Four-dimensional variational assimilation of Special Sensor Microwave/Imager total column water vapour in the ECMWF model

Special Sensor Microwave/Imager (SSM/I) radiances have been used to infer a total column water-vapour amount using a one-dimensional variational retri eval in the European Centre for Medium-Range Weather Forecasts (ECMWF) fore cast model. These retrievals are then assimilated using the operational, fo ur-dimensional variational (4DVar) analysis at ECMWF. The impact of assimil ating the total column water vapour on the ECMWF analyses and forecasts is assessed. The primary impact is on the model analysed humidity fields in th e tropics and southern hemisphere where 2 to 4% more water vapour is includ ed in the lower troposphere. The addition of moisture in the model also has an impact on the meridional circulation. No impact is seen in the mean ext ratropical forecast performance but a case study is presented of the improv ed forecast of low-pressure systems in the Southern Ocean, when SSM/I data are used in 4DVar.