Identify the data you want to retrieve

• 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 closer closest to the surface (127/to/137)
• Area: Europe
• Output grid: regular lat/lon 0.5/0.5

Use MARS "list" verb to find out the size

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of your request

You can use Using the MARS catalogue and the "View MARS request" functionality you can create a list request of all the data you want which would tell you the size and distribution of the raw data in the archive. Note the LIST verb and  to get a MARS request and modify the keywords with the information that you need. Once you have the request prepared, you should replace RETRIEVE by LIST and add the OUTPUT = COST keyword. Any post-processing keyword will be ignored by the LIST verb.

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 this is the output:verb, the following output is written into 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;

Information on how to interpret this output:

• The size is in bytes
• online_size is the number of bytes that are in the MARS disk
• offline_size is the number of bytes that are in the MARS tapes
• number_of_tapes tells how the off_line data is split in tapes at this particular time. This may change in the future if for some reason there is more data in the MARS disk

Use the MARS catalogue to

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find out how the data

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are distributed in files and tapes

Using the MARS catalogue we , browse to the data we you want to get until we get 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 we can choose for 1 day, 1 time, , a different "Parameter", "Level" and "Step" . This means that all can be selected for a specific date and time. All the fields you can choose in this page are in should be stored on the same tape file. Therefore users

You can use the "Estimated download size" feature or you can run with the list verb of the sub section of data that you need appearing in this final stage page.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

...

Now that we know the size of the data that we want to retrieve in one go from the same hypercube block, we can start the study of the best way to iterate to get the full time and datedata you need.

Split the request in sensible chunks iterating through the correct keywords

Using the browser we know The browser can now be used to find out how the data is are distributed in the MARS tree. Now we focus If you have a look to on the "Current selection" section . In this casefor 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 1 time/ 1 day in one go for one monthretrieving data for one date and time at a time for a full month.

Bash script example

#!/bin/bash

#this# 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 degrees)
AREA="73.5/-27/33/45"
GRID="0.5/0.5"
 
# fixed selection from the same block
PARAMS="130/131/132"
LEVELIST="127/128/129/130/131/132/133/134/135/136/137"
STEP="0/to/90/by/1"
 
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}${MONTHm}${my_date}

      #time lop
      for my_time in ${TIMES}; do
        cat << EOF > my_request_${my_date}_${my_time}.mars
REQUESTRETRIEVE,
    CLASS      = OD,
    TYPE       = FC,
    STREAM     = OPER,
    EXPVER     = 0001,
    LEVTYPE    = ML,
    GRID       = ${GRID},
    AREA       = ${AREA},
    LEVELIST   = ${LEVELIST},
    PARAM      = ${PARAMS},
    DATE       = ${my_date},
    TIME       = ${my_time},
    STEP       = 0/to/90/by/1${STEP},
    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      

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