Re-examining the effect of particle phase functions on the remote-sensing reflectance

Abstract

Even though it is well known that both the magnitude and detailed angular shape of scattering (phase function, PF), particularly in the backward angles, affect the color of the ocean, the current remote-sensing reflectance (R) models typically account for the effect of its magnitude only through the backscattering coefficient (b). Using 116 volume scattering function (VSF) measurements previously collected in three coastal waters around the U.S. and in the water of the North Atlantic Ocean, we re-examined the effect of particle PF on R in four scenarios. In each scenario, the magnitude of particle backscattering (i.e., b) is known, but the knowledge on the angular shape of particle backscattering is assumed to increase from knowing nothing about the shape of particle PFs to partially knowing the particle backscattering ratio (B), the exact backscattering shape as defined by β˜(γ≥90°) (particle VSF normalized by the particle total scattering coefficient), and the exact backscattering shape as defined by the χ factor (particle VSF normalized by the particle backscattering coefficient). At sun zenith angle=30°, the nadir-viewed R would vary up to 65%, 35%, 20%, and 10%, respectively, as the constraints on the shape of particle backscattering become increasingly stringent from scenarios 1 to 4. In all four scenarios, the R variations increase with both viewing and sun angles and are most prominent in the direction opposite the sun. Our results show a greater impact of the measured particle PFs on R than previously found, mainly because our VSF data show a much greater variability in B, β˜(γ≥90°), and χ than previously known. Among the uncertainties in R due to the particle PFs, about 97% can be explained by χ, 90% by β˜(γ≥90°), and 27% by B. The results indicate that the uncertainty in ocean color remote sensing can be significantly constrained by accounting for χ of the VSFs.

A Bidirectional Subsurface Remote Sensing Reflectance Model Explicitly Accounting for Particle Backscattering Shapes

“…Recent technological advancement (Lee & Lewis, ; Sullivan & Twardowski, ) has greatly expanded the collection, and the knowledge of natural variability, of the VSFs of oceanic particles (Chami et al, ; Sullivan & Twardowski, ; Zhang et al, ). These measurements showed that the natural variability of VSFs is much greater than what had been known or assumed previously (Xiong et al, ). In this study, we will utilize the new VSF dataset to parameterize r rs in an attempt to better understand the effect of VSF shapes on the BRDF.…”

“…Recent technological advancement (Lee & Lewis, 2003;Sullivan & Twardowski, 2009) 2017JC013313 2012. These measurements showed that the natural variability of VSFs is much greater than what had been known or assumed previously (Xiong et al, 2017). In this study, we will utilize the new VSF dataset to parameterize r rs in an attempt to better understand the effect of VSF shapes on the BRDF.…”

“…Studies have shown that the angular structure of bidirectional r rs varies with the shape of the actual volume scattering functions (VSFs; β , m −1 sr −1 ) in the ocean (Chami et al, ; Gordon, ; Mobley et al, ; Morel et al, ; Morel & Gentili, ; Xiong et al, ; Zaneveld, ); however, the current r rs models are built primarily using one particle phase function (Hoge et al, ; Morel & Gentili, ) or a linear combination of two (Hlaing et al, ; Lee et al, ; Morel et al, ; Park & Ruddick, ), because the VSFs had been scarcely measured (Morel et al, ). Recent technological advancement (Lee & Lewis, ; Sullivan & Twardowski, ) has greatly expanded the collection, and the knowledge of natural variability, of the VSFs of oceanic particles (Chami et al, ; Sullivan & Twardowski, ; Zhang et al, ).…”

“…Studies have shown that the angular structure of bidirectional r rs varies with the shape of the actual volume scattering functions (VSFs; b, m 21 sr 21 ) in the ocean (Chami et al, 2006a;Gordon, 1989;Mobley et al, 2002;Morel & Gentili, 1991;Xiong et al, 2017;Zaneveld, 1982Zaneveld, , 1995; however, the current r rs models are built primarily using one particle phase function (Hoge et al, 2003;Morel & Gentili, 1991, 1996 or a linear combination of two (Hlaing et al, 2012;Lee et al, 2011;Park & Ruddick, 2005), because the VSFs had been scarcely measured . Recent technological advancement (Lee & Lewis, 2003;Sullivan & Twardowski, 2009) 2017JC013313 2012.…”

Ocean Color Analytical Model Explicitly Dependent on the Volume Scattering Function

“…The Zhang et al [84] results are also inconsistent with the phase functions used in previous BRDF models, such as M02, which have been shown to have satisfactory performance in validation efforts [17,18], even in many Case II coastal waters [19,90]. The recent theoretical r rs modeling study of Xiong et al [91] based on the phase function shapes presented in [84], where up to 50% disagreement with L11 was observed, consequently appear unrealistic.…”