Clara Lázaro (Universidade do Porto, Portugal); Telmo Vieira (Universidade do Porto, Portugal); M. Joana Fernandes (Universidade do Porto, Portugal)

The Surface Water and Ocean Topography (SWOT) satellite, equipped with a 120-km-wide swath Ka-band radar interferometer (KaRIn) and a Poseidon-3C type altimeter, uses advanced radar technology to provide high-resolution observations of the ocean, coastal and inland waters. However, radar signals are delayed due to the water vapor and cloud liquid water content in the troposphere, so the correctness of the radar surface height measurements is strongly influenced by the accuracy of the wet tropospheric correction (WTC) to account for those delays. SWOT WTC at each location is currently being derived by the linear interpolation of the two ~70 km spaced measurements at the center of each KaRIn half-swath, acquired by two independent onboard Advanced Microwave Radiometers (SWOT AMR - SAMR). The fact that only two SAMR are available to retrieve the WTC over the whole KaRIn swath and that these observations are optimized for ocean conditions, various improvements to the current SWOT WTC can be foreseen. In particular, areas close to land (< 25 km) and inland waters still present some challenges for acquiring valid and accurate WTC observations. In this study, the GNSS-derived Path Delay Plus (GPD+) algorithm adapted for SWOT is implemented to estimate the WTC at locations along the Poseidon-3C altimeter track and the KaRIn swath, which are subsequently compared with the WTC values available in the SWOT products and those derived from the ERA5 model. The focus of the current study is the impact of the new GPD+ WTC in the coastal regions. The results show that GPD+ allows the retrieval of WTC at locations where these values are absent or flagged as invalid in the SWOT products. In addition, GPD+ also improves the WTC accuracy, particularly at locations close to land and whenever the interpolation between observations is not possible because one of the two onboard SAMR measurements is invalid. The comparison with global navigation satellite system (GNSS) data, although not independent from GPD+, provides relevant information about land contamination in the various analyzed WTC. Thus, the root mean square (RMS) of the WTC differences between GNSS and the values provided by the various considered datasets (SWOT products, GPD+ and ERA5) were analyzed as a function of distance from the coast. Results for latitudes in the range of 50°N-50°S show that GPD+ can reduce the RMS of the differences up to 0.5 cm, particularly closer to land. The impact of GPD+ on the WTC accuracy is also explored through the sea level anomaly (SLA) variance analysis. These findings highlight the worth of the GPD+ methodology to increase the number of locations with valid WTC information and its respective accuracy. The outcome of this improved WTC is then reflected in the sea level anomaly calculation which, in turn, will impact the solidness of future sea level variability analysis. Ongoing and future work envisages the impact of GPD+ on the spatial variability of the WTC and corresponding SLA values along the KaRIn swath, over the coastal and open ocean. Since GPD+ incorporates a set of external observations, apart from the already reported increase in the number of valid observations, it is expected to observe variability not present in the current SWOT WTC.