CLARA-A2: CM SAF cLoud, Albedo and surface RAdiation dataset from AVHRR data - Edition 2

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)

The CLARA-A2 record provides cloud properties, surface albedo and surface radiation parameters derived from the AVHRR sensor onboard polar orbiting NOAA and METOP satellites. This second edition is the improved and extended follow-up of the first version of the record (Karlsson et al., 2013; CLARA-A1 DOI: 10.5676/EUM_SAF_CM/CLARA_AVHRR/V001) which now covers a 34 year time period (1982-2015). Original visible radiances were inter-calibrated and homogenised, using MODIS data as a reference, before applying the various parameter retrievals. The inter-calibration was based on an original method introduced by Heidinger et al. (2010) which now has been updated (MODIS Collection 6) and extended (six years have been added). CLARA-A2 features a range of cloud products: cloud mask, cloud top temperature/pressure/height, cloud thermodynamic phase, and (for liquid and ice clouds separately) cloud optical thickness, particle effective radius and cloud water path. Cloud products are available as monthly and daily averages and also as daily resampled global products (Level 2b) for individual satellites. Cloud parameter results are also presented as multi-parameter distributions (i.e., joint frequency histograms of cloud optical thickness, cloud top pressure and cloud phase) for daytime conditions. Surface albedo is presented as monthly and pentad (5 day) averages and is derived using all available data during the studied period. Surface radiation products are provided as monthly averages for the downwelling shortwave (including also daily averages) and the down- and upwelling longwave components. The monthly and daily averages are available on a 0.25°x0.25° global grid. Surface albedo and cloud products are also provided in two equal area grids with a resolution of 25 km x 25 km covering the polar regions. Daily resampled cloud products (level 2b) are provided in a global grid with a resolution of 0.05°x0.05°. For the latter, also a probabilistic cloud mask is added as an experimental product. The advancements compared to CLARA-A1 can be summarized as follows: (1) Extended AVHRR measurement record (six years). (2) Upgraded inter-calibration (using MODIS Collection 6 as reference). (3) Significantly improved cloud screening (utilizing CALIPSO-CALIOP data during development). (4) Improved efficiency of cloud top height retrievals (i.e., reduced number of pixels with no retrieval solutions). (5) Improved cloud phase discrimination and ice cloud property retrieval. (6) Improved atmospheric correction using dynamic aerosol optical depths for the full surface albedo data record. (7) Extended product portfolio (i.e., adding daily resampled Level 2b cloud products including experimental cloud probabilities). A summary of the CLARA-A2 characteristics and a comprehensive evaluation of the results are provided by Karlsson et al., (2016). Along with the data, a comprehensive documentation including user guide, algorithm descriptions, reprocessing layout and extensive validation studies, is provided.
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