The ESA DUE GlobAlbedo Land Surface Albedo Products from European Sensors and their Analysis and Validation
Muller, Jan-Peter¹; López, Gerardo¹; Watson, Gill¹; Shane, Neville¹; Kennedy, Tom¹; Lewis, Philip¹; Fischer, Jürgen²; Guanter, Luis²; Domenech, Carlos³; Preusker, Réné³; North, Peter⁴; Heckel, Andreas⁴; Danne, Olaf⁵; Krämer, Uwe⁵; Zühlke, Marco⁵; Brockmann, Carsten⁵; Cescatti, Alessandro⁶; Pinnock, Simon^7
1UCL MSSL, UNITED KINGDOM; ²FUB, GERMANY; ³FUB, UNITED KINGDOM; ⁴Swansea University, UNITED KINGDOM; ⁵Brockmann Consult, GERMANY; ⁶JRC Ispra, ITALY; ⁷ESA-ESRIN, ITALY

A land surface broadband albedo map of the entire Earth's land surface, including snow, is required for use in Global Climate Model initialisation and verification as well as in hydrological modelling of Soil-Vegetation Transfer Schemes (SVATS). A group of 10 users were selected to work with the GlobAlbedo* Implementation team to define requirements and drive the project towards practical applications of the product.

To generate such a map by temporal compositing at 1km and lower resolution on both equal area and latitude, longitude grid requires both sufficient directional looks and the very precise correction of top-of-atmosphere radiances to ''at surface'' directional reflectances (SDRs). In addition, such a map requires precise radiometric calibration and inter-calibration of different sensors [1] and the computation of radiative transfer coefficients to derive broadband SDRs from different input narrowband SDRs and given sufficient angular sampling from all the directional looks within a given temporal window, derive a suitable BRDF and albedo products such as DHR (Direct Hemispherical Reflectance known as ''black-sky'') and BHR (BiHemispherical Reflectance, known as ''white-sky'') [2]. The final albedo product has been integrated in three spectral broadband ranges, namely the solar spectrum (400-3000nm), the visible (400-700nm) and the near- and shortwave-infrared (700-3000nm). In addition, maps of normalized difference vegetation index (NDVI) and fraction of absorbed photosynthetically active radiation (fAPAR) are being generated consistent with the albedo product to complement the Globalbedo data set for analysis of vegetation-related processes [3].

To achieve the aim of deriving independent estimates using European only assets, GlobAlbedo set out to create a 15 year time series by employing SPOT4-VEGETATION and SPOT5-VEGETATION2 as well as ENVISAT-MERIS. Legacy algorithms for deriving SDRs using an optimal estimation approach are outlined in [2] as well as the use of a prior estimation dataset called a ''prior'' using a novel system for gap-filling using ten year mean estimates derived from US sensors [2]. Each and every output pixel albedo value has an estimated uncertainty associated with it and the corresponding BRDF has a full variance-covariance matrix for each and every pixel.

Results from the processing of 14 years (1998-2012) will be shown together with an assessment of the accuracy of this prototype dataset using contemporaneous satellite and tower-based albedometer measurements from the FLUXNET network [3]. The processing chain has been set-up so that the product could, with suitable support, be carried forward using SPOT5-VEGETATION2 and Proba-Vuntil the launch of SENTINEL-3 when SLSTR and OLCI could be employed to take over.

The final GlobAlbedo product is available from http://www.GlobAlbedo.org/ including a detailed validation assessment, an ATBD and a Product User Guide, browse products of each and every 10° by 10° tile as well as animations of different products from all 1km 10° x 10° tiles as well as global products. The website also includes the ability to download products by subsetted point, tile or globally. Products are provided in netCDF (CF) format for easy ingest into GCM and SVATS. They are produced on 8-daily and monthly time-steps. The DHR-BHR can also be converted to ''Blue Sky'' given an AOD derived from satellite or from a ground-based sun photometer. Examples of this will be shown. The GlobAlbedo dataset has been analysed for different features associated with land surface changes such as droughts, impact of ephemeral snow and

References cited
[1] D. Potts, S. Mackin, J-P. Muller, N. Fox (2012). Sensor Intercalibration over Dome C for the ESA GlobAlbedo Project. IEEE Trans. Geosci. Rem. Sens. DOI: 10.1109/TGRS.2012.2217749

[2] GlobAlbedo_ATBD_V3.0 (2011). GlobAlbedo: Algorithm Theoretical Basis Document. Authors: P. Lewis, C. Brockmann, O. Danne, J. Fischer, L. Guanter, A. Heckel, O. Krueger, G. López, J-P. Muller, P. North, D. Potts, R. Preusker. Available from http://www.GlobAlbedo.org/

[3] Pinty, B., Jung, M., Kaminski, T., Lavergne, T., Mund, M., Plummer, S., Thomas, E., Widlowski, J.L., 2011. Evaluation of the JRC-TIP 0.01° products over a mid-latitude deciduous forest site. Remote Sens. Environ. 115, 3567-3581.

[4] Cescatti, A., Marcolla, B., Vannan, S.K.S., Pan, J.Y., Roman, M.O., Yang, X., Ciais, P., Cook, R.B., Law, B.E., Matteucci, G., Migliavacca, M., Moors, E., Richardson, A.D., Seufert, G., Schaaf, C.B., 2012. Intercomparison of MODIS albedo retrievals and in situ measurements across the global FLUXNET network. Remote Sens. Environ. 121, 323-334.

* work supported under ESA/ESRIN contract 22390/09/I-OL