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compute will perform mathematical operations on retrieved grid point fields in GRIB format, see section 10 MARS compute.
list queries the MARS archive about the data availability for a given request, without actually retrieving the data. It is suitable for interactive or batch use to check availability of certain datasets and its collocation in the archive, see section 9 MARS list.
read can be used to filter or manipulate data already retrieved into a file, see read section 5.4.
write is mainly used to save data from fieldsets, a temporary storage, into files, see section 5.2.
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