Improving the ISCCP data record for use in climate studies: Retroactive calibration of the narrowband visible imagers

The International Satellite Cloud Climatology Project (ISCCP) has used visible and infrared imager data collected from meteorological satellites around the globe to produce radiation and cloud data since 1983. Because of the long duration of this record, ISCCP data is frequently used in climate analysis. Although ISCCP was not originally planned to provide data for monitoring climate change, its current visible calibration stability level of 3-5% is much better than originally available. Since ISCCP was started, new radiance calibration specifications for climate monitoring measurements have been established – stability better than 1% per decade – prompting a re-evaluation of the ISCCP narrowband imager calibrations to determine whether this requirement can be met.

Several techniques recently developed for the NASA Earth Observing System (EOS) program demonstrate the capacity to improve the calibration of the historical ISCCP cloud and related surface radiation budget (SRB) records. The Clouds and the Earth's Radiant Energy (CERES) team has pioneered the use of optically thick deep convective clouds as bright invariant targets and the use of MODIS as a reference to improve the stability of narrowband imager data such as geostationary data. In a related NASA-sponsored activity, the USGS has used the Robotic Lunar Observatory (ROLO) to develop a model of the expected reflectance of sunlight from the lunar surface as a function of lunar-solar geometry. SeaWiFS has used comparisons of its lunar observations to the ROLO model to achieve exceptional (~0.1%) stability in narrowband imager channels.

A new project applies these techniques retroactively to ISCCP imager data. This activity has the potential to significantly improve the ISCCP radiances, which would enhance the usefulness of the ISCCP and SRB data sets for climate change studies. This presentation will discuss progress in applying these new techniques to the ISCCP radiances from geostationary and AVHRR satellites, including specifics of the methods as used and examples of results.