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The surface albedo quantifies the fraction of the sunlight reflected by the surface of the Earth. Following the definition of the black-sky albedo as an Essential Climate Variable by GCOS we offer this type of albedo as retrieved from SPOT data here. The black-sky albedo or directional albedo or directional-hemispherical reflectance is the integration of the bi-directional reflectance over the viewing hemisphere. It assumes all energy is coming from a direct radiation from the sun, it is computed for a specific time.
Daily top-of-canopy reflectances are normalized of a 30-day long synthesis period by inversion of a semi-empirical linear kernel-driven reflectance model; the used weighting function favours the most recent daily observations of the compositing 30-day period. Spectral directional and hemispherical albedos are estimated using the spectral coefficients resulting from the model inversion and the angular integral of kernel functions pre-computed and stored in look-up-tables. Finally broadband albedos are calculated by linear relationship with spectral albedos over visible ([0.4µm-0.7µm]), near-infrared ([0.7µm-4µm]), and total shortwave ([0.3µm-4µm]) bands, weighted by the spectral irradiance.
For more information we refer to the ATBD mentioned in the references section.
This is version 1.4/1.5 of this data set provided by COPERNICUS Global Land Service / VITO.
Data are provided by VITO as global netCDF files with 1/112° grid cell resolution. Data were block-averaged onto a 0.5° x 0.5° plate carree grid. Data are available for the latitude range 60°S to 80°N. Note, however, that limited solar illumination reduces the maximum northern latitude to smaller values during winter.
Last update of data set at ICDC: September 6, 2019.
Period and temporal resolution:
Spatial coverage and resolution:
The original data set at 1/112° grid resolution contains a bit-wise to read quality flag. The content of the flag can be read under http://land.copernicus.eu/global/products/sa under "Technical". The information given here kind of overlaps with the flag values given in the data products giving the three choices: invalid pixels <--> values above the maximum physical value <--> values below the minimum physical value.
The 0.5° x 0.5° data set offered here does not contain a separate flag layer because it was found difficult to translate the fine-resolution flag information to the coarser grid. Hence simply only the valid 1/112° x 1/112° grid points are used to compute the average value of the albedo values and their uncertainties. As can be seen in the parameters section we give number of land pixels, number of valid pixels, and number of usable satellite data to convey information about reliability. We recommend to read the text given in the netCDF files.
The SPOT / PROBA-V albedo products have been validated following the recommendations of the CEOS Land Product Validation (LPV) group. The validation activities comprise inter-comparison with independent albedo products on global and regional scale (e.g. with the BELMANIP-2 network) and with surface-based observations. Both, the spatiotemporal consistency and the accuracy are good. The spatial distribution is highly reliable. There are reliable seasonal and inter-annual variations and also the magnitude between low and high values is reliable. Over snow-free areas the RMSE is about 0.03. Temporal profiles are found to be very smooth with an inter-annual precision of 0.005. Data gaps and less reliable retrievals are found in high northern latitudes, and over ice and snow-covered surfaces. Also over desert sites the seasonal variation is not reliable. More information about the validation efforts can be found in the validation report listed under references.
Copernicus' Scientific and Technical Support:
Upon using this data please cite as follows:
The product was generated by the land service of Copernicus, the Earth Observation programme of the European Commission. The research leading to the current version of the product has received funding from various European Commission Research and Technical Development programmes. The product is based on SPOT/VGT 1km Collection 2 and PROBA-V 1km Collection 1 data ((c) CNES and distributed by VITO), last access date: 05/08/2019, provided on 0.5 degree x 0.5 degree plate carree grid in NetCDF file format by the Integrated Climate Data Center (ICDC, icdc.cen.uni-hamburg.de) University of Hamburg, Hamburg, Germany.