EPS Surface Albedo (ETAL, LSA-103)

NRT Product Availble since Jan 2015

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Land surface albedo is a key variable for characterising the energy balance in the coupled soil-vegetation-atmosphere system. The albedo quantifies the part of the energy that is absorbed and transformed into heat and latent fluxes. Owing to strong feedback effects the knowledge of albedo is important for determining weather conditions at the atmospheric boundary layer. Climate sensitivity studies with Global Circulation Models have confirmed the unsteady nature of the energy balance with respect to small changes in surface albedo. Other domains of applications are in hydro-meteorology, agro-meteorology and environment-related studies.

 

Product Documentation

This operational product is documented in the following documents:

Please see Product Peer-Review publications in References.

The use of LSA SAF products in publications is kindly requested to be duly acknowledged:
ETAL albedo was provided by the EUMETSAT Satellite Application Facility on Land Surface Analysis (LSA SAF; Trigo et al., 2011)
http://lsa-saf.eumetsat.int

Trigo, I. F., C. C. DaCamara, P. Viterbo, J.-L. Roujean, F. Olesen, C. Barroso, F. Camacho-de Coca, D. Carrer, S. C. Freitas, J. García-Haro, B. Geiger, F. Gellens-Meulenberghs, N. Ghilain, J. Meliá, L. Pessanha, N. Siljamo, and A. Arboleda, 2011: The Satellite Application Facility on Land Surface Analysis. Int. J. Remote Sens., 32, 2725-2744, doi: 10.1080/01431161003743199

The ETAL (EPS Ten-daily ALbedo) provides time composites of the surface albedo based on clear-sky measurements from the Advanced Very High Resolution Radiometer (AVHRR) on-board EUMETSAT polar system satellites, the Metop series. The ETAL albedo product is generated each 10 days in a 1km global sinusoidal grid centred at (0°N, 0°W). An iterative scheme allows the composition of the information with a characteristic time scale of 20 days. The ETAL filename date refers to the end date of this 20 days period.

The product is based on the AVHRR three short-wave channels (VIS 0.63 µm, NIR 0.865 µm, SWIR 1.6 µm). In addition to the corresponding narrowband estimates, broadband albedo is derived for the visible, near-infrared and total short-wave wavelength ranges. Information on cloud cover is obtained from the output of the Nowcasting and Very Short Range Forecasting Satellite Application Facility (NWC SAF) software. Dynamic information on the atmospheric pressure and water vapour content comes from the ECMWF numerical weather prediction model. Climatological values are currently used for ozone concentration and aerosol optical thickness.

 

Algorithm Description

In a first step the cloud-free reflectance observations of each slot are corrected for atmospheric effects using the simplified radiative transfer code SMAC (Rahman and Dedieu, 1994). Afterwards, the linear kernel-driven BRDF model from Lucht et al. (2000) is fitted to a daily time series of the resulting top-of-canopy reflectance factor values. Albedo estimates are then obtained by suitably integrating the BRDF model functions. Generated products are the directional-hemispherical (or "black-sky") albedo at local solar noon for the visible, near-infrared and total short-wave wavelength ranges as well as the bi-hemispherical (or "white-sky") albedo for the total shortwave range.

 

Data Characteristics

The ETAL product is available every 10-days (at the 5th, 15th, and 25th of each month) in a 1km global sinusoidal grid centred at (0°N, 0°W). For each release, the albedo quantities, their respective error estimates, and a processing flag are disseminated in HDF5 format. The relevant information concerning the data fields is included in the HDF5 attributes.

 

Product Uncertainties

For each albedo quantity the algorithm delivers theoretical estimates for the non-correlated (random) error contribution by propagating estimates for the error of the top-of-canopy reflectance factors through the linear model inversion. In practice non-detected clouds and systematic errors in the aerosol optical thickness values used for the atmospheric correction are the most important error sources. The specification for the overall accuracy of the albedo product is 10%. Results of detailed validation studies are given in the Validation Report.

Carrer, D., Smets, B., Ceamanos, X., RoujeanW. H. , J.-L., and R. Lacaze (2018), ‘Copernicus Global Land SPOT/VEGETATION and PROBA-V surface albedo products - 1km Version 1’, Algorithm Theoretical Basis Document, Issue 2.11, Copernicus Global Land Operations “Vegetation and Energy” ”CGLOPS-1”, Framework Service Contract N° 199494 (JRC).


Carrer, D., Roujean, J.-L. & Meurey, C. Comparing operational MSG/SEVIRI land surface albedo products from Land SAF with ground measurements and MODIS. IEEE Transactions on Geoscience and Remote Sensing 48, 1714–1728 (2010).


Geiger, B., Carrer, D., Franchisteguy, L., Roujean, J.-L. & Meurey, C. Land surface albedo derived on a daily basis from Meteosat second generation observations. IEEE Transactions on Geoscience and Remote Sensing 46, 3841–3856 (2008).

Rahman, H., & Dedieu, G. (1994). SMAC: a simplified method for the atmospheric correction of satellite measurements in the solar spectrum. Remote Sensing15(1), 123-143.