Cloud Screening Method for Ocean Color Observation Based on the Spectral Consistency

The study discusses a new supplementary method of masking cloud-affected pixels in satellite ocean color imageries. Pixels, typically found around cloud edge, sometimes have anomalous features either in chlorophyll a concentration or in-water reflectance estimates caused by residual error of inter-band registration correction, or more generally, by differences in the band-wise field-ofview of the detectors. Our method is to check the pixel-wise consistency over the spectral water reflectance RW retrieved by the atmospheric correction. We define two spectral ratio between water reflectance, IRR1 and IRR2, each defined as RW(B1)/RW (B3) and as RW(B2)/RW(B4) respectively, where B1~B4 stand for 4 consecutive visible bands. We show that almost linear relation holds over log-scaled IRR1 and IRR2 for ship-measured RW data of SeaBAM in situ data set. Similar relation with a little more variability is also shown for SeaWiFS and GLI Level 2 sub-scenes. We then introduce a new cloud screening criterion that identifies those pixels that have significant discrepancy from the relationship. We apply this method to ADEOS-II/GLI ocean color data to evaluate the performance over Level-2 data, showing that it saves significant portion of near-cloud pixels yet giving chlorophyll a concentration averages in the near-cloud area that is very close to the ones in “far-cloud” pixels, or pixels that are 5 or more pixels far from the cloud edge. The method will be applicable to other satellite ocean color sensors. ∗ Corresponding author.