ERA-Interim Data assimilation system

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

• 12 hour 4D-Var. u 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: u 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.