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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 - think of it as a globe with a series of evenly spaced data points along each parallel (latitude), and parallels spaced at quasi-regular intervals. So near the poles you have only few points along a parallel, but close to the equator you have many data points along a parallel.
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The ERA5 HRES (High Resolution) and EDA (Ensemble) data are archived in these grid representations:
- Some (mostly model levelupper 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 (mostly pressure level and surface levelincluding 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).
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