Based on Precipitable Water Vapor (PWV) obtained from five coastal Global Navigation Satellite System (GNSS) stations, this study investigates the characteristics and interrelationships of sea surface waves and atmospheric parameters during seawater encroachment in the Bohai and Yellow Seas of China in mid-October 2024. Results indicate that the correlation coefficient between GNSS PWV and ERA5 PWV data is 0.97 (p< 0.01), with a mean bias of -3.28 mm and a standard deviation of 2.70 mm. During the seawater encroachment, three notable increases in PWV were observed, with peaks ranging from 30 to 55 mm, all accompanied by precipitation (less than 10 mm). The PWV change rates varied spatially, with the Yellow Sea coast experiencing faster changes than the Bohai Sea coast. Under strong wind conditions, sea surface wave characteristics were manifested by increased wave heights and longer wave periods, alongside an increased whitecap coverage of breaking waves (3%–5%). The temporal variation curves reveal an inverse relationship between PWV and wave variables, with PWV lagging both breaking and non-breaking waves by approximately 3–12 h. Meanwhile, the process involves enhanced water vapor evaporation and intensified air-sea heat exchange. These factors affected the weighted mean temperature and caused a continuous decrease in PWV. Consequently, zonal differences in the spatial distribution of PWV were observed. By linking GNSS PWV with wave variables, this research introduces a novel insight for the study of air-sea interaction processes and coastal hazard monitoring under extreme weather events.

Global Navigation Satellite System-Acoustic (GNSS-A) technology has been widely used in ocean engineering and ocean environmental science. Accurate sound travel time modeling is essential for GNSS-A seafloor positioning. Currently, half of the two-way travel time (TWTT) has been used as an approximation for the one-way travel time (OWTT). In this work, the time error of the approximate OWTT is investigated under different survey configurations, and a sequential GNSS-A seafloor positioning method using the extended Kalman filter (EKF) is developed to investigate the impact of sound travel time modeling. Simulations show that the time error induced under the static survey configuration is less than 0.6 ms; the time error induced under the circle survey configuration with a stable inclination angle is stable, but the time error of the line survey configuration can reach 28 ms. As confirmed through field experiments, sequential GNSS-A seafloor positioning using TWTT modeling is more stable than OWTT modeling. The positioning residuals of TWTT modeling are similar to those of OWTT modeling under the circle configuration but at least 2 times less than those of OWTT modeling under the line survey configuration. Furthermore, the average positioning residuals of OWTT and TWTT modeling can be greatly reduced for a survey configuration combining circular and linear tracks. These findings provide a feasible method for improving the precision and efficiency of GNSS-A seafloor positioning.

Water vapor over oceans is significant for numerical weather prediction (NWP) and climate research. Ocean platform-based global navigation satellite system (GNSS) which can sense the atmospheric water vapor is becoming an important supplement for water vapor measurements over oceans. However, the application of ocean platform-based GNSS meteorology is normally based on geodetic GNSS receivers, which implies the high cost of hardware. In this contribution, we investigate the potential of retrieving real-time water vapor over oceans with a low-cost receiver (u-blox F9P), and a geodetic GNSS receiver (Trimble NetR9) is also equipped in the experiment vessel. The post-processed Trimble NetR9 zenith total delay (ZTD) estimates and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 precipitable water vapor (PWV) products are used for the validation of real-time ZTDs and PWV values. The results show that the real-time ZTDs derived from the low-cost multi-GNSS (GPS + Galileo) observations obtain a difference of over 2.13 cm in root-mean-square (RMS) compared to the post-processed ZTDs with an averaged initialization time of approximately 40 mins. In addition, compared to ERA5 PWV, the real-time PWV derived from u-blox F9P multi-GNSS observations shows a difference in RMS of approximately 4 mm. Although u-blox F9P multi-GNSS performs relatively worse than Trimble NetR9 multi-GNSS in real-time ZTD/PWV estimates, the accuracy of low-cost GNSS receivers derived water vapor over oceans can still meet the requirements for NWP and nowcasting, which demonstrates promising prospects in supplementing the measurements of water vapor over oceans.

The wet tropospheric correction (WTC) retrieved from the onboard calibration microwave radiometer (CMR) of Haiyang-2A (HY-2A) is critical in monitoring the global sea level. However, the CMR WTC became significantly biased from June 2017 due to the failure of the 18.7-GHz band, which caused massive errors in the sea surface height (SSH) measurements. We investigate the accuracy of the CMR WTC derived from the two remaining bands to address this problem. A comprehensive evaluation using multisource data demonstrates that the dual-band + backscattering coefficient (BC) algorithm achieves comparable accuracy to the three-band algorithm, and it does not suffer from any large errors when the equipment works well. Hence, we calibrated the HY-2A CMR data with the dual-band + BC algorithm when the 18.7-GHz band failed, and the accuracy of the CMR WTC is improved from 2.34 to 1.39 cm compared with European Center for Medium-Range Weather Forecasts (ECMWF) ERA5 data. In addition, the SSH measurements are improved significantly by a maximum of 2 cm in mean value using the dual-band + BC WTC during the failure period of HY-2A CMR. Compared with Jason-3 SSH measurements, the HY-2A with dual-band + BC shows a slightly larger difference than HY-2A with three-band by 0.1 cm in rms. This method prolongs the operational lifetime of the HY-2A CMR and could be used in the reprocessing of HY-2A observations.

Atmospheric water vapor plays an essential role in climate change and weather forecasting. However, monitoring water vapor with high spatial and temporal resolutions remains a challenge, especially over ocean regions where observations are insufficient. Shipborne global navigation satellite systems (GNSSs) contribute to enriching water vapor measurements over oceans and also can help validate satellite observations. Due to the lack of long-time serial observations, the performance of shipborne GNSS-derived precipitable water vapor (PWV) is inadequately evaluated on the global ocean scale. In this study, an overall assessment of shipborne GNSS PWV over global oceans is performed based on six voyages from 2014 to 2018. In coastal areas, the PWV differences of shipborne GNSS with respect to (w.r.t.) ground-based GNSS and ground-launched radiosonde data are 2.64 and 2.85 mm in the root mean square (rms), respectively. In open oceans, compared to ship-launched radiosonde profiles and satellite measurements, shipborne GNSS PWV shows the rms of differences of 2.54 and 2.53 mm, respectively. In addition, the rms of PWV differences between the whole track of shipborne GNSS PWV and National Centers for Environmental Prediction (NCEP) Climate Forecast System Version 2 (CFSv2) products is 2.96 mm. The intertechnique validations demonstrate that the accuracy of shipborne GNSS PWV is superior to 3 mm, which meets the requirements of climate research and numerical weather prediction (NWP).