August 25, 2014

Land Surfaces Products

In addition to the generation of Aerosols and Clouds products by Icare, the multi-angular Polder-3/Parasol measurements are also processed by the Service Centre of the Land Surface Thematic Centre POSTEL to assess the radiative properties of continents, and a vegetation index, indicator of the ecosystems phonological state. The Land Surface Polder-3/Parasol products can be downloaded through the POSTEL web site.

The Land Surface algorithms are specified by LSCE/CEA and CNRM/Météo-France, the Expertise Centres of POSTEL. An algorithmic document is available.

First, the clouds are removed and the effects of atmospheric components (aerosols, water vapour, and ozone) absorption and diffusion are corrected to retrieve the land surface reflectances. Acquired under viewing angles up to 60°-70°, and for the full azimuth range, these data provide an unique accurate sampling of the land surface bi-directional reflectance distribution function (BRDF). They will be exploited soon to create a Polder-3/Parasol BRDF databases.

Then, the multi-angular spectral surface reflectances are normalized using the kernel-driven reflectance model of Maignan et al. (2004). The inversion is carried out over a synthesis period of 30 days with a sliding window of 10 days. The inversion yields three coefficients: a nadir-zenith reflectance, a geometric and a volumic coefficients. They are used with the angular kernels integrated over viewing directions and over solar directions to calculate the directional (see image below) and the hemispheric spectral albedo, successively. Values of integrated kernels are specified by CEA/LSCE. The spectral integrations over visible broadband [400 - 700nm], near-infrared broadband [700, 4000nm], and whole spectrum broadband [300 - 4000nm] are then carried out using the 5 narrow bands (490, 565, 670, 765 and 865nm). The conversion coefficients are provided by the team MATIS from CNRM/Météo-France. The validation of Polder-3/Parasol albedo products is in progress at CNRM/Météo-France.

Spectral albedo

At last, the NDVI is calculated from the directional albedos at 670nm and 865nm, in order to remove the variations due to changes in angular conditions. This NDVI is corrected for directional effects.

One year of Parasol NDVI
One year of Parasol NDVI

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