The Web API uses MARS to transform and interpolate the input fields to the user requested grid.
From 29 January 2019 10AM (UTC) MARS uses the interpolation provided by the MIR library. For more information see: https://confluence.ecmwf.int/display/UDOC/MARS+interpolation+with+MIR |
Please report any issue you may have with the retrieved data to servicedesk@ecmwf.int