A Doppler spectral method of identifying water and ice in marginal ice zone imagery

Abstract

Complex synthetic aperture radar imageries at C band and X band acquired during March 1989 in the Greenland Sea are specially processed. This processing yields Doppler spectra from two backscatter regions near the marginal ice zone: (1) newly forming ice inside a tongue‐shaped region called the Odden and (2) open water adjacent to the Odden. We observe a consistent broadening of the Doppler bandwidth in the water spectra compared to the ice spectra. Short ocean waves (wavelengths <∼10 m) are attenuated as they travel into the ice, whereas long ocean gravity waves (wavelengths ≫10 m) exhibit little attenuation. A model is derived describing the Doppler bandwidth in terms of the platform velocity, the short waves, and the long waves. The short subresolution waves and the long gravity waves contribute to the Doppler broadening in the ocean. However, for the gentle ocean swell observed here we show that the long‐wave contribution can be neglected. Therefore the differential broadening measured between the water and the ice is primarily due to the short random waves. The Doppler bandwidth broadening is Hz at C band and at X band, whereas the measured bandwidth due to the platform velocity (155 m/s) is approximately 75 Hz at both C band and X band. The broadening can also be given in terms of the scene correlation time; 2τc ≈ 22 ms and 16 ms at C band and X band, respectively, under certain assumptions.