This knowledge base article shows you how to calculate daily total precipitation using ERA-Interim data. If you just want monthly means, then you can simply download it from http://apps.ecmwf.int/datasets/data/interim-mdfa/levtype=sfc/.

Step-by-step guide

1. Use script below to download daily total precipitation ERA-Interim data for 1st January 2018. This script will download total precipitation from two forecasts at 00 UTC and 12 UTC, both at step 12. This will give you 24 hours coverage.
   
   1. Time 00 UTC and step 12 will give you 00 - 12 UTC total precipitation data
   2. Time 12 UTC and step 12 will give you 12 - 24 UTC total precipitation data

   #!/usr/bin/env python from ecmwfapi import ECMWFDataServer server = ECMWFDataServer() server.retrieve({ "class": "ei", "dataset": "interim", "date": "2018-01-01", "expver": "1", "grid": "0.75/0.75", "levtype": "sfc", "param": "228.128", "step": "12", "stream": "oper", "time": "00:00:00/12:00:00", "type": "fc", "format": "netcdf", "target": "tp_20180101.nc", })

   
   

2. Run a second script to calculate daily total precipitation. All it does is to add up the two values for 00-12 UTC and 12 - 24 UTC for the given day.

   #!/usr/bin/env python from ecmwfapi import ECMWFDataServer server = ECMWFDataServer() server.retrieve({ "class": "ei", "dataset": "interim", "date": "2018-01-01", "expver": "1", "grid": "0.75/0.75", "levtype": "sfc", "param": "228.128", "step": "12", "stream": "oper", "time": "00:00:00/12:00:00", "type": "fc", "format": "netcdf", "target": "tp_20180101.nc", })

    

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