J. A. Johannessen et al., Direct ocean surface velocity measurements from space: Improved quantitative interpretation of Envisat ASAR observations

Geophys. Res. Lett., 35, L22608; doi:10.1029/2008GL035709

Direct ocean surface velocity measurements from space: Improved quantitative interpretation of ENVISAT ASAR observations

Direct ocean surface velocity measurements from space: Improved quantitative interpretation of Envisat ASAR observations

J. A. Johannessen (Nansen Environmental and Remote Sensing Center, Bergen, Norway), B. Chapron (Institute Francais de Recherche pour l'Exploitation de la Mer, Plouzané, France), F. Collard (BOOST Technologies, Brest, France), V. Kudryavtsev (Nansen International Environmental and Remote Sensing Center, St. Petersburg, Russia), A. Mouche (BOOST Technologies, Brest, France), D. Akimov (Nansen International Environmental and Remote Sensing Center, St. Petersburg, Russia), and K.-F. Dagestad (Nansen Environmental and Remote Sensing Center, Bergen, Norway)

Previous analysis of Advanced Synthetic Aperture Radar (ASAR) signals collected by ESA's Envisat has demonstrated a very valuable source of high-resolution information, namely, the line-of-sight velocity of the moving ocean surface. This velocity is estimated from a Doppler frequency shift, consistently extracted within the ASAR scenes. The Doppler shift results from the combined action of near surface wind on shorter waves, longer wave motion, wave breaking and surface current. Both kinematic and dynamic properties of the moving ocean surface roughness can therefore be derived from the ASAR observations. The observations are compared to simulations using a radar imaging model extended to include a Doppler shift module. The results are promising. Comparisons to coincident altimetry data suggest that regular account of this combined information would advance the use of SAR in quantitative studies of ocean currents.

(Received 15 August 2008; accepted 2 October 2008; published 29 November 2008.)

Citation: Johannessen, J. A., B. Chapron, F. Collard, V. Kudryavtsev, A. Mouche, D. Akimov, & K.-F. Dagestad (2008), Direct ocean surface velocity measurements from space: Improved quantitative interpretation of Envisat ASAR observations, Geophys. Res. Lett., 35, L22608; doi:10.1029/2008GL035709.

Link to abstract: http://www.agu.org/pubs/crossref/2008/2008GL035709.shtml

Improving measurements of ocean surface velocity from space

Advanced synthetic aperture radar (ASAR) signals collected by the European Space Agency's Envisat help scientists map surface currents in the world oceans. As demonstrated, ASAR is a valuable source of high-resolution information on the line-of-sight roughness and velocity of the moving ocean surface. The velocity is estimated from careful examination and analysis of residual Doppler frequency shifts extracted from the ASAR scenes at a spatial resolution of about 5-10 km (3-6 miles). Using this methodology, Johannessen et al. consistently derive both kinematic and dynamic properties of the ocean surface roughness. The authors compare the observations with advanced simulations as well as altimeter-based surface current measurements, finding good agreement to motivate further research to integrate the derived characteristics of ocean surface motion into broader models. This can help scientists learn more about mesoscale and submesoscale upper ocean dynamics, coupled physical-biogeochemical processes, and air-sea interactions in intense current regimes.