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It is beyond the scope of the Metview documentation to provide explanations to the MARS Retrieval parameters. Instead, instead, it is strongly recommended to consult the MARS keyword keywords documentation find to found out details about the parameters used by this iconMARS keywords..
The MARS Catalogue
The MARS Catalogue is an extremely useful page site where you can browse the contents of the full archive. Here, when When you reach the page in the catalogue where the actual parameters, steps and levels to be retrieved appear you will find a View the MARS Request link.
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