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It is beyond the scope of this documentation to provide explanation to the MARS Retrieval parameters and in general about MARS. Instead, it is strongly recommended to consult the MARS user documentation, especially the MARS keywords documentation to find out details about how to define MARS retrievals.
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This is how it works: 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.
Clicking on this link and selecting the Metview macro tab you will see the Macro code defining the retrieval. This can be straight used in Macro and can also help editing the MARS Retrieval icon.
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