Estimation of the Upper Tropospheric Relative Humidity Field from METEOSAT Water Vapor Image Data

A retrieval method is described for estimating a mean column value of the upper tropospheric relative humidity (UTH) from radiance measurements in the 6.3 μm channel of the geostationary satellite METEOSAT. The physical retrieval method is based on an efficient radiative transfer scheme which uses the temperature forecast profiles from the European Centre for Medium Range Weather Forecasts (ECMWF) as ancillary data. Theoretical radiances for the given temperature profile and a set of fixed upper tropospheric humidities are employed to relate the observed radiance to a mean humidity for a layer between 600 and 300 hPa. The retrieval is confined to areas with neither medium- nor high-level clouds. A calibration procedure of the 6.3 μm channel is described which uses the radiative transfer scheme with measured radiosonde profiles of temperature and humidity and collocated satellite measurements. An example of the UTH product and a comparison with radiosondes is presented. An estimate of the error of the UTH is obtained from a sensitivity test of the radiation scheme to errors in the input profiles. Both the sensitivity test and the comparison with radiosondes yield absolute error estimates for the UTH of 10%–15%.