UPPER-TROPOSPHERIC HUMIDITY OBSERVATIONS FROM METEOSAT COMPARED WITH SHORT-TERM FORECAST FIELDS

Monthly mean brightness temperature observations from the water vapor channel (WV: 5.7 - 7.1 mu m) aboard the geostationary weather satellite METEOSAT-4 are presented for July 1992. The WV channel is sensitive to the atmospheric column water vapor in the upper troposphere above 600 to 500 hPa. The WV channel has been recalibrated with a radiative transfer model using quality controlled radiosondes. The observations are compared with brightness temperature calculations based on short-term forecast fields (12 and 24 h) of the general circulation model of the European Centre for Medium Range Weather Forecasts (ECMWF). While the model humidity field closely resembles the observed large-scale distribution it is too moist in the subtropics and slightly too dry in areas associated with the Inter Tropical Convergence Zone (ITCZ). A simple method is suggested that quantifies the moisture bias in the general circulation model by adjusting the model based brightness temperatures.