ISCCP CATALOG OF DATA AND PRODUCTS Section 6

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6. CLOUD CLIMATOLOGY PRODUCT (C AND D) DATA SETS AND HOLDINGS

Stage C data were constructed from the combination of the original B3 radiances, the results of the three parts of the experimental cloud algorithm, and the correlative data used in the analysis. The three fundamental parts of the cloud algorithm are cloud detection, radiative transfer model analysis, and statistical analysis. The first part determines whether a particular radiance measurement is associated with cloudy or clear conditions. The second part compares the measured radiances, together with other correlative information about the atmosphere and surface, to a radiative model to retrieve several cloud and surface parameters. The third part accumulates spatial distribution information about the radiances and retrieved cloud and surface parameters to summarize the analysis.

Stage C1 data represent the global merger of results from all available geostationary and polar orbiting satellites, reported every three hours (from 0000 UTC, onwards) with a spatial resolution of approximately 250 km. Variables reported at each location include frequency of cloud occurrence and an error estimate, the spatial mean and standard deviations of cloudy and clear sky radiances (for 0.6 and 11 microns), the surface temperature and solar reflectance, surface type and topography, atmospheric temperature and humidity profiles, and ozone column abundance. The frequency distribution of cloudiness with top pressure and with optical thickness are also provided to indicate the types of clouds present. These variables can be read in either an equal-area or an equal latitude-longitude map grid with a 2.5 degree increment.

The Stage C2 data are the monthly averages and summary statistics of the Stage C1 quantities. In addition to the time variances of some quantities, the mean diurnal variation of the cloud and surface properties are reported. Stage C2 data are reported in the same format as Stage C1 data.

The algorithm used to produce the C1 data has steadily evolved. For an early description of the cloud algorithm and its design see Rossow and Schiffer, 1991. The cloud algorithm is fully described in Rossow and Garder, 1993a. This paper also includes a detailed description of the C1 data content and format.

In generating the B3, C1, and C2 data, the GPC uses two correlative data sets: the atmospheric (TV) data set and the ice/snow (IS) data set. Although these data sets are available from the original sources covering a longer time period, the versions used during the processing of the satellite data are archived with the cloud climatology for two reasons: 1) to document the complete ISCCP data analysis procedure, and 2) to provide the data in a format especially suited to satellite data processing. Other researchers may also find this form of the data more convenient to use than the original.

The first version of ISCCP cloud products, the C-series, covered the period from July 1983 through June 1991. C-data production was terminated as of June 30, 1991. No more of these products will be produced. On-going research indicated that a number of improvements and refinements of the analysis and reported statistics were possible. These changes have been implemented (D-series) for processing of data in the period beyond June 1991; all older data will be reprocessed into the newer version.

Highlights of the differences between the C-series and D-series cloud products are as follows:

Radiance Calibrations

• Revised VIS and IR calibrations to eliminate spurious changes between different reference polar orbiters (Brest et al. 1996)
• Revised normalizations of geostationary satellite calibrations to eliminate occasional short-term deviations (Brest et al. 1996)

Cloud Detection

• Improved cirrus detection over land by lowering IR threshold from 6 K to 4 K
• Improved polar cloud detection over ice and snow surfaces by lowering VIS threshold from 0.12 to 0.06 microns and by using threshold test on 3.7 micron radiances
• Improved detection of low clouds at high latitudes by changing to VIS reflectance threshold test

Radiative Model

• Improved treatment of cold (top temperature < 260 K) clouds by using ice polycrystal scattering phase function to retrieve optical thickness and top temperature
• Improved retrieval of cloud optical thicknesses over ice and snow surfaces using 3.7 micron radiances
• Improved retrieval of cloud top temperatures by including effects of IR scattering
• Improved retrieval of surface and cloud top temperatures by adopting new treatment of water vapor continuum absorption in IR

Gridded Product Contents

• Better resolved variations of optically thicker clouds by adding sixth optical thickness category
• Added correct cloud water path parameter
• Reported actual average values of cloud top temperature, pressure, optical thickness and water path for each of nine cloud types defined by cloud top pressure and optical thickness in the 3-hourly data set
• Reported separate cloud properties for liquid and ice forms of low and middle-level clouds
• Provided conversion of cloud top pressures to cloud top heights above mean sea level based on atmospheric temperature profile
• Added cloud amount frequency distribution to monthly data set

Increase Resolution

• Archived pixel-level cloud products with resolution of 30 km and 3 hours