Sensitivity of multiangle, multispectral polarimetric remote sensing over open oceans to water-leaving radiance: Analyses of RSP data acquired during the MILAGRO campaign
Highlights
►A hydrosol model for numerical studies of polarized radiance emerging from open oceans is updated. ►Studies show this radiance to vary with viewing angle, wavelength λ, and Chlorophyll a amount [Chl]. ►Waterleaving radiance computations are reproduced in airborne photopolarimetric measurements. ►Bio-optical variation of polarized waterleaving radiance with [Chl] cannot be ignored if λ ≤ 470 nm. ►Natural variation of polarized waterleaving radiance with CDOM is negligible for [Chl] ≤ 1 mg/m3.
Introduction
The polarized intensity of light scattered by particles exhibits features as a function of wavelength and scattering angle that are distinctly different from those of the total scattered intensity (Coulson, 1988, Ulaby and Elachi, 1990, Videen et al., 2004). The polarized and total intensity features also exhibit very different sensitivities to particle properties such as size relative to the wavelength, shape, and composition (Hansen and Travis, 1974, Mishchenko et al., 2002). Furthermore, both sensitivities vary with particle properties. For example, the polarized component of light scattered by particles having a refractive index close to that of the surrounding medium, such as particulates suspended in the ocean, shows fewer features with scattering angle and less variation with shape and size than the corresponding features of light scattered by particles that have a strong refractive-index contrast with the surrounding medium such as atmospheric aerosols. Finally, the features in polarized intensity of singly scattered light are much less likely to be washed out by those of multiply scattered light than is the case for the features in total intensity (Hansen and Travis, 1974, Hovenier et al., 2004, Mishchenko et al., 2006, van de Hulst, 1980). This is because the magnitude of polarized intensity of light scattered n times decreases rapidly with n as compared with the magnitude of total intensity. These differences cause the retrieval of aerosol properties from remotely sensed polarization to be much more accurate than the corresponding retrievals from remotely sensed intensity, as has been demonstrated in theoretical studies (Hasekamp and Landgraf, 2005, Mishchenko and Travis, 1997a, Mishchenko and Travis, 1997b). Analyses of actual polarimetric remote sensing data obtained by the Polarization and Directionality of the Earth's Reflectance (POLDER; Deschamps et al., 1994) instrument confirm these studies – whether these analyses focus exclusively on POLDER observations (e.g., Bréon and Goloub, 1998, Herman et al., 2005), or on a comparison of aerosol retrievals from POLDER and from intensity-only remote sensing such as from the moderate resolution imaging spectrometer (MODIS; Barnes et al., 1998) instrument (Gérard et al., 2005).
The recognition of the advantages of polarimetric remote sensing (Hansen and Travis, 1974, Hovenier et al., 2004, Mishchenko et al., 2004, Mishchenko et al., 2006, Ulaby and Elachi, 1990, Videen et al., 2004) has led to the development of a new generation of satellite scanning instruments, such as the Aerosol Polarimetry Sensor (APS) instrument (Mishchenko et al., 2004, Mishchenko et al., 2007; see specifications in Section 2), that combine, and in many respects surpass (except for cross ground-track swath), the polarimetric and multiangle measurement strengths of POLDER and the multispectral measurement strengths of MODIS. The launch of such instruments will lead to more accurate retrievals of aerosols as demonstrated by Chowdhary et al. (2001), but it also puts more stringent requirements on the modeling of remote sensing data. For example, Chowdhary et al. (2002, 2005) show that for remote sensing over oceans the polarization of light measured in the visible part of the spectrum yields valuable information about the complex refractive index of fine mode aerosols provided that the contributions of polarized water leaving radiance are appropriately dealt with. They demonstrate that such contributions become very small for principal plane observations and consequently limited their analyses to such viewing geometries. Similar arguments were made by Chami et al., 2001, Harmel and Chami, 2008, although it was argued there from analyses for a single wavelength in the visible that polarized water leaving radiance over open oceans can be ignored for most viewing geometries available from space borne observations.
To provide a lower boundary condition for aerosol retrievals from polarized reflectance observations outside the principal plane over open oceans, Chowdhary et al. (2006, henceforth referred to as C2006) developed a hydrosol model that reproduces empirically observed variations in ocean albedos as a function of biomass concentration in the ocean and of the wavelength of observation, and that provides the corresponding angular variation in intensity and polarization of water leaving radiance. The primary purpose of this hydrosol model is therefore not for retrieval of marine particulates but to account for oceanic contributions in polarimetric remote sensing of aerosols as in Hasekamp et al. (2011). Nevertheless, one can change the microphysical and bio-optical properties prescribed for such particles in this model to mimic local (natural) variations and anomalous cases. The objectives of this paper are 1) to revisit this model and assess changes caused by variations in colored dissolved organic matter (CDOM) and in the scattering matrix of marine particulates, 2) to validate the water-leaving total and polarized radiance computed with this model against actual data obtained from aircraft at various altitudes and azimuth angles, and 3) to assess the sensitivity of spaceborne polarimetry over open oceans to variations in polarized water-leaving radiance. The organization of this paper is as follows. Section 2 provides a description of the airborne polarization data analyzed in this work, and of the field campaign in which these data were obtained. In Section 3, we evaluate the hydrosol model used in the analysis of underwater-light contributions to these data. The multiple scattering computations that link the hydrosol model to the polarization data are briefly reviewed in Section 4. Section 5 applies these computations to sensitivity studies of underwater light scattering, to analyses of the airborne polarization data, and to simulations of spaceborne polarization data. Finally, we summarize our results in Section 6.
Section snippets
Measurements
The primary measurements used in this work are obtained by the Research Scanning Polarimeter (RSP) instrument (Cairns et al., 1999), which is an airborne version of the APS satellite instrument (Peralta et al., 2007). The objectives of APS-like measurements are discussed and detailed in Mishchenko et al. (2007), and can be summarized as the retrieval of the optical thickness, size distribution, complex refractive index, and shape information for fine and coarse mode aerosols, as well as of the
Bulk ocean
To compute the scattering matrix and single scattering albedo of ocean waters, we consider these waters to be a bulk mixture of seawater, with scattering matrix Fw and scattering coefficient bw, and a particulate component, with scattering matrix Fp and scattering coefficient bp (see Table 1 for notation, dimension, and definition of variables). We follow the convention that a scattering matrix F has its scattering function referred to using the italic font face, i.e. F ≡ F11 (C2006), and the
Multiple-scattering computations
The total and polarized reflectance of radiation emerging from the top of an atmosphere–ocean system (AOS) are calculated as in C2006, where various numerical recipes used to increase the efficiency of computations are described. That is, we first use the doubling/adding method for radiative transfer computations of polarized light (de Haan et al., 1987, Hovenier et al., 2004) to obtain the reflection and transmission properties of the atmosphere and the ocean body, and the geometric-optics
Irradiance ratio sensitivity analysis
The graphs in Fig. 3a depict model variations of the bulk ocean irradiance ratio Ablk, and in Fig. 3b of the bulk ocean absorption coefficient ablk, with wavelength λ for various concentrations of Chlorophyll a. The spectral range evaluated is from 400 to 700 nm which covers those spectral bands used for aerosol remote sensing by RSP and APS-like instruments, and also the spectral bands typically used for remote sensing of ocean color (Esaias et al., 1998). Note however that our models do not
Summary
In a previous paper (Chowdhary et al., 2006), we developed a variable detritus-plankton (D–P) hydrosol model to compute the multi-angle and multi-spectral behavior of polarized light emerging from open oceans as a function of Chlorophyll a concentration [Chl]. The purpose of that model was to account for the polarized underwater light contribution to measurements made by spaceborne polarimeters. In this work, we reviewed the properties of this hydrosol model and made a correction for the
Acknowledgements
The MILAGRO/INTEX-B Campaign is a collaborative effort of a large number of participants with the support of multi-national agencies. We thank the governments of the Federal District, the States of Mexico, Hildalgo and Veracruz, and the Mexican Ministries of the Environment, Foreign Relations, Defense and Finance for their logistical support; IMP, U-Tecámac, and Rancho La Bisnega for hosting the supersites; and many other Mexican institutions for their support. We further extend our gratitude
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