October 18, 2013



POLDER 2 advanced algorithms

The "color" of the ocean is determined by the interactions of incident light with the different substances present in water. Besides pure sea water, the optically most significant substances are free-floating photosynthetic organisms (phytoplankton), detritus, dissolved material and inorganic particulates. Since the success of the first ocean color instrument, (CZCS), the interpretation of ocean color in terms of phytoplankton pigments (the chlorophyll, Chl) is now well recognized. The Chl data collected by CZCS between 1978 and 1986, then by OCTS and POLDER from 08/96 to 06/97, and now by SeaWiFS since September 1997, have been used in many research areas such as global biogeochemical cycles studies, dynamical processes studies, and fisheries. This wide field of application can be attributed to the fact that observation of ocean color from space has the advantage to offer a synoptic and repetitive coverage over a wide area of the ocean surface.

About the ocean color processing

The processing transforms the Level-1 data, normalized radiances observed by the POLDER radiometer, into geophysical properties, at Level-2, daily, and Level-3, a composite over a 10-day period (a "decade"), or a month. The derived geophysical properties are available in two level 2 products : the directional product (OC2A) contains directional marine reflectances at 443, 490, 565 and 670 nm. The non-directional product (OC2B) contains marine reflectances at 443, 490, 565 and 670 nm, chlorophyll-a concentration of the surface water, bio-optical parameters such as absorption and backscattering coefficients. By-products of the atmospheric correction algorithm like aerosol optical thickness and angstrom exponent are also made available in the two products. For more details, see List of Level2 and List of Level3 parameters.

It should be noticed that the POLDER-2 algorithm includes some new important improvements:

    a new way to deal with the so-called "black pixel assumption" for atmospheric correction over eutrophic or case-2 waters,
    new L2 parameters like water absorption and backscattering coefficients.

    The main steps of the processing (see flowchart for details) are presented in:

      Calibration adjustment
      Cloud detection
      Gaseous absorption and stratospheric aerosol correction
      Atmospheric scattering correction to derive the marine reflectances, and the aerosol parameters as by-products
      Bio-optical algorithm to derive Chlorophyll-a concentrations and bio-optical parameters
      Composite of Level-2 data over a 10-day period and a month

      ATBDs in pdf format are available for atmospheric scattering correction and bio-optical algorithm.

      Those algorithms have been developped by the Laboratoire d'Optique Atmosphérique (University of Lille) and by the Laboratoire des Sciences du Climat et de l'Environnment (CEA, Saclay).

      Any questions about them should be adressed to :

      Pierre-Yves Deschamps (at LOA)
      Jean-Marc Nicolas (nicolas@loa.univ-lille1.fr)
      Cyril Moulin (moulin@lsce.saclay.cea.fr)
      Hubert Loisel (loisel@mren2.univ-littoral.fr)

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