A PRELIMINARY INVESTIGATION INTO THE POTENTIAL OF HY-4A/MICAP DATA FOR SOIL MOISTURE RETRIEVAL

H. Cui¹, H. Liu¹, J. Shi¹, T. Zhao², Z. Peng³

¹National Space Science Center, Chinese Academy of Sciences, ²Aerospace Information Research Institute, Chinese Academy of Sciences, 3Department of Earth System Science, Institute for Global Change Studies, Tsinghua University

Soil moisture is a critical state variable in the global water cycle, carbon cycle, and land–atmosphere energy exchange, playing an essential role in applied research fields such as hydrology, ecology, meteorology, and agriculture. China’s first ocean salinity satellite, HY-4A, was successfully launched on November 14, 2024. One of its key payloads is the Microwave Imager Combined Active and Passive (MICAP), which for the first time combines an L/C/K-band Microwave Interferometric Radiometer (MIR), capable of both single-angle-per-grid-cell and along-track variable-angle observations, with an L-band scatterometer (Liu et al., 2015). This multi-frequency active-passive observation system is equipped with on-orbit radio frequency interference detection and suppression capabilities (Han et al., 2024), significantly improving ocean salinity measurement accuracy while also holding potential for terrestrial soil moisture retrieval. However, its potential for soil moisture retrieval requires further investigation and comprehensive evaluation. Therefore, this study investigates the potential of MICAP for soil moisture retrieval by utilizing multi-angle brightness temperature observations at the L-band. The Multi-Channel Collaborative Algorithm (MCCA) integrates soil moisture and vegetation as a coupled system, enabling simultaneous retrieval of both soil moisture content (SMC) and vegetation optical depth using flexible combinations of multi-frequency, multi-angle, and multi-polarization observations (Shi et al., 2008; Zhao et al., 2021; Peng et al.,). Based on the MCCA, this study improves the SMC retrieval algorithm, particularly the components involving incidence angle information, to accommodate the single-angle-per-grid-cell with along-track variable-angle observation capability of MICAP at the L-band. Soil moisture and vegetation optical thickness were successfully retrieved using the refined algorithm, with MICAP L-band TB observations combined with auxiliary data (such as soil texture and ERA5 land surface temperature), and validated against the International Soil Moisture Network data (primarily covering North America and Europe) collected between February and March 2025. The results demonstrate strong consistency between MICAP soil moisture and ground-based measurements. Moreover, the spatial distribution patterns of MICAP soil moisture also show strong consistency with SMOS and SMAP satellite products, with all datasets exhibiting distinct regional characteristics on a global scale. These results demonstrate the significant potential of MICAP for soil moisture retrieval. However, the validation period remains relatively short. To further explore the capabilities of the MICAP payload over land, subsequent efforts will focus on developing and systematically validating an improved soil moisture retrieval algorithm that integrates multi-frequency brightness temperature observations over extended time series. This algorithm will be systematically compared with internationally recognized mainstream algorithms, aiming to further improve retrieval accuracy and provide reliable data support for operational applications in hydrology, ecology, meteorology, and agriculture.

Keywords: Microwave Imager Combined Active and Passive (MICAP), Multi-Channel

Collaborative Algorithm (MCCA), Soil moisture retrieval, L band, International Soil Moisture Network, Validation

References

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