ERA‐40 is a re‐analysis of meteorological observations from September 1957 to August 2002 produced by the European Centre for Medium‐Range Weather Forecasts (ECMWF) in collaboration with many institutions. The observing system changed considerably over this re‐analysis period, with assimilable data provided by a succession of satellite‐borne instruments from the 1970s onwards, supplemented by increasing numbers of observations from aircraft, ocean‐buoys and other surface platforms, but with a declining number of radiosonde ascents since the late 1980s. The observations used in ERA‐40 were accumulated from many sources. The first part of this paper describes the data acquisition and the principal changes in data type and coverage over the period. It also describes the data assimilation system used for ERA‐40. This benefited from many of the changes introduced into operational forecasting since the mid‐1990s, when the systems used for the 15‐year ECMWF re‐analysis (ERA‐15) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re‐analysis were implemented. Several of the improvements are discussed. General aspects of the production of the analyses are also summarized.

Enhanced wind datasets of the European satellite Meteosat are now provided every 90 mins together with the quality indicator (QI) derived by the quality control of the Meteorological Product Extraction Facility (MPEF) at the European Organisation for the Exploitation of Meteorological Satellites. All three channel cloud motion winds and clear sky water vapor motion winds have been passively monitored by comparison with the European Centre for Medium-Range Weather Forecasts model background field. The evaluation of the relationship between the MPEF QI and the observation - background differences indicate possible benefits to be gained from the use of the QI within the observation screening of the assimilation system. The MPEF quality indicator is used as a selection criterion within the screening. The applied thresholds are restricted in the Tropics compared to the extratropical regions where the threshold for high-level winds has been relaxed below the automatic quality control at MPEF. The wind data derived from imagery of both Meteosat platforms at 0 degrees and 118 degreesE are used in this study. The overall effect is an increase of active Meteosat winds by a factor of 2. This means a considerably increased impact of Meteosat winds on the tropospheric analyses. The assessment of mean wind increments indicates that the increased temporal sampling together with the use of the quality indicator within the observation screening leads to an improvement of the consistency of the atmospheric motion wind data actively used within the four-dimensional variational assimilation system. The averaged impact on the short- and medium-range forecasts is found to be neutral in the Northern Hemisphere and positive in the Southern Hemisphere. In a selected synoptic case study the use of the new Meteosat wind product indicates a considerable improvement of the medium-range forecasts for the North Atlantic and European areas.

Special Sensor Microwave/Imager (SSM/I) radiances have been used to infer a total column water‐vapour amount using a one‐dimensional variational retrieval in the European Centre for Medium‐Range Weather Forecasts (ECMWF) forecast model. These retrievals are then assimilated using the operational, four‐dimensional variational (4DVar) analysis at ECMWF. the impact of assimilating the total column water vapour on the ECMWF analyses and forecasts is assessed. the primary impact is on the model analysed humidity fields in the tropics and southern hemisphere where 2 to 4% more water vapour is included 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 extratropical forecast performance but a case study is presented of the improved forecast of low‐pressure systems in the Southern Ocean, when SSM/I data are used in 4DVar.

During August-September 1995 new near-surface wind datasets over the tropical Atlantic from both the ERS-1 scatterometer and Meteosat satellites were available at the European Centre for Medium-Range Weather Forecasts. At this time there was an unusually high number of hurricanes present in the tropical Atlantic and so the impact of these data on analyzing and forecasting the main cyclones was investigated. Assimilation experiment using a new variational scheme, with the ERS-1 winds, showed clear improvements both in the analyses and short range forecasts. compared with the optimal interpolation scheme without these data. For example, the forecast positions for Hurricane Iris were reduced by almost 50% when the scatterometer data iras included. For Hurricane Luis the improvement was for a higher percentage of cases when the model identified the cyclone in the 24- and 48-h forecasts. For the 72-h forecasts 80% of the reported cyclones were detected compared with only 33% for the analyses without ERS-1 data.The impact of the Meteosat lower-tropospheric cloud motion winds was found to be small due to lack of coverage in the vicinity of the center of the hurricanes at this time. The impact of one profile from a ship in the vicinity of Hurricane Luis just before its approach to the Caribbean Islands was clearly demonstrated by large improvements to both analysts with and without the scatterometer winds.

The current use of satellite remote sensing data in the European Centre for Medium Range Weather Forecasts (ECMWF) atmospheric forecast model is outlined. The method of assimilation used for the diverse observations is based on variational analysis and at present a 3 dimensional system (SD-Var) is in operation at Cloud motion winds from geostationary satellites, upwelling infrared and microwave radiances from polar orbiters and scatterometer sea surface winds are all assimilated at present. Assimilation of these new observation types have been shown to have a positive benefit on the model analyses and forecasts.