Hennings, Ingo, Metzner, M. and De Loor, G. P. (2002) The influence of quasi resonant internal waves on the radar imaging mechanism of shallow sea bottom topograhy. Oceanologica Acta, 25 . pp. 87-99. DOI 10.1016/S0399-1784(02)01186-6.

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Abstract

During previous field experiments in the North Sea it was often assumed that the water column in such shallow coastal tidal waters is
vertically well mixed and stratification was neglected when discussing the Normalized Radar Cross Section modulation caused by the sea
floor. In this paper the influence of quasi resonant internal waves with the sea bed on the radar imaging mechanism of submarine sand waves
itself is investigated. In situ data of the tidal current velocity and several water quality parameters such as sea surface temperature,
fluorescence, and beam transmittance were measured in the Southern Bight of the North Sea in April 1991. Simulations of the total NRCS
modulation caused by sand waves and internal waves as a function of the current gradient or strain rate induced by the internal wave current
field at the sea surface have been carried out using the quasi-steady approximation and linear internal wave theory. As a first approximation
the strain rate depending on stratification was calculated using the two-layer model. These simulations demonstrate that at least a density
difference between the two layers of the order of Δρ ≈ 1 kg m–3 is necessary for a sinusoidal thermocline to effect the total NRCS
modulation considerably. The NRCS modulation as a function of wind friction velocity has been calculated independently and is discussed
with regard to the strain rate of the surface current field caused by the superimposed imaging mechanisms of sand waves and internal waves.
It turned out that the existence of a surface roughness-wind stress feedback mechanism cannot be excluded.

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