The error
You try to retrieve data from the ECMWF data archive in NetCDF format but the retrieval fails with this message:
ECCODES ERROR : Wrong number of fields ECCODES ERROR : File contains 806 GRIBs, 806 left in internal description, 745 in request ECCODES ERROR : The fields are not considered distinct! ECCODES ERROR : Hint: This may be due to several fields having the same validity time. ECCODES ERROR : Try using the -T option (Do not use time of validity)
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 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, which supports multiple data values at any one validity time. 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-Interim:
- first with time = 00:00 and all required steps
- then with time = 12:00 and all required steps
