Ice Contamination of Meteosat/SEVIRI Implied by Intercalibration Against Metop/IASI

The intercalibration of the infrared channels of the geostationary Meteosat/Spinning Enhanced Visible and InfraRed Imager (SEVIRI) satellite instruments shows that most channels are radiometrically consistent with those of Metop/IASI (Infrared Atmospheric Sounding Interferometer), which is used as a reference instrument. However, the 13.4-μm channel shows a cold bias of ~1 K in warm scenes, which changes with time. This is shown to be consistent with the contamination of SEVIRI by a layer of ice ~1 μm thick building up on the optics, which is believed to have condensed from water outgassed from the spacecraft. This ice modifies the spectral response functions and, hence, the weighting functions of the channels in stronger atmospheric absorption bands, thus introducing an apparent calibration error. Analysis of the radiometer's gain using an onboard black body source and a view of cold space confirms a loss consistent with transmission through a layer of comparable thickness, which also increases the radiometric noise-particularly for channels near the 12-μm libration band of water ice. Intercalibration, such as the Global Space-based Inter-Calibration System Correction, offers an empirical method to correct this bias.