Pierre Broche

Experimental study of the Rhone plume. Part I: physics and dynamics

Abstract The complicated dynamic processes occurring when fluvial waters mix with marine waters control the nature and the fluxes of materials exported by rivers to the sea. Understanding these processes is of primary importance in evaluating budgets. In wide-open estuarine situations these processes take place under the influence of an intense turbulence induced by tides. Conversely, the Rhone waters spread into the Mediterranean Sea in the form of an easily distinguishable buoyant plume often extending far offshore from the mouth of the river. The aim of this study is to describe the dynamic and hydrological fields on the basis of eulerian VHF radar mapping of surface currents coupled with lagrangian in situ physical or geochemical measurements. This paper focuses mainly on physical processes. Data analysis provides an insight into the typical scales of variability of the phenomena, either vertically or horizontally. It is shown that morphological fluctuations can occur (mainly in orientation and offshore extent) according to wind and outflow forcing conditions, and that the vertical structure variations can range from an almost unaltered two-layer distribution to an evolving and deepening mixed layer situation, or even to a more complex superimposed multi-layered structure. The simultaneous examination of radar maps and lagrangian drifter tracking allows the main dynamic tendencies of the Rhone plume to be sketched out.

Inversion of reflectance spectra of nonchlorophyllous turbid coastal waters

Abstract This article deals with the inversion of reflectance spectra measured in the Rhone River mouth area during a flood event in terms of parameters characterizing the turbid water components. The optically active constituents considered are the total suspended matter, mainly composed of sediment, of known concentration ranging from 3.7 mg L −1 to 234 mg L −1 , and yellow substance. A simple reflectance model is presented. Scattering is treated by Mie theory applied to a collection of particles of real refractive index m p and obeying a Junge size distribution law. A sensitivity study performed on the model shows that both concentration and refractive index and both cutoff particle diameters of the Junge size distribution law cannot be accurately determined from a given reflectance spectrum. Two model parameters, among a total of six, are estimated from model inversion: m p and a parameter characterizing the yellow substance absorption spectrum, a y0 =a y (440 nm). The derived values are realistic as compared to the values that can be found in literature. Other combinations of parameters could be determined by inversion but measurement of m p and a y0 from reflectance spectra already has interesting applications in a satellite remote sensing perspective. Increasing the number of parameters of the model is feasable, but much work is needed to envisage the inverse problem.

Effect of mixing on microbial communities in the Rhone River plume

Abstract The biological processes involved during mixing of a river plume with the marine underlying water were studied off the Rhone River outlet. Samples of suspended and dissolved matter were collected while tracking a drifting buoy. Three trajectories were performed, at 2-day intervals, under different hydrological and meteorological situations. A biological uptake was evidenced from ammonium (NH 4 ) and phosphate (PO 4 ) shortage, indicating an early “NH 4 -dependent” functioning occurring before the well-known “NO 3 -based” cycle. The different ratios between NH 4 , NO 3 and PO 4 , as a function of salinity, were discussed to detail the preferential use in PO 4 and NH 4 . Salinity zones with enhanced bacterial production, high chlorophyll a concentration, as well as DOC, NH 4 and PO 4 consumption were evidenced from 20 to 35 in salinity. It was shown that the successive abundance of bacteria and phytoplankton during transfer reflected the competition for PO 4 of both communities. On the Rhone River plume, the role played by temperature, light conditions and suspended matter upon biological activity seems relatively minor compared to salinity distribution and its related parameter: nutrient availability. It can be concluded that biological uptake in the Rhone River plume was closely related to the dilution mechanism, controlled itself by the dynamics of the plume. In windless conditions and close to the river mouth, the density gradient between marine and river water induced limited exchanges between the nutrient-rich freshwater and the potential consumers in the underlying marine water. Consequently, little biological activity is observed close to the river mouth. Offshore, mixing is enhanced and a balance is reached between salinity tolerance and nutrient availability to form a favourable zone for marine phytoplankton development. This can be quite far from the river mouth in case of a widely spread plume, corresponding to high river discharge. Under windy and wavy conditions, the plume freshwater is early and rapidly mixed, so that the extension of the “enhanced production zone” is drastically reduced and even bacteria could not benefit from the fast mixing regime induced.

Comparison of measurements of sea currents by HF radar and by conventional means

Abstract An analysis of remote measurements of sea surface currents during MARSEN is presented. Remote measurements were made by high-frequency (HF) radars in the German Bight from September to October.1979. Moored current meters made concurrent in situ measurements several metres below the surface. Comparisons of instantaneous data sets show differences, which may be explained by vertical current shear. The computed tidal coefficients from HF and current meter measurements agree well.

Reflectance sensitivity to solid suspended sediment stratification in coastal water and inversion: A case study

A two-flow radiative transfer model is used to study the sensitivity of spectral reflectance to variations of solid suspended matter (SSM) concentration with depth. Examples are given of the variations of reflectance spectra with depth profiles of SSM. We developed an inversion method for the model restricted to the simple case of a two-layer ocean. The method was tested on various documented experimental data. A majority of stratified cases were detected by the method. Reasonably good estimates of the mean sediment concentration of the upper layer were obtained. Only the order of magnitude of the thickness of the upper layer and the mean concentration of the lower layer were recovered. The influence of chlorophyll (CHL) concentration profile on reflectance spectra was found to be weak for such high SSM concentrations. Our inverse algorithm, of low computational cost, could be envisaged in a remote-sensing perspective provided complementary studies.

On remote sensing of vertical shear of ocean surface currents by means of a Single-frequency VHF radar

We propose and test experimentally a new way of probing vertical shear of ocean surface currents by means of a single frequency VHF radar. The key idea is to use additional information provided by the so called secondary peaks in radar echo spectra which appear due to nonlinearity. Results of the field experiment carried out in the Rhones delta show that the new method does enable one to probe the vertical shear with a reasonable accuracy and has potential for further development.

Slicks, waves, and fronts observed in a sea coastal area by an X-band airborne synthetic aperture radar☆

Abstract This article reports observations made in a sea coastal zone by an airborne X-band SAR. This zone includes a basin connected to the sea and the vicinity of the Rhone river estuary. We present the experiment and the data processing procedures including, in particular, a method for removing speckle effects from image spectra of waves. Observations of slick patterns in the basin are reported and used to relate their spatial distribution to features of the circulation as predicted by modelling. Then, we present the wave phenomena observed at sea. Off-shore, the observations are found to be consistent with actual wind conditions. Differences in wave visibility between the images are discussed using the ocean wave SAR imaging theory. The significant wave height is calculated from the image presenting the best geometry for wave detection. Along the coast, the wave transformations observed are discussed on the basis of the results of a simple wave model initialized by the main wave parameters measured off-shore. The complicated refraction effects predicted by the model can only be partially detected on the radar images because of the geometry of the observations. Finally, a frontal event is detected and interpreted as the signature of the boundary of the river plume. The wave-current model of Skop and Leipold (1988) explains the brightness of the radar signature of the front. Among other results, the conclusion emphasizes the good adequation of airborne SAR systems for remote sensing of the coastal environment.

Off-shore wind measurements by HF doppler ground-wave radar

Remote measurements of wind velocity are performed, up to 100 km away from the shore, by using some features of the Doppler spectrum of HF radio waves backscattered by the sea. The variation with radio-frequency of the ratio of the amplitudes of the two Bragg lines in this spectrum shows a discontinuity occurring at a value simply related to the wind velocity. The validity of these measurements is discussed by comparison with meteorological data obtained by conventional methods.

USE OF HF DOPPLER RADARS FOR THE INVESTIGATION OF THE WIND-CURRENT RELATIONSHIP

Abstract The theoretical behaviour of the near-surface wind-driven currents is strongly dependent upon the models used to describe the mixed surface layer (especially the kinematic viscosity profile). This paper investigates measurements of ocean currents by HF Doppler radars in order to shed some light on this problem. It is found that the relative directions of the wind and current velocities and the variation of the current speed with respect to radar frequency (i.e. with respect to depth) are the most sensitive parameters to analyse for discriminating among the various models. The results of a preliminary experiment which was conducted in the western Mediterranean during the autumn of 1981 are presented. In this experiment the current was measured on 8 consecutive days at two radar frequencies corresponding to integration depths of 1 and 2 m, respectively. A special study shows that the near-surface current is only weakly influenced by the proximity of the coast (the measurements were obtained between...

L-band doppler radar echoes of the sea surface in coastal zone

A remote sensing experiment using an L-Band radar was conducted in a nearshore environment. The results obtained contribute to identify some aspects of the coherent Lband response of the sea, specified by the Doppler spectrum, in neargrazing conditions. At low winds the radar response is of Bragg type. The asymmetry of Doppler spectra varies with wind direction. Surface current measurements were performed from Doppler spectra and assessed by in situ measurements. At high winds the radar response is of Bragg type for VV polarization but not for HH. We show that fast scatterers detected in HH are linked to the dominant waves, which do not propagate in direction of the wind in the complex coastal area considered. Measurements of attenuation with distance of radar signals are also presented.