Remote sensing of absorption and scattering coefficient using neural network model: Development, validation, and application

Abstract

The total absorption (a(λ)) and backscattering (bb(λ)) coefficients of natural waters are the most significant factors affecting light propagation within water columns, and thus play indispensable roles in the estimation of aquatic biomass, primary production, and carbon pools. Despite its importance, no accurate retrieval model has been specifically developed for both oceanic and coastal waters, but significant efforts have been made in regard to oceanic inversion models. The objectives of the present study are to evaluate the applicability of the quasi-analytical algorithm (QAA) in deriving a(λ) and bb(λ) from oceanic and coastal waters, and to improve it using a neural network-based semi-analytical algorithm (NNSAA). Based on a comparison of the a(λ) and bb(λ) predicted by these models with field measurements taken from the national aeronautics and space administration bio-optical marine algorithm dataset (NOMAD), the Yellow Sea and China East Sea, it is shown that the NNSAA model (R2>0.82 and mean relative error, MRE=20.6–35.5%) provides a stronger performance than the QAA model (R2<0.73 and MRE=32.2–69.6%). The model was also applied to MODIS data after atmospheric correction using a near-infrared-based and shortwave infrared-based combined model. Through validation by field measurements, it was shown that the NNSAA model can predict a(λ) and bb(λ) with high accuracy (R2>0.77 and MRE<39.9%). Finally, the NNSAA model was used to map the global climatological seasonal mean a(443) and bb(555) for the time range of July 2002 to September 2013. Except the coastal zones, it was shown that the a(443) and bb(532) in some high-latitude areas are much higher than in the mid- and low-latitude regions, due to the effects of spurious signals from neighboring sea-ice. In the equatorial oceans, the a(443) value in the surface water is considerably higher in the equatorial Pacific than in the equatorial Atlantic in the upwelling region, while the integrate a(443) is much higher in the Atlantic than in the throughout the entire tropical gyre areas. The difference between a(443) and bb(532) in the subsurface water is due to a pronounced deep biomass maximum existing in the equatorial Atlantic, which is associated with the higher nitrate in the lower euphotic zone.