ERA-Interim

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ECMWF has in the past produced three major reanalyses: FGGE, ERA-15 (ECMWF Newsletter No 73) and ERA-40 (ECMWF Newsletter No 101). The last of these consisted of a set of global analyses describing the state of the atmosphere and land and ocean-wave conditions from mid-1957 to mid-2002. ERA-Interim as initially described in ECMWF Newsletter No 110 is an 'interim' reanalysis of the period 1989-present in preparation for the next-generation extended reanalysis to replace ERA-40. ECMWF Newsletter Nos 111 and 115 contain further information. An open-access journal article describing the ERA-Interim reanalysis is now available from the Quarterly Journal of the Royal Meteorological Society. ERA-Interim was recently extended by a decade, to the year 1979, and it continues to be updated forward in time.

ERA-Interim analysis daily products from 1 Jan 1979 can be accessed by MARS users (expver=1, class=ei). Also available are twice daily ten-day forecasts and monthly means. The ERA-Interim archive is more extensive than that for ERA-40, e.g. the number of pressure levels is increased from ERA-40's 23 to 37 levels and additional cloud parameters are included. ERA-Interim products are also publicly available on the ECMWF Data Server, at a 1.5° resolution, including several products that were not available for ERA-40.

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Differences in data assimilation and use of observations between ERA-40 and ERA-Interim

• Data assimilation

The main advances in the ERA-Interim data assimilation compared to ERA-40 are:

• 12 hour 4D-Var
• T255 horizontal resolution
• Better formulation of background error constraint
• New humidity analysis
• Improved model physics
• Data quality control that draws on experience from ERA-40 and JRA-25
• Variational bias correction of satellite radiance data, and other improvements in bias handling
• More extensive use of radiances, and improved fast radiative transfer model
• Observations

ERA-Interim uses mostly the sets of observations acquired for ERA-40, supplemented by data for later years from ECMWF's operational archive. There are, however, a few noteworthy exceptions:

• Altimeter wave-heights
• A new ERS altimeter wave-height dataset has been acquired from ESA, providing data of more uniform quality than the Fast Delivery Dataset used from August 1991 onwards in ERA-40
• Winds and clear-sky radiances. EUMETSAT provided reprocessed winds and clear-sky radiances from Meteosat-2 (1982-1988) for ERA-40 and has reprocessed later Meteosat data for ERA-Interim
• Ozone profiles: Reprocessed GOME data from the Rutherford Appleton Laboratory will provide ozone profile information from 1995 onwards
• Radio occultation measurements. CHAMP, GRACE and COSMIC data since 2006
• Boundary forcing fields

Boundary forcing fields are taken from ERA-40 until 2001, and from ECMWF operations for later dates

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