This data set is based on observations of two vertically profiling satellite sensors: a radar (CloudSat) and a lidar (CALIPSO-CALIOP). The CloudSat radar operates at a frequency of 94 GHz (wavelength: about 2 mm). The CALIPSO lidar operates with three channels: two at 532 nm which perpendicular to each other polarization and one at 1064 nm.
CloudSat and CALIPSO (both launched April 28, 2006) are part of the so-called "A-Train" constellation and fly along the same orbit at a distance of only 10-15 seconds (CloudSat leads) so that footprints of both sensors overlap, allowing to sense the same volume with the combinations of different frequencies involved.
CloudSat provides vertical profiles of the radar cross section and thus the refraction index, which is a measure for the concentration of cloud and precipitation particles in the sensed part of the atmospheric column, at a range resolution of about 240 m. CALIPSO-CALIOP provides three vertical profiles of the laser energy scattered back by aerosol and cloud particles at a range resolution of about 15 m; again the amount of the energy scattered back is a measure of the concentration of scattering particles in the sensed part of the atmospheric column.
Combination of the data obtained with both sensors allows to obtain an almost complete characterization of the vertical cloud structure because the two sensors couple with their different wavelengths used (CloudSat: about 2 mm, CALIPSO: 532 nm and 1064 nm) to different cloud and precipitation particles of both phases, liquid and solid. While CloudSat is rather insensitive to thin, high cirrus clouds CALIPSO can reliably detect these even if these are very thin. In contrast, some clouds can be opaque enough to prevent CALIPSO from seeing what is below, while CloudSat can penetrate these clouds, resolving what is underneath. Another advantage is, that in contrast to most other vertical profiling sensors, both sensors used here provide temperature independent results.
A big disadvantage is, however, the footprint size (about 1 km for CloudSat, about 70 m for CALIPSO) - similar to satellite altimeters used to remotely sense the ocean surface (see SSH and sea ice thickness). Because of this, data from many satellite overpasses have to be averaged / combined to a product with comparably coarse spatio-temporal resolution (in this case: 2° grid resolution and monthly temporal resolution) in order to obtain a reasonable statistics.
The data set contains the cloud fraction per grid cell. Different data sets exist for low, medium high and high clouds as well as the total cloud cover; we also offer the mean vertical profile for each grid cell. Each file contains the number of cloud pixels (per grid cell), the total number of pixels (per grid cell), and the ratio of both = cloud fraction of the respective grid cell.