Abstract
Radiometric water products from the neural network (NNv2) in the alternative atmospheric correction (AAC) processing chain of Ocean and Land Colour Instrument (OLCI) data were assessed over different marine regions. These products, not included among the operational ones, were custom-produced from Copernicus Sentinel-3 OLCI Baseline Collection 3. The assessment benefitted of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> reference data from the Ocean Color component of the Aerosol Robotic Network (AERONET-OC) from sites representative of different water types. These included clear waters in the Western Mediterranean Sea, optically complex waters characterized by varying concentrations of total suspended matter and chromophoric dissolved organic matter (CDOM) in the northern Adriatic Sea, and optically complex waters characterized by very high concentrations of CDOM in the Baltic Sea. The comparison of the water-leaving radiances <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L_{\text {WN}}(\lambda)$ </tex-math></inline-formula> derived from OLCI data on board Sentinel-3A and Sentinel-3B with those from AERONET-OC confirmed consistency between the products from the two satellite sensors. However, the accuracy of satellite data products exhibited dependence on the water type. A general underestimate of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${L}_{\text {WN}}(\lambda)$ </tex-math></inline-formula> was observed for clear waters. Conversely, overestimates were observed for data products from optically complex waters with the worst results obtained for CDOM-dominated waters. These findings suggest caution in exploiting NNv2 radiometric products, especially for highly absorbing and clear waters.
Highlights
THE Ocean and Land Colour Instrument (OLCI, [1]) operates on board the Sentinel-3A (OLCI-A) and Sentinel3B (OLCI-B) satellites since February 2016 and April 2018, respectively
Large overestimates and inconsistent spectral shapes affect LNWNN( ) for the highly absorbing waters at Gustaf Dalen Lighthouse Tower (GDLT). This inconsistency of the spectral shape was already documented for the NNv1 products in the Baltic Sea by Kyryliuk and Kratzer [16], who reported a shift in the RRS( ) peak from 560 to 490 nm in near-coastal areas explained by a poorer performance of the atmospheric correction in the blue spectral region
Ocean and Land Colour Instruments (OLCI) NNv2 LNWNN( ) products in the 400-665 nm spectral region have been assessed using in situ AERONET-OC data representing different water types
Summary
THE Ocean and Land Colour Instrument (OLCI, [1]) operates on board the Sentinel-3A (OLCI-A) and Sentinel3B (OLCI-B) satellites since February 2016 and April 2018, respectively. The AAC combines two distinct neural networks (NNs), one performing the atmospheric correction to retrieve water reflectance ρWNNN ( ) and the other one quantifying the water inherent optical properties (IOPs) from ρWNNN ( ) [3] These IOP data products are used to determine the so-called OLCI NN operational products, i.e., the concentrations of total chlorophyll-a (CHL_NN) and of total suspended matter (TSM_NN), and the total absorption coefficient of chromophoric dissolved organic matter and nonpigmented particles at the 443 nm center-wavelength (ADG_443_NN). Even though the ρWNNN ( ) is not included among the OLCI operational data products, it can be generated by any user through the Case 2 Regional CoastColour (C2RCC) processor [3] and ideally applied in combination with local biooptical algorithms to generate regional data products This potential application of ρWNNN ( ) data indicates the need for a thorough assessment to support their confident exploitation
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