An Investigation of non-Bragg Scattering from the Sea Surface

2003 ◽  
Author(s):  
Ellen E. Lettvin ◽  
William J. Plant
Keyword(s):  
Sea Surface ◽  
Bragg Scattering

An Investigation of Non-Bragg Scattering from the Sea Surface

2001 ◽  
Author(s):  
Ellen E. Lettvin ◽  
William J. Plant
Keyword(s):  
Sea Surface ◽  
Bragg Scattering

scholarly journals The Role of Micro Breaking of Small-Scale Wind Waves in Radar Backscattering from Sea Surface

2020 ◽  
Vol 12 (24) ◽  
pp. 4159
Author(s):  
Irina A. Sergievskaya ◽  
Stanislav A. Ermakov ◽  
Aleksey V. Ermoshkin ◽  
Ivan A. Kapustin ◽  
Olga V. Shomina ◽  
...  
Keyword(s):  
Wind Waves ◽  
Breaking Waves ◽  
Sea Surface ◽  
Small Scale ◽  
Breaking Wave ◽  
Generation Process ◽  
Bragg Scattering ◽  
Radar Backscattering ◽  
Radar Sensing ◽  
The Impact

The study of the microwave scattering mechanisms of the sea surface is extremely important for the development of radar sensing methods. Some time ago, Bragg (resonance) scattering of electromagnetic waves from the sea surface was proposed as the main mechanism of radar backscattering at moderate incidence angles of microwaves. However, it has been recently confirmed that Bragg scattering is often unable to correctly explain observational data and that some other physical mechanisms should be taken into consideration. The newly introduced additional scattering mechanism was characterized as non-polarized, or non-Bragg scattering, from quasi-specular facets appearing due to breaking wave crests, the latter usually occurring in moderate and strong winds. In this paper, it was determined experimentally that such non-polarized radar backscattering appeared not only for rough sea conditions in which wave crests strongly break and “white caps” occur, but also at very low wind velocities close to their threshold values for the wave generation process. The experiments were performed using two polarized Doppler radars. The experiments demonstrated that a polarization ratio, which characterizes relative contributions of non-polarized and Bragg components to the total backscatter, changed slightly with wind velocity and wind direction. Detailed analysis of radar Doppler shifts revealed two types of scatterers responsible for the non-polarized component. One type of scatterer, moving with the velocities of decimeter-scale wind waves, determined radar backscattering at low winds. We identified these scatterers as “microbreakers” and related them to nonlinear features in the profile of decimeter-scale waves, like bulges, toes and parasitic capillary ripples. The scatterers of the second type were associated with strong breaking, moved with the phase velocities of meter-scale breaking waves and appeared at moderate winds additionally to the “microbreakers”. Along with strong breakers, the impact of microbreaking in non-polarized backscattering at moderate winds remained significant; specifically the microbreakers were found to be responsible for about half of the non-polarized component of the radar return. The presence of surfactant films on the sea surface led to a significant suppression of the small-scale non-Bragg scattering and practically did not change the non-Bragg scatterer speed. This effect was explained by the fact that the nonlinear structures associated with dm-scale waves were strongly reduced in the presence of a film due to the cascade mechanism, even if the reduction of the amplitude of dm waves was weak. At the same time, the velocities of non-Bragg scatterers remained practically the same as in non-slick areas since the phase velocity of dm waves was not affected by the film.

scholarly journals Study on Non-Bragg Microwave Backscattering from Sea Surface Covered with and without Oil Film at Moderate Incidence Angles

2021 ◽  
Vol 13 (13) ◽  
pp. 2443
Author(s):  
Honglei Zheng ◽  
Jie Zhang ◽  
Ali Khenchaf ◽  
Xiao-Ming Li
Keyword(s):  
Cross Sections ◽  
Sea Surface ◽  
Scale Model ◽  
Bragg Scattering ◽  
Total Scattering ◽  
Microwave Scattering ◽  
Wind Speeds ◽  
Geophysical Model ◽  
Radar Echoes ◽  
Radar Cross Sections

In the past decades, Bragg scattering has been considered to be an important scattering mechanism of microwave backscattering from sea surfaces. However, as reported in many recent literatures, non-Bragg scattering (which is often attributed to wave breaking) also makes a significant impact on radar scattering, especially for Horizontal–Horizontal (HH) polarized radar signals. To date, we know far less about non-Bragg scattering than Bragg scattering. Herein, this paper carries out an investigation on non-Bragg scattering and its effect on radar echoes at moderate incidence angles, both for oil-free and oil-covered sea surfaces. This paper firstly presents a systematic comparison of several sea spectra commonly used for the simulation of microwave scattering from sea surfaces. It is found that none of them perform well for the description of Bragg waves. Then, the “pure” Bragg wave spectra are inverted in the framework of the two-scale model (TSM) and geophysical model functions (GMFs). The normalized radar cross sections (NRCS) related to total scattering, non-Bragg scattering, and “pure” Bragg scattering in C, X, and Ku-bands are simulated under various conditions (i.e., incidence angles, wind speeds, and wind directions). Quantitative assessments of the relative contributions of non-Bragg scattering to total scattering are conducted. We also perform a survey on the non-Bragg scattering from the oil-covered sea surface. This article provides some new insights for a better understanding of the non-Bragg microwave scattering from rough sea surfaces at moderate incidence angles.

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