Measurements
The AVHRR is a passive radiation-detection imager mounted on polar-orbiting NOAA and EUMETSAT satellites, that measures upwelling radiation in specific band between 0.6 to 12 μm. Measurements from the Advanced Very High Resolution Radiometer (AVHRR) radiometer on board the polar orbiting NOAA satellites and the EUMETSAT MetOp satellites have been performed since 1978. The instrument only measured in four spectral bands in the beginning (AVHRR/1), but from 1982 a fifth channel was added (AVHRR/2) and in 1998 even a sixth channel (1.6 μm) was made available (AVHRR/3), although only accessible if switched with the previous third channel at 3.7 μm. The native pixel size (field of view) at nadir is 1.1km along with a sampling distance of 0.87km. Basis for the presented dataset however is the (Global Area Coverage) GAC format of the data which is of reduced resolution (approx. 1x4km) at a sampling distance of 4km. The swath width is 1464 km allowing for seamless scanning of the complete Earth at least 2 times a day (higher observation frequencies at higher latitudes). More information on AVHRR can be found at: https://www.wmo-sat.info/oscar/instruments/view/61.
Local Observation Time and Length of Data Record
CLARA is based on all available AVHRR measurements. Thus, measurements from both observation nodes (i.e., satellites operating in both morning and afternoon orbits) are used.
The second edition of CLARA (CLARA-A2) has a temporal coverage of the period 1982-2015. Observations in the afternoon orbit have here Local Solar Time at observation (at nadir) near 13:30 in the afternoon and near 01:30 at night. For observations in the morning orbit a shift in observation times occurred in 2002 with the launch of the NOAA-17 satellite followed by METOP satellites. Consequently, observations before 2002 occurred near 07:30 in the morning and near 19:30 in the evening. After 2002, corresponding times were 09:30 and 21.30.
Note that satellite orbits have been exposed to substantial orbital drift throughout the years, especially satellites in afternoon orbit. Only morning orbits from the METOP satellites (first satellite launched in 2007) have been kept stable.
Spatial Resolution
1×5 km at nadir with 4km sampling distance (AVHRR GAC)
Cloud Detection
Cloud screening was based on the multispectral thresholding approach utilising all available AVHRR channels. This method was first introduced by Dybbroe et al., 2005 but several upgrades of the method has been made prior to the release of CLARA-A2. The most important improvement was the tuning of thresholds tests utilising the observations from the CALIPSO-CALIOP sensor. The achieved results have been examined in detailed by Karlsson and Håkansson (2018).
Retrieval Methodology for Cloud top height parameters and Cloud physical properties
The retrieval of cloud top temperature, height and pressure for opaque clouds (with all parameters retrieved simultaneously) was based on radiance matching of observed and simulated radiances in the 11 micron channel. Simulations were performed using the RTTOV radiative transfer model with input from ERA-Interim profiles of temperature and humidity. For thin (semi-transparent) clouds a split-window histogram approach with 11 and 12 micron channel data was used based on radiances collected within coarse resolution image segments (typically 30×30 km).
Retrieval of cloud optical thickness (COT) and cloud effective radius (CER) was based on a LUT approach following the Nakajima-King methodology (Roebeling et al., 2006). Cloud phase was also retrieved using the method introduced by Pavolonis et al. (2005). Finally, cloud water paths for both liquid and frozen clouds were calculated based on upgraded versions of the original parameterisation by Stephens et al., (1978).
Based on an AVHRR Fundamental Climate Data Record (FCDR), the CLARA-A2 data record was generated based on all available 5-channel AVHRR data since 1982. No orbital drift correction was applied. A full description of the data record was given by Karlsson et al. (2017).
Ancillary input
- Atmospheric T and H2O profiles, surface T from ECMWF meteorological reanalysis data (ERA Interim)
- Radiative transfer using the RTTOV RTM
- Vis/NIR LUTS derived using the DAK RTM
- Spectral surface emissivities (monthly climatology) from MODIS
- MODIS MCD43C1 surface BRDF product
- USGS 1.0 km AVHRR-based Land Cover Type data
- ERA-Interim snow and ice information used to modify the surface albedo
- Single scattering properties of ice crystals (Baum Ice crystals)
References
The CLARA-A2 data record:
Karlsson, K.-G., Anttila, K., Trentmann, J., Stengel, M., Meirink, J. F., Devasthale, A., Hanschmann, T., Kothe, S., Jääskeläinen, E., Sedlar, J., Benas, N., van Zadelhoff, G.-J., Schlundt, C., Stein, D., Finkensieper, S., Håkansson, N., and Hollmann, R.: CLARA-A2: the second edition of the CM SAF cloud and radiation data record from 34 years of global AVHRR data, Chem. Phys., 17, 5809-5828, doi:10.5194/acp-17-5809-2017, 2017
Related papers:
Dybbroe, A., Thoss, A., and Karlsson, K.-G.: NWCSAF AVHRR cloud detection and analysis using dynamic thresholds and radiativetransfer modelling – Part I: Algorithm description, Appl. Meteorol., 44, 39–54, 2005.
Karlsson, K.-G. and Håkansson, N.: Characterization of AVHRR global cloud detection sensitivity based on CALIPSO-CALIOP cloud optical thickness information: demonstration of results based on the CM SAF CLARA-A2 climate data record, Meas. Tech., 11, 633–649, https://doi.org/10.5194/amt-11-633-2018, 2018.
Pavolonis, M. J., Heidinger, A. K., and Uttal, T.: Daytime global cloud typing from AVHRR and VIIRS: Algorithm description, validation, and comparison, Appl. Meteorol., 44, 804–826, doi:10.1175/JAM2236.1, 2005.
Roebeling, R. A., Feijt, A. J., and Stammes, P.: Cloud property retrievals for climate monitoring: implications of differences between SEVIRI on METEOSAT-8 and AVHRR on NOAA-17, Geophys. Res., 111, D20210, doi:10.1029/2005JD006990, 2006.
Stephens, G.: Radiation profiles in extended water clouds, II: Parameterization schemes, Atmos. Sci., 35, 2123–2132, 1978.
Data Distribution
Officially released CLARA-A2 data are accessible at: https://doi.org/10.5676/EUM_SAF_CM/CLARA_AVHRR/V002