Soil moisture comparison through triple and quadruple collocation between: Metop, ERA, SMOS and in-situ data

Three years of soil moisture data derived from: the Advanced SCATterometer (ASCAT) and the Soil Moisture and Ocean Salinity (SMOS) radiometer are analyzed and compared. The comparison has been performed within the framework of an activity aiming at validating the EUMETSAT Hydrology Satellite Application Facility (H-SAF) soil moisture products derived from ASCAT over Europe and North Africa. A strategy has been set up in order to enable the comparison between products representing a volumetric soil moisture content, as those derived from SMOS, and a relative saturation index, as those derived from ASCAT. For specific sites, which belong to the International Soil Moisture Network, satellite products have been compared to the ground measurements of moisture gauges and to the ERA Interim/Land model output. The triple collocation technique has been exploited, which is useful to estimate the relative amount of error affecting different sources of soil moisture measurements. A preliminary approach with a quadruple collocation has been also investigated to jointly analyses the four sources of data.