CDS request fails with "request not valid" message

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If your CDS request (web or API) for ERA5 data in  netCDF format data ends up with a failure (no error message is given except that the request is not valid/no reason given), this may well be because the size of the data request is too large for the grib to netCDF conversion to work properly

If the same request for the data in grib works ok, then the conversion to netCDF is likely to be the cause of the issue.

Generally, requests for data larger than 10GB can trigger this error, so if you need data in netCDF either reduce the size of your request, or download the data in grib abd convert to netCDF on your local system.