ERA-Interim production stopped on 31st August 2019
For the time being and until further notice, ERA-Interim (1st January 1979 to 31st August 2019) shall continue to be accessible through the ECMWF Web API.
ERA5 is now available from the Climate Data Store (CDS) (What are the changes from ERA-Interim to ERA5?) and users are strongly advised to migrate to ERA5 (How to download ERA5).
At ECMWF, the variables TCWV (Total Column Water Vapour) and VIWV (Vertical Integral of Water Vapour) are essentially the same, but are produced by different software.
TCWV is generally produced by the ECMWF NWP forecast system (using the IFS) and is available in the analyses and forecasts.
VIWV is produced by the ERA post processing budget software and, for ERA-Interim, is only available in the analyses.
More information is available in the ERA-Interim archive document:
http://www.ecmwf.int/en/elibrary/8174-era-interim-archive-version-20
The units of both TCWV and VIWV are kgm-2.