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You can either convert your existing OpenIFS installation or create a new installation for OpenIFS/AC. We recommend the latter and in this case you will also need the sources for the OpenIFS model (oifs43r3v2) which can be found on the ftp server.
Connecting How to establish a connection to the ECMWF ftp server and how to obtain OpenIFS 43r3 sources is described in more detail here in the OpenIFS User Guide.
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