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To help users to improve S2S CMA MARS requests performance via the WebAPI

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 (CMA, 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)
        • HindcastDates (2014-01-01 or 2014-01-02 or 2014-01-03, ...)
          •  time-steps
            • members (for perturbed forecast)
              • levels (for pl or pt)
                • parameters

The idea is to request as much data as possible from the same tape file, all time-steps, all members, all parameters for a type of level, a type, a HindcastDate

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

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 HindcastDates in one go, since the overall request will not be so big.

for HindcastDate in HindcastDate-list (eg, 2010-03-01 to 2010-03-31)
     S2S-request(HindcastDate)

You may also have a look on some CMA re-forecast examples or on the ECMWF Web API Home

for HindcastYear in HindcastYears 
    for HindcastMonth in HindcastYear
        for HindcastDay in HindcastMonth
             HindcastDate = HindcastYear-HindcastMonth-HindcastDays

                 S2S-request(HindcastDate)

for each HindcastYear from 2010 to 2014

    for HindcastMonth in 04, 06

        for HindcastDay in HindcastMonth

             HindcastDate = HindcastYear-HindcastMonth-HindcastDay

             S2S-request(HindcastDate) (see below an example)

#!/usr/bin/env python
from ecmwfapi import ECMWFDataServer
server = ECMWFDataServer()
server.retrieve({
    "class": "s2",
    "dataset": "s2s",
    "date": "2014-05-01",
    "expver": "prod",
    "hdate": HindcastDate, (ie the selected HindcastDate eg "2014-04-01"),
    "levtype": "sfc",
    "origin": "babj",
    "param": "165",
    "step": "0",
    "stream": "enfh",
    "target": "CHANGEME",
    "time": "00",
    "type": "cf",
})