ERA-Interim

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. Now progress is being made in producing 'ERA-Interim' as described in ECMWF Newsletter No 110.

ERA-Interim has now progressed beyond the end of ERA-40. After completion of the first four years it was decided to revise the configuration of the system, as discussed in ECMWF Newsletter No. 111, and to rerun the initial segment. Other shorter reruns for later periods have been completed as needed for technical reasons.

The first ten years, 1989-1998, of the validated ERA-Interim analysis daily products, comprising the merged production run and the reruns can now be accessed by MARS users (expver=1, class=ei). Also available are the 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. It is expected that the next five years of ERA-Interim, 1999-2003 will be released after validation around June 2008 and that the production will catch up with real time during the second half of this year. ERA-Interim products is also publicly available on the ECMWF Data Server, at a 1.5° resolution, including several products than were not available for ERA-40.

• 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 are currently reprocessing 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 GPS radio occultation measurements, processed and archived at UCAR, have been obtained to cover the period from mid 2001 to mid 2006. Subsequent occultation data, from the constellation of CHAMP, GRACE and COSMIC receivers, has been received operationally
• Boundary forcing fields

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