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

Abstract

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 bluegreen 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 clearsky 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.

Marine boundary layer cloud property retrievals from high-resolution ASTER observations: case studies and comparison with Terra MODIS

“…ASTER band 3N reflectances R 0.86,A need to exceed distinct thresholds. Similar tests to identify clear-sky pixels have been reported by Ackerman et al (1998), Ackerman et al (2008), Frey et al (2008), and Banks and Mélin (2015) for MODIS observations, generally establishing thresholds of R 0.86,M < 0.03 for confidently clear and R 0.86,M > 0.065 for cloudy pixels.…”

“…This ratio utilizes the rather constant spectral behavior of clouds in the VNIR, which leads to their white appearance. Ackerman et al (1998) found thresholds of 0.8 < r 1 < 1.1, while Ackerman et al (2008) and Banks and Mélin (2015) reported adjusted lower thresholds of 0.85 and 0.95 for confidently clear and cloudy pixels, respectively. The upper threshold is usually set to 1.1, in part to exclude land surfaces.…”

Two decades observing smoke above clouds in the south-eastern Atlantic Ocean: Deep Blue algorithm updates and validation with ORACLES field campaign data

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An Evaluation of Autonomous In Situ Temperature Loggers in a Coastal Region of the Eastern Mediterranean Sea for Use in the Validation of Near-Shore Satellite Sea Surface Temperature Measurements

“…Satellite monitoring provides an essential part of this global observing network but also requires a comprehensive global network of trustworthy and accurate in situ measurements for validation and calibration. The present reality is that the actual sampling of a number of important climate-related variables, including temperature, is unevenly distributed in space or time with a large part of the coastal regions being poorly sampled [18][19][20].…”