CMA re-forecast efficient retrieval

To help users to improve S2S CMA MARS requests performance.

A good understanding of the MARS efficiency issues is essential especially for users that are interested in downloading large amounts of data.

In general it is organised, 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)
        • Hdates (2015-01-01 or 2015-01-05 or 2015-01-08, ...)
          •  time-steps
            • members (for perturbed forecast)
              • levels (for pl or pt)
                • parameters

The idea is to have in the same tape file, all time-steps, all members, all parameters for a type of level, a type, a Hdate

Following the previous paragraph,  the natural way to group requests would be:
all parameters, all levels, all members, all time-steps for 1 Hdate.

Note the following:

1. 'all' means 'all' that the user wants. It doesn't have to be all parameters.
2. If a user is interested only on z500,  he may request more Hdates in one go, since the overall request will not be so big.

for Hdate in Hdate-list
     your-s2S-request(Hdate)

for each Hdate from 2010-03-01 to 2010-03-31
    call your-s2s-request(Hdate) 

The best approach is to iterate over the Hyears you wish. For each Hyear iterate over all Hmonths and for each Hmonth iterate over all its Hdays.

for Hyear in Hyears 
    for Hmonth in Hyear
        for Hdays in Hmonth
             Hdate = Hyear-Hmonth-Hday

                 your-s2S-request(Hdate)

for Hyear in Hyears 
    for Hmonth in Hyear
        for Hdays in Hmonth

             Hdate = Hyear-Hmonth-Hday

             your-S2S-request(Hdate)

for each Hyear from 2010 to 2014

    for Hmonth in April, June

        for Hday in Hmonth

             Hdate = Hyear-Hmonth-Hday

             your-S2S-request(Hdate)