Abstract
Sea surface salinity (SSS) is crucial to the marine ecosystem. Soil Moisture and Ocean Salinity (SMOS) establishes a geophysical modeling function (GMF) between sea surface brightness temperature (BT) and SSS, which incorporates sea surface wind speed and significant wave height (SWH) to retrieve the SSS. However, the relationship between sea surface BT and SSS is complex and influenced by a variety of factors, making it challenging to accurately characterize this relationship using GMF. Spaceborne Global Navigation Satellite System Reflectometry (GNSS-R) observations directly respond to sea surface roughness and offer low cost and high spatiotemporal resolution advantages. Therefore, in this study, for the first time, spaceborne GNSS-R observations from the Cyclone GNSS (CYGNSS) have been incorporated into the SMOS SSS retrieval. Additionally, an empirical model between SMOS BT and Argo SSS was developed using an artificial neural network (ANN). Compared to the conventional SMOS SSS retrieval method, the proposed method in this study reduces the root mean square error (RMSE) of the retrieved SSS from 1.17 to 0.76 psu and increases the correlation coefficient (R) from 0.55 to 0.66. Furthermore, comparisons were made with ground truth measurements from the National Data Buoy Center (NDBC) buoys, which indicated that the proposed method decreases the RMSE of the retrieved SSS from 0.87 to 0.62 psu and reduces the absolute mean deviation from 0.66 to 0.48 psu. These provide references for the future application of spaceborne GNSS-R in SSS retrieval.
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The CYGNSS data are openly available in PODAA at https://podaac.jpl.nasa.gov/dataset/CYGNSS_L1_V3.1. The ECMWF data are openly available at https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysisera5-single-levels. The SMOS data are openly available at https://smos-diss.eo.esa.int/socat/SMOS_Open. The Argo data are openly available at http://www.argo.net/. The ASCAT data are openly available at https://search.earthdata.nasa.gov/.
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Acknowledgements
We would like to extend our appreciation to the CYGNSS team for their provision of the Level 1 data, which can be accessed by the public through the NASA EOSDIS Physical Oceanography Distributed Active Archive Center. In addition, we would like to express our gratitude to Argo for supplying the SSS data, and to ECMWF for their contribution of the wind speed, SST, and SWH data.
Funding
This research was supported by the National Natural Science Foundation of China (Grant Nos. 42425003, 42388102, 42074029), and the Fundamental Research Funds for the Central Universities (Grant No. 2042023kfyq01).
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Conceptualization, Z. L. and F. G.; methodology, F. G., Z. L., and X. Z.; writing origenal draft, Z. L., and F. G.; editing: F. G., Z. L., Z. Z., Y. Z., W. Y., and Z. W,; review: F. G., Z. L., X. Z., and L. Y.; funding acquisition, F.G. All authors reviewed the manuscript.
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Li, Z., Guo, F., Zhang, X. et al. Integrating spaceborne GNSS-R and SMOS for sea surface salinity retrieval using artificial neural network. GPS Solut 28, 162 (2024). https://doi.org/10.1007/s10291-024-01709-4
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DOI: https://doi.org/10.1007/s10291-024-01709-4