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Grid geometry

ERA5 data is produced and archived not as spectral coefficients or on a Cartesian grid (a two-dimensional rectangular grid), but on a reduced Gaussian grid - think of it as a globe with , which has quasi-uniform spacing over the globe.  Reduced Gaussian grids have a series of evenly spaced data points along each parallel (latitude), and parallels spaced at quasi-regular intervals. So near Near the poles you have there are only a few points along a parallel, but close to the equator you have 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 (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). 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.

For a list of spectral, Gaussian and equivalent lat/lon grids see the Open IFS FAQ > , section "OpenIFS questions: general and runtime >   ", and there "What does the 'T' mean in 'T511', 'T1279' etc?"  and "How do I know the grid from from the 'T' number?"

When you download ERA5 data, the grid geometry of your output data depends on the selected data format:

• If you request the data in the native GRIB format it is delivered with the above grid geometry.
• If you request the data in NetCDF format, it is automatically converted and interpolated from the above grids to a regular lat/lon grid.

Grid resolution

The ERA5 HRES (High Resolution) data has a native resolution of 0.28125 degrees (31km), and the EDA (Ensemble) has a resolution of 0.5625 degrees (62km).

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• .

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All data is made available in spherical coordinates, i.e. unprojected. Some GIS software applications call this 'Geographic Coordinate Systems'.

Coordinate system

All gridded data is made available in Decimal Degrees, lat/lon, with latitude values in the range [-90;+90] referenced to the equator and longitude [-180;180values in the range [0;360] referenced to the Greenwich Prime Meridian

Earth model

For data in GRIB1 format the earth model is a sphere with radius = 6367.47 km, as defined in the WMO GRIB Edition 1 specifications, GDS Octet 17

For data in GRIB2 format the earth model is...GRIB2 allows additional spheroids, including custom ones. For more information see the WMO GRIB2 specifications, section 2.2.1 .

For data in NetCDF format, the earth model is inherited from the GRIB data.. Some software applications automatically display these longitudes in the range [-180;+180].

Interpolation

When you download ERA5 data you optionally can have the option to interpolate the data interpolated to a custom grid and horizontal resolution (eg. 'grid':'0.5/0.5'). The default interpolation method is bilinear for continuous parameters (e.g. Temperature) and nearest neighbour for discrete parameters (eg. Vegetation).

Note that with the NetCDF format, our NetCDF implementation only supports regular grids, so when you request data in NetCDF format ('format':'netcdf'), the data is automatically interpolated from the native Gaussian grid to a regular lat/long grid, which yuo have to specify (eg. 'grid':'0.5/0.5').

The lat/long equivalent of T639 is 0.28125 deg (360/(2*(639+1))). However, the GRIB1 format only supports three decimals, so we recommend that you in any case round the resolution ; we recommend to at least 0.3x0.3 deg.You can in principle specify 25 deg (giving 360/0.25=1440 equally-spaced grid points). Specifying a higher resolution is technically possible, e.g. as 'grid':'0.1/0.1', but  this only oversamples the data and does not improve the accuracy of the data. Specifying a coarser resolution is also possible, but in this case, care should be taken to avoid aliasing.

If you request data in NetCDF format ('format':'netcdf'), interpolation to a regular grid is mandatory, because ECMWF's NetCDF implementation only supports regular grids. Specify the desired lat/lon grid with the 'grid' keyword, for example 'grid':'0.25/0.25'.

Visualisation of regular lat/lon data

If you use ERA5 data in a regular lat/lon grid

, many software applications by default 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

values, with a regular spacing, as shown

on the right

in

plot

(

b

a): here global ERA5 data was

downloaded with

interpolated to a regular lat/lon grid

and a

with resolution r

of

= 0.

3 deg, and plotted on top of a satellite image with 0.25 degree image resolution

25 deg. 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. Many software applications by default visualise regularly spaced data as a continuous tiled surface, as in (b). If you use this visualisation, think of coordinates as referencing the centroids of the tiles. However, the ECMWF interpolation software does not conserve area integrals, so this visualisation can be misleading. We recommend you conceptualise ERA5 data as in (a).

(a) Visualisation of regular lat/lon data as

a continuous tiled surfaceImage Removed

point matrix Image Added (b) Visualisation of regular lat/lon data as

point matrixImage Removed

a continuous tiled surface Image Added

For GIS users

Some software applications do not recognise the spatial reference information embedded in the data file and may require you to manually assign a spatial reference. In this case use a 6367.47km sphere for all data if possible. This GRIB1 sphere does not have an EPSG code.

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• ERA-Interim is  produced and stored as spectral coefficients with a truncation of T639 T255 or on the N320 N128 reduced Gaussian grid (depending on the parameter). 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.
• ERA-15 has a resolution of T106, N80.

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