...
| Note | ||
|---|---|---|
| ||
For the time being and until further notice, ERA-Interim (1st January 1979 to 31st August 2019) shall continue to be accessible through the ECMWF Web API. Keeping in mind that ERA-Interim is published with an offset of about three months from the dataset's reference date, ERA-Interim August 2019 data will be made available towards the end of October 2019. ERA5 is now available from the Climate Data Store (CDS) (What are the changes from ERA-Interim to ERA5?) and users are strongly advised to migrate to ERA5 (How to download ERA5). |
...
Longitudes range from 0 to 360, which is equivalent to -180 to +180 in Geographic coordinate systems.
Spatial reference systems
The ECMWF model assumes the Earth is a perfect sphere, but the geodetic latitude/longitude of the surface elevation datasets are used as if they were the spherical latitude/longitude of the ECMWF model.
ECMWF data is referenced in the horizontal with respect to the WGS84 ellipse (which defines the major/minor axes) but in the vertical it is referenced to the Geoid (EGM96).
Earth model
For data in GRIB1 format (as is the case with ERA-Interim data) the earth model is a sphere with radius = 6367.47 km, as defined in the WMO GRIB Edition 1 specifications, Table 7, GDS Octet 17
For data in NetCDF format (i.e. converted from the native GRIB format to NetCDF), the earth model is inherited from the GRIB data.
Temporal frequency
Analyses of atmospheric fields on model levels, pressure levels, potential temperature and potential vorticity, are available every 6 hours at 00, 06, 12, and 18 UTC. Forecasts run twice at 00 and 12 UTC and provide 3 hours output for surface and pressure level parameters up to 24 hours, with decreasing frequency to 10 days.
...
- https://www.the-cryosphere.net/11/923/2017/tc-11-923-2017.pdf
- https://www.ecmwf.int/sites/default/files/elibrary/2013/13946-using-reanalyses-studying-eurasian-snow-cover-and-its-relationship-circulation-variability.pdf
- https://journals.ametsoc.org/doi/pdf/10.1175/JHM-D-12-012.1
Surface elevation and orography
In ERA-Interim, and often in meteorology, altitudes (the altitude of the land and sea surface, or specific altitudes in the atmosphere) are not represented as geometric altitude (in metres above the spheroid), but as geopotential height (in metres above the geoid). However, ECMWF archive the geopotential (in m2/s2), not the geopotential height.
In order to calculate the geopotential height of the land and sea surface (the so called surface geopotential height, or orography):
- First, download the surface geopotential (i.e. the geopotential of the land and sea surface), http://apps.ecmwf.int/datasets/data/interim-full-invariant/?param=129.128
- Then divide the surface geopotential by g=9.80665 to obtain the surface geopotential height in metres.
In order to define the surface geopotential in ERA-Interim, the ECMWF model uses surface elevation data interpolated from GTOPO30, with some fixes for Antarctica and Greenland. See Chapter 10 Climatological data, of Part IV. Physical processes, of the ERA-Interim model documentation at https://www.ecmwf.int/search/elibrary/part?solrsort=sort_label%20asc&title=part&secondary_title=31r1.
Notes
- The surface geopotential is also available on model levels (at level=1), where it is archived in spectral form.
- Over oceans the surface geopotential field shows 'spectral ripples' (Know issue KI6, see ERA-Interim known issues page for details), which are a reflection of the fact that the ERA-Interim model is a spectral model and the grid point surface geopotential has been spectrally fitted.
- The model levels are hybrid pressure/sigma, eg for ERA-Interim. See Chapter 2 Basic equations and discretization of Part III. Dynamics and numerical procedures, of the ERA-Interim model documentation at https://www.ecmwf.int/search/elibrary/part?solrsort=sort_label%20asc&title=part&secondary_title=31r1.
- The definition of the 60 model levels, for ERA-Interim, and the corresponding half-level, ph, and full-level, pf, values of pressure (for a standard atmosphere with a surface pressure of 1013.250 hPa), geopotential and geopotential heights can be found at http://www.ecmwf.int/en/forecasts/documentation-and-support/60-model-levels
Known issues
Please see the ERA-Interim known issues page for guidance and workarounds.
...