This data set is only available for a restricted user group, please contact us if you want to access these data.
RESTRICTED only accessable in CEN/MPI net or via CliSAP login What does that mean?
CLARA-A2 stands for "CM SAF cLouds, Albedo RAdiation data record, AVHRR-based, Edition 2" and comprises a suite of radiation and cloud parameters derived within the EUMETSAT Climate Monitoring Satellite Application Facility (CM-SAF) from Advanced Very High Resolution Radiometer (AVHRR) data.
Data from AVHRR channels 1 and 2 are used to compute the top-of-atmosphere (TOA) reflectances and apply the required cloud masking by utilizing the AVHRR-PPS package (see Dybbroe et al., 2005 under references). Depending on surface type: water, ice, land (snow-covered, bare, different vegetation zones) different processing steps are carried out. Various data sets are used to carry out the required atmospheric correction (e.g. for aerosol optical depth: TOMS and/or OMI data; for surface wind speed over water: SMMR through SSMIS data; for other parameters: ERA-Interim atmospheric re-analysis data). Over land also topographic corrections are applied. Details about the processing steps can be found in the respective ATBD as well as in the papers by Jin et al. (2011), Xiong et al. (2002), and Liang (2000) in the references.
We offer edition 2 of the CLARA data set, i.e. CLARA-A2, for internal use.
The data set was included into the ICDC data base on May 24 2018.
|Mean broadband shortwave (0.25 … 2.5µm) surface albedo (black sky albedo)||percent|
|As above but median||percent|
|As above but standard deviation of the mean||percent|
|As above but 3rd statistical moment: skewness||percent|
|As above but 4th statistical moment: kurtosis||percent|
|Mean sun zenith angle for valid albedo values||degrees|
|As above but standard deviation of the mean||degrees|
|Number of valid albedo values per grid cell||--|
Period and temporal resolution:
- 1982-01 to 2015-12
Coverage and spatial resolution:
- Spatial resolution: 0.25 degrees x 0.25 degrees, equally spaced lat/lon grid (Climate Modeling Grid, CMG)
- Geographic longitude:-179.875°E to 179.875°E
- Geographic latitude: -89.875°N to 89.875°N
- Dimension: 1440 columns x 720 rows
- Altitude: following terrain
The data set does not include separate uncertainty estimates. It does contain, however, information about the standard deviation per grid cell which could be taken as a measure of uncertainty. It contains also information about solar zenith angle and the number N of valid albedo values used to compute the statistical moments. It is recommended to use only data with N larger than 20.
Because a few of the additional data required for the processing (see description) are missing in 2015, it is recommended to not over-interpret results that seem to be specific for that particular year.
Evaluation of the product confirms that the target accuracy of 25% difference relative to the in-situ observation is met for nearly all sites used for the evaluation within the period 1992-2014. Mean absolute differences to surface albedo from evaluation data sets are often below 5%.
Browsing through the data set reveals that areas of a highly variable aerosol load, i.e. aerosol optical depth (AOD) and/or frequent cloud cover, can have artificially large surface albedo values. South China and Indonesia are candidates here.
Limitations and artifacts may also originate from extremely clear atmospheres (Antarctic interior), mixed cloud / snow covers, and land cover classifications not representing the actual conditions.
For more information about the quality and limitations of the data set we refer to the CM-SAF Product User Guide (PUM) and the CM-SAF Validation report in the references.
ICDC / CEN / University of Hamburg
email: stefan.kern (at) uni-hamburg.de
- Algorithm Theoretical Basis Document (ATBD)
- Product User Manual (PUM)
- Validation Report
- Dybbroe, A., et al, 2005, SAFNWC AVHRR cloud detection and analysis using dynamics thresholds and radiative transfer modelling part I: Algorithm description. Journal of Applied Meteorology, 44, 39-54.
- Jin, Z., et al., 2004, A parameterization of ocean surface albedo. Geophysical Research Letters, 31, 22301.
- Jin, Z., et al., 2011, A new parameterization of spectral and broadband ocean surface albedo. Optics Express, 19(27), 26,429-26,443.
- Liang, S., 2000, Narrowband to broadband conversions of land surface albedo: Algorithms. Remote Sensing of Environment, 76, 213-238.
- Xiong, X., et al., 2002, Surface albedo over the Arctic Ocean derived from AVHRR and its validation with SHEBA data. Journal of Applied Meteorology, 41, 413-425.
The citation for this data set is:
Karlsson, Karl-Göran; Anttila, Kati; Trentmann, Jörg; Stengel, Martin; Meirink, Jan Fokke; Devasthale, Abhay; Hanschmann, Timo; Kothe, Steffen; Jääskeläinen, Emmihenna; Sedlar, Joseph; Benas, Nikos; van Zadelhoff, Gerd-Jan; Schlundt, Cornelia; Stein, Diana; Finkensieper, Stephan; Håkansson, Nina; Hollmann, Rainer; Fuchs, Petra; Werscheck, Martin (2017): CLARA-A2: CM SAF cLoud, Albedo and surface RAdiation dataset from AVHRR data - Edition 2, Satellite Application Facility on Climate Monitoring, DOI:10.5676/EUM_SAF_CM/CLARA_AVHRR/V002, https://doi.org/10.5676/EUM_SAF_CM/CLARA_AVHRR/V002.
All intellectual property rights of the CM SAF products belong to EUMETSAT. The use of these products is granted to every interested user, free of charge. If you wish to use these products, EUMETSAT copyright credit must be shown by displaying the words "copyright (year) EUMETSAT" on each of the products used.