Modeling of atmospheric effects on the angular distribution of a backscattering peak

Powers, B.J.; Gerstl, S.A.W.

doi:10.1109/36.7691

Cited by 14 publications

(3 citation statements)

References 9 publications

(2 reference statements)

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“…The widths of the hotspots in Fig. 3 are very similar, echoing the argument of Powers and Gerstl (1988) that the hotspot width is expected to be relatively invariant to atmospheric perturbations. Lines of different colors correspond to simulations using different values of N STREAMS ; in general, differences between these lines are pretty small, especially in the atmosphere without aerosol and when the viewing angle is less than 60°.…”

Section: Hotspot Comparisons and Brdf Reconstruction Accuracysupporting

confidence: 66%

An improved BRDF hotspot model and its use in VLIDORT for studying the impact of atmospheric scattering on hotspot directional signatures in the atmosphere

Xiong,

Liu,

Spurr

et al. 2024

Atmos. Meas. Tech.

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Abstract. The term “hotspot” refers to the sharp increase in the reflectance occurring when incident (solar) and reflected (viewing) directions almost coincide in the backscatter direction. The accurate simulation of hotspot directional signatures is important for many remote sensing applications. The RossThick–LiSparse–Reciprocal (RTLSR) bidirectional reflectance distribution function (BRDF) model is widely used in radiative transfer simulations, and the hotspot model mostly used is from Maignan–Bréon, but it typically requires large values of numerical quadrature and Fourier expansion terms in order to represent the hotspot accurately for its use coupled with atmospheric radiative transfer modeling (RTM). In this paper, we have developed a modified version based on the Maignan–Bréon's hotspot BRDF model that converges much faster numerically, making it more practical for use in the RTMs that require Fourier expansion of BRDF to simulate the top-of-atmosphere (TOA) hotspot signatures, such as in the RTM models using the Doubling–Adding or discrete ordinate method. Using the vector linearized discrete ordinate radiative transfer model (VLIDORT), we found that reasonable TOA–hotspot accuracy can be obtained with just 23 Fourier terms for clear atmosphere and 63 Fourier terms for atmosphere with aerosol scattering. In order to study the impact of molecular and aerosol scattering on hotspot signatures, we carried out a number of hotspot signature simulations with VLIDORT. We confirmed that (1) atmospheric molecule scattering and the existence of aerosol tend to smooth out the hotspot signature at the TOA and that (2) the hotspot signature at the TOA in the near-infrared is larger than in the visible, and its impact by surface reflectance is more significant. As the hotspot amplitude at the TOA with aerosol scattering included is smaller than that with molecular scattering only, the amplitude of hotspot signature at the surface is likely underestimated in the previous analysis based on the POLDER measurements, where the atmospheric correction was based on a single-scatter Rayleigh-only calculation. This modified model can calculate the amplitude of the hotspot accurately, and, as it agrees very well with the original RossThick model away from the hotspot region, this model can be simply used in conditions with and without hotspots. However, there are some differences in this modified model compared to the original Maignan–Bréon model for the scattering angles close to the hotspot point; thus, it may not be appropriate for those who need an exact representation of the hotspot angular signature.

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Section: Hotspot Comparisons and Brdf Reconstruction Accuracysupporting

confidence: 66%

An improved BRDF hotspot model and its use in VLIDORT for studying the impact of atmospheric scattering on hotspot directional signatures in the atmosphere

Xiong,

Liu,

Spurr

et al. 2024

Atmos. Meas. Tech.

View full text Add to dashboard Cite

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“…On the other hand, the reduced atmospheric transmission due to the aerosol load may lead to an underestimate of A and, in turn, the leaf reflectance. No significant effect is expected on the retrieved width, as was pointed out by Powers and Gerstl [1988]. For an aerosol optical thickness of 0.1, the underestimate of A and/or the leaf reflectance would be on the order of 30%, depending on the solar zenith angle.…”

Section: Atmospheric Effectsmentioning

confidence: 68%

“…This in turn should lead to the leaf chemical properties such as chlorophyll concentration [Jacquemoud and Baret, 1990]. It should be noted also that the hot spot width is expected to be relatively invariant to atmospheric perturbations, such as aerosol loading [Powers and Gerstl, 1988]. Several shadow-hiding models have been elaborated, in the context of studies of planetary regolith [Hapke, 1986] or terrestrial vegetation [Kuusk, 1985[Kuusk, , 1995Jupp and Strahler, 1991;Chen and Leblanc, 1997].…”

Section: Introductionmentioning

confidence: 99%

Analysis of hot spot directional signatures measured from space

Bréon

2002

J. Geophys. Res.

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[1] Reflectance measurements from the spaceborne Polarization and Directionality of Earth Reflectances (POLDER) instrument are used to analyze the so-called hot spot directional signature in the backscattering direction. The hot spot is measured with an angular resolution better than half a degree using the directional capabilities of the radiometer, with some assumptions on the spatial homogeneity of the surface. The analysis yields the first quantitative observation of the hot spot signature of vegetated surfaces, with such angular resolution. The measurements show that the hot spot reflectance is a function of the phase angle x rather than a function of a parameter Á, often used in hot spot modeling, that quantifies the horizontal distance between Sun and view directions. The observed directional signature is very accurately fitted by a linear ratio of the phase angle, as predicted by a simple theory of radiative transfer within the canopy foliage. Most of the measured hot spot half widths are between 1°and 2°. Some dispersion occurs for the cases belonging to the forest and desert International Geosphere-Biosphere Program (IGBP) classes, in the range 1°to 5°. Theory predicts that the width is independent of wavelength. Our measurements indicate that the widths at 670 and 865 nm are very close, but with a significant scatter in regards to the rather small variability. The distribution of the width as a function of the IGBP surface classification shows a variability within the classes that is larger than between the classes, except for the ''evergreen broadleaf'' class. The hot spot reflectance amplitude is generally on the order of 0.10-0.20 at 865 nm and 0.03-0.18 at 670 nm, although the full range of values is wider. For thick canopies, it may be interpreted in terms of foliage element (leaf ) reflectance. Retrieved values are on the order of 0.4 in the near infrared and in the range 0.05-0.20 at 670 nm. At 440 nm, the amplitude of the signature is very small, as is expected from the small surface reflectance. This confirms that the atmospheric contribution to the reflectance increase at the backscattering direction is negligible.

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Radiative transfer in three-dimensional atmosphere-vegetation media

Myneni

Asrar²

1993

Journal of Quantitative Spectroscopy and Radiative Transfer

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Modeling of atmospheric effects on the angular distribution of a backscattering peak

Cited by 14 publications

References 9 publications

An improved BRDF hotspot model and its use in VLIDORT for studying the impact of atmospheric scattering on hotspot directional signatures in the atmosphere

An improved BRDF hotspot model and its use in VLIDORT for studying the impact of atmospheric scattering on hotspot directional signatures in the atmosphere

Analysis of hot spot directional signatures measured from space

Radiative transfer in three-dimensional atmosphere-vegetation media

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