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- If you specify GRIB as output format you will get a GRIB2 file (for model level parameters) or GRIB1 file (for other parameters, unless otherwise indicated). By default the output will be on the native T639 grid, but you can specify a different Gaussian grid or a lat/long grid:
- If you specify a Gaussian grid other than the default T639 the data will be interpolated to your chosen resolution. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation).
- If you specify a lat/lon grid the data will be interpolated to your chosen resolution. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation). The lat/long equivalent of T639 is 0.28125 deg (360/(2*(639+1))). You could use this resolution for parameters stored in GRIB2 format, but GRIB1 format only supports three decimals, so we recommend you round the resolution; we recommend to 0.3x0.3 deg.
- If you specify NetCDF as output format: Our NetCDF implementation only supports regular grids, so when you extract data it is automatically interpolated from the native Gaussian grid to a regular lat/long grid. The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation). Regarding resolution, the lat/long equivalent of T639 is 0.28125 deg (360/(2*(639+1))). You could use this resolution for parameters stored in GRIB2 format, but GRIB1 format only supports three decimals, so we recommend you in any case round the resolution; we recommend to 0.3x0.3 deg.
To visualise the data If you use ERA5 data in a regular lat/lon grid, many software applications by default plot regular lat/lon visualise the data as a continuous tiled surface, as in plot (a) on the right. However, you might prefer to think of the ERA5 data as point data with a regular spacing, as shown on the right in plot (b): here global ERA5 data was downloaded with a regular lat/lon grid and a resolution r of 0.3 deg, and plotted on top of a satellite image with 0.25 degree image resolution. The 'top left' ERA5 data point is always at Longitude=0 ; Latitude=90, with further grid points spaced by r, and the 'bottom right' grid point at Longitude=360-r ; Latitude=-90. | (a) Visualisation of regular lat/lon data as a continuous tiled surface (b) Visualisation of regular lat/lon data as point matrix |
For ERA5 data in all representations the assumed underlying earth model (the geodetic datum) is a sphere with with radius 6367.47km. The surface of the ECMWF model is defined by the orography which has been interpolated from a combination of SRTM30 and other elevation datasets (for details see Part IV. Physical processes, of the IFS model documentation, Chapter 11.2.2 Surface elevation data at 30 arc seconds) The SRTM30 orography data is referenced in the horizontal with respect to the WGS84 ellipse (which defines the data major/minor axis) ...
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