ISCCP Description

Measurements

The basic measurements analysed to produce the ISCCP Cloud Products are the "window" infrared (wavelength 10-11 μm) and visible (wavelength about 0.6 μm) radiances (augmented by 3.7 μm radiances over snow and ice) measured by the imaging radiometers on all of the operational polar orbiting and geostationary weather satellites to resolve the diurnal to interannual, mesoscale to global cloud property variations. All radiances are cross-calibrated to the NOAA-9 AVHRR, which in turn is connected to a NIST-traceable standard by NASA ER2 underflights (visible) or onboard calibration target (infrared).

Local Observation Time and Length of Data Record

For the GEWEX Cloud Assessment version of the ISCCP monthly products, the original 8-times daily results were averaged to represent observation periods centered on 3:00 AM, 9:00 AM, 3:00 PM and 9:00 PM local time at the equator. The GEWEX Cloud Assessment collection includes data for 1984–2007; the full D-version ISCCP products cover the period from July 1983 to December 2009.

Spatial Resolution

The radiances are measured for individual image pixels about 4–7 km in size (higher resolution visible measurements are averaged to match infrared measurements), sampled at 25–30 km intervals.

Cloud Detection

The cloud detection procedure analyzes the infrared and visible radiances (as well as the near-infrared over snow and ice) separately; cloud is reported present in a pixel if any channel indicates cloud presence. The presence of cloud is determined by comparing the observed radiance against an estimate of clear sky radiances for each pixel (location) and time, where the threshold difference in radiance is the estimated uncertainty of the clear sky radiances. The clear sky radiances are identified by a series of tests that examine radiance difference in space and time: the hypothesis is that clear sky exhibits smaller space-time radiance variations than cloudy sky but the magnitude of the clear sky variation assumed is situation-surface type dependent. Statistics of the clear sky radiances from these space-time-contrast tests are collected over short-term and long-term time intervals and compared to provide the final estimate of clear sky radiances. Cloud amount (CA) is determined only for the gridded products that include results from many pixels (about 70 on average) in the original 2.5° equal-area mapping; these results are remapped to a 1.0° equal-angle mapping for the GEWEX Cloud Assessment version.

Retrieval Methodology

Cloud top temperatures (CT) are retrieved for each cloudy pixel in two ways: assuming a blackbody cloud emission at all times of day and correcting for transmitted thermal radiation based on the visible optical thickness during daytime. This retrieval compares the measured radiances against results from a radiative transfer model that pre-calculates infrared brightness temperatures as a function of viewing angle and cloud top pressure for each location based on the surface temperature, retrieved from the clear sky radiance estimate, and an atmospheric temperature-humidity profile from the NOAA operational TOVS product. The TOVS profiles are also used to associated CT with CP (and CZ for the GEWEX Cloud Assessment version). For the GEWEX Cloud Assessment, the blackbody results are reported at night to illustrate the differences but in the actual ISCCP monthly product, there is a correction of the nighttime results for the lack of visible radiance measurements determined by interpolating the daytime differences of the two results over the nighttime.
Cloud optical thicknesses (COD) are retrieved from visible radiances, assuming a specific particle microphysics (size distribution, shape and phase), where a CT= 260K separates the two phases. COD retrieval employs a pre-calculated look-up-table that gives COD as a function of visible radiance, surface reflectance (retrieved from the clear sky radiance estimate), CP, and viewing geometry (ozone absorption is also accounted for). The actual retrievals of CT and COD are iterative to account for the mutual dependence. In the original ISCCP products, CWP is determined from COD and the assumed particle sizes (cloud effective radius, CRE= 10 μm for liquid and 30 μm for ice particles), but for the GEWEX Cloud Assessment version, the COD, CRE and CWP values come from an experimental analysis of AVHRR data using 0.6 and 3.7 μm wavelengths to retrieve COD and CRE from which CWP is calculated.

Ancillary Input

  • Land-water mask
  • topography from very early version of what is now GTOPO
  • 5-day snow cover based on the weekly NOAA operational product
  • 5-day sea ice cover from the weekly NOAA-Navy ice charts
  • daily ozone and temperature-humidity profiles from operational NOAA TOVS product
  • References

  • Rossow, W. B., and R. A. Schiffer, 1991: ISCCP Cloud Data Products. Bull. Amer. Meteor. Soc., 72, 1–20.
  • Rossow, W. B., and R. A. Schiffer, 1999: Advances in understanding clouds from ISCCP. Bull. Amer. Meteor. Soc., 80, 2261–2287.
  • Distribution of L2 and L3 dataset

    The official ISCCP archives reside at NOAA/NESDIS/NCDC. Please, use this link to obtain the data.