Cloud Masking Over Land

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.
Four 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. The third 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.