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The atmospheric model used for ERA-40 is known as IFS CY23r4. A comprehensive documentation about the ECMWF forecasting system (the code of which has been developed jointly with Météo-France.) can be found on the ECMWF website at httpat https://www.ecmwf.int/researchen/ifsdocs/CY23r4/index.html.publications/ifs-documentation
ERA-40 had the following resolution:
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