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

• Published in: IEEE transactions on geoscience and remote sensing Vol. 50; no. 7; pp. 2901 - 2909
• Main Authors: Mouche, A. A., Collard, F., Chapron, B., Dagestad, K., Guitton, G., Johannessen, J. A., Kerbaol, V., Hansen, M. W.
• Format: Journal Article
• Language: English
• Published: New York, NY IEEE 01.07.2012; Institute of Electrical and Electronics Engineers
• Subjects: Applied geophysics; Doppler; Doppler radar; Doppler shift; Earth sciences; Earth, ocean, space; Exact sciences and technology; Internal geophysics; Sciences of the Universe; Sea surface; surface wind; synthetic aperture radar (SAR); Wind speed; waves; models; inverse problem; surface wind; cross sections; strength; low pressure; velocity; anomalies; incidence angle; in situ; currents; synthetic aperture radar (SAR); winds; displacements; polarization; calibration; uncertainties; radar methods; Synthetic aperture radar; Doppler; direction
• ISSN: 0196-2892; 1558-0644
• DOI: 10.1109/TGRS.2011.2174998

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.