The Rhine outflow plume studied by the analysis of synthetic aperture radar data and numerical simulations

The dynamics of the Rhine outflow plume in the proximity of the river mouth is investigated by using remote sensing data and numerical simulations. Th e remote sensing data consist of 41 synthetic aperture radar (SAR) images a cquired by the First and Second European Remote Sensing satellites ERS-1 an d ERS-2 over the outflow region of the river Rhine. Most of them show sea s urface signatures of oceanic phenomena, for example, surface current and wi nd variations, ship wakes, and oil slicks. In particular, in 36 of these im ages pronounced frontal features are visible as narrow zones of mainly enha nced, sometimes enhanced/reduced radar backscatter that can be associated w ith the Rhine surface front. Within the area enclosed by the frontal line, large zones characterized by a lower radar backscatter than in the outer ar ea are often visible. The analysis of the ERS SAR images suggests that the form and the location of the frontal features are mainly linked to the semi diurnal tidal phase in the outflow region, although their variability sugge sts also that they weakly depend on river discharge, residual currents, and neap-spring tidal cycle. In order to test this observational hypothesis, t he results obtained from the analysis of the ERS SAR images are compared wi th the results obtained from the numerical simulation of the hydrodynamics of the Rhine outflow region carried out using a two-layer, frontal model, w hich is based on the nonlinear, hydrostatic shallow-water equations on an f plane. The model is forced by prescribing tidal and residual currents and river discharge at the open boundaries. Several simulations are performed b y varying the values of these forcing parameters. The numerical results cor roborate the observational conjecture: It is found that the form and the lo cation of the simulated interface outcropping lines in the proximity of the river mouth are mainly determined by the semidiurnal tidal phase in the ou tflow region and that river discharge, residual currents, and neap-spring t idal cycle contribute only secondarily to their determination. Inserting th e simulated surface velocity field into a simple radar-imaging model that r elates the modulation of the backscattered radar power to the surface veloc ity convergence in radar look direction, narrow, elongated bands of enhance d radar backscatter emerge near the model frontal line while patches of low radar backscatter appear within the simulated Rhine plume area. The consis tency of the model results with the results obtained from the analysis of t he SAR images enables one to infer a mean spatial and temporal evolution of the Rhine outflow plume over a semidiurnal tidal cycle from the analysis o f spaceborne SAR images acquired during different tidal cycles over the Rhi ne outflow area and suggests the possibility of using numerical modeling, i n conjunction with the analysis of spaceborne measurements, for monitoring the oceanic variability in the Rhine outflow area.