October 18, 2013



POLDER 2 algorithms

The POLDER instrument presents original features since it is not only multispectral but also multidirectional and multipolarization. Algorithms dedicated to "Earth Radiation Budget, Water Vapor, and Clouds" (hereafter noted "ERB, WV & Clouds") were developed, taking into account these capabilities. More particularly,

    the multi-polarization allows to determine the cloud thermodynamic phase and the cloud top pressure,
    the multi-directionality improves the derivation of the cloud optical thickness and the estimate of the reflected flux,
    the multi-spectrality allows to derive the cloud middle pressure and the clear-sky water vapor content.

    The first algorithms of the "ERB, WV & Clouds" processing line were applied to the eight months of ADEOS 1 - POLDER data. These algorithms are described in Buriez et al. (1997). First results are presented in Parol et al. (1999). Further publications can be found in the bibliography. An improved version of the algorithms will be applied to ADEOS 2 - POLDER data. This second version has already been applied to some ADEOS 1-POLDER data. The major improvements are :

      a better spatial resolution of the "ERB, WV & clouds" products : In the first version, calculations were made at the full resolution but results were averaged over 9 x 9 pixels. Now, all of the products are provided at the 3 x 3 pixels resolution (18.5 km x 18.5 km). The reference Earth grid is thus the same for the cloud parameters and for the aerosol parameters derived from the POLDER "Aerosol" processing lines.
      a better cloud detection, especially in presence of dust events and over ice/snow surfaces.
      a reduction of the bias in total water vapor content by determining the reflectance ratio to water vapor parameterization directly with respect to SSMI water vapor observations and by taking into account the effect of surface spectral variability.
      a more complete determination of the cloud thermodynamic phase and introduction of a microphysical index (only for ice clouds) based on the polarization signature at 865 nm.
      a better retrieval of the ice cloud optical thickness by using an inhomogeneous hexagonal microphysics model instead of a liquid droplet microphysics model.
      a drastic reduction of the number of abnormally high values of retrieved cloud top pressure.
      a more accurate derivation of the spectral albedoes and the calculation of a monthly averaged reflected shortwave flux.

      The level 2 (orbital swath) products contain about thirty non-directional parameters and ten directional parameters (for each of the 14 viewing directions) (see list of level 2 parameters). The level 3 (monthly) products contain about forty parameters (see list of level 3 parameters).

      These "ERB, WV & clouds" products are presented in the following sections:

        CLOUD PROPERTIES (including cloud phase and microphysical index, cloud optical thickness, and cloud pressures)

        A more complete description can be downloaded from the "ERB, WV & clouds" algorithm documents for POLDER-2.

        Development of the algorithms results from a joint effort of Laboratoire d’Optique Atmosphérique (LOA), Laboratoire des Sciences du Climat et de l’Environnement (LSCE) and Laboratoire de Météorologie dynamique (LMD). It has been supported by CNES (Centre National d’Etudes Spatiales), CNRS (Centre National de la Recherche Scientifique) and Région Nord-Pas de Calais.

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