Analysis of ASCAT, SMOS, in-situ and land model soil moisture as a regionalized variable over Europe and North Africa

A comparison of soil moisture products derived from satellite data, in-situ measurements and land models was performed in the frame of the EUMETSAT H-SAF project. In particular, soil moisture retrievals of ASCAT/H-SAF and SMOS were compared with two other independent data sets, that are the NCEP/NCAR volumetric soil moisture content reanalysis developed by NOAA, and the ERA-Interim/Land soil moisture produced by ECMWF. In situ data available through the International Soil Moisture Network and distributed in regions comprising Denmark, France, Germany, Italy, Poland and Spain, were also included in the comparison. The whole H-SAF region of interest, including Europe and North Africa, was considered and the period between January 2010 and December 2012 was analysed.The Triple Collocation (TC) approach was adopted to perform the comparison exercise. TC was critically reviewed to compare different solutions proposed in the literature and to discuss the possibility of performing a pointwise TC, or a global TC, which considers each system as a whole, with unique gains and error standard deviations in the whole area. The TC results showed a very good behaviour of the ERA land model, while SMOS satellite slightly outperformed ASCAT or vice versa, depending on factors like the geographical area or the consideration of the whole dynamic range of soil moisture or only the anomalies with respect to the seasonal variability.