Satellite-Based Remote Sensing of Atmospheric Water Vapor Over Oceans: An Inter-Comparison Against Shipborne GNSS Observations

Water vapor over oceans is integral to climate research, weather prediction, and various scientific disciplines. Owing to challenges in deploying in situ instruments, water vapor over oceans is predominantly measured using satellite-borne sensors such as the satellite-borne scanning microwave radiometer (SMWR) and satellite-borne optical imager (SOI). Nevertheless, evaluations of satellite-based remote sensing of atmospheric water vapor over oceans using alternative independent techniques remain limited. In this study, we examine the performance of satellite-borne sensors in measuring water vapor over oceans using shipborne global navigation satellite system (GNSS) precipitable water vapor (PWV) from 2014 to 2021. The comprehensive evaluation of satellite-based PWV over oceans reveals that SMWR PWV outperforms SOI PWV by approximately 2 mm in root-mean-square (rms). Among all satellite-borne SMWRs, Fengyun (FY)-3 C Microwave Radiation Imager-1 exhibits superior agreement with a mean value of 0.26 mm and an rms of 2.20 mm. Both SMWRs and SOIs exhibit diminished agreement in wetter areas, especially for SOIs, attributable to heightened sensitivity to cloudy and moist weather conditions. Temporally, most satellite-borne sensors demonstrate stable performance in PWV retrieval over oceans, with no apparent observation drift. In addition, MODIS exhibits slightly better stability performance compared to SMWRs. The inter-technique validations affirm the elevated accuracy and stability of satellite-based PWV over oceans. Nonetheless, long-term calibration and refinement of algorithms remain imperative, particularly in tropical regions.