Estimation of Surface Current Divergence from Satellite Doppler Radar Scatterometer Measurements of Surface Ocean Velocity

Abstract

Abstract The ability to estimate surface current divergence and vorticity from space is assessed from simulated satellite Doppler radar scatterometer measurements of surface velocity with an effective footprint diameter of 5 km across an 1800-km measurement swath. The focus is on non-internal-wave contributions to divergence and vorticity. This is achieved by simulating Doppler radar measurements of surface velocity from a numerical model in which internal waves are weak because of high dissipation, seasonal cycle forcing, and the lack of tidal forcing. Divergence is much more challenging to estimate than vorticity because the signals are weaker and restricted to smaller scales. With the measurement noise that was anticipated based on early engineering studies, divergence cannot be estimated with useful resolution. Recent advances in the understanding of how the noise in measurements of surface currents depends on the ambient wind speed have concluded that measurement noise will be substantially smaller in conditions of wind speed greater than 6 m s−1. A reassessment of the ability to estimate non-internal-wave contributions to surface current divergence in this study finds that useful estimates can be obtained in such wind conditions; the wavelength resolution capability for divergence estimates in the middle of the measurement swaths will be better than 100 km in 16-day averages. The improved measurement accuracy will also provide estimates of surface current vorticity with a resolution nearly a factor of 2 higher than was previously thought, resulting in wavelength resolutions of about 50, 30, and 20 km in snapshots, 4-day averages, and 16-day averages, respectively. Significance Statement The divergence of surface ocean velocity is of great interest to oceanographers because of its direct relation to the near-surface vertical velocity that has important implications for air–sea exchanges of CO2 and other gases, as well as the supply of nutrients from depth that are critical to biological productivity. Observational estimates of surface divergence are challenging because of the weakness of the divergence signals and the technical difficulties in acquiring two-dimensional observations of velocity with sufficient accuracy and spatial resolution to obtain accurate estimates of the divergence. The analysis presented here concludes that useful estimates of surface current divergence can be obtained from a future Doppler radar satellite mission that is in the early stages of development by NASA.