Update of Radiance Calibrations for ISCCP

Abstract

Since July 1983 ISCCP has collected, normalized, and calibrated radiance data (visible and thermal infrared) from the imaging radiometers on the National Oceanic and Atmospheric Administration polar orbiters and from the geostationary satellites GOES, Meteosat, and GMS. Although analyzed by the International Satellite Cloud Climatology Project (ISCCP) to obtain information about clouds, this global radiance dataset also represents a valuable resource for other remote sensing studies. Examination of the 8-yr cloud climatology produced with the first version of the ISCCP calibration revealed artifacts in the global means that coincided with the changes in the afternoon polar orbiters (used as a reference standard), as well as some localized anomalies related to occasional errors in the geostationary normalizations. This paper reports the changes to the ISCCP normalization and calibration procedures (originally reported in Brest and Rossow) that have been made to reduce these artifacts and errors and to produce a revised calibration. The key assumption, made after examining more than 10 years of global data, is that the mean properties of the earth are more nearly constant over this timescale than are the calibration of these radiometers. The authors conclude that the relative calibrations of the radiances used by ISCCP are now uncertain on average by no more than ±0.01?0.02 absolute, ±3%?5% relative, for visible (VIS) radiances and ±l?2 K absolute, ±0.3%?1.0% relative, for infrared (IR) radiances. The absolute calibration uncertainty is estimated to be about 10% for VIS and 2% for IR. The history of efforts to calibrate the Advanced Very High Resolution Radiometer (AVHRR) points to some lessons important to future spacecraft observations of climate change: Real decadal-scale changes of the earth are much smaller in magnitude than uncertainties in calibration changes and cannot be reliably detected without significant improvements of instrument calibration; infrequent aircraft calibration results for AVHRR with their attendant uncertainties make it difficult to distinguish real interannual variability from short-term calibration changes and suggest that something more will be required to obtain the needed accuracy; and finally, even calibrations based on onboard targets may not be sufficient, given that the authors still find differences in IR calibrations of more than 1 K. These results suggest, in particular, that reliance on one method of calibration for future spacecraft missions is unlikely to reduce calibration uncertainties enough for climate change monitoring. Even with improvements in all of these areas, the relative accuracy attained will only be apparent in the context of a long data record.