You can use a Python program to extract ERA-Interim and other data remotely from ECMWF's archive system MARS. For an introduction see here.
When extracting data from daily data you can specify:
A single date, for example the 1st of January 2015:
"date": "2015-01-01",
Multiple distinct dates, for example the 1st of each month in 2015:
"date": "2015-01-01/2015-02-01/2015-03-01/2015-04-01/2015-05-01/2015-06-01/2015-07-01/2015-08-01/2015-09-01/2015-10-01/2015-11-01/2015-12-01",
A date range, for example all days from 1st of January to 28th of February 2015:
"date": "2015-01-01/to/2015-02-28",
When you extract monthly means, the monthly means are timestamped to the first day of the month, and you have to extract for exactly these dates. So to extract monthly means for all of 2015 you need to use this syntax to specify dates:
"date": "20150101/20150201/20150301/20150401/20150501/20150601/20150701/20150801/20150901/20151001/20151101/20151201",
In all cases you can write a date either as YYYY-MM-DD or as YYYYMMDD.
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