Remote sensing of normalized diffuse attenuation coefficient of downwelling irradiance

Abstract

The diffuse attenuation of downwelling irradiance, Kd (m−1), is an important property related to light penetration and availability in aquatic ecosystems. The standard Kd(490) product (the diffuse attenuation coefficient at 490 nm) of the global oceans from satellite remote sensing has been produced with an empirical algorithm, which limits its reliability and applicability in coastal regions. More importantly, as an apparent optical property (AOP), Kd is a function of the angular distribution of the light field (e.g., solar zenith angle). The empirically derived product thus contains ambiguities when compared with in situ measurements as there is no specification regarding the corresponding solar zenith angle associated with this Kd(490) product. To overcome these shortcomings, we refined the Kd product with a product termed as the normalized diffuse attenuation coefficient (nKd, m−1), that is equivalent to the Kd in the absence of the atmosphere and with the sun at zenith. Models were developed to get nKd from both in situ measurements and ocean color remote sensing. Evaluations using field measurements indicated that the semianalytically derived nKd product will not only remove the ambiguities when comparing Kd values of different light fields but will also improve the quality of such a product, therefore maximizing the value offered by satellite ocean color remote sensing.

Decreasing Atmospheric CO2 During the Late Miocene Cooling

“…The calculation of light at depth was done using a model of penetration of PAR from surface to depth (Buiteveld, 1995; Murtugudde et al, 2002), monthly climatologies of PAR, and the diffuse attenuation coefficient for downwelling irradiance at 490 nm (Kd490). This is combined with Equation 1 in Lin et al (2016) to calculate PAR at each depth. For Site 1088, light is assumed to be at a constant value of 25 μE/m 2 /s, which corresponds to a depth during the growing season of 40–50 m. For late Pleistocene data from Site 1266, a value of 50 μE/m 2 /s is assumed, in agreement with doubled incident light at this latitude in modern‐day observations (Frouin et al, 2018).…”

Reconciling Between Optical and Biological Determinants of the Euphotic Zone Depth

“…Separately, because the wavelengths most relevant for phytoplankton photosynthesis are blue-green (Dubinsky et al, 1986), which is also the spectral domain penetrating deepest in the ocean (Lee et al, 2013;Nielsen, 1952;Nielsen & Hansen, 1961), vertical profiles of usable solar radiation at time t (USR(z,t), λ = 400-560 nm) were also calculated from E d (λ,z,t) following Lee et al (2014). USR basically collectively describes this photosynthetically significant radiometric energy in aquatic environment (Lee et al, 2014;Lin et al, 2016).…”

Wind and rainfall regulation of the diffuse attenuation coefficient in large, shallow lakes from long‐term MODIS observations using a semianalytical model

“…Deriving a and b b is the first step of this semianalytical method. A quasi-analytical algorithm (QAA) [Lee et al, 2002] developed for estimation of a and b b was proposed to retrieve K d by Lee et al [2005bLee et al [ , 2013Lee et al [ , 2014 and has been widely used to estimate K d (e.g., K d (490), K d (USR), and K d (λ)) in global ocean waters [Lee et al, 2005b[Lee et al, , 2007[Lee et al, , 2013[Lee et al, , 2014Lin et al, 2016]. However, the poor performance of QAA in b b (λ) prediction (estimation uncertainty of >50%) in highly turbid coastal waters may lead to inaccuracy in estimations of absorption and K d (490) [Zhang et al, 2010;Chen et al, 2014].…”