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ERA stands for 'European Reanalysis' and refers to a series of research projects at ECMWF which produced various datasets (ERA-Interim, ERA-40, etcetera).
ERA-Interim uses a fixed version of a numerical weather prediction (NWP) system (IFS - Cy31r2) to produce reanalysed data. The fixed version ensures that no spurious trends are caused by an evolving NWP system, though the changing observing system can create such trends. The NWP system blends, or assimilates observations with a previous forecast to obtain the best fit to both. The result of this blending is called an analysis and is the starting point for the next forecast. In this manner, data is produced at increasingly later times.
Analysed data is described as instantaneous, though it does represent an average over the model time step (30 minutes for ERA-Interim). Depending on the parameter, forecast data in ERA-Interim is either instantaneous or accumulated from the beginning of the forecast. Parameters such as precipitation and radiation are accumulated. The accumulated parameters in ERA-Interim are listed in Table 9 (and 13, 14 and 15) of the ERA-Interim archive document:
http://www.ecmwf.int/en/elibrary/8174-era-interim-archive-version-20
Analyses are available every 6 hours and the twice daily forecasts (from 00 and 12 UTC) provide output for surface and pressure level parameters, at forecast steps every 3 hours to 24 hours, then with decreasing frequency to 10 days.
Table 8 in the ERA-Interim archive document lists the non-accumulated surface parameters (but not the invariants, see Table 7) and indicates whether they are available from the analyses and/or forecasts.
Product description
- The ERA-Interim dataset contains atmospheric and surface parameters:
- 6-hourly atmospheric fields on model levels, pressure levels, potential temperature and potential vorticity
- 3-hourly surface fields and daily vertical integrals
- Monthly averages of daily means
Synoptic monthly averages at 0 UTC, 6 UTC, 12 UTC, 18 UTC
- The temporal coverage is from 1 January 1979 to present
- The spatial coverage is global
- The spectral resolution is T255 (~80 km spatial resolution) on 60 vertical levels from the surface up to 0.1 hPa.
The data assimilation system used to produce ERA-Interim is based on a 2006 release of the Integrated Forecast System (IFS - Cy31r2). The system includes a 4-dimensional variational analysis (4D-Var) with a 12-hour analysis window.
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