Ice contamination of Meteosat/SEVIRI IR13.4 channel implied by Inter-Calibration against Metop/IASI

The inter-calibration of the infrared channels of the geostationary Meteosat/SEVIRI satellite instruments shows most channels are radiometrically consistent with Metop-A/IASI, 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 modifies the spectral response functions and hence the weighting functions of channels in stronger atmospheric absorption bands, thus introducing an apparent calibration error. Analysis of the radiometer's gain using views of the on board black body source and cold space confirm a loss consistent with transmission through a layer of comparable thickness, which also increases the radiometric noise - especially for channels near the 12 μm libration band of water ice. Inter-calibration, such as the Global Space-based Inter-Calibration System (GSICS) Correction, offers an empirical method to correct this bias.