Retrievals of tropospheric aerosol properties and vegetation cover over continental surfaces are only possible over cloud-free regions. The first step in satellite data processing thus requires a reliable cloud detection scheme based on the specific radiometric signature of clouds. Such a scheme is implemented in POLDER processing and takes advantage of POLDER multi-spectral and multi-directional specific capabilities. The scheme consists in a simple cloud filtering based on a threshold method.

Five tests are applied to each pixel at POLDER Level 2 (full resolution grid). These tests are non-exclusive and one proved positive is sufficient to declare the pixel cloudy. The first one compares the apparent pressure in oxygen band to the surface pressure. The second one is a threshold on the reflectance in the blue channel. Directional variability of the surface reflectance is also considered in a third test to eliminate partial cloud covers. The fourth consists in detection of the presence of polarized rainbow, indicative of the presence of water droplets. Once all these tests applied, some of the pixels declared cloudy by the blue channel test are considered again in order to prevent from abusive elimination of actually clear pixels. It is highly probable to be the case above snow covers. This kind of difficulty is also encountered with the apparent pressure test above dense forests. Finally, pixels adjacent to a detected cloud are considered to be contaminated and are rejected for further processing.

Performances of this cloud detection scheme have been evaluated against ground-based sky observations. For more details about this validation or about the detection method itself, the reader is referred to Bréon and Colzy, 1999. Hereafter follows a brief presentation of each one of these cloud detection tests implemented in Polder Level 2 data processing for Land Surfaces thematic.

Principle

This test involves 2 POLDER channels of different width (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. A large difference is indicative of 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
In POLDER Class 1, a=60 and b=140. For Class 2, a finest matching is applied with b=120.

Use of meteorological surface pressure

In POLDER Class 1, the surface pressure considered the simplified hypothesis of a uniform standard atmosphere, accounting only for the surface altitude. Class 2 makes use of a more realistic surface pressure derived from meteorological data (ECMWF).

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 (443 nm) that is the shortest wavelength POLDER channel. However, this test cannot be applied to raw R443 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 443 nm reflectance corrected from molecular diffusion is above a given threshold, 443:
R443-R443mol > 443

A reference base of ground R443min

Whereas in Class 1, the blue channel test was rather simple with only a coarse distinction between dense vegetation covers (443=0.10) and deserts (443=0.15), Class 2 introduces a finer tuning of the threshold taking into account the spatial variability of surface reflectance. A base of surface reflectances R443min has been constructed using the hemispheric reflectances calculated at Polder Level 3 over the 8 months of POLDER-1 data. The pixel is declared cloudy when the observed blue reflectance, corrected for molecular scattering, is larger than a spatially varying threshold:
R443-R443mol > R443min+'443 where '443 is fixed to 0.05, supposing that reflectance increase due to vegetation growth during annual cycle do not exceed this rate.

Principle

If TOA reflectance is rather isotropic in presence of a continuous cloud cover, this is no more the case with broken cloudiness. Such a mixed meteorological situation, when occurring at Polder pixel scale (6x6 km²), is difficult to capture with the sole other four cloud tests. Adding in the cloud detection algorithm a criteria based on directional variability of earth reflectance (in the blue channel) provides a good tool for recognition of these sub-pixel cloud cover. This method is only possible thanks to POLDER multi-directional acquisition.

Empirical directional RMS threshold

Variability of directional reflectance measurements is estimated through the RMS of the distance to a polynomial fit established over all viewing geometry configurations (hereafter referred as RMSdir). The criteria applied to statute whether the considered pixel contains partial clouds or not is a simple threshold. This threshold has been empirically determined by confrontation of the calculated POLDER directional variability and meteorological measurements in points of synoptic stations. If the RMSdir overcomes the given threshold, the pixel is classified as cloudy.

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.

Since snow has a large radiance in the 443 nm 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 test;
negative polarized rainbow test;
positive R443 test;
large reflectance at 670 nm;
pixel identified snow or ice by meteorological data;

The pixel is put back to clear.

Tropical forests show a specific spectral signature. The surface reflectance, averaged over the narrow "763 nm" channel is significantly larger than that on the wide "765 nm" channel, when the apparent surface pressure algorithm assumes a spectrally neutral surface. This signature yields false cloud detection. To correct this problem, the pixels that are declared cloudy by the Apparent Pressure test are reconsidered:

In configuration of:

positive Papp test;
negative polarized rainbow test;
negative R443 test;
large NDVI;

The pixel is put back to clear.