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 μm) 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°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%.