An assessment of cloud masking schemes for satellite ocean colour data of marine optical extremes

One of the most important steps in utilizing ocean colour remote-sensing data is subtracting the contribution of the atmosphere from the signal at the satellite to obtain marine water-leaving radiance. To be carried out accurately, this requires clear-sky conditions, i.e. all clouds need to be excluded or masked from the data prior to atmospheric correction. The standard cloud mask used routinely in the processing of NASA global ocean colour data is based on a simple threshold applied to the Rayleigh-corrected top-of-atmosphere (TOA) radiance. The threshold is kept purposefully low to ensure high-quality processing at a global scale. As a consequence, the standard scheme can sometimes inadvertently mask important extreme optical events such as intense blue-green algal (cyanobacteria) blooms or the outflow of sediment-rich waters from some of the world's largest rivers. However, the importance of these extreme conditions, both for ecological and hydrological applications, requires that they should be appropriately monitored. Therefore, an assessment of existing cloud masking schemes that could provide valuable alternatives was carried out. A new hybrid cloud mask was also proposed and similarly tested. The selected schemes were systematically assessed over a full annual cycle of satellite ocean colour data on three example regions: the Baltic Sea, the Black and Azov Seas, and the Amazon River delta. The results indicate that the application of alternative cloud masking schemes produces a significant increase in clear-sky diagnostics that varies with the scheme and the region. Major occurrences of extreme optical conditions, such as cyanobacteria blooms, or river deltas formerly excluded from any processing may be recovered, but some schemes may underestimate the amount of thin clouds potentially detrimental to ocean colour atmospheric correction.