Optical Algorithm for Cloud Shadow Detection Over Water

The application of ocean color product retrieval algorithms for pixels containing cloud shadows leads to erroneous results. Thus, shadows are an important scene type that should be identified and excluded from the set of clear-sky pixels. In this paper, we present an optical cloud shadow-detection technique called the Cloud Shadow Detection Index (CSDI). This approach is for homogeneous water bodies such as deep waters where shadow detection is very challenging due to the relatively small differences in the brightness values of the shadows and neighboring sunlit or some other regions. The CSDI technique is developed based on the small differences between the total radiances reaching the sensor from the shadowed and neighboring sunlit regions of similar optical properties by amplifying the differences through integrating the spectra of the two regions. The Integrated Value (IV) is then normalized by the mean of the IVs within a spatial adaptive sliding box where atmospheric and marine optical properties are assumed homogeneous. Assuming that the true color and the IV images represent accurate shadow locations, the results were visually compared. The CSDI images agree reasonably well with the corresponding true color and the IV images over open ocean. Also, the shape of the cloud shadow particularly for the isolated cloud closely follows that of the cloud, as expected, reconfirming the potential of the CSDI technique.