Measurements
The CALIOP instrument onboard the NASA/CNES CALIPSO satellite detects clouds based on vertically resolved profiles of lidar backscatter CALIOP consists of a polarized laser transmitter operating at 532 nm and 1064 nm and a receiver which samples the backscattered laser pulses at a vertical range of 30 m (532 nm) or 60 m (1064 nm). Cloud altitude is derived from measurements of the time-of-flight of the laser pulses. The 532 nm channel of the lidar receiver is polarization sensitive and provides profiles of backscatter depolarization. Multiple cloud layers can be profiled, although the lidar signal becomes fully attenuated at an optical depth of about three.
Local Observation Time and Length of Data Record
1:30 AM and 1:30 PM ; 2007-2016
Spatial Resolution
0.06 km (vertical) x 0.34 km (horizontal)
Cloud Detection
Lidar VIS and NIR vertical backscatter profiles
Horizontal averaging used as necessary to improve sensitivity
Retrieval Methodology
Cloud altitudes are determined from laser backscatter time-of-flight. Cloud thermodynamic phase is identified using the vertically resolved depolarization profiles. Water clouds and dense ice clouds are detected at single shot resolution (0.34 km). Horizontal averaging is used to reduce noise to allow detection of weakly scattering cirrus. The Version 2 product for GEWEX is based on CALIOP Version 4.2 data products, which feature improved calibration, improved aerosol-cloud discrimination, and an improved ice-water classification algorithm.
Ancillary Input
- MERRA-2 molecular density profiles
- Digital Elevation Map
Products for updated GEWEX cloud assessment database
TOP_LAYER_CALIPSO-ST
COLUMN_CALIPSO-ST
PASSIVE_CALIPSO-ST
OPAQUE_CALIPSO-ST
The optical depth required to reach this level of attenuation depends on surface reflectivity and on lighting conditions, so that opaque clouds can have somewhat lower optical depths during daytime, when surface detections are more difficult, than at night. Garnier et al. (2021) present statistics on emissivities of co-located CALIPSO IIR and CALIOP single-layer ice clouds over global oceans which are opaque or semi-transparent to CALIOP. Based on one year of data (2008), 98% of opaque clouds have COD > 3 at night while 97% of semi-transparent clouds have COD < 3. During daytime, 95% of opaque clouds have COD > 2 while 97% of semi-transparent clouds have COD < 2.
References
Avery, M. A., R. A. Ryan, B. J. Getzewich, M. A. Vaughan, D. M. Winker, Y. Hu, A. Garnier, J. Pelon and C. A. Verhappen, 2020: “CALIOP V4 Cloud Thermodynamic Phase Assignment and the Impact of Near-Nadir Viewing Angles”, Atmos. Meas. Tech., 13, 4539–4563, doi:10.5194/amt-13-4539-2020.
Liu, Z., J. Kar, S. Zeng, J. Tackett, M. Vaughan, M. Avery, J. Pelon, B. Getzewich, K.-P. Lee, B. Magill, A. Omar, P. Lucker, C. Trepte and D. Winker, 2019: “Discriminating Between Clouds and Aerosols in the CALIOP Version 4.1 Data Products”, Atmos. Meas. Tech., 12, 703–734, doi:10.5194/amt-12-703-2019.
Vaughan, M., K. Powell, R. Kuehn, S. Young, D. Winker, C. Hostetler, W. Hunt, Z. Liu, M. McGill, B. Getzewich, 2009: Fully Automated Detection of Cloud and Aerosol Layers in the CALIPSO Lidar Measurements, J. Atmos. Oceanic Technol., 26, 2034-2050, doi:10.1175/2009JTECHA1228.1
Winker, D. M., M. A. Vaughan, A. H. Omar, Y. Hu, K. A. Powell, Z. Liu, W. H. Hunt, and S. A. Young, 2009: Overview of the CALIPSO Mission and CALIOP Data Processing Algorithms. J. Atmos. Oceanic Technol., 26, 2310-2323.
Winker, D. M., J. Pelon, J. A. Coakley, Jr., and co-authors, 2010: The CALIPSO Mission: A Global 3D View Of Aerosols And Clouds, Bull. Amer. Met. Soc., 91, 1211-1229, doi:10.1175/2010BAMS3009.1.