CLOUD DETECTION DESCRIPTION
First, the cloud detection method is based on a series of threshold tests applied to each individual pixel and to every viewing direction. Four tests aim at detecting clouds and a pixel is declared cloudy if one of these tests proves positive:
- An apparent pressure Papp is derived from the ratio of reflectance measured in the channels centered at 763 nm and 765 nm. The pixel is labeled cloudy if Papp is markedly lower than the meteorological surface pressure (Vanbauce et al., 1998).
- A pixel is declared cloudy if the measured reflectance at wavelength , ( = 865 nm over ocean and = 443 nm over land) is significantly larger than its clear-sky estimate . In the new algorithm, a threshold value depending on spatial variability is chosen in order to avoid classifying aerosols as clouds.
- For scattering angles less than 140°, the molecular optical thickness of the atmospheric layer above the observed surface (cloud or sea-surface) is directly derived from the polarized reflectance at 443 nm. The pixel is declared cloudy when this molecular optical thickness is smaller than that of clear atmosphere.
- The polarized radiance at 865 nm presents features different for clear-sky and for liquid water cloud conditions notably in the rainbow direction. A pixel is identified as cloud-contaminated if the actual polarized radiance is outside the expected range for clear-sky conditions.
If all of the previous tests prove negative, two more tests are added in order to identify the clear pixels:
- A pixel that has not been declared cloudy is labeled as clear if - ( = 865 nm over ocean and = 443 nm over land) is weak enough (< 2 %).
- Finally, a pixel is expected to be clear if its reflectance presents a large spectral variability. Over ocean, a pixel is declared cloud-free if the R865/R443 ratio is less than 0.35. This ratio has to be more than 2.2 over land surface.
Over snow-covered or iced surfaces test (ii) is replaced by another one combining measured reflectance at 765nm and apparent pressure Papp.
Moreover, the tests (ii) to (vi) are only applied when the viewing direction is outside the sunglint region.
If a POLDER pixel fails all the tests described above, it remains unclassified for a given viewing direction. A multidirectional approach is then used in order to labeled the undetermined pixels as cloudy or clear. Finally the pixel remaining unclassified is labeled as clear or cloudy depending on the classification of the neighboring pixels and on the spatial variability of ( = 865 nm over ocean and = 443 nm over land).
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