The influence of quasi resonant internal waves on the radar imaging mechanism of shallow sea bottom topography

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. Précédemment, en mer du Nord, la stratification, pratiquement inexistante dans cette région peu profonde et à fortes marées, a été négligée lors de la discussion de la modulation par le fond marin de la section efficace normalisée du radar. Dans le présent article, nous examinons l'influence des vagues internes quasi-résonantes et du fond sous-marin sur les images radar du fond sableux. Les données in situ ont été recueillies en avril 1991 dans la partie sud de la mer du Nord : vitesse du courant de marée, température de surface, fluorescence et transmission. Des simulations de la modulation de la section efficace normalisée du radar produite par les ondulations du fond sableux et les ondes internes en fonction du gradient de courant ou de la tension induits par le champ de courants des ondes internes à la surface de mer ont été réalisées. L'approximation quasi-station