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ERA5 data is produced and archived not on a Cartesian grid (a two-dimensional rectangular grid), but as spectral coefficients or on a reduced Gaussian grid, which has quasi-uniform spacing over the globe. Reduced Gaussian grids grids have a series of evenly spaced data points along each parallel (latitude), and parallels spaced at quasi-regular intervals. Near the poles there are only a few points along a parallel, but close to the equator there are many more data points along a parallel.
A reduced Gaussian grid is quite different from a Cartesian grid (a two-dimensional rectangular grid).
The ERA5 HRES (High Resolution) and EDA (Ensemble) data are archived in these grid representations:
- Some (upper air) parameters are archived as spectral coefficients (indicated in the data as 'sh', Spherical Harmonics) with a triangular truncation of T639 (HRES) and T319 (EDA)
- Some (including all the surface) parameters are archived on a reduced Gaussian grid with a resolution of N320 (HRES) and N160 (EDA). These grids are so called "linear grids", sometimes referred to as TL639 (HRES) and TL319 (EDA). N320, for example, refers to the number of latitude circles from pole to equator.
For wave data, both HRES and EDA are archived on a reduced latitude/longitude grid.
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ERA-Interim is produced and stored as spectral coefficients with a truncation of T255 or on the N128 reduced Gaussian grid (depending on the parameter), with 60 model levels. See ERA-Interim: What is the spatial reference
- ERA-40 has a resolution of T159 (triangular truncation of 159), N80 (80 latitude circles, pole to equator), L60 (model levels).
- ERA-15 has a resolution of T106 with 31 vertical hybrid levels, N80.
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