Shear-flow induced secondary circulation in parallel underwater topographic corrugation and its application to satellite image interpretation

Abstract

This study aims to figure out satellite imaging mechanisms for submerged sand ridges in the shallow water region in the case of the flow parallel to the topography corrugation. Solving the disturbance governing equations of the shear-flow yields the analytical solutions of the secondary circulation. The solutions indicate that a flow with a parabolic horizontal velocity shear and a sinusoidal vertical velocity shear will induce a pair of vortexes with opposite signs distributed symmetrically on the two sides of central line of a rectangular canal. In the case of the presence of surface Ekman layer with the direction of Ekman current opposite to (coincident with) the mean flow, the two vortexes converge (diverge) at the central line of canal in the upper layer and form a surface current convergent (divergent) zone along the central line of the canal. In the case of the absence of surface Ekman layer, there is no convergent (divergent) zone formed over the sea surface. The theoretical results are applied to interpretations of three convergent cases, one divergent case and statistics of 27 cases of satellite observations in the submerged sand ridge region of the Liaodong Shoal in the Bohai Sea. We found that the long, finger-like, bright patterns on SAR images are corresponding to the locations of the canals (or tidal channels) formed by two adjacent sand ridges rather than the sand ridges themselves.Academician Foundation of China; Shanghai Science and Technology Committee Program - Special for EXPO [10DZ0581600]; Shanghai Institute of Urban Ecology and Sustainability [SHUES2011A07]; US National Science Foundation [0962107, 1061998

Mechanisms of SAR Imaging of Shallow Water Topography of the Subei Bank

“…Considering the sidewall friction, the surface, and the bottom Ekman layers, the authors proposed a physics model to analyze the secondary circulation induced by the flow parallel to underwater topographic corrugation (see Figure 9) [17]. The analytical solutions show that in the case where the direction of the surface Ekman current is opposite to the mean flow, there is a surface current convergent zone along the central line of a canal.…”

“…All of these studies have shown that under the condition of a tidal current perpendicular to topographic features, the underwater topography can be imaged by SAR. However, recent satellite observations show that when the tidal current is parallel to topographic corrugations such as underwater sand ridges, sand bars, or tidal channels, the shallow water topography can also appear on SAR imagery [9,[17][18][19]. These observations cannot be explained using the existing 1-D radar imaging model.…”

“…Considering the tidal convergence, Li et al [9] developed a two-dimensional (2-D) analytical model for the interpretation of SAR imaging of underwater sand ridges parallel to the tidal current. Recently, Zheng et al [17] analyzed the secondary circulation induced by the flow parallel to the topographic corrugation by solving the three-dimensional (3-D) disturbance governing equations of the shear-flow. The theoretical results were applied to interpret SAR imaging of tidal channels.…”

Underwater Topography Detection and Analysis of the Qilianyu Islands in the South China Sea Based on GF-3 SAR Images

“…is a function related to radar parameters such as incident angle and polarization mode, and G(u, ϕ) is a function related to the modulation of large-scale ocean currents on sea surface microscale waves. Formula (9) reflects the relationship between the gray value of the SAR image and the change in tidal current caused by topography, which can be used as the calculation expression of the SAR gray value of the topography simulation. The topography inversion model for shallow water is composed of tidal current motion equations, appropriate initial boundary conditions, and SAR image gray value expressions.…”

“…Zheng [7] showed that SAR can detect underwater topography when the current is perpendicular to the topographic features. However, recent satellite observations have shown that shallow water topography can also appear on SAR images when currents are parallel to topographic features such as underwater sand ridges, sandbars, or tidal troughs [8][9][10][11]. Zhang [12] used 25 scenes of ASAR and ERS-2 SAR images from 2006 to 2010 to study and count the radar backscatter characteristics of the underwater topography of the Subei Shoal, and combined the relationship between the Ekman current and tidal current, finally proposing two possible SAR underwater topography imaging mechanisms in shallow water.…”