Validation of Remote-Sensing Algorithms for Diffuse Attenuation of Downward Irradiance Using BGC-Argo Floats

Estimates of the diffuse attenuation coefficient (K-d) at two different wavelengths and band-integrated (PAR) were obtained using different published algorithms developed for open ocean waters spanning in type from explicit-empirical, semi-analytical and implicit-empirical and applied to data from spectral radiometers on board six different satellites (MODIS-Aqua, MODIS-Terra, VIIRS-SNPP, VIIRS-JPSS, OLCI-Sentinel 3A and OLCI-Sentinel 3B). The resultant K(d)s were compared to those inferred from measurements of radiometry from sensors on board autonomous profiling floats (BGC-Argo). Advantages of BGC-Argo measurements compared to ship-based ones include: 1. uniform sampling in time throughout the year, 2. large spatial coverage, and 3. lack of shading by platform. Over 5000 quality-controlled matchups between K(d)s derived from float and from satellite sensors were found with values ranging from 0.01 to 0.67 m(-1). Our results show that although all three algorithm types provided similarly ranging values of K-d to those of the floats, for most sensors, a given algorithm produced statistically different K-d distributions from the two others. Algorithm results diverged the most for low K-d (clearest waters). Algorithm biases were traced to the limitations of the datasets the algorithms were developed and trained with, as well as the neglect of sun angle in some algorithms. This study highlights: 1. the importance of using comprehensive field-based datasets (such as BGC-Argo) for algorithm development, 2. the limitation of using radiative-transfer model simulations only for algorithm development, and 3. the potential for improvement if sun angle is taken into account explicitly to improve empirical K-d algorithms. Recent augmentation of profiling floats with hyper-spectral radiometers should be encouraged as they will provide additional constraints to develop algorithms for upcoming missions such as NASA's PACE and SBG and ESA's CHIME, all of which will include a hyper-spectral radiometer.

Automated summary

This study compared different algorithms for estimating the diffuse attenuation coefficient (K-d) using data from various satellites and measurements from autonomous profiling floats. The results showed that although the algorithms provided similar values to the floats, they produced statistically different distributions. The discrepancies were attributed to the limitations of the datasets used for algorithm development and the neglect of sun angle in some algorithms. The study highlights the importance of using comprehensive field-based datasets and considering sun angle for algorithm improvement.