Calibration of the geostationary satellite water vapor channels

The Meteosat geostationary satellites carry a filtered radiometer channel centered at 6.2 µm for the measurement of upper tropospheric humidity. The operational calibration is derived from radiative transfer simulations applied to radiosonde measurements. Here, we propose a new method based on coincidence with NOAA/HIRS-12 measurements. Radiative transfer simulations show that Meteosat measurement can be accurately predicted from HIRS-12 brightness temperature. Besides, the HIRS instrument is well calibrated, which makes it suitable as a reference for the calibration of other spaceborne radiometers. The method has been applied to 3 years of data (February 1994 to February 1997). The proposed calibration coefficient is smaller by about 10-15%, which results in equivalent brightness temperatures biased by about 3-4K, and 35-50% relative error on the upper tropospheric humidity. The calibration value shows that the sensor sensitivity degrades during the Northern hemisphere winter, possibly an effect of ice cover on the optics, and is fairly level during the rest of the year. The new method shows fluctuations much smaller than those provided by the "operational" method. This result indicates that the short-term stability of the instrument calibration is much better than 5%, the variability given by the operational calibration.