, but it will also be provided over water surfaces and clouds.

For more information, see ATBD 2.4.

The MERIS Level 2 products are coded in such a way as to take the full advantage of the storage space allocated to them. As a consequence, in order to convert the data to geophysical units, an offset and gain correction shall be applied.

In addition to the level 2 reference product which contains all data sets of the distributed product table, three smaller extracted products are available:

· Reduced resolution (1.2 km) column water vapour content product.

· Reduced resolution (1.2 km) vegetation product.

· Low resolution (4.8km) column water vapour content product.

2.7.2.2.6 Flags

Flags are provided on a pixel by pixel basis.

The list of flags is as follows:

Surface Classification :

· land pixel

· cloud pixel

· water pixel

Product Confidence :

· validity for MDS 1 to 13

· validity for MDS 14

· validity for MDS 15

· validity for MDS 16

· validity for MDS 17

· validity for MDS 18

· validity for MDS 19

Flag meaning :

· Coastline: From Level 1b

· Cosmetic: From Level 1b

· Suspect: From Level 1b

· Continental absorbing aerosol

· Dust-Line absorbing aerosol

· Turbid water

· Anomalous scattering water

· Yellow substance loaded water

· Ice or high aerosol load

· Corrected for glint

· Contaminated by glint

· Dense dark vegetation

· Cloud Top Pressure and Surface pressure do not match.

· Pressure product Lower than ECMWF pressure over land or bright scattering over sea.

2.7.2.2.7 Annotation data set

Annotation data provided in the Level 2 product are identical to those of the Level 1b product.

All annotation data will be provided at equally spaced tie points every 16 by 16 pixels in Reduced Resolution and every 64 by 64 Full Resolution pixels, which is equivalent to a 19km by 19 km grid on ground.

The following meteorological data provided by ECMWF will be resampled to the tie point grid: :

· Mean sea level atmospheric pressure

· Surface wind speed

· Relative Humidity

· Total column ozone

The following geographical information will be provided with respect to the Earth reference model WGS84:

· Latitude, Longitude

· Altitude and topographically corrected localisation

· Bathymetry

The following geometrical information will be provided with respect to the Earth reference model WGS84 :

· Solar illumination zenithal angle

· Solar illumination azimuthal angle

· Viewing zenithal angle

· Viewing azimuthal angle

The annotation data povided in the near real time products will consist of meteorological forcasts and, geographical and geometric information computed using a predicted satellite orbit state vector.

The annotation data povided in the off-line (consolidated) products will consist of meteorological analysis data and, geographical and geometric information computed using a restituted satellite orbit state vector.

For further details regarding the MERIS Level 2 data products see Product Spreadsheets.