SWOT, The Surface Water and Ocean Topography Satellite Mission [INVITED]

Surface fresh water is essential for life, yet we have surprisingly poor knowledge of its variability in space and time. Similarly, ocean circulation fundamentally drives global climate variability, yet the ocean current and eddy field that affects ocean circulation and heat transport at the sub-mesoscale resolution and particularly near coastal and estuary regions, is poorly known. The Surface Water and Ocean Topography satellite mission concept (SWOT) is a swath-based interferometric-altimeter designed to acquire elevations of ocean and terrestrial water surfaces at unprecedented spatial and temporal resolutions. SWOT will provide measurements of storage changes in lakes, reservoirs, and wetlands as well as estimates of discharge in rivers. These measurements are important for global water and energy budgets, constraining hydrodynamic models of floods, carbon evasion through wetlands, and water management, especially in developing nations. Perhaps most importantly, SWOT measurements will provide a fundamental understanding of the spatial and temporal variations in global surface waters, which for many countries are the primary source of water. An on-going effort, the “virtual mission” (VM) is designed to help constrain the required height and slope accuracies, the spatial sampling (both pixels and orbital coverage), and the trade-offs in various temporal revisits. Example results include the following: (1) Ensemble Kalman filtering of VM simulations recover water depth and discharge, reducing the discharge RMSE from 23.2% to 10.0% over an 84-day simulation period, relative to a simulation without assimilation. (2) Ensemble-based data assimilation of SWOT like measurements yields bathymetric slope and depth to within 3.0 microradians and 50 cm, respectively. (3) SWOT measurements of storage changes in lakes larger than 1 km2 will have relative errors generally less than 5% whereas errors for one-hectare size lakes will be about 20%. (4) SWOT estimates of discharge compared to a one-year model-based “truth” data set suggest that instantaneous discharge estimates will have a median relative RMSE of 10.9% and that 86% of all instantaneous errors will be less than 25%. (5) Based on a global distribution of gauge-based discharge and candidate SWOT orbits with varying spatial and temporal sampling, SWOT errors in instantaneous discharge will be less than 25% for rivers wider than 50 m, whereas errors in monthly discharge will be less than 20% for rivers with drainage areas larger than 7000 km2. (6) We estimate that currently available satellite nadir altimetry data cannot monitor more than 15% of the global lake volume variation, whereas from 50% to more than 65% of the global lake volume variation will be observed by SWOT.