The cause of the hot spot in vegetation canopies and soils: Shadow-hiding versus coherent backscatter

Hapke, Bruce; Dimucci, D.; Nelson, Robert M.; Smythe, W. D.

doi:10.1016/0034-4257(95)00257-x

Cited by 122 publications

(54 citation statements)

References 18 publications

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“…This partially comes from terrestrial limb and directionality of the vegetation reflectance effects. The latter are complicated (Privette et al 1995;Hapke et al 1996), due for instance to mutual shadow effects between trees.…”

Section: Discussionmentioning

confidence: 99%

A test for the search for life on extrasolar planets

Arnold

Gillet²,

Lardière³

et al. 2002

A&A

152

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Abstract. We report spectroscopic observations (400−800 nm, R ≈ 100) of Earthshine in June, July and October 2001 from which normalized Earth albedo spectra have been derived. The resulting spectra clearly show the blue colour of the Earth due to Rayleigh diffusion in its atmosphere. They also show the signatures of oxygen, ozone and water vapour. We tried to extract from these spectra the signature of Earth vegetation. A variable signal (4 to 10 ± 3%) around 700 nm has been measured in the Earth albedo. It is interpreted as being due to the vegetation red edge, expected to be between 2 to 10% of the Earth albedo at 700 nm, depending on models. We discuss the primary goal of the present observations: their application to the detection of vegetation-like biosignatures on extrasolar planets.

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Section: Discussionmentioning

confidence: 99%

A test for the search for life on extrasolar planets

Arnold

Gillet²,

Lardière³

et al. 2002

A&A

152

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“…Another aspect is the difference in the wavelength regions, which cause higher MODIS values, between 1.5% and 15%, compared to TROPOLEX measurements depending on the surface type (according to ASTER). Furthermore the lidar configuration always measures the "hotspot" (view angle is equal to the light incident angle) (Hapke et al, 1996;Bréon et al, 2002), whereas the MODIS measurement geometry normally differs from this particular case, since the MODIS viewing angle related to the sun zenith angle is variable. For most surface types this leads to higher TROPOLEX values (see below).…”

Section: Comparison To Modis Datamentioning

confidence: 99%

Airborne lidar reflectance measurements at 1.57 μm in support of the A-SCOPE mission for atmospheric CO<sub>2</sub>

et al. 2009

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Abstract. The characteristics of the lidar reflectance of the Earth's surface is an important issue for the IPDA lidar technique (integrated path differential absorption lidar) which is the proposed method for the spaceborne measurement of atmospheric carbon dioxide within the framework of ESA's A-SCOPE project. Both, the absolute reflectance of the ground and its variations have an impact on the measurement sensitivity. The first aspect influences the instrument's signal to noise ratio, the second one can lead to retrieval errors, if the ground reflectance changes are strong on small scales. The investigation of the latter is the main purpose of this study. Airborne measurements of the lidar ground reflectance at 1.57 µm wavelength were performed in Central and Western Europe, including many typical land surface coverages as well as the open sea. The analyses of the data show, that the lidar ground reflectance is highly variable on a wide range of spatial scales. However, by means of the assumption of laser footprints in the order of several tens of meters, as planned for spaceborne systems, and by means of an averaging of the data it was shown, that this specific retrieval error is well below 1 ppm (CO 2 column mixing ratio), and so compatible with the sensitivity requirements of spaceborne CO 2 measurements. Several approaches for upscaling the data in terms of the consideration of larger laser footprints, compared to the one used here, are shown and discussed. Furthermore, the collected data are compared to MODIS ground reflectance data.

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“…Coherent backscatter is a wavelength-scale hot spot mechanism that is important for lunar soil and other barren solar system surfaces [Hapke et al, 1993]. The very few measurements of the relative importance of shadow hiding and coherent backscatter for terrestrial vegetated surfaces suggest that while shadow hiding dominates for most plants and clumpy moist soils, coherent backscatter can dominate for finely structured plants such as moss and for fine dry soils [Hapke et al, 1996]. Laboratory studies indicate that coherent backscatter generates a peak of width less than 0.5°varying proportionally to wavelength [Hapke et al, 1993].…”

Section: Coherent Backscattermentioning

confidence: 99%

“…It is based on the fact that portions of wave fronts that are multiply scattered within a nonuniform medium and follow the same path, but in opposite directions, combine constructively at zero phase angle. Laboratory experiments suggest that shadow hiding dominates the hot spot for most terrestrial surfaces [Woessner and Hapke, 1987;Hapke et al, 1996].…”

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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The cause of the hot spot in vegetation canopies and soils: Shadow-hiding versus coherent backscatter

Cited by 122 publications

References 18 publications

A test for the search for life on extrasolar planets

A test for the search for life on extrasolar planets

Airborne lidar reflectance measurements at 1.57 μm in support of the A-SCOPE mission for atmospheric CO<sub>2</sub>

Analysis of hot spot directional signatures measured from space

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The cause of the hot spot in vegetation canopies and soils: Shadow-hiding versus coherent backscatter

Cited by 122 publications

References 18 publications

A test for the search for life on extrasolar planets

A test for the search for life on extrasolar planets

Airborne lidar reflectance measurements at 1.57 μm in support of the A-SCOPE mission for atmospheric CO&lt;sub&gt;2&lt;/sub&gt;

Analysis of hot spot directional signatures measured from space

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Product

Resources

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Airborne lidar reflectance measurements at 1.57 μm in support of the A-SCOPE mission for atmospheric CO<sub>2</sub>