ISCCP FIELD CAMPAIGN DATA VARIABLES

Selected data variables from ISCCP are available for on-line viewing or downloading. The full datasets may be obtained from our archives. The data provided on-line here may be from one of two versions of the ISCCP analysis: the first version is called the C-series and the revised version is called the D-series. Analysis results are reported in two forms: a "pixel-level" form (CX or DX) that has a resolution of 3 hr and 30 km, and a "gridded" form (C1 or D1) that has a resolution of 3 hr and 280 km.

Cloud Amount (C1/D1 only)

This variable represents the frequency of occurrence of cloudy conditions in individual satellite image pixels, each of which covers an area of about 4 to 49 square kilometers. Comparisons to other measurements confirm that this quantity also represents fractional areal coverage at one time for the larger 280 km grid cell areas. Clouds are detected by tests of infrared radiances in nighttime and by separate infrared and visible radiance tests in daytime.

Cloud Types (C1/D1 only)

In the gridded ISCCP products, several cloud types are defined to give more detailed information on the variations of cloud properties. These cloud types are defined by the VIS/IR cloud top pressure and optical thickness or by the IR cloud top pressure alone. In both the C-series and D-series datasets, there are three IR cloud types: low, middle and high: low clouds have top pressures greater than or equal to 680 mb, high clouds have top pressures less than 440 mb, and middle clouds are in between. The VIS/IR cloud types divide clouds in the three pressure levels by optical thickness range and are slightly different in the C-series and D-series datasets as shown in Figure 1. In the D-series datasets, low and middle cloud types can be either liquid or ice depending on temperature. The names used to refer to particular combinations of cloud top pressure and optical thickness are the classical morphological cloud types, but the association of particular ranges of cloud properties with these morphological types is only qualitative.

Cloud/Clear Flag (CX/DX only)

This quantity indicates which individual satellite pixels have been determined to contain cloud by the ISCCP cloud detection algorithm (Rossow, W.B., and L.C. Garder, 1993a: Cloud detection using satellite measurements of infrared and visible radiances for ISCCP. J. Climate, 6, 2370-2393). Basically, these pixels have either an infrared or visible radiance that deviates from the estimated clear sky values by more than some threshold amount: 2.5 K or 4 K for infrared brightness temperatures over ocean or land and 0.03 or 0.06 for visible reflectances over ocean or land, respectively.

Radiance counts (CX/DX only)

The count values are code values from 0 - 254 (255 is reserved to represent no data) that represent the actual observed radiances. Infrared counts are related to brightness temperatures, low counts corresponding to low temperatures. Visible counts are scaled visible counts that vary with solar illumination. The standard infrared and visible counts are calibrated radiances; the extra wavelength channels have the original pre-launch calibrations.

Clear sky radiance counts (CX/DX only)

These count values are estimates of the radiances that would be observed at each location and time if there were no clouds present and come from the ISCCP cloud detection analysis. By comparing these values to the observed radiances with other threshold values, the cloud detection procedure can be re-done.

IR Cloud Top Temperature/Pressure

These variables are determined from the infrared radiances measured by the satellites in cloudy conditions and the correlative data that indicate the temperatures and humidities at various pressure levels in the atmosphere. The temperature represents the amount of infrared radiation emitted by the cloud and the pressure represents the vertical location above mean sea level of the cloud top that corresponds to its temperature. Since some clouds are partially transparent to infrared radiation, their actual top temperatures may be somewhat colder and their estimated cloud top pressures may be somewhat lower than given by these values.

VIS-adjusted Cloud Top Temperature/Pressure

In daytime conditions when cloud optical thickness values are obtained from the visible radiances, the cloud top temperature/pressure values can be corrected for transmission of radiation through partially transparent clouds. The resulting values of temperature/pressure are lower than the IR cloud top temperature/pressure values. The magnitude of the difference between these values is larger for smaller optical thicknesses.

Cloud Optical Thickness

This variable is determined from the visible radiances measured by the satellite in cloudy conditions and represents the amount of solar radiation at visible wavelengths reflected by the clouds (the amount of infrared radiation absorbed by the clouds is also related to this variable). Since some clouds are partially transparent to solar radiation, this variable is obtained by accounting for the radiation reflected from the surface using the surface visible reflectances obtained from clear scenes. No results are reported for nighttime conditions or in the un-illuminated (winter) regions near the poles. The cloud optical thickness, together with the surface reflectance and cloud amount, determine how much solar radiation is reflected back to space by each location on Earth.

Cloud optical thickness is retrieved from observed visible radiances using one microphysical model in the C-series datasets and using two models in the D-series datasets. One model is a liquid water cloud composed of spherical droplets with a cross-section-weighted average radius of 10 microns and a variance of the size distribution of 0.15. This model is used for all clouds in the C-series datasets and for clouds with top temperatures greater than or equal to 260K in the D-series datasets. The second model is an ice cloud composed of fractal polycrystals with a cross-section-weighted radius of 30 microns and a variance of the size distribution of 0.1. This model is used in the D-series datasets for clouds with top temperatures less than 260K.

Surface Temperature

This variable is retrieved from the IR radiances measured under clear conditions and correlative data describing the temperature and humidity profile in the atmosphere. The surface temperature represents the amount of IR radiation emitted by the liqiud or solid surface and is equal to the actual temperature of the surface when the surface emissivity is equal to one. Most surfaces have emissivities less than but nearly equal to one, so the actual surface temperatures are slightly larger than the values given here: water surface temperature will be about 1.0 K larger and land surface temperatures will be 1.0 - 3.0 K larger depending on the amount of vegetation.

Surface Reflectance

This variable is retrieved from the VIS radiances measured under clear conditions and correlative data describing the ozone column abundance. The surface reflectance, particularly of water surfaces, varies with viewing and illumination geometry. Brighter areas over oceans at lower latitudes are caused by the occurrence of sunglint. Large reflectances in polar areas are caused by snow and ice cover.

ISCCP TOGA/COARE