| Summary: | The marine atmospheric boundary layer (MABL) has a profound impact on sensible heat and moisture exchanges between the surface and the free troposphere. The goal of this study is to develop an alternative technique for retrieving MABL-specific humidity (<i>q</i>) using GNSS-RO data in deep-refracted signals. The GNSS-RO signal amplitude (i.e., signal-to-noise ratio or SNR) at the deep straight-line height (<i>H<sub>SL</sub></i>) was been found to be strongly impacted by water vapor within the MABL. This study presents a statistical analysis to empirically relate the normalized SNR (<i>S<sub>RO</sub></i>) at deep <i>H<sub>SL</sub></i> to the MABL <i>q</i> at 950 hPa (~400 m). When compared to the ERA5 reanalysis data, a good linear <i>q</i>–<i>S<sub>RO</sub></i> relationship is found with the deep <i>H<sub>SL</sub></i> <i>S<sub>RO</sub></i> data, but careful treatments of receiver noise, SNR normalization, and receiver orbital altitude are required. We attribute the good <i>q</i>–<i>S<sub>RO</sub></i> correlation to the strong refraction from a uniform, horizontally stratiform and dynamically quiet MABL water vapor layer. Ducting and diffraction/interference by this layer help to enhance the <i>S<sub>RO</sub></i> amplitude at deep <i>H<sub>SL</sub></i>. Potential MABL water vapor retrieval can be further developed to take advantage of a higher number of <i>S<sub>RO</sub></i> measurements in the MABL compared to the Level-2 products. A better sampled diurnal variation of the MABL <i>q</i> is demonstrated with the <i>S<sub>RO</sub></i> data over the Southeast Pacific (SEP) and the Northeast Pacific (NEP) regions, which appear to be consistent with the low cloud amount variations reported in previous studies.
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