Validating SWOT in the coastal zone: a radar altimetry and tide gauge case study in the Bristol Channel and Severn River-Estuary system

The Bristol Channel and Severn River-Estuary system is highly dynamic with one of the largest tidal ranges in the world. The Severn is the longest river in the UK, with the second largest mean flow.  The coastline surrounding these waters and upriver is covered by a network of tide gauges (TGs), which has been continuously operational for a period of decades. This makes it an ideal area for the validation of new satellite altimetry sensors such as those on the Surface Water and Ocean Topography (SWOT) mission,  which for the first time collects 2D maps of water level, and to explore quality of SWOT data in the coastal zone. The SWOT-UK project  used this area for these purposes, for a comprehensive programme of field campaigns and multidisciplinary research in a coastal and estuarine context, as part of the UK contribution to the international SWOT Science Team validation work. For the SWOT-UK project, a set of TG, CryoSat-2 and Sentinel-3 data has been gathered to validate total water level and surface slope during the 1-day repeat SWOT CAL/VAL mission phase. Extra in situ GNSS-IR instruments were deployed to fill gaps in the existing TG network. This combined data set was used to assess the consistency and quality of the TG network and develop a validation scheme for the SWOT L2 and L3 altimetry data, in coastal and estuarine settings. This will highlight issues of how the coastal dynamics, hydrology and morphology affect the comparison of satellite altimetry and TGs, and how these features may be seen in the SWOT data. The slope along the satellite passes (across-channel), near-shore coastal dynamics and intertidal morphology have been seen to affect the comparison of satellite altimetry and TG data, and these geographic characteristics are expected to influence the uncertainty in the comparison with the SWOT data. The validation data from Bristol Channel and Severn Estuary has been used for an assessment of the performance of these novel 2D altimeter measurements. Cross-comparisons with in situ and satellite data will be presented, with an exploration of the effect of coastal dynamics and morphology on these data. This will lead to an improvement of satellite altimetry in the coastal zone. In addition, the SWOT data will be relevant to the improvement of numerical models, by providing a quality of validation and assimilation data not previously available, and help understand and reduce uncertainties due to changing intertidal morphology.