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IFS scientific & technical documents
ECMWF has many publications online, including user manuals, tutorials, seminar proceedings, technical reports and the newsletter, amongst other documents. The full list of ECMWF publications can be found at: http://www.ecmwf.int/en/research/publications.
A more general complete description of the ECMWF Integrated Forecast System (IFS) can be found in the ECMWF Forecast User Guide.