On the determination of characteristics of the interior ocean dynamics from radar signatures of oceanic internal solitary waves

In this paper we discuss two different methods of inferring characteristics of the interior ocean dynamics from radar signatures of internal solitary waves visible on synthetic aperture radar (SAR) images. The first one consists in the recognition and the interpretation of sea surface patterns of internal solitary waves; the second one consists in the analysis of the modulation depth of the normalized radar backscattering cross section (NRCS) associated with internal solitary waves. For this purpose we consider a data set composed of SAR and in situ measurements carried out from 1991 to 1997 in the region of the Strait of Messina. The recognition and the interpretation of sea surface patterns of internal solitary waves in the Strait of Messina can be used to study characteristics of the density distribution in the area: The internal wave field varies with seasonal variations in the vertical density stratification and with remotely induced variations, i.e., variations induced by the larger-scale circulation, in the horizontal density distribution. In order to inquire into the possibility of inferring parameters of the interior ocean dynamics by analyzing the modulation of the NRCS associated with internal solitary waves, several numerical simulations are carried out using a radar imaging model. These simulations are performed by assuming different wind conditions and internal wave parameters. It is shown that an accurate knowledge of wind conditions is crucial for deriving internal wave parameters and hence parameters of the interior ocean dynamics from the modulation of measured NRCS associated with internal solitary waves

Fine-Scale Features on the Sea Surface in SAR Satellite Imagery - Part 2: Numerical Modeling

With the advent of the new generation of synthetic aperture radar (SAR) satellites, it has become possible to resolve fine-scale features on the sea surface on the scale of meters. The proper identification of sea surface signatures in SAR imagery can be challenging, since some features may be due to atmospheric distortions (gravity waves, squall lines) or anthropogenic influences (slicks), and may not be related to dynamic processes in the upper ocean. In order to improve our understanding of the nature of fine-scale features on the sea surface and their signature in SAR, we have conducted high-resolution numerical simulations combining a three-dimensional non-hydrostatic computational fluid dynamics model with a radar imaging model. The surface velocity field from the hydrodynamic model is used as input to the radar imaging model. The combined approach reproduces the sea surface signatures in SAR of ship wakes, low-density plumes, and internal waves in a stratified environment. The numerical results are consistent with observations reported in a companion paper on in situ measurements during SAR satellite overpasses. Ocean surface and internal waves are also known to produce a measurable signal in the ocean magnetic field. This paper explores the use of computational fluid dynamics to investigate the magnetic signatures of oceanic processes. This potentially provides a link between SAR signatures of transient ocean dynamics and magnetic field fluctuations in the ocean. We suggest that combining SAR imagery with data from ocean magnetometers may be useful as an additional maritime sensing method. The new approach presented in this work can be extended to other dynamic processes in the upper ocean, including fronts and eddies, and can be a valuable tool for the interpretation of SAR images of the ocean surface

Numerical simulations of SAR microwave imaging of the Brazil current surface front

Abstract This paper analyzes the hydrodynamic and atmospheric instability modulation mechanisms which influence the Brazilian Current's (BC) thermal front signature in Synthetic Aperture Radar (SAR) images. Simulations were made using the M4S SAR imaging model. Two SAR images of the Brazilian Southeastern Coast depicting the BC's thermal front were selected including a VV (ASAR/Envisat) and a HH polarization (RADARSAT-1) image. Conditions of current shear and divergence were reproduced for the fronts imaged, using in situ (Acoustic Doppler Current Profilers) current velocities. Wind velocity fields were simulated based on QuikSCAT data. Results showed that SAR imaging of the BC front may be influenced both by atmospheric instabilities and hydrodynamic modulations. The first mechanism prevailed on the RADARSAT image and the latter on the ASAR/Envisat image. When atmospheric instabilities prevailed, the contribution of shear and divergence was almost negligible. When hydrodynamic modulations prevailed, a better agreement between the simulated responses and SAR image responses was obtained by inforcing a reduction of 88% in the relaxation rate, and higher divergence values, of the order of 10-4 s-1. Results indicate that, for some specific cases, local increases in shear and divergence may allow the detection of the BC thermal front

Instantaneous sea ice drift speed from TanDEM-X interferometry

The drift of sea ice is an important geophysical process with widespread implications for the ocean energy budget and ecosystems. Drifting sea ice can also threaten marine operations and present a hazard for ocean vessels and installations. Here, we evaluate single-pass along-track synthetic aperture radar (SAR) interferometry (S-ATI) as a tool to assess ice drift while discussing possible applications and inherent limitations. Initial validation shows that TanDEM-X phase-derived drift speed corresponds well with drift products from a ground-based radar at Utqiagvik, Alaska. Joint analysis of TanDEM-X and Sentinel-1 data covering the Fram Strait demonstrates that S-ATI can help quantify the opening/closing rate of leads with possible applications for navigation. S-ATI enables an instantaneous assessment of ice drift and dynamic processes that are otherwise difficult to observe. For instance, by evaluating sea ice drift through the Vilkitsky Strait, Russia, we identified short-lived transient convergence patterns. We conclude that S-ATI enables the identification and analysis of potentially important dynamic processes (e.g., drift, rafting, and ridging). However, current limitations of S-ATI are significant (e.g., data availability and they presently only provide the cross-track vector component of the ice drift field) but may be significantly reduced with future SAR systems

Instantaneous sea ice drift speed from TanDEM-X interferometry

The drift of sea ice is an important geophysical process with widespread implications for the ocean energy budget and ecosystems. Drifting sea ice can also threaten marine operations and present a hazard for ocean vessels and installations. Here, we evaluate single-pass along-track synthetic aperture radar (SAR) interferometry (S-ATI) as a tool to assess ice drift while discussing possible applications and inherent limitations. Initial validation shows that TanDEM-X phase-derived drift speed corresponds well with drift products from a ground-based radar at Utqiaġvik, Alaska. Joint analysis of TanDEM-X and Sentinel-1 data covering the Fram Strait demonstrates that S-ATI can help quantify the opening/closing rate of leads with possible applications for navigation. S-ATI enables an instantaneous assessment of ice drift and dynamic processes that are otherwise difficult to observe. For instance, by evaluating sea ice drift through the Vilkitsky Strait, Russia, we identified short-lived transient convergence patterns. We conclude that S-ATI enables the identification and analysis of potentially important dynamic processes (e.g., drift, rafting, and ridging). However, current limitations of S-ATI are significant (e.g., data availability and they presently only provide the cross-track vector component of the ice drift field) but may be significantly reduced with future SAR systems.</p

The Inner-Shelf Dynamics Experiment

17 USC 105 interim-entered record; under review.The article of record as published may be found at http://dx.doi.org/10.1175/BAMS-D-19-0281.1The inner shelf, the transition zone between the surfzone and the midshelf, is a dynamically complex region with the evolution of circulation and stratification driven by multiple physical processes. Cross-shelf exchange through the inner shelf has important implications for coastal water quality, ecological connectivity, and lateral movement of sediment and heat. The Inner-Shelf Dynamics Experiment (ISDE) was an intensive, coordinated, multi-institution field experiment from September–October 2017, conducted from the midshelf, through the inner shelf, and into the surfzone near Point Sal, California. Satellite, airborne, shore- and ship-based remote sensing, in-water moorings and ship-based sampling, and numerical ocean circulation models forced by winds, waves, and tides were used to investigate the dynamics governing the circulation and transport in the inner shelf and the role of coastline variability on regional circulation dynamics. Here, the following physical processes are highlighted: internal wave dynamics from the midshelf to the inner shelf; flow separation and eddy shedding off Point Sal; offshore ejection of surfzone waters from rip currents; and wind-driven subtidal circulation dynamics. The extensive dataset from ISDE allows for unprecedented investigations into the role of physical processes in creating spatial heterogeneity, and nonlinear interactions between various inner-shelf physical processes. Overall, the highly spatially and temporally resolved oceanographic measurements and numerical simulations of ISDE provide a central framework for studies exploring this complex and fascinating region of the ocean.U.S. Office of Naval Research (ONR)ONR Departmental Research Initiative (DRI)Inner-Shelf Dynamics Experiment (ISDE

A new interpretation of multifrequency/multipolarization radar signatures of the Gulf Stream front

Radar signatures which are observed on SIR-C/X-SAR multifrequency/multipolarization synthetic aperture radar images of the Gulf Stream off the U.S. east coast are compared with results of simulations with a numerical radar imaging model. Based on in situ data, current and wind variations are included into the model as well as a variation of the thermal stability of the marine atmospheric boundary layer across the Gulf Stream front. According to our model predictions, all of these parameter variations can cause radar signatures of similar shape and modulation depth. But, due to specific dependencies of radar signatures on variations of surface currents and winds, we show that it is possible to distinguish between radar signatures of oceanic and atmospheric origin in multifrequency/multipolarization images and to estimate the corresponding current and wind variations independently. For one set of radar images we derive a most likely scenario of oceanic and atmospheric parameters during the time of the image acquisition for which good overall agreement between observed and simulated radar signatures is obtained at most radar channels

Current measurements by airborne along-track InSAR: measuring technique and experimental results

Since the first demonstration of high-resolution mapping of surface currents by airborne along-track interferometric synthetic aperture radar (along-track InSAR) in the late 1980s, theoretical models of the along-track InSAR imaging mechanism and recommendations for ideal instrument parameters, measuring strategies, and data processing and interpretation techniques have been discussed in a number of publications. However, due to the experimental nature of existing instruments and algorithms and a very limited reference database from actual experiments, potential users have not recognized the along-track InSAR as a readily available tool for current measurements until now. In order to promote the use of InSAR and to validate and demonstrate current measurements on the basis of instrument parameters and data processing techniques proposed earlier, the authors have carried out experiments with an airborne X-band along-track InSAR over spatially varying current fields at two test sites in the German Bight of the North Sea. In this paper, an overview of the experimental scenarios and the acquired data is given, and the newly implemented algorithms for the retrieval of two-dimensional (2-D) surface current fields from the InSAR raw data are described. Using acoustic Doppler current profiler (ADCP) data and predictions of a numerical circulation model as reference, a root mean square (rms) error of spatial variations in the InSAR-derived current fields on the order of 0.1 m/s at an effective resolution of about 100 m is obtained, which is consistent with theoretical expectations. Furthermore, it is shown that the proposed iterative correction scheme for nonlinearities of the InSAR imaging mechanism on the basis of numerical simulations works well and leads to a significant improvement. Altogether, it is concluded that the proposed technique for current measurements by along-track InSAR is efficient, robust, and sufficiently mature for applications that require high-resolution snapshots of surface current fields within areas of some square kilometers, such as the monitoring of bathymetric changes in coastal waters

On the potential of current measurements by spaceborne along-track InSAR for river runoff monitoring

The global monitoring of volume transports of large rivers is a challenging technological problem with a number of important applications. It is highly desirable to use satellite-based remote sensing techniques for consistent worldwide measurements on a regular basis. While water level measurements in rivers with spaceborne altimeters have already been demonstrated a few years ago, instruments for routine current measurements from space have not been available until now. This will change with the advent of along-track interferometric synthetic aperture radars (along-track InSARs) on upcoming satellites such as the German TerraSAR-X, which is scheduled for launch in 2006. In this paper we evaluate the potential of such instruments for current measurements in rivers. We consider fundamental theoretical issues as well as existing InSAR data from the Shuttle Radar Topography Mission (SRTM) and numerical simulation results. We find that currents in the Elbe river estuary (Germany) can be derived from SRTM data quite well. A similar data quality is predicted for TerraSAR-X. We discuss the potential of TerraSAR-X for routine current measurements in rivers and propose the development of a comprehensive data synthesis system which combines water level and current data from satellites and other sources in such a way that a maximum amount of information on river is obtained

Doppler spectra of the radar backscatter from the sea surface obtained from a three-scale composite surface model

The Doppler shift of the backscattered radar signal from the sea surface can be used for determining the line-of-sight velocity of the scatterers and thus for measuring surface currents and ocean wave spectra. The bandwidth of the instantaneous Doppler spectrum, which is associated with the distribution of the line-of-sight velocity within the radar resolution cell, is a measure for the scene coherence time which enters into the SAR imaging mechanism for ocean scenes. Experimental results indicate that the measured Doppler shift can significantly exceed the values expected from simple first-order considerations. Furthermore, the Doppler bandwidth has been found to vary along long ocean waves, and a difference between the Doppler bandwidths for upwind and downwind looking radars has been observed. The author presents a new model for the computation of Doppler spectra which includes the hydrodynamic modulation of short waves by longer waves. The proposed model can reproduce the experimental results at least qualitatively at the present stage.<