Measurements
CALIPSO-GOCCP was developed from the CALIPSO level 1 attenuated backscatter measurements for evaluating clouds in climate models.
The CALIOP lidar (onboard CALIPSO) is a nadir-pointing instrument and acquires lidar profiles at 532 nm and 1064 nm along the CALIPSO groundtrack. The laser footprints have a diameter of about 70 m with a center-to-center spacing of 1/3 km. The data is averaged vertically and horizontally on-board the satellite so that downlinked data has a resolution of 1/3 km horizontally and 30 m below 8 km altitude, and 1 km and 60 m from 8 to 20 km.
Local Observation Time and Length of Data Record
1:30 AM and 1:30 PM ; 2007-2016
Spatial Resolution
0.48 km x 0.34 km
Cloud Detection
Clouds are detected by constructing profiles of attenuated Scattering Ratio, SR(z), defined as SR(z) = ATB(z)/ATBmol(z), where ATB(z) is the calibrated 532 nm lidar return signal and ATBmol(z) is the 532 nm molecular return signal that would be measured in a cloud-free and aerosol-free atmosphere. Clouds are detected from the SR profiles on a fixed vertical grid (480 m), using a single constant threshold: SR>5 is declared cloudy. The cloud cover is derived statistically by accumulating profiles at a grid scale of 1°x1°.
CALIPSO-GOCCP algorithm consists of the following steps: (i) compute the ATBmol by normalization to a non-cloudy area within the stratosphere, (ii) average the CALIPSO-level1 ATB to 40 vertical levels equidistant of 480 m, retaining the full horizontal along-track resolution (1/3 km), (iii) compute the SR profile, (iv) for each profile detect the presence of clouds at each 480 m level (the pixel is declared cloudy when SR>5 and ATB‑ATBmol > 2.5.10-3 km-1sr-1), (v) accumulate the cloudy and clear pixels within each grid box (typically 1° latitude x 1° longitude).
Retrieval methodology
Cloud Top height corresponds to the top of the first cloudy level from the top of the averaged profiles onto the 1°x1° grid.
Cloud Temperatures are derived from the GMAO temperature profiles, averaged onto the 1°x1° grid.
GCM-Oriented CALIPSO Cloud Products (CALIPSO-GOCCP) reduces noise during day time and close to the surface:
– Daytime profiles that are considered too noisy based on the normalization ratio in the stratosphere are rejected (30% of day-time profiles). This may result in an underestimate of cloudy pixels in daytime, as the solar photons reflected from bright clouds increase the noise in the lidar signal.
– Close to the surface, the attenuated backscatter signal at 30m vertical resolution can increase significantly because of the reflection on the surface produced by snow or by a change in ground altitude. This increased ATB at 30 m vertical resolution can contaminate the value of ATB at 480 m vertical resolution. To avoid an artificial increase of ATB (at 480 m vertical resolution) due to this surface echo, all the pixels (at 30 m vertical resolution) located below the strong backscatter signal (ATBstrong) and 90 m above it, are rejected.
Ancillary Input
GMAO temperature profiles
References
Chepfer H., G. Cesana, D. Winker, B. Getzewich, M. Vaughan, and Z. Liu, 2012: Comparison of two different cloud climatologies derived from CALIOP attenuated backscattered measurements (Level 1): the CALIPSO-ST and the CALIPSO-GOCCP. J. Atmos. Oceanic Technol., doi: 10.1175/JTECH-D-12-00057.1
Chepfer H., S. Bony, D. Winker, G. Cesana, J. L. Dufresne, P. Minnis, C. J. Stubenrauch, and S. Zeng, 2010: The GCM Oriented Calipso Cloud Product (CALIPSO-GOCCP). J. Geophys. Res., 115, D00H16, doi:10.1029/2009JD012251
H. Chepfer, S. Bony, D. M. Winker, M. Chiriaco, J.-L. Dufresne, and G. Seze, 2008: Use of CALIPSO lidar observations to evaluate the cloudiness simulated by a climate model, Geophys. Res. Lett., vol. 35, L15704, doi: 10.1029/2008GL034207