HUMIDITY IN THE ECMWF MODEL - MONITORING OF OPERATIONAL ANALYSES AND FORECASTS USING SSM I OBSERVATIONS/

Over oceans, total column water vapour (TCWV) estimates derived from b rightness 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 i n real-time, and so they are not assimilated into the ECMWF model. The y can thus be used for independent validation of this important aspect of the model's hydrological cycle. The quality of ECMWF operational a nalyses of humidity is assessed for the period between August 1992 and November 1994. At that time, assimilation was performed through an Op timal Interpolation scheme. For this work, 2-week averages of TCWV hav e been computed from analysis humidity fields of the ECMWF model and c ompared 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 analyse d humidity in the tropics can reach 30 to 50%, whereas oceanic subside nce 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 sch eme on analyses and forecasts.