Ocean wind fields retrieved from radar-image sequences

An algorithm is presented for retrieving wind vectors from radar-image sequences acquired by a standard nautical radar near at near grazing incidence. The radar operates at X-band (9.5 GHz) with horizontal and vertical polarization in transmit and receive. The algorithm consists of two parts, one for wind direction and another for wind speed retrieval. Wind directions are locally extracted from wind induced streaks, which are approximately in line with the mean wind direction. The algorithm assumes wind direction as normal to the gradient of the amplitude image, which is approximated by finite differences over an appropriate length. The resulting wind direction is taken as normal to the retrieved local gradients. Wind speeds are derived from the radar cross section, by parameterization of its dependency on the wind vector using a neural network. The algorithm was tested and validated using data from a radar mounted in the North Sea. The applicability of nautical radars for wind retrieval is shown for both tower based and ship borne (moving) instruments.