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.

The reanalysis programs of numerical weather prediction (NWP) centers provide global, comprehensive descriptions of the atmosphere and of the earth's surface over long periods of time. The high realism of their representation of key NWP parameters, like temperature and winds, implies some realism for less emblematic parameters, such as cloud cover, but the degree of this realism needs to be documented.This study aims to evaluate the high clouds over open oceans in the ECMWF 15- and 45-yr reanalyses. The assessment is based on a new 23-yr climatology of monthly frequencies of high-cloud occurrence retrieved from the infrared radiances measured by operational polar satellites. It is complemented by data from the International Satellite Cloud Climatology Project.It is shown that the 45-yr ECMWF reanalysis dramatically improves on the previous 15-yr reanalysis for the realism of seasonal and interannual variations in high clouds, despite remaining systematic errors. More than 60% of the observed anomalies during the January 1979-February 2002 period over large oceanic basins are captured by the latest reanalysis. However the realism of the analyses in the areas and in the years with sparse observations appears to be poor. Consequently. the interannual variations may not be reliable before January 1979 in most parts of the world. Possible improvements of the handling of assimilated satellite observations before and after this date are suggested.

The re-analysis programmes of numerical weather prediction (NWP) centres provide global, comprehensive descriptions of the atmosphere and of the Earth surface over long periods of time. The high realism of their representation of key NWP parameters, like temperature and winds, implies some realism for less emblematic parameters, such as cloud cover, but the degree of this realism needs to be documentated. This study aims to evaluate the high clouds over open oceans in the ECMWF 15-yr and 45-yr re-analyses. The assessment is based on a new 23-yr climatology of monthly frequencies of high cloud occurrence retrieved from the infrared radiances measured by operational polar satellites. It is complemented by data from the International Satellite Cloud Climatology Project. It is shown that the 45-yr ECMWF re-analysis dramatically improves on the previous 15-yr re-analysis for the realism of seasonal and interannual variations in high clouds, despite remaining systematic errors. More than 60% of the observed anomalies during the January 1979 February 2002 period in large oceanic basins are captured by the latest re-analysis. However the realism of the analyses in the areas and in the years with sparse observations appears to be poor. Consequently, the interannual variations may not be reliable before January 1979 in most parts of the world. Possible improvements of the handling of assimilated satellite observations before and after this date are suggested.