Abstract
In recent years, different space agencies have launched satellite missions that carry passive microwave instruments on-board that can measure surface soil moisture. Three currently operational missions are the Soil Moisture and Ocean Salinity (SMOS) mission developed by the European Space Agency (ESA), the Advanced Microwave Scanning Radiometer 2 (AMSR2) developed by the Japan Aerospace Exploration Agency (JAXA), and the Microwave Radiation Imager (MWRI) from China’s National Satellite Meteorological Centre (NSMC). In this study, the quality of surface soil moisture anomalies derived from these passive microwave instruments was sequentially assessed over the mainland of the People’s Republic of China. First, the impact of a recent update in the Land Parameter Retrieval Model (LPRM) was assessed for MWRI observations. Then, the soil moisture measurements retrieved from the X-band observations of MWRI were compared with those of AMSR2, followed by an internal comparison of the multiple frequencies of AMSR2. Finally, SMOS retrievals from two different algorithms were also included in the comparison. For each sequential step, processing and verification chains were specifically designed to isolate the impact of algorithm (version), observation frequency or instrument characteristics. Two verification techniques are used: the statistical Triple Collocation technique is used as the primary verification tool, while the precipitation-based Rvalue technique is used to confirm key results. Our results indicate a consistently better performance throughout the entire study area after the implementation of an update of the LPRM. We also find that passive microwave observations in the AMSR2 C-band frequency (6.9 GHz) have an advantage over the AMSR2 X-band frequency (10.7 GHz) over moderate to densely vegetated regions. This finding is in line with theoretical expectations as emitted soil radiation will become masked under a dense canopy with stricter thresholds for higher passive microwave frequencies. Both AMSR2 and MWRI make X-band observations; a direct comparison between them reveals a consistently higher quality obtained by AMSR2, specifically over semi-arid climate regimes. Unfortunately, Radio Frequency Interference hampers the usefulness of soil moisture products for the SMOS L-band mission, leading to a significantly reduced revisit time over the densely populated eastern part of the country. Nevertheless, our analysis demonstrates that soil moisture products from a number of multi-frequency microwave sensors are credible alternatives for this dedicated L-band mission over the mainland of the People’s Republic of China.
Highlights
Surface soil moisture is an important variable in hydrological and climate systems as it controls the interaction between the land surface and atmosphere
In response to the need for a long-term remotely sensed soil moisture record, the European Space Agency (ESA) established the Climate Change Initiative (CCI) for soil moisture, which has the goal of merging single-sensor soil moisture datasets into a consistent, multi-decadal soil moisture dataset (ECV-SM; [9])
Even though progress towards consistency between satellite paths was recently demonstrated [27], this study only focuses on the descending path of Microwave Radiation Imager (MWRI) and Advanced Microwave Scanning Radiometer 2 (AMSR2) and the ascending path of Soil Moisture and Ocean Salinity (SMOS)
Summary
Surface soil moisture is an important variable in hydrological and climate systems as it controls the interaction between the land surface and atmosphere. In response to the need for a long-term remotely sensed soil moisture record, the European Space Agency (ESA) established the Climate Change Initiative (CCI) for soil moisture (www.esa-soilmoisture-cci.org), which has the goal of merging single-sensor soil moisture datasets into a consistent, multi-decadal soil moisture dataset (ECV-SM; [9]). Nowadays, this ECV-SM has found its way to more than 3000 users and has been readily adopted in a wide range of disciplines
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