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Europe operates a network of ground-based cloud-profiling active remote sensing stations as part of the Aerosol, Clouds and Trace Gas Infrastructure Network (ACTRIS). These stations operate vertically-pointing cloud radar and lidar/ceilometer and use the Cloudnet processing scheme (Illingworth et al., 2007) for the continuous evaluation of cloud profile properties. The Cloudnet scheme combines the cloud radar and lidar measurements at a temporal resolution of 30 s and a vertical resolution of 30 m to create a Level-2 classification product containing cloud base height, cloud top height, and classifications per altitude bin (no cloud, aerosol, liquid cloud droplets, ice cloud, ...). The Cloudnet processing scheme was also applied to similar cloud-profiling measurements from the US Department of Energy Atmospheric Radiation Measurement (ARM) sites. Cloudnet products are freely available for download from the Cloudnet database.


Geographical distribution of the ACTRIS/Cloudnet and ARM instruments contributing cloud (top) height correlative measurements to the validation of Sentinel-5p: CLOUDNET (in blue), ARM (in red), and NOAA/ARM (in green). (© Update of Compernolle et al., 2021/02/18.)

Correlative data is obtained via the following channels:

  • the ACTRIS RI and the ACTRIS-2 project (European Commission contract H2020-INFRAIA, grant no. 654109) for providing ground-based data from the Cloudnet sites, produced by the Finnish Meteorological Institute,
  • the Cloudnet database, mirrored at EVDC for validation purposes,
  • the Atmospheric Radiation Measurement (ARM) user facility, managed by the Office of Biological and Environmental Research for the U.S. Department of Energy Office of Science, for providing data for the ARM sites.



Illingworth, A. J. et al., Cloudnet, Bull. Am. Meteorol. Soc., 88, 883-898 2007, doi:10.1175/bams-88-6-883