19 October 2016 Impact of sea surface temperature on satellite retrieval of sea surface salinity
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Currently, global sea surface salinity (SSS) can be retrieved by the satellite microwave radiometer onboard the satellite, such as the Soil Moisture and Ocean Salinity(SMOS) and the Aqurius. SMOS is an Earth Explorer Opportunity Mission from the European Space Agency(ESA). It was launched at a sun-synchronous orbit in 2009 and one of the payloads is called MIRAS(Microwave Imaging Radiometer using Aperture Synthesis), which is the first interferometric microwave radiometer designed for observing SSS at L-band(1.41 GHz).The foundation of the salinity retrieval by microwave radiometer is that the sea surface radiance at L-band has the most suitable sensitivity with the variation of the salinity. It is well known that the sensitivity of brightness temperatures(TB) to SSS depends on the sea surface temperature (SST), but the quantitative impact of the SST on the satellite retrieval of the SSS is still poorly known. In this study, we investigate the impact of the SST on the accuracy of salinity retrieval from the SMOS. First of all, The dielectric constant model proposed by Klein and Swift has been used to estimate the vertically and horizontally polarized brightness temperatures(TV and TH) of a smooth sea water surface at L-band and derive the derivatives of TV and TH as a function of SSS to show the relative sensitivity at 45° incident angle. Then, we use the GAM(generalized additive model) method to evaluate the association between the satellite-measured brightness temperature and in-situ SSS at different SST. Moreover, the satellite-derived SSS from the SMOS is validated using the ARGO data to assess the RMSE(root mean squared error). We compare the SMOS SSS and ARGO SSS over two regions of Pacific ocean far from land and ice under different SST. The RMSE of retrieved SSS at different SST have been estimated. Our results showed that SST is one of the most significant factors affecting the accuracy of SSS retrieval. The satellite-measured brightness temperature has a higher sensitivity with SSS variation and better accuracy of SSS retrieval at higher SST. For the most open oceans where surface salinity is typically greater than 32 psu, the sensitivity is around 0.2-0.25 K/psu for both vertical polarization and horizontal polarization when SST is 5°C,and the TB is more sensitive to the SSS for vertical polarization than horizontal polarization with the increase of SST. When SST increases to 30°C, the sensitivity is around 0.8 K/psu for vertical polarization which is about 40% larger than it of the horizontal polarization. In addition, the result of GAM model indicates that satellite-measured brightness temperature has better correlation with in-situ SSS at higher SST. The mean absolute error of the SMOS-derived SSS is around 0.9 psu when SST is 15°C, and decreases to 0.4 psu when the SST is 30°C.
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Xuchen Jin, Qiankun Zhu, Xianqiang He, Peng Chen, Difeng Wang, Zengzhou Hao, Haiqing Huang, "Impact of sea surface temperature on satellite retrieval of sea surface salinity", Proc. SPIE 9999, Remote Sensing of the Ocean, Sea Ice, Coastal Waters, and Large Water Regions 2016, 99990W (19 October 2016); doi: 10.1117/12.2240841; https://doi.org/10.1117/12.2240841

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