Analysis of hot spot directional signatures measured from space

Abstract

[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.

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

“…In contrast to the ocean, land surfaces usually provide higher reflectance values, although the directional characteristic (e.g. hot spot over vegetation) is less pronounced (Baldridge et al, 2009;Bréon et al, 2002). However, the brightness of the various surface types can vary substantially for different wavelengths as shown for example by the results from 1.6 µm channel of the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument (Justice et al, 2002).…”

“…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).…”

Evaluation of various observing systems for the global monitoring of CO<sub>2</sub> surface fluxes

“…Over land, we used the MODIS albedo product, which is a good approximation of the reflectance for typical viewing conditions. For the particular case of A-SCOPE, the albedo was multiplied by a factor of 2, because the backscatter (or Hot-Spot) effect has to be taken into account for the lidar viewing geometry (Bréon et al, 2002).…”

End-to-End Simulation for a Forest-Dedicated Full-Waveform Lidar Onboard a Satellite Initialized from Airborne Ultraviolet Lidar Experiments

“…The second one is the desired phase function P whereas the third one explicitly accounts for the hot spot [1 + R(γ)]. This model was tested on many data sets from airborne and spaceborne measurements performed with the Polarization and Directionality of the Earth Reflectance (POLDER) instrument [80,81]. As demonstrated by Bicheron and Leroy [82] and Maignan et al [83], it is quite reliable on the bidirectional observations.…”