Normalized Diffuse Attenuation Coefficient of Downwelling Irradiance for Satellite Ocean Color Remote Sensing

The diffuse attenuation of downwelling irradiance, K_d (m^-1), is an important property related to light availability in aquatic ecosystems. The standard K_d(490) product (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 to coastal regions. More importantly, as an apparent optical property (AOP), K_d is affected by the angular distribution of light field (e.g. solar zenith angle). The empirically derived product thus contains ambiguities when compared with in situ measurements. To overcome these shortcomings, we proposed a new term named as normalized diffuse attenuation coefficient (nK_d, m^-1), which is the K_d in the absence of the atmosphere and with the sun at the zenith. Models were developed to get nK_d for application with in situ measurements and from satellite ocean color remote sensing. Results using field measurements indicated that the new nK_d product will improve the quality and applicability of K_d and maximize the value offered by satellite ocean color remote sensing.