Abstract
Accurately removing atmospheric interferences and retrieving water-leaving reflectance are decisive for subsequent ocean color applications. Over turbid waters, the black pixel assumption at the near-infrared (NIR) spectral region does not hold, and shortwave infrared (SWIR)-based atmospheric correction algorithm should be applied. Turbidity index is proposed to detect turbid waters and worked as a criterion for NIR-SWIR combined algorithm. However, studies demonstrated that turbidity index did not work well as expected. This study, using simulated data and satellite images, aimed to revisit the effectiveness of turbidity index for the switching scheme of NIR-SWIR algorithm. The simulated data were obtained from aerosol look-up tables, and the Aqua MODIS images were used. The variations of turbidity index calculated from aerosol reflectance and Rayleigh-corrected reflectance were explored. Results showed that turbidity index did not obey the assumption that it should be close to one over clear waters with negligible NIR water-leaving reflectance; its value calculated from simulated aerosol reflectance ranged from 0.7 to 2.2; and the turbidity index values varied depending on fine-mode fraction, aerosol optical thickness, relative humidity and observing geometries. Therefore, more effective switching scheme should be developed for the NIR-SWIR combined atmospheric correction algorithm.
NSTL
Elsevier