I heard about soil moisture data produced by ECMWF. How can I access this data?
Soil moisture data is available as part of two public datasets produced by ECMWF: ERA-Interim/Land (1979 to 2010) and ERA-Interim (1979 to present). For soil moisture data ERA-Interim/Land should be preferred over ERA-Interim.
The main difference between ERA-interim and ERA-Interim/Land is that to produce ERA-Interim/Land, near-surface meteorological fields from ERA-Interim were used to force the latest version of the HTESSEL land-surface model (Hydrology-Tiled ECMWF Scheme for Surface Exchanges over Land). This scheme is an extension of the TESSEL scheme (van den Hurk et al. 2000) that was used in ERA-Interim, which was based on a 2006 version of ECMWF’s operational Integrated Forecasting System (IFS). HTESSEL includes an improved soil hydrology (Balsamo et al. 2009), a new snow scheme (Dutra et al. 2010), a multi-year satellite-based vegetation climatology (Boussetta et al. 2011), and a revised bare-soil evaporation (Balsamo et al. 2011; Albergel et al. 2012a).
For further information on ERA-Interim/Land see Balsamo et al 2012: ERA-Interim/Land: A global land-surface reanalysis based on ERA-Interim meteorological forcing. ECMWF ERA Report Series 13
Download ERA-Interim/Land. For soil moisture make sure in the left hand menu 'Analysis' is selected. Volumetric soil water is available for four soil depth layers. Values for soil moisture are typically 0.1 to 0.4, units are m^3^ water / m^3^ volume (cubic meter water per cubic meter volume).
For detailed documentation of the underlying models please see the documentation of ECMWF's forecasting system IFS, C31R1, Part IV: Physical processes, in particular Chapter 8 Surface parametrization.