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Identification of soil moisture retrieval errors: Learning from the comparison of SMOS and ASCAT

Due to the strong contrast of the dielectric properties of dry and wet soil at microwave frequencies soil moisture can be retrieved on a global scale from active and passive microwave remote sensing instruments. Recent validation studies carried out over a number of in situ networks in Europe, the US and Australia have demonstrated that the soil moisture retrieval skill achieved with the Soil Moisture and Ocean Salinity (SMOS) mission and the Advanced Scatterometer (ASCAT) compares well in many regions of the world. But of course, there are also areas where the retrieved soil moisture values from these instruments do not match well. In this study global SMOS and ASCAT soil moisture data are compared with the aim to identify the areas of agreement and disagreement. The analysis confirms the findings of the previous studies that over most regions worldwide the temporal evolution of SMOS and ASCAT retrievals compare quite well. Notable exceptions are arid environments where the ASCAT retrievals fail to reproduce the real soil moisture trends, while the SMOS retrievals perform as expected. More work is required to understand the contrasting behavior of the ASCAT and SMOS soil moisture retrievals in these environments.