Statistical Merging of Active and Passive Microwave Observations Into Long-Term Soil Moisture Climate Data Records

Satellite observations from active and passive microwave sensors are a valuable means to measure surface soil moisture at the global scale. More than a dozen historical and currently active missions have been used for this purpose, together spanning a period from 1978 to present. Within the Climate Change Initiative (CCI) of the European Space Agency (ESA), these separate missions are systematically combined into homogenized Climate Data Records (CDRs). Currently, the operational production and the near-real-time updating of these CDRs is being transferred to the EU Copernicus Climate Changes Services (C3S). In this study, we provide an overview of the characteristics of the most recent ESA CCI and C3S soil moisture products. In particular, we focus on the new merging procedure, which optimally weighs the individual missions based on their uncertainties estimated with triple collocation analysis.