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Cause

This is a known issue when downloading forecast and occurs if

• you request data in NetCDF format,

• and
• you request forecast data, and
• your data request contains overlapping "time"+"step" specifications

For example, for the ERA-Interim dataset there are two daily forecasts (00:00, 12:00), with 3-hourly forecast steps. So one could specify in a data retrieval script:

"date":"2016-12-01"
"type":"fc"
"time": "00:00/12:00",
"step": "3/6/9/12/15",
"format": "netcdf"

• time 00:00 + step 3 → validity time 2016-12-01, 03:00
• time 00:00 + step 6 → validity time 2016-12-01, 06:00
• time 00:00 + step 9 → validity time 2016-12-01, 09:00
• time 00:00 + step 12 → validity time 2016-12-01, 12:00
• time 00:00 + step 15 → validity time 2016-12-01, 15:00
• time 12:00 + step 3 → validity time 2016-12-01, 15:00
• time 12:00 + step 6 → validity time 2016-12-01, 18:00
• time 12:00 + step 9 → validity time 2016-12-01, 21:00
• time 12:00 + step 12 → validity time 2016-12-02, 00:00
• time 12:00 + step 15 → validity time 2016-12-02, 03:00
  

In this example you get two data values at the same validity time 2016-12-01, 15:00.  The NetCDF format does not support multiple data values at a single time, hence the creation of the output NetCDF file fails, triggering the error message.

Workarounds

• Retrieve the data in its native GRIB format, then convert the GRIB files to NetCDF following these guidelines. The GRIB format which supports multiple data values at any one validity time.
• If you are only interested in the analysis, but not forecasts, retrieve data only for step 0
• Then convert the data from GRIB to NetCDF format:
  • For CAMS Global data use ECMWF's ecCodes, and then the grib_to_netcdf tool with the -T option.
  • For CAMS Regional data see here.
• Retrieve data for each forecast "time" If you need overlapping forecast, retrieve for each time option separately, for example for ERA-Interim:
  • first with time = 00:00 and all required steps
  • then with time = 12:00 and all required steps