Intercalibration of the Meteosat-7 water vapor channel with SSM/T-2

A new method has been developed to intercalibrate the Meteosat-7 water vapor (WV) channel (5.7-7.1 mu m) with collocated and calibrated satellite radiances at 183.3 +/- 1 GHz from the microwave instrument SSM/T-2. With radiative transfer calculations it is shown that this microwave channel is sensitive to radiation from the upper troposphere, very similar to the Meteosat WV channel. Radiative transfer simulations with a set of representative climatological profiles for both channels provide a basis for the relation between Meteosat WV and SSM/T-2 183.3 +/- 1 GHz brightness temperatures. The intercalibration method is used in a case study to assess a systematic error of the Meteosat-7 WV channel which rather relies on vicarious calibration technique. Results for the limited period under investigation suggest that the operational calibration coefficient for the Meteosat-7 WV channel is positively biased by about 12%, causing a bias of about 3 degrees K in brightness temperature. It is shown that such a bias would have a significant impact on the retrieval of upper tropospheric humidity (UTH) with relative errors up to 30%.