The albedo is the fraction of the incoming solar radiation reflected by the land surface, integrated over the whole viewing directions. The albedo can be directional (calculated for a given sun zenith angle) or hemispheric (integrated over the whole illumination directions), spectral (for each narrow band of the sensor) or broadband (integrated over the whole solar spectrum).
MEDIAS-France and EARS assess the various albedo products using different approaches applied to VEGETATION and METEOSAT measurements.

Broadband hemispheric surface albedo derived from VEGETATION data over South-West of France, August 2000

MEDIAS-France generates many albedo products from VEGETATION / SPOT data using the FP5 / CYCLOPES processing line. Observations acquired during a compositing period are first cloud-screened and corrected for atmospheric effects. Then, they are directionnally normalized by inversion of a 3-parameter linear bidirectional reflectance model (Roujean et al., 1992) following the approach presented in Hagolle et al. (2005). The inversion yields three coefficients, a nadir-zenith reflectance, a geometric and a volumetric coefficients, used to estimate directional, hemispheric, spectral and broadband albedos.
MEDIAS-France defines the customization specifications to fit the ONC requirements which needs 0.5°x0.5° tiled averages (mean and standard deviation values) maps of broadband visible and near-infrared hemispheric albedos for 8 vegetation classes ("conifer evergreen forest", "deciduous forest", "broadleaf evergreen forest","grassland C3","grassland C4", "crops C3", "crops C4"). The number of valid pixels available to compute the average is also provided, because it is useful to assess the product quality.

Broadband visible surface albedo derived from METEOSAT images over Euro-Mediterranean region, central 10-day period of July 1998.

The surface albedo is also a product of the EWBMS (Energy Water Balance Monitoring System) database provided by EARS. It is derived from METEOSAT visible (0.3-1.5 µm) images, so its spatial resolution is 5 km sub-satellite. The first step of the methodology consists in calibrating the noon visible METEOSAT image to obtain the daily planetary albedo. The planetary albedo is related to the surface albedo by means of a two-flux radiation transmission model, modified after Kondratyev (1969). In this model the turbidity of the atmosphere is parameterized by the atmospheric optical depth. One pair of corresponding planetary and surface albedo values is required to determine the atmospheric optical depth. A darkest pixel approach is used, which relates the minimum planetary albedo of land pixels occurring in the image, to the lowest land surface albedo, usually present over forest. The value of atmospheric optical depth thus obtained is assumed to apply as a first order atmospheric correction to the whole image. Knowing this value, the actual atmospheric correction consists in converting all planetary albedo to surface albedo. In this way, a daily surface albedo map is obtained. Finally, as the daily surface albedo is not expected to change fast, but may be sub-pixel cloud contaminated, a 10-day lowest value surface albedo composite is generated, which is supposed to represent the cloud free situation.

References

Hagolle, O., A. Lobo, P. Maisongrande, F. Cabot, B. Duchemin, A. De Peyreira, Quality assessment and improvement of temporally composited products of remotely sensed imagery by combination of VEGETATION 1&2 images, Remote Sensing of Environment, vol.94, 2, 172-186, 2005.

Kondratyev, K.Y., Radiation in the atmosphere, New York, London: Academic Press, 1969.

Roujean J.L., M. Leroy, and P.Y. Deschamps, A bidirectional reflectance model of the Earth’s surface for the correction of remote sensing data, Journal of Geophysical Research, 97, D18, 20,455-20,468, 1992.

Surface Albedo

References