Keping Du

An inherent-optical-property-centered approach to correct the angular effects in water-leaving radiance

Remote-sensing reflectance (R(rs)), which is defined as the ratio of water-leaving radiance (L(w)) to downwelling irradiance just above the surface (E(d)(0⁺)), varies with both water constituents (including bottom properties of optically-shallow waters) and angular geometry. L(w) is commonly measured in the field or by satellite sensors at convenient angles, while E(d)(0⁺) can be measured in the field or estimated based on atmospheric properties. To isolate the variations of R(rs) (or L(w)) resulting from a change of water constituents, the angular effects of R(rs) (or L(w)) need to be removed. This is also a necessity for the calibration and validation of satellite ocean color measurements. To reach this objective, for optically-deep waters where bottom contribution is negligible, we present a system centered on waters inherent optical properties (IOPs). It can be used to derive IOPs from angular Rrs and offers an alternative to the system centered on the concentration of chlorophyll. This system is applicable to oceanic and coastal waters as well as to multiband and hyperspectral sensors. This IOP-centered system is applied to both numerically simulated data and in situ measurements to test and evaluate its performance. The good results obtained suggest that the system can be applied to angular R(rs) to retrieve IOPs and to remove the angular variation of R(rs).

Usable solar radiation and its attenuation in the upper water column

University of Massachusetts Boston; NASA Ocean Biology and Biogeochemistry and Water and Energy Cycle Programs; JPSS VIIRS Ocean Color Cal/Val Project; National Natural Science Foundation of China [41071223, 40976068, 41121091]; Ministry of Science and Technology of China [2013BAB04B00]

Remote sensing of normalized diffuse attenuation coefficient of downwelling irradiance

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), which 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 semi-analytically 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. This article is protected by copyright. All rights reserved.

Attenuation of visible solar radiation in the upper water column: A model based on IOPs

For many oceanic studies, it is changes horizontally with constituents in the water required to know the distribution of visible solar (1, 5), but also changes with depth for any water (6, radiation (EPAR) in the upper water column. 7). One way to reach this is by remote sensing. This To represent the steeper than exponential includes two components: First, EPAR at surface reduction of EpAR with depth, multiple exponential is calculated based on atmosphere properties terms (6, 7) were usually adopted, with an along with the position of the Sun. Second, the attenuation coefficient (or attenuation depth) vertical attenuation of EPAR (KPAR) is derived assigned for each term. These attenuation from products of ocean-color remote sensing. coefficients are kept vertically constant, but Currently, KPAR is estimated based on horizontally vary with Jerlov (5) water types. chlorophyll concentration ((C)) from ocean Recently, simple and explicit models have been color. This kind of approach works well for developed to incorporate satellite-derived waters where all optical properties can be chlorophyll concentrations ((C)) into the adequately described by values of (C), but will description of the attenuation of EpAR. When (C) result in large uncertainties for coastal waters values are provided via satellite observations of where (C) alone cannot accurately describe the ocean color (8, 9), the partition factors and optical properties. In this paper, we present an attenuation coefficients of the terms could be innovative model that describes KPAR as a calculated (4). function of waters inherent optical properties Such kind of approach works for Case-i (lOP).

Secchi disk observation with spectral-selective glasses in blue and green waters

Radiative transfer modeling of Secchi disk observations has historically been based on conjugated signals of eye response and radiance, where waters attenuation in the entire visible band is included in the attenuation when deciding the Secchi disk depth in water. Aas et al. [Ocean Sci.10(2), 177 (2014)Remote Sens. Environ.169, 139 (2015)] hypothesized that it is actually the attenuation in waters transparent window that matters to the observation of a Secchi disk in water. To test this hypothesis, observations of Secchi disks in blue and green waters were conducted via naked eyes, blue-pass glasses, and green-pass glasses. Measurement results indicate that for blue waters, the observed Secchi depths via naked eyes match the depths obtained with blue-pass glasses and much deeper than the depths with green-pass glasses, although the green-pass glasses match the highest response of human eyes. These observations experimentally support the hypothesis that our eye-brain system uses the contrast information in the transparent window to make a judgement decision regarding sighting a Secchi disk in water.