54 / 2023-07-24 15:08:10

Estimation of absorption coefficient of oceanic waters via band difference of remote sensing reflectance and its applications

Absorption coefficient; chlorophyll concentration; multi-band difference; remote sensing reflectance; correction of residual error

Absorption coefficient (a) is an important optical property of the aquatic environment, which can be estimated from the spectrum of water color, commonly measured by the remote sensing reflectance (Rrs). In the past decades, the estimation of a was achieved via band ratio of Rrs or semi-analytical inversion of Rrs, approaches that are sensitive to noises in Rrs. Here we extended the band-difference scheme for chlorophyll concentration (Chl) of oceanic waters developed a decade ago to the estimation of a. As demonstrated earlier for the estimation of Chl, the product of a from the band difference of Rrs showed much smoother spatial pattern than that from a semi-analytical algorithm. More importantly, it is found that the upper limit of using band difference of Rrs can be extended from -0.0005 sr-1 (the upper limit set a decade ago for the estimation of Chl) to ~0.0005 sr-1 (corresponding to a(440) ~0.08 m-1), which covers ~91% of the global ocean. When a is known, there could be many applications in satellite ocean color remote sensing, which include the estimation of Chl as well as the correction of residual errors in Rrs. In particular, for Chl estimation, the band-difference approach can extend the upper limit from ~ 0.2 mg/m3 to ~0.5 mg/m3, thus to obtain smooth Chl image product for ~90% of the global ocean. Further, with known a, residual errors in Rrs can be well corrected, which then not only helps to provide better estimation of the backscattering coefficient, but also recovers more observations from satellite ocean color remote sensing.