The Rhine Outflow Plume Studied by the Analysis of Synthetic Aperture Radar Data and Numerical Simulations

Abstract

The dynamics of the Rhine outflow plume in the proximity of the river mouth is investigated by using remote sensing data and numerical simulations. The remote sensing data consist of 41 synthetic aperture radar (SAR) images acquired by the First and Second European Remote Sensing satellites ERS-1 and ERS-2 over the outflow region of the river Rhine. Most of them show sea surface signatures of oceanic phenomena, for example, surface current and wind variations, ship wakes, and oil slicks. In particular, in 36 of these images pronounced frontal features are visible as narrow zones of mainly enhanced, sometimes enhanced/reduced radar backscatter that can be associated with the Rhine surface front. Within the area enclosed by the frontal line, large zones characterized by a lower radar backscatter than in the outer area 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 semidiurnal tidal phase in the outflow region, although their variability suggests also that they weakly depend on river discharge, residual currents, and neap-spring tidal cycle. In order to test this observational hypothesis, the results obtained from the analysis of the ERS SAR images are compared with the results obtained from the numerical simulation of the hydrodynamics of the Rhine outflow region carried out using a two-layer, frontal model, which 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 by varying the values of these forcing parameters. The numerical results corroborate the observational conjecture: It is found that the form and the location of the simulated interface outcropping lines in the proximity of the river mouth are mainly determined by the semidiurnal tidal phase in the outflow region and that river discharge, residual currents, and neap-spring tidal cycle contribute only secondarily to their determination. Inserting the simulated surface velocity field into a simple radar-imaging model that relates the modulation of the backscattered radar power to the surface velocity convergence in radar look direction, narrow, elongated bands of enhanced radar backscatter emerge near the model frontal line while patches of low radar backscatter appear within the simulated Rhine plume area. The consistency of the model results with the results obtained from the analysis of the 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 of spaceborne SAR images acquired during different tidal cycles over the Rhine outflow area and suggests the possibility of using numerical modeling, in conjunction with the analysis of spaceborne measurements, for monitoring the oceanic variability in the Rhine outflow area.

Observations at the tidal plume front of a high‐volume river outflow

“…In some cases, there are as many as 10 undulation cycles. Similar SAR backscatter patterns have been identified to be internal waves using theory and in situ data at other locations [ Hessner et al , 2001]. J. D. Nash and J. N. Moum (River plumes as a source of large amplitude internal waves in the ocean, submitted to Nature , 2005) present detailed observations of internal waves or solitons generated at Columbia plume fronts.…”

“…We estimate the local front velocity and direction from repeated measurements of front location from two vessels and a helicopter, then verify that these estimates are reasonable using synthetic aperture radar (SAR) images for similar ocean, tide and wind conditions. Banded regions of elevated SAR backscatter are generally found in association with strongly convergent river plume fronts (Figure 1a), due to modulation of sea surface roughness caused by the convergence at the leading edge of the front and divergence after its passage [ Hessner et al , 2001].…”

A model for the spreading and sinking of the Deep Ice Shelf Water in the Ross Sea

“…In this paper we investigate aspects of the hydrodynamics of the Ross Sea related to the spreading and sinking of the DISW in two ways. First, by means of in situ observations and then by the results of a nonlinear, reduced gravity, frontal layered numerical model which is able to simulate the dynamics of isolated ("frontal") water masses and hence the dynamics of a bottom trapped current over realistic topography (Hessner et al 2001, Rubino et al 2002.…”

Visibility of River Plume Fronts with an X-Band Radar

“…Kilcher and Nash [1] gave a comprehensive review on "tidal plume fronts" (in their terminology) which are formed by ebb discharge from a river's mouth. Visual observations [2], instrumental observations [1,3,4], marine radar observations at grazing angle (e.g., [1,5,6]), and radar observations from space (e.g., [7,8]) are the major means of detecting and following river plume fronts with the main purposes of elucidating the spatial and temporal behavior of the fronts, mixing processes, flow structures of river and sea water, and propagation dynamics. Garvine and Monk [2] made field observations of vertical distributions of current velocity, water temperature, and salinity at the mouth of the Connecticut River, USA.…”

“…Images of synthetic aperture radar (SAR) have been analyzed to understand the behavior of outflows of large rivers. Hessner et al [7] analyzed the behavior of river plume fronts observed along the Rhine River outflow into the North Sea by European radar satellites ERS-1 and ERS-2. They assert that the formation and the locations of the frontal features are mainly linked to the semidiurnal tidal phase in the outflow region.…”