Penetration of UV‐visible solar radiation in the global oceans: Insights from ocean color remote sensing

Abstract

[1] Penetration of solar radiation in the ocean is determined by the attenuation coefficient (K d ()). Following radiative transfer theory, K d is a function of angular distribution of incident light and water's absorption and backscattering coefficients. Because these optical products are now generated routinely from satellite measurements, it is logical to evolve the empirical K d to a semianalytical K d that is not only spectrally flexible, but also the sunangle effect is accounted for explicitly. Here, the semianalytical model developed in Lee et al. (2005b) is revised to account for the shift of phase function between molecular and particulate scattering from the short to long wavelengths. Further, using field data collected independently from oligotrophic ocean to coastal waters covering >99% of the K d range for the global oceans, the semianalytically derived K d was evaluated and found to agree with measured data within $7-26%. The updated processing system was applied to MODIS measurements to reveal the penetration of UVA-visible radiation in the global oceans, where an empirical procedure to correct Raman effect was also included. The results indicated that the penetration of the blue-green radiation for most oceanic waters is $30-40% deeper than the commonly used euphotic zone depth; and confirmed that at a depth of 50-70 m there is still $10% of the surface UVA radiation (at 360 nm) in most oligotrophic waters. The results suggest a necessity to modify or expand the light attenuation product from satellite ocean-color measurements in order to be more applicable for studies of ocean physics and biogeochemistry.

Determination of the Downwelling Diffuse Attenuation Coefficient of Lake Water with the Sentinel-3A OLCI

“…Extensive research has been conducted to develop algorithms for estimating K d for waters. Most of this research focuses on oceanic waters and can be classified into three types: (1) The type I research concerns the establishment of an empirical relationship between K d and the chlorophyll a (Chla) concentration [16] based on the observation that the optical properties of oceanic waters are mainly affected by phytoplankton; (2) The type II research focuses on the establishment of relationships between K d (490) and the inherent optical properties and boundary conditions (e.g., the solar elevation angle and water surface conditions) of water using the radiative transfer theory-based semi-analytical algorithm [4,[17][18][19][20]; (3) The type III research involves the establishment of statistical relationships between K d (490) and the apparent optical properties (e.g., the water-leaving radiance, L w , and remote sensing reflectance, R rs ) [15,[20][21][22]. The algorithms developed in these three types of research are mainly applicable to open oceanic waters and slightly turbid coastal waters.…”

Joint retrieval of aerosol and water-leaving radiance from multispectral, multiangular and polarimetric measurements over ocean

“…Accurate modeling of these processes is necessary (Mobley, 2008;Zhai et al, 2015) but requires additional inputs and computations that can significantly slow down the retrievals (Mobley, 2011b). To optimize the trade-off between computational efficiency and numerical accuracy, the correction scheme proposed by Lee et al (2013) is used to quantify the contribution …”

“…Assuming an isotropic distribution of the radiance contributed by Raman scattering, the corrected reflection matrix of ocean and air-water interface system for use by Eq. (1) is Lee et al (2013). Fluorescence is neglected in our RT model due to its tiny contribution to TOA signals over open ocean, though it is known to have some impact on nLw at 685 nm (Gordon, 1979).…”

Differences in Rate and Direction of Shifts between Phytoplankton Size Structure and Sea Surface Temperature

“…Then, R rs (λ) was resampled at 10 MODIS bands (412, 443, 469, 488, 531, 547, 555, 645, 667, and 678 nm) from the original wavelength of the spectroradiometers using spline interpolation [25]. To estimate a ph (λ) from in situ R rs (λ), the latest version of the quasi-analytical algorithm version 6 (QAA-v6) [33] was applied to the observed R rs (λ).…”