by Zecchetto, S. and F. De Biasio
SAR images of the sea surface often show roll-vortex structures and other features which, in general, are spread out over several length scales, and may present spatial periodicity as well as intermittence. Standard techniques, as the two-dimensional Fourier analysis, are unsuitable both when it is of interest to detect intermittent phenomena and to analyse the spatial disposition of the backscatter structures inside the SAR images. For the above reasons, the two-dimensional continuous wavelet transform (CWT2) analysis has been applied to two ERS SAR images over the Mediterranean Sea, showing wind rolls and atmospheric gravity waves. Through the evaluation of the wavelet variance map, which ideally corresponds to the two-dimensional Fourier spectrum, it has been possible to assess the presence of two main energy areas at large (from 7 to 28 km) and small (from 0.5 to 2 km) spatial scales. While the large scale fluctuations may be ascribed to atmospheric gravity waves and other features induced by the surroundings, the small scale fluctuations reveal the inner structure of the atmospheric wind rolls. The SAR-like maps, obtained adding the wavelet coefficient maps pertaining to the small scales, have permitted to highlight the high and low intensity backscatter cells associated to the wind rolls. These cells have been statistically characterised by means of the frequency distributions of the size of the cells maximum and minimum axes, of the orientation of the maximum axis and of their area. The results indicate that high and low intensity backscatter cells have similar characteristics in both cases studied: they appear of elliptic shape, with the major axis along the wind roll direction; the average axes ratio is 2.5:1. The frequency distributions of the cells area indicate a continuous distribution of sizes, without significant gaps.