Cause

This is a known issue 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

Cause of the error:

This is known issue when downloading data in the NetCDF format:

For example, you select all times (00:00, 06:00, 12:00, 18:00) and you select all steps (0 to 120 in 3-hour steps).

In this case you will get data for:

2016-12-01, 00:00 (time 00:00 + step 0)

2016-12-01, 03:00 (time 00:00 + step 3)

2016-12-01, 06:00 (time 00:00 + step 6)

2016-12-01, 06:00 (time 06:00 + step 0)

2016-12-01, 09:00 (time 06:00 + step 3)

... and so on.

So in this example you get two values at validity time 2016-12-01, 0615:00.  The NetCDF format only supports does not support multiple data values at a single , one-dimensional time dimension, so a specific time can not have two Ozone values say.

This is why trying to retrieve this data in NetCDF format results in an error.

Possible workarounds

, hence the creation of the output NetCDF file fails, triggering the error message.

Workarounds

• Retrieve the data in its native GRIB format, which Use the data's native GRIB format. This format supports multiple data values at any one validity time.
• If you are only interested in the analysis, but not forecasts, download 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" separately, for example for ERA-InterimIf you need overlapping forecast, download for each time option separately, for example:
  • first with time = 00:00 and all required steps
  • then with time = 06:00 and all required stepsthen with time = 12:00 and all required stepsthen with time = 18:00 and all required steps