On Shape, Orientation, and Structure of Atmospheric Cells Inside Wind
Rolls in Two SAR Images
Abstract
Synthetic aperture radar (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, such as two-dimensional
(2-D) Fourier analysis, are unsuitable both when it is of interest to
detect intermittent phenomena and to analyze the spatial disposition of
the backscatter structures inside the SAR images. For the above reasons,
the 2-D continuous wavelet transform analysis has been applied to two
European Remote Sensing mission 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 2-D
Fourier spectrum, it has been possible to assess the presence of two
main energy areas at large (from 7–28 km) and small (from 0.5–2 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 by adding the
wavelet coefficient maps pertaining to the small scales, have permitted
to highlight the high- and low-intensity backscatter cells associated
with the wind rolls. These cells have been statistically characterized
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.