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Cause

This is known issue when downloading forecast data in the NetCDF format:, with multiple data values at the same validity time.

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:

No Format
"date":"2016-12-01"
"type":"fc"
"time": "00:00/12:00",
"step": "0/3/6/9/12",
"format": "netcdf"

With the above specification you get data for the following validity times:

  • time 00:00 + step 0 → validity time 2016-12-01, 00:00
  • 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 12:00 + step 0 → validity time 2016-12-01, 12:00
  • 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

So in In this example you get two data values at the same validity time 2016-12-01, 12:00.  The NetCDF format (generated by MARS) only supports a single, one-dimensional time dimension, so a specific time can not have two values.This is why trying to retrieve this data in NetCDF format results in an errordoes not support multiple data values at a single time, hence the creation of the output NetCDF file fails, triggering the error message.

Workarounds

  • Download the data in its native GRIB format, then convert the GRIB files to NetCDF following these guidelines. The GRIB 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
  • If 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 steps
    • then with time = 12:00 and all required steps
    • then with time = 18:00 and all required steps

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