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The error
You try to retrieve data from the ECMWF data archive in NetCDF format but the retrieval fails with this message:
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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:
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"date":"2016-12-01"
"type":"fc"
"time": "00:00/12:00",
"step": "3/6/9/12/15",
"format": "netcdf" |
With the above specification you get data for the following validity times:
- 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:
- 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
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