Identify the data you want

• ML, 3 parameters
• Time: 00 and 12
• Dates: 1 to 30 April 2017
• first 90 steps, every hour (0/to/90/by/1)
• 10 levels closest to the surface (127/to/137)
• Area: Europe
• Output grid: regular lat/lon 0.5/0.5

Use MARS list to find out the size

Using the MARS catalogue and the "View MARS request" functionality a list request of all the data you want can be created which can be used to find out the size and distribution of the raw data in the archive. Note the LIST verb and the OUTPUT = COST keyword.

LIST,
    OUTPUT     = COST,
    CLASS      = OD,
    TYPE       = FC,
    STREAM     = OPER,
    EXPVER     = 0001,
    LEVTYPE    = ML,
    LEVELIST   = 127/128/129/130/131/132/133/134/135/136/137,
    PARAM      = 130/131/132,
    DATE       = 20170401/20170402/20170403/20170404/20170405/20170406/20170407/20170408/20170409/20170410/20170411/20170412/20170413/20170414/20170415/20170416/20170417/20170418/20170419/20170420/20170421/20170422/20170423/20170424/20170425/20170426/20170427/20170428/20170429/20170430,
    TIME       = 0000/1200,
    STEP       = 0/to/90/by/1,
    TARGET     = list.txt

When we run the list action, the following output is written to the file specified with the TARGET keyword (in this example, 'list.txt'):

size=591270399720;
number_of_fields=180180;
online_size=206846193282;
off_line_size=384424206438;
number_of_tape_files=60;
number_of_disk_files=546;
number_of_online_fields=63033;
number_of_offline_fields=117147;
number_of_tapes=8;

Use the MARS catalogue to find out how the data are distributed in files and tapes

Using the MARS catalogue, browse to the data you want to retrieve until you reach the final stage which gives us a selection and several options: http://apps.ecmwf.int/mars-catalogue/?class=od&stream=oper&expver=1&type=fc&year=2017&month=apr&levtype=ml&date=2017-04-01&time=00:00:00

For this particular case, a different "Parameter", "Level" and "Step" can be selected for a specific date and time. All the fields you can choose in this page are stored on the same tape file. You should get as much data as possible from this page in a single MARS retrieval request.

If we run the list of the data that we need and appears in the final stage page we get:

LIST,
    CLASS      = OD,
    TYPE       = FC,
    STREAM     = OPER,
    EXPVER     = 0001,
    LEVTYPE    = ML,
    LEVELIST   = 127/128/129/130/131/132/133/134/135/136/137,
    PARAM      = 130/131/132,
    DATE       = 20170401,
    TIME       = 0000,
    STEP       = 0/to/90/by/1,
    OUTPUT     = cost,
    TARGET     = list2.txt

size=9854506662;
number_of_fields=3003;
online_size=6211981722;
off_line_size=3642524940;
number_of_tape_files=1;
number_of_disk_files=12;
number_of_online_fields=1893;
number_of_offline_fields=1110;
number_of_tapes=1;

Now that we know the size of the data that we want to retrieve in one go from the same hypercube we can start the study of the way to iterate to get the full time and date.

Split the request in sensible chunks iterating through the correct keywords

The browser can now be used to find out how the data are distributed in the MARS tree. Focussing on the "Current selection" section for this specific example shows:

From the top to the bottom we have to start iterating from the inner loop to the outer loop "time", "date", etc.

This is an example BASH script to loop retrieving data for one date and time at a time for a full month.

#!/bin/bash

# this example will filter the area of Europe (N/W/S/E) and interpolate the final fields to
# a 0.5x0.5 regular lat-lon grid (GRID=0.5/0.5)
AREA="73.5/-27/33/45"
GRID="0.5/0.5"
PARAMS="130/131/132"
LEVELIST="127/128/129/130/131/132/133/134/135/136/137"
TIMES="0000 1200"
YEAR="2017"
MONTH="04"

#date loop
for y in ${YEAR}; do

  for m in ${MONTH}; do
    #get the number of days for this particular month/year
    days_per_month=$(cal ${m} ${y} | awk 'NF {DAYS = $NF}; END {print DAYS}')

    for my_date in $(seq -w 1 ${days_per_month}); do
      my_date=${YEAR}${MONTH}${my_date}

      #time lop
      for my_time in ${TIMES}; do
        cat << EOF > my_request_${my_date}_${my_time}.mars
RETRIEVE,
    CLASS      = OD,
    TYPE       = FC,
    STREAM     = OPER,
    EXPVER     = 0001,
    LEVTYPE    = ML,
    GRID       = ${GRID},
    AREA       = ${AREA},
    LEVELIST   = ${LEVELIST},
    PARAM      = ${PARAMS},
    DATE       = ${my_date},
    TIME       = ${my_time},
    STEP       = 0/to/90/by/1,
    TARGET     = oper_ml_${my_date}_${my_time}.grib
EOF
      mars my_request_${my_date}_${my_time}.mars
      if [ $? -eq 0 ]; then
        rm -f my_request_${my_date}_${my_time}.mars
      fi
      done
    done
  done
done