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Grid geometry

ERA5 data is produced and archived as spectral coefficients or on a reduced Gaussian grid, 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. 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.

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 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.

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 values in the range [0;360] referenced to the Greenwich Prime Meridian. Some software applications automatically display these longitudes in the range [-180;+180].

Interpolation

When you download ERA5 data you have the option to interpolate the data to a custom grid and horizontal resolution (eg. 'grid':'0.5/0.5').

The native horizontal spatial reference for ERA5 HRES data is a T639, N320 reduced Gaussian grid, equivalent to a horizontal resolution of about 31km or 0.3deg. When you download ERA5 data this is the default horizontal resolution, but optionally you can have the data interpolated to a custom horizontal resolution.

The native file format for ERA5 is GRIB, but when you download ERA5 data you can choose between the native GRIB format or have the data converted to NetCDF format. This determines your options for the output horizontal 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))).

...

However, the GRIB1 format only supports three decimals, so we recommend that you round the resolution

...

to 0.

...

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

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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.

(a) Visualisation of regular lat/lon

Many software applications by default visualise regularly spaced data as a continuous tiled surface

Image Removed

, as in (b)

Visualisation of regular lat/lon data as point matrixImage Removed

 

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) ...

<adapted for ERA5 until here>

but the geodetic lat/lon of the orography dataset are used as if they were the spherical lat/lon of the ECMWF model. In the vertical the data is referenced to the Geoid (EGM96).

 

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

In practice, considering that global meteorological models operate in spatial resolutions of at least multiple kilometres and with significant spatial uncertainty, for most users assigning one of the commonly used geodetic datums (WGS1984, ETRS1989, etc.) to the downloaded data is a 'good enough' solution.

Further information:

The different file formats handle spatial reference information differently:

• GRIB1 only allows two possibilities - a spherical earth with radius 6367.47 km or an oblate spheroid earth with the major axis 6378.160 km, minor axis 6356.775 km and f = 1/297.0, as specified in the WMO GRIB Edition 1 specifications.
• GRIB2 allows additional spheroids, including custom ones. For more information see the WMO GRIB2 specifications, section 2.2.1 .
• ERA-Interim data provided in NetCDF format is converted on demand from the native GRIB format to NetCDF, so the spatial reference is inherited from the GRIB file. 

At ECMWF the ERA-Interim data is  produced and stored as a set of grid points on a reduced Gaussian grid on a sphere with radius 6367.47 km, as specified in the WMO GRIB Edition 1 specifications.

...

Comparison with other ERA products

• 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.

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

• , N80.

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