Evaluation of Ocean Color Remote Sensing Algorithms for Diffuse Attenuation Coefficients and Optical Depths with Data Collected on BGC-Argo Floats

Abstract

The vertical distribution of irradiance in the ocean is a key input to quantify processes spanning from radiative warming, photosynthesis to photo-oxidation. Here we use a novel dataset of thousands local-noon downwelling irradiance at 490 nm (Ed(490)) and photosynthetically available radiation (PAR) profiles captured by 103 BGC-Argo floats spanning three years (from October 2012 to January 2016) in the world’s ocean, to evaluate several published algorithms and satellite products related to diffuse attenuation coefficient (Kd). Our results show: (1) MODIS-Aqua Kd(490) products derived from a blue-to-green algorithm and two semi-analytical algorithms show good consistency with the float-observed values, but the Chla-based one has overestimation in oligotrophic waters; (2) The Kd(PAR) model based on the Inherent Optical Properties (IOPs) performs well not only at sea-surface but also at depth, except for the oligotrophic waters where Kd(PAR) is underestimated below two penetration depth (2zpd), due to the model’s assumption of a homogeneous distribution of IOPs in the water column which is not true in most oligotrophic waters with deep chlorophyll-a maxima; (3) In addition, published algorithms for the 1% euphotic-layer depth and the depth of 0.415 mol photons m−2 d−1 isolume are evaluated. Algorithms based on Chla generally work well while IOPs-based ones exhibit an overestimation issue in stratified and oligotrophic waters, due to the underestimation of Kd(PAR) at depth.

Chlorophyll‐Based Model to Estimate Underwater Photosynthetically Available Radiation for Modeling, In‐Situ, and Remote‐Sensing Applications

“…The sea‐surface iPAR(0 − ) is retrieved following Xing et al. (2020), and the layer‐averaged diffuse attenuation coefficient at depth z , K d (iPAR, z ), is defined as: …”

“…Finally, only Flag-1 (18,359, 51.4%) and Flag-2 (5,315, 14.9%) profiles are used in this study. The sea-surface iPAR(0 − ) is retrieved following Xing et al (2020), and the layer-averaged diffuse attenuation coefficient at depth z, K d (iPAR, z), is defined as:…”

“…Xing et al. (2020) have analyzed its performance in the global oceans with an extensive BGC‐Argo data set and found it works well near the sea surface, but exhibits a significant underestimation in subsurface oligotrophic waters. This is not surprising as Lee05's algorithm assumes constant IOPs independent of depth, and in oligotrophic waters the presence of a deep chlorophyll maxima (DCM) increases the attenuation of light at depth.…”

Radiometry on Argo Floats: From the Multispectral State-of-the-Art on the Step to Hyperspectral Technology

“…Argo radiometric data plays further a significant role in increasing the amount of data available for satellite matchups in remote areas (Organelli et al, 2017a;Wojtasiewicz et al, 2018), as well as for evaluating the performance of ocean color remote sensing retrievals (Organelli et al, 2017a;Xing et al, 2020). Thus, a further exploitation of the Argo platform capabilities is reasonable to enhance the utilization of satellite data in terms of product retrievals (with less uncertainties).…”

Deep chlorophyll maximum and nutricline in the Mediterranean Sea: emerging properties from a multi-platform assimilated biogeochemical model experiment

“…calibration and measurement errors (Bittig et al, 2019;Xing et al, 2020). Data assimilation (DA) aims to increase knowledge and representation of processes by integrating models with information obtained from observations.…”