ON THE CALIBRATION OF THE METEOSAT WATER-VAPOR CHANNEL

The calibration of the Meteosat water vapor channel image data is of p rime importance for a quantitative use. The vicarious calibration tech nique employs the use of data from radiosonde ascents. A screening tec hnique, removing spurious radiosonde information, has been improved an d used operationally since February 4, 1994. It involves new quality c ontrol checks on satellite zenith angle and the upper tropospheric rel ative humidity measured by the radiosonde, leading to a lower (about 8 %) and more stable calibration. The operational calibration technique uses averages for the clear-sky (above 700 hPa) satellite observations over large areas and relates them to expected radiances at the top of the atmosphere, determined from radiosonde data. The approach, valid owing to the small gradients in the water vapor field in such areas, r estricts the number of usable radiosonde observations severely. An alt ernative calibration method is outlined, which uses a smaller clear-sk y area, thus accepting more radiosonde ascents. This method further de creases the variability of the calibration coefficient by about 20%.