Principle
This test involves 2 POLDER channels of different wide (a narrow one of 10 nm and a broader one of 40 nm) centered on the oxygen « A » absorption band at 765 nm. The ratio of the reflectances measured in each of theses 2 channels is a good indicator of atmospheric oxygen absorption (Bréon and Bouffiès, 1996). From this ratio, an apparent pressure of the reflector can be derived that is a physical quantity directly comparable to surface pressure. This comparison allows concluding about the presence of a cloud layer since these scattering layers (in particular high clouds) have a large effect on the apparent pressure (Bréon and Colzy, 1999). The pixel is declared cloudy if the apparent pressure is markedly lower than surface pressure:
Psurf-Papp>
P
where Papp is result of average over all available viewing angles such as to reduce random uncertainties.
An NDVI dependant threshold
A clear surface shows a spectral signature that affects the apparent pressure (lower apparent pressure over the vegetation). As a consequence, the threshold depends on the vegetation coverage, quantified by the NDVI. Bréon and Bouffiès, 1996 highlighted that the difference Psurf-Papp is NDVI dependant. An empirical representation of Psuf-Papp=f(NDVI) for clear sky cases enabled to advance a linear relation:
P = a*NDVI + b
with a=60 and b=120.
Use of meteorological surface pressure
In POLDER first algorithms, the surface pressure considered the simplified hypothesis of a uniform standard atmosphere, accounting only for the surface altitude. Advanced algorithm makes use of a more realistic surface pressure derived from meteorological data (ECMWF).
Blue channel reflectance
In presence of liquid water clouds, the reflected radiance shows a maximum in the 142° scattering direction and this maximum is highly polarized. This property is exploited through multi-directional and polarized POLDER data. The test on the presence of a polarized rainbow is realized in the 865 nm channel, the less contaminated by atmosphere (molecules and aerosols) single scattering. If R865pol within 142° is significantly larger than R865pol away from this direction, the pixel is declared cloudy.
Principle
While ground reflectance increases with wavelength in visible and near-infrared, cloud reflectance is roughly uniform within these windows. Thus the contrast between ground and cloud reflectances is the largest in the blue channel that is the shorter wavelength POLDER channel. However, this test cannot be applied to raw Rblue measurements as long as the molecular diffusion dominates the signal and screens the ground signature. Also, the observed pixel is declared cloudy by this test if blue reflectance corrected from molecular diffusion is above a given threshold, blue :
Rblue-Rblue_mol > blue
A reference base of ground Rblue_min
Whereas in first algorithm, the blue channel test was rather simple with only a coarse distinction between dense vegetation covers (blue=0.10) and deserts (blue=0.15), advanced algorithm introduces a finer tuning of the threshold taking into account the spatial variability of surface reflectance. A base of surface reflectances Rblue_min has been constructed using the hemispheric reflectances calculated at POLDER Level 3 over the 8 months of POLDER1/ADEOS1 data. The pixel is declared cloudy when the observed blue reflectance, corrected for molecular scattering, is larger than a spatially varying threshold :
Rblue-Rblue_mol > Rblue_min+’blue
where ’blue is fixed to 0.05, supposing that reflectance increase due to vegetation growth during annual cycle do not exceed this rate.
Polarization rainbow
In presence of liquid water clouds, the reflected radiance shows a maximum in the 142° scattering direction and this maximum is highly polarized. This property is exploited through multi-directional and polarized POLDER data. The test on the presence of a polarized rainbow is realized in the 865 nm channel, the less contaminated by atmosphere (molecules and aerosols) single scattering. If R865pol within 142° is significantly larger than R865pol away from this direction, the pixel is declared cloudy.
Snow covers
Since snow has a large radiance in the blue wavelength and shows a spectral signature very similar to that of the clouds (in the spectral range covered by POLDER), the blue channel test fails, declaring snow pixels as cloudy. Although is appears not possible to unambiguously distinguish thin clouds from partial snow, it is useful to distinguish at least thick clouds from uniform snow covered region.
In configuration of :
negative Papp testnegative polarized rainbow testpositive Rblue testlarge reflectance at 670 nmpixel identified snow or ice by meteorological data
The pixel is put back to clear.