When downloading ERA-Interim data the default horizontal resolution is 0.75x0.75 degrees (Note that ERA-Interim Ocean-Wave data are stored on the wave model’s reduced 1.0x1.0 degrees latitude/longitude grid). If you specify a higher resolution the data will be interpolated and appear smoother. However, the underlying data is still the same, so the improvement is only a visual effect.
We use a bilinear interpolation for all ECMWF products except for vegetation, precipitation type, soil type and Wave 2D spectra, where we use nearest-neighbour.
There is some info about our grids and interpolations in this presentation:
https://software.ecmwf.int/wiki/download/attachments/55122669/intro-interpolation-2016.pdf?api=v2
More detail:
The ERA-Interim Most CAMS and C3S data data is produced and archived not on a Cartesian grid (a two-dimensional rectangular grid), but on a reduced Gaussian grid - think of it as a globe with a series of evenly spaced data points along each parallel, and parallels spaced at 0.7 degreesregular intervals. So near the poles you have only few points along a parallel, but close to the equator you have about 500 many data points along a parallel. For example for ERA-Interim non-wave data this interval is about 0.75 degrees[1], and for ERA5 the default resolution is about 0.3 degrees.
When you download ERA-interim download CAMS and C3S data, the grid points are projected into you can specify to get the output data in a Cartesian lat/long grid and at a custom resolution.
For ERA-Interim you can specify a custom grid on the data server. In this case the . The default horizontal resolution of the output is 0.75x0.75 degrees (about 80km), approximating the irregular grid spacing on the native Gaussian grid
With ERA5 you can specify a custom grid using a Python script. In this case you should set the horizontal resolution to maximum 0.3x0.3 degrees (about 30km), approximating the irregular grid spacing on the native Gaussian grid
In principle you can specify a higher
If you specify a different output resolution, but the system does not use more input points, it merely interpolates the same data to a higher resolution. This makes the output look smoother, but does not increase the accuracy or the precision of the data.
When data is interpolated, all continuous fields (e.g. precipitation, temperature) are interpolated by bilinear interpolation, and discrete fields (e.g. vegetation, precipitation type, soil type) and Wave 2D spectra are interpolated by nearest-neighbour.
For more information about our grids and interpolations see in this presentation https://software.ecmwf.int/wiki/download/attachments/55122669/intro-interpolation-2016.pdf?api=v2
1 For ERA-Interim non-wave data the horizontal resolution is about 0.75 degrees, but ERA-Interim Ocean-Wave data are stored on the wave model’s reduced 1.0x1.0 degrees latitude/longitude grid
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