Contribution of a Wide-Swath Altimeter in a Shelf Seas Assimilation System: Impact of the Satellite Roll Errors

Abstract

The authors investigate the potential qualitative improvement brought by wide-swath, interferometry-based ocean altimetry measurements with respect to classical nadir altimeters in a coastal/shelf data assimilation system. In addition, particular attention is paid to roll errors, which could significantly reduce the expected benefits of wide-swath altimetry. A barotropic, nonlinear free-surface model is set up over the European shelf as part of an ensemble Kalman filter. Experiments assimilating simulated data are performed over the North Sea to test the ability of altimeter configurations to reduce model errors due to the action of meteorological forcing in the presence of bathymetric uncertainties. A simplified wide-swath observation scheme is used, composed of nadir altimeter height plus a nadir-centered cross-track sea level slope measurement. The simplified wide-swath measurements are found to be able to constrain events unsampled by a single nadir altimeter owing to a wider domain of influence in the cross-track direction and the ability to detect cross-track gradients. Since the satellite-borne interferometer is highly sensitive to the platform behavior, especially satellite roll, experiments taking roll errors into account are then carried out. Whereas observational errors are considered independent in most data assimilation studies, the roll of the platform correlates those errors along the path of the satellite. Despite the large amplitude of the roll errors, the contribution of the wide-swath altimeter in coastal zones remains valuable as long as the roll frequency is known (within Gaussian error) and the assimilation scheme is designed to take observational error correlations into account.