Through systematic increases of computing power, 4-dimensional variational assimilation (4D-Var) became feasible and part of ECMWF operations since 1997, paving the way to base ERA-Interim on 4D-Var (rather than 3D-Var as in ERA-40).
Enhanced computing power also allowed horizontal resolution to be increased from T159 (N80) to T255 (N128), and the latest cycle of the atmospheric model (IFS CY31r1 and CY31r2) to be used, taking advantage of improved model physics.
ERA-interim retains the same 60 model levels used for ERA-40 with the highest level being 0.1 hectopascal.
In addition, data assimilation of ERA-Interim also benefits from quality control that draws on experience from ERA-40, variational bias correction of satellite radiance data, and more extensive use of radiances with an improved fast radiative transfer model.
ERA-Interim production to stop on 31st August 2019
As ERA5 is now available (What are the changes from ERA-Interim to ERA5?), we are preparing to stop the production of ERA-Interim on 31st August 2019. This means that the complete span of ERA-Interim data will be from 1st January 1979 to 31st August 2019.
Keeping in mind that ERA-Interim is published with an offset of about three months from the dataset's reference date, ERA-Interim August 2019 data will be made available towards the end of 2019.
For the time being and until further notice, ERA-Interim shall continue to be accessible through the ECMWF Web API. ERA5 is available from the Climate Data Store (CDS).