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. 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. 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