INverting consistent surface CUrrent fields from SAR (INCUSAR)

This project started 1 June 2009, and is supported by ESA.

The primary objective of the project is retrieval of high resolution ocean surface currents from SAR imagery. Within the last decade it has been recognised that the SAR Doppler Centroid anomaly contains information about the velocity of the ocean surface in the line of sight of the radar, though at a coarser resolution than the traditional SAR roughness measurements (~5 km compared to ~100 m). The retrieved surface Doppler velocity includes a contribution from the ocean surface current, and a contribution from wind waves, through orbital motion caused by longer waves, phase velocity of the shorter Bragg waves, and motion of irregular ocean surface elements originated from wave breaking. The Doppler velocity is therefore a new resource in addition to the roughness for both wind and surface current retrieval from SAR.

It is recognised that retrieval of wind, waves and current from SAR is a highly coupled problem. This first part of the study will focus on a method to improve the SAR wind retrieval, which will benefit the second part which is a consistent inversion of wind, waves and the surface current. The main challenge for SAR wind retrieval has been to obtain a correct wind direction, which is needed for subsequent retrieval of the wind speed. The wind direction can be taken from a forecast model, a scatterometer, or sometimes from linear features in the SAR image itself. However, this does not always work well in cases with sharp wind gradients, which are not uncommon close to the coasts where several ocean current systems are found. The Doppler Centroid anomaly has recently proven to be an additional resource for this task, and promising results have been obtained with a Bayesian wind inversion scheme where the Doppler information is incorporated. The subsequent inversion of consistant wind, waves and surface current will combine both SAR Doppler and roughness measurements, and will utilise a complex radar imaging model (forward model) to simulate the radarsignatures for given wind, wave and current fields. An iterative approach will be attempted to converge to a consistent solution.

WP 1: Bayesian wind retrieval

WP 2: Surface current inversion