Morten W. Hansen

On the Use of Doppler Shift for Sea Surface Wind Retrieval From SAR

The synthetic aperture radar (SAR) Doppler centroid has been used to estimate the scatter line-of-sight radar velocity. In weak to moderate ocean surface current environment, the SAR Doppler centroid is dominated by the directionality and strength of wave-induced ocean surface displacements. In this paper, we show how this sea state signature can be used to improve surface wind retrieval from SAR. Doppler shifts of C-band radar return signals from the ocean are thoroughly investigated by colocating wind measurements from the ASCAT scatterometer with Doppler centroid anomalies retrieved from Envisat ASAR. An empirical geophysical model function (CDOP) is derived, predicting Doppler shifts at both VV and HH polarization as function of wind speed, radar incidence angle, and wind direction with respect to radar look direction. This function is used into a Bayesian inversion scheme in combination with wind from a priori forecast model and the normalized radar cross section (NRCS). The benefit of Doppler for SAR wind retrieval is shown in complex meteorological situations such as atmospheric fronts or low pressure systems. Using in situ information, validation reveals that this method helps to improve the wind direction retrieval. Uncertainty of the calibration of Doppler shift from Envisat ASAR hampers the inversion scheme in cases where NRCS and model wind are accurate and in close agreement. The method is however very promising with respect of future SAR missions, in particular Sentinel-1, where the Doppler centroid anomaly will be more robustly retrieved.

Retrieval of Sea Surface Range Velocities From Envisat ASAR Doppler Centroid Measurements

The processing steps and error corrections needed to retrieve estimates of sea surface range Doppler velocities from Envisat Advanced Synthetic Aperture Radar Wide Swath Medium resolution image products are presented. Retrieval accuracies based on examination of the corrected Doppler shift measurements are assessed. The root-mean-square errors of the Doppler shift after bias corrections are found to be 4.7 and 3.9 Hz in VV and HH polarizations, respectively. At 35° incidence angle, this corresponds to horizontal Doppler velocities of 23 and 19 cm/s.

Wave Breaking in Slicks: Impacts on C-Band Quad-Polarized SAR Measurements

Radarsat-2 C-band quad-polarization SAR observations of crude oil, emulsion, and plant oil slicks acquired in the wind speed range from 4 to 8 m/s and incidence angles from 30° to 50° are analyzed to yield new insights into the attenuation of short waves and breaking waves by surface slicks in real conditions. To provide a direct quantitative assessment of the surface wave damping, the measurements are decomposed into polarization difference (PD), polarization ratio (PR), nonpolarized (NP), and cross-polarized (CP) components. The PD signals relate to the extent of Bragg damping in the slick areas, over which the PR is systematically higher than over the ambient sea surface. Attenuation of the breaking waves is revealed to affect both the CP and the NP signals, with distinct but weaker contrasts compared to that of the PD. A revised physical model description is proposed to provide consistent interpretation of the polarized and NP signals. The results suggest that the different slick types and look-alikes can be efficiently discriminated and classified.

INCUSAR—A Method to Retrieve Temporal Averages of 2D Ocean Surface Currents from Synthetic Aperture Radar Doppler Shift

Within the last decade the Doppler anomaly retrieved from the Synthetic Aperture Radar (SAR) has been demonstrated to be a useful resource for detection of ocean surface currents. As with any Doppler-based method, only the component along the instrument look direction can be retrieved at a time. It is here demonstrated how a time-averaged two-dimensional surface current field can be obtained from corresponding averages of surface current components detected along two non-orthogonal view angles, from respectively ascending and descending passes of the Envisat satellite. By a quantitative assessment of the involved uncertainties, an improved surface current field is obtained by merging the Doppler current with geostrophic current calculated from a climatological Mean Dynamic Topography.