POLDER 2 - OCEAN COLOR

Cloud Masking

Over the ocean, cloud detection is much easier than over land surfaces because the surface is a relatively uniform and dark background. This background generates a nice contrast with the clouds. As a consequence, a simple reflectance based threshold on the longer wavelength channel measurement eliminates most of the clouds. On the other hand, it is desirable for aerosol objectives to keep aerosol contaminated pixels. Aerosol, and in particular large dust events, may yield a significant reflectance, comparable to that generated by thin or broken clouds. It is then not possible to distinguish clouds and aerosol with a simple threshold on the reflectance.
Analysis of satellite reflectance images show that the aerosol fields are much more spatially homogeneous than the cloud fields. This observation suggests the use of a simple test based on the spatial variability of the reflectance. We make use of a simple threshold on the 3x3 pixels standard deviation. The cloud detection over the ocean is based on the combination of a simple rough test on the top of the atmosphere reflectance, and another test on the spatial variability of the reflectance. We found that the later is the primary test (i.e. few clouds are detected only by the former).