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

A method has been developed to intercalibrate the Meteosat-7 water vapor channel 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, similar to the Meteosat WV channel. Radiative transfer simulations with climatological TIGR profiles for both channels provide a basis from which the relation between Meteosat WV and SSM/T-2 brightness temperatures can be derived. This method is used to assess systematic error of the Meteosat-7 WV channel which has no onboard calibration system but rather relies on a vicarious calibration technique. Results for the period under investigation suggest that the operational calibration coefficient for the Meteosat-7 WV channel is positively biased by about 12-13%, causing 3-4K bias of brightness temperature. It is shown that such a bias has a significant impact on the retrieval of UTH with relative errors up to 30%.