First you need to understand how the S2S data is organised in MARS. In general it is organised in MARS , as a huge tree, with the indentation below, showing different levels down that tree: - centre (ECMWF, NCEP, JMA, ...)
- realtime or reforecast
- type of data (control forecast or perturbed forecast)
- type of level (single level or pressure level or potential temperature)
- dates (2015-01-01 or 2015-01-05 or 2015-01-08, ...)
- time-steps
- members (for perturbed forecast)
 The idea is to be have in the same tape file, all time-steps, all members, all parameters for a type of level, a type, a date
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