Operational Wind Field Retrieval using C-band Synthetic Aperture Radars

An algorithm is introduced that enables to retrieve high resolution ocean surface wind fields on an operational basis using space borne synthetic aperture (SAR). The algorithm was designed for the European satellites ERS and ENVISAT as well as for the Canadian satellite RADARSAT-1. The SARs aboard these satellites operate at C-band in either vertical (VV) or horizontal (HH) polarization in transmit and receive.

Derivation of the sea surface wind field from SAR is a two step process: In the first step wind directions are extracted, which are a necessary input into the second step to retrieve wind speeds. SAR wind directions are retrieved from wind-induced phenomena, which are aligned in wind direction at a typical spacing of 0.2 to 3 km. The orientation of these features is derived by determining local gradients from smoothed SAR amplitude images. The remaining directional ambiguities are removed by considering the results of atmospheric numerical forecast models. In the second step wind speeds are retrieved utilizing geophysical model function (GMF) that describes the dependency of the normalized radar cross section of the ocean surface on the local near-surface wind speed, wind direction versus antenna look direction and incidence angle for VV polarized images. For wind speed retrieval from C-band HH polarized SAR images a hybrid model function is applied consisting of the GMF and a C-band polarization ratio.

The algorithms performance is validated utilizing RADARSAT-1 and ENVISAT SAR images and comparing the resulting wind fields to atmospheric numerical model results and in situ data. The limitations of the algorithm are discussed with respect to operational use. Recently the algorithm has been implemented at the Center for Southeastern Tropical Advanced Remote Sensing (CSTARS) of the University of Miami were it generate wind field maps from SAR images on an operational basis.