Cloud masking of SeaWiFS images over coastal waters using spectral variability

Cloud masks developed in the frame of ocean color missions are usually based on the assumption that the marine reflectance is close to zero in the near-infrared (NIR). This is valid over the open ocean, but coastal (Case-2) waters may have a higher NIR reflectance due to suspended matter and non-maritime aerosols. Cloud-free pixels are sometimes classed as clouds, leading to a loss of data. We present an algorithm, based on standard ocean color wavelengths, that makes use of the lower spectral variability of clouds compared to water. Images from different coastal areas have been used to develop and test the algorithm and a radiative transfer model has been used for a numerical sensitivity analysis. The algorithm shows a good performance in many of the tested scenes, and using this algorithm instead of the standard SeaWiFS NIR threshold will increase the amount of data over Case-2 waters.