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
1:30 AM and 1:30 PM (orbits/observation times drift in the course of most AVHRR-carrying satellites ; 1982-2016
Spatial Resolution
1×5 km at nadir with 4km sampling distance (AVHRR GAC)
Cloud Detection and phase
Artificial Neural Networks (ANN) trained against CALIOP cloud optical thickness (for cloud detection) and CALIOP clod top phase (for cloud phase). Scene-dependent thresholds applied to ANNs’ output to infer binary cloud/phase masks.
Retrieval Methodology
The Community Cloud retrieval for CLimate (CC4CL) is an optimal estimation retrieval scheme for the derivation of the properties of clouds from top-of-atmosphere (TOA) radiances measured by satellite-borne visible-IR radiometers as described in greater detail by McGarragh et al. (2018), Sus el al. 2018 and Stengel et al. (2020). Heritage of CC4CL is ORAC which was originally designed for application to SEVIRI and was subsequently adapted for the ATSR instrument, which was used to produce the GRAPE (Global Retrieval of ATSR cloud Parameters and Evaluation) data set. For the generation of the presented Cloud_cci AVHRR-PMv3 dataset the algorithm has been adopted to AVHRR channels 0.67, 0.87, 1.6, 3.7, 11 and 12 µm in the framework of the Cloud_cci project (www.esa-cloud-cci.org). CC4CL simultaneously retrieves cloud optical thickness, cloud effective radius and cloud top pressure. The retrievals are further processed to infer cloud top temperature/height and liquid/ice water path among others. The CC4CL retrieval system is version controlled and accessible at GitHub: https://github.com/ORAC-CC/orac/wiki
Based on an AVHRR Fundamental Climate Data Record (FCDR) and the physical retrieval system CC4CL, the multi-annual, global dataset of cloud and radiation properties, named Cloud_cci AVHRR-PMv3 (Stengel et al., 2020), was generated, which is exclusively based on AVHRR measurement from NOAA-7, NOAA-9, NOAA-11, NOAA-14, NOAA-16, NOAA-18 and NOAA-19 satellites. No orbital drift correction is applied.
Ancillary input
- Atmospheric T and H2O profiles, surface T from ECMWF meteorological reanalysis data (ERA Interim)
- Radiative transfer using RTTOV
- Vis/NIR LUTS derived using DISORT RTM
- Spectral surface emissivities (monthly climatology) from MODIS
- MODIS MCD43C1 surface BRDF product
- USGS 0.5km MODIS based Land Cover Type 2 data
- ERA-Interim snow and ice information used to modify the surface albedo
- Single scattering properties of ice crystals (Baum Ice crystals)
References
McGarragh, G. R., Poulsen, C. A., Thomas, G. E., Povey, A. C., Sus, O., Stapelberg, S., Schlundt, C., Proud, S., Christensen, M. W., Stengel, M., Hollmann, R., and Grainger, R. G.: The Community Cloud retrieval for CLimate (CC4CL) – Part 2: The optimal estimation approach, Atmos. Meas. Tech., 11, 3397-3431, doi:10.5194/amt-11-3397-2018, 2018.
Sus, O., Stengel, M., Stapelberg, S., McGarragh, G., Poulsen, C., Povey, A. C., Schlundt, C., Thomas, G., Christensen, M., Proud, S., Jerg, M., Grainger, R., and Hollmann, R.: The Community Cloud retrieval for CLimate (CC4CL) – Part 1: A framework applied to multiple satellite imaging sensors, Atmos. Meas. Tech., 11, 3373-3396, doi:10.5194/amt-11-3373-2018, 2018
Stengel, M., Stapelberg, S., Sus, O., Finkensieper, S., Würzler, B., Philipp, D., Hollmann, R., Poulsen, C., Christensen, M., and McGarragh, G.: Cloud_cci Advanced Very High Resolution Radiometer post meridiem (AVHRR-PM) dataset version 3: 35-year climatology of global cloud and radiation properties, Earth Syst. Sci. Data, 12, 4160, doi:10.5194/essd-12-41-2020, 2020.
Data Distribution
Officially released Cloud_cci AVHRR-PMv3 data are accessible at: https://doi.org/10.5676/DWD/ESA_Cloud_cci/AVHRR-PM/V003