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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.
The manuals on 'Dynamics & Numerical Procedures', 'Physical Processes' and 'Technical & Computational Procedures' are the ones most appropriate for OpenIFS.
ECMWF provide a number of tutorials useful for learning about aspects of OpenIFS related software. Please see Publications on main ECMWF website and follow links or follow specific links below.