Over oceans, total column water vapour (TCWV) estimates derived from brightness temperatures of the Special Sensor Microwave Imager (SSM/I) have been shown to give accuracies similar to radiosonde measurements. Data provided by the SSM/I on the DMSP satellites are not available in real‐time, and so they are not assimilated into the ECMWF model. They can thus be used for independent validation of this important aspect of the model's hydrological cycle. The quality of ECMWF operational analyses of humidity is assessed for the period between August 1992 and November 1994. At that time, assimilation was performed through an Optimal Interpolation scheme. For this work, 2‐week averages of TCWV have been computed from analysis humidity fields of the ECMWF model and compared with simultaneous SSM/I estimates. Discrepancies between model and SSM/I estimates associated with different meteorological regimes are discussed. According to the season, the underestimation of analysed humidity in the tropics can reach 30 to 50%, whereas oceanic subsidence areas located off western coasts of subtropical continents appear too wet by as much as 60%. Mid‐latitudes are generally too moist, but to a lesser extent. An example is also shown of the utilization of SSM/I TCWV to evaluate the impact of an experimental prognostic cloud scheme on analyses and forecasts.
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M. Vesperini (1998) studied this question.