TCWV (Total Column Water Vapour) and VIWV (Vertical Integral of Water Vapour) are essentially the same, but produced by different software.
TCWV is produced by the IFS (ECMWF forecast system) 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
Both TCWV and VIWV are in kg m^-2.
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).
