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. 2019 Oct 22;6:223. doi: 10.1038/s41597-019-0236-x

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© The Author(s) 2019

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Fig. 4 — Logic and thresholds for assigning pixel quality level (QL) flags to L2P SSTs. “Invalid data” means input data are flagged as invalid. “Land” means the centre of the pixel is not over ocean. “Bad NWP” means tests on the integrity of the prior fields are failed. P_clear is the posterior probability of the pixel being clear-sky over ice-free ocean. “sens” is the evaluation of sensitivity of the retrieval to true changes in SST. χ² is the channel-normalised result of a test on the goodness of fit achieved in retrieval (applies to optimally estimated SST only). θ_sat is the satellite zenith angle of the retrieval. “twilight” means the solar zenith angle at the target pixel is within 5° of 90°. “DDI” refers to an index for presence of desert (mineral) dust aerosol (available for ATSRs only). The P_clear threshold of 0.90 for designation as QL > 3 is for the ATSRs and AVHRRs at night-time, and for AVHRRs observations during daytime is instead 0.99.