ESA CCI SM MODEL-INDEPENDENT Long-term Climate Data Record of Surface Soil Moisture from merged multi-satellite observations

This dataset was produced with funding from the European Space Agency (ESA) Climate Change Initiative (CCI) Plus Soil Moisture Project (CCN4 to ESRIN Contract No: 4000126684/19/I-NB "ESA CCI+ Phase 1 New R&D on CCI ECVS Soil Moisture").  Project website: https://climate.esa.int/en/projects/soil-moisture/

This dataset contains information on the Surface Soil Moisture (SM) content derived from satellite observations in the microwave domain.

Abstract

The MODEL-INDEPENDENT (or "modelfree") product of ESA CCI SM v9.2 provides -- similar to the COMBINED product -- global, harmonized daily satellite soil moisture measurements from both radar and radiometer observations at 0.25-degree spatial grid resolution. It currently covers the period from 2002-2024.

Soil moisture is derived from observations of 13 different active and passive satellites operating across various frequency bands (K, C, X, and L-band). Unlike the COMBINED product -- for which all satellite time series are scaled to GLDAS Noah soil moisture simulations before merging, in order to remove biases between them -- the MODEL-INDEPENDENT product utilizes a satellite-only scaling reference dataset. This reference is based on harmonized L-band (SMOS and SMAP) retrievals from 2010 onwards (Madelon et al., 2022), which were extended further back with C-band retrievals from AMSR-E (2002-2010). Gaps in the scaling reference dataset are filled using the algorithm from the GAPFILLED product (Preimesberger et al., 2025). The merging algorithm employed is consistent with that of the v9.2 COMBINED product. The new scaling reference leads to significantly different absolute soil moisture values, especially in latitudes above 60°N. Data from the SMMR, SSMI and ERS missions are not included in this product.

This product is in its early development stage and should be used with caution, as it may contain incomplete or unvalidated data.

Summary

• 2002-06-19 to 2024-12-31, global, 0.25 deg. regular grid

• GLDAS Noah (model) is replaced with a purely satellite-based scaling reference (based on SMOS)

• Different absolute value range compared to the COMBINED product is expected due to the different scaling reference used

Changelog

• Compared to v9.1, the scaling method to harmonize AMSR-E and SMOS/SMAP data for the scaling reference was changed and uses a third-degree polynomial for CDF matching now.

Known issues

• A temporal inconsistency is observed between the AMSR-E and SMOS period (at 01-2010). This affects mainly data above 60°N and can affect long-term trend detection

Technical Details

The dataset provides global daily estimates for the 2002-2024 period at 0.25° (~25 km) horizontal grid resolution. Daily images are grouped by year (YYYY), each subdirectory containing one netCDF image file for a specific day (DD), month (MM) in a 2-dimensional (longitude, latitude) grid system (CRS: WGS84). The file name has the following convention:

ESACCI-SOILMOISTURE-L3S-SSMV-COMBINED_MODELFREE-YYYYMMDD000000-fv09.2.nc

Each netCDF file contains 3 coordinate variables (WGS84 longitude, latitude and time stamp), as well as the following data variables:

• sm: (float) The Soil Moisture variable reflects estimates of daily average volumetric soil moisture content (m3/m3) in the soil surface layer (~0-5 cm) over a whole grid cell (0.25 degree).
• sm_uncertainty: (float) The Soil Moisture Uncertainty variable reflects the uncertainty (random error) of satellite observations. Derived using triple collocation analysis.
• dn_flag: (int) Indicator for satellite orbit(s) used in the retrieval (day/nighttime). 1=day, 2=night, 3=both
• flag: (int) Indicator for data quality / missing data indicator. For more details, see netcdf attributes.
• freqbandID: (int) Indicator for frequency band(s) used in the retrieval. For more details, see netcdf attributes.
• mode: (int) Indicator for satellite orbit(s) used in the retrieval (ascending, descending)
• sensor: (int) Indicator for satellite sensor(s) used in the retrieval. For more details, see netcdf attributes.
• t0: (float) Representative time stamp, based on overpass times of all merged satellites.

Additional information for each variable is given in the netCDF attributes.

Software to open netCDF files

These data can be read by any software that supports Climate and Forecast (CF) conform metadata standards for netCDF files, such as:

• Xarray (python)
• netCDF4 (python)
• esa_cci_sm (python)
• Similar tools exists for other programming languages (Matlab, R, etc.)
• Software packages and GIS tools can open netCDF files, e.g. CDO, NCO, QGIS, ArCGIS
• You can also use the GUI software Panoply to view the contents of each file

References

R. Madelon et al., “Toward the Removal of Model Dependency in Soil Moisture Climate Data Records by Using an L-Band Scaling Reference," in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 15, pp. 831-848, 2022, doi: 10.1109/JSTARS.2021.3137008.

Related Records

This record and all related records are part of the ESA CCI Soil Moisture science data records community.