Occurrence and Energy Dissipation of Breaking Surface Waves in the Nearshore Studied with Coherent Marine Radar

Wave breaking influences air-sea interactions, wave induced forces on coastal structures, sediment transport and associated coastline changes. A good understanding of the process and a proper incorporation of wave breaking into earth system models is crucial for a solid assessment of the impacts of climate change and human influences on coastal dynamics. However, many aspects are still poorly understood which can be attributed to the fact that wave breaking is difficult to observe and study because it occurs randomly and involves multiple spatial and temporal scales. Within this doctoral work, a nearshore field experiment was planned and conducted on the island of Sylt in the North Sea to investigate the dynamics of wave breaking. The study combines in-situ observations, numerical simulations and remote sensing using shore-based coherent marine radar. The field measurements are used to investigate the coherent microwave backscatter from shoaling and breaking waves. Three major developments result from the study. The first one is a forward model to compute the backscatter intensity and Doppler velocity from known wave kinematics. The second development is a new classification algorithm to identify dominant breakers, whitecaps and radar imaging artifacts within the radar raw data. The algorithm is used to infer the fraction of breaking waves over a sub- and an inter-tidal sandbar as well as whitecap statistics and results are compared to different parameterizations available in literature. The third development is a new method to deduce the energy of the surface roller from the Doppler velocity measured by the radar. The roller energy is related to the dissipation of roller energy by the stress acting at the surface under the roller. From the spatial gradient of roller energy, the transformation of the significant wave height is computed along the entire cross-shore transect. Comparisons to in-situ measurements of the significant wave height from two bottom mounted pressure gauges and a wave rider buoy show a total root-mean-square-error of 0.20 m and a bias of −0.02 m. It is the first time that measurements of the spatio-temporal variation of the bulk wave energy dissipation together with the fraction of breaking waves are achieved in storm conditions over such a large distance of more than one kilometer. The largest dissipation rates (> 300 W/m² ) take place on a short distance of less than one wave length (≈ 50 m) at the inter-tidal sandbar. However, during storm conditions 50 % of the incoming wave energy flux is already dissipated at the sub-tidal sandbar. The simultaneous measurements of the occurrence frequency and the energy dissipation facilitate an assessment of the bulk dissipation of individual breaking waves. For the spilling-type breakers in this area, the observed dissipation rate is about 30 % smaller than the dissipation rate according to the generally used bore analogy. This must be considered within nearshore wave models if accurate predictions of the breaking probability are required

The Application of Proper Orthogonal Decomposition to Numerically Modeled and Measured Ocean Surface Wave Fields Remotely Sensed by Radar

Phase-resolved ocean surface wave elevation maps provide important information for many scientific research areas (e.g., rogue waves, wave-current interactions, and wave evolution/growth) as well as for commercial and defense applications (e.g., naval and shipping operations). To produce these maps, measurements in both time and space are necessary. While conventional wave sensing techniques are limited spatially, marine radar has proven to be a complex yet promising remote sensing tool capable of providing both temporal and spatial wave measurements. The radar return from the sea surface is complex because it contains contributions from many sources only part of which provide information about the ocean surface wave field. Most existing techniques used to extract ocean wave fields from radar measurements implement fast Fourier transforms (FFTs) and filter this energy spectrum using the linear dispersion relationship for ocean waves to remove non-wave field contributions to the radar signal. Inverse Fourier transforms (IFFTs) return the filtered spectrum to the spatial and temporal domain. However, nonlinear wave interactions can account for a non-negligible portion of ocean wave field energy (particularly in high sea states), which does not completely adhere to the linear dispersion relationship. Thus, some nonlinear wave energy is lost using these FFT dispersion-filtering techniques, which leads to inaccuracies in phase-resolved ocean surface wave field maps. This deficiency is significant because many of the aforementioned research areas and applications are most concerned with measurement and prediction of such anomalous wave conditions. Proper orthogonal decomposition (POD) is an empirical technique used in scientific fields such as fluid mechanics, image processing, and oceanography (Sirovich, 1987). This technique separates a signal into a series of basis functions, or modes, and time or spatial series coefficients. Combining a subset of the modes and coefficients can produce a reduced order representation of the measured signal; this process is referred to as a reconstruction. This research applies POD to radar Doppler velocity measurements of the sea surface and uses the leading modes as a filter to separate wave contributions to the radar measurement from non-wave contributions. In order to evaluate the robustness of this method, POD is applied to ocean wave radar measurements obtained using three different radar systems as well as to numerically modeled radar data for a variety of environmental conditions. Due to the empirical nature of the POD method, the basis functions have no innate physical significance, therefore the shape and content of leading POD modes is examined to evaluate the linkage between the mode functions and the wave field physics. POD reconstructions and FFT-based methods are used to compute wave field statistics that are compared with each other as well as to ground truth buoy measurements. Correlation coefficients and root mean squared error are used to evaluate phase-resolved wave orbital velocity time series reconstructions from POD and FFT-based methods relative to ground truth buoy velocity time series measurements. Results of this study show that when POD is applied to radar measurements of the sea surface: (i) the leading mode basis functions are oscillatory and linked to the physics of the measured wave field; (ii) POD performs comparably to FFT-based dispersion filtering methods when calculating wave statistics; and (iii) phase-resolved POD orbital velocity maps show higher correlations with buoy velocity time series relative to orbital velocity time series based on FFT dispersion filtering methods when high group line energy is present (i.e., in the presence of steep and breaking waves)

The importance of scale structure in scattering from random, rough surfaces

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1998.Includes bibliographical references (leaves 237-244).by Vincent Lupien.Ph.D

Using High-Resolution Glider Data and Biogeochemical Modeling to Investigate Phytoplankton Variability in the Ross Sea

As Earth’s climate changes, polar environments experience a disproportionate share of extreme shifts. Because the Ross Sea shelf has the highest annual productivity of any Antarctic continental shelf, this region is of particular interest when striving to characterize current and future changes in Antarctic systems. However, understanding of mesoscale variability of biogeochemical patterns in the Ross Sea and how this variability affects assemblage dynamics is incomplete. Furthermore, it is unknown how the Ross Sea may respond to projected warming, reduced summer sea ice concentrations, and shallower mixed layers during the next century. to investigate these dynamics and explore their consequences over the next century, high-resolution glider observations were analyzed and used in conjunction with a one-dimensional, data-assimilative biogeochemical-modeling framework. An analysis of glider observations from two latitudinal sections in the Ross Sea characterized mesoscale variability associated with the phytoplankton bloom and highlighted potential mechanisms driving change in the assemblage. In particular, an observed increase in the ratio of carbon to chlorophyll (C:Chl) suggested a marked transition from a phytoplankton assemblage dominated by Phaeocystis antarctica- to one dominated by diatoms. The expected control of phytoplankton variability by Modified Circumpolar Deep Water and mixed layer depth were shown to be insignificant relative to the effects of wind and sea surface temperature on the temporal/spatial scales measured by the glider. Additional glider measurements were used to force the Model of Ecosystem Dynamics, nutrient Utilisation, Sequestration and Acidification, which was adapted for use in the Ross Sea (MEDUSA-RS) to include both solitary and colonial forms of Phaeocystis antarctica. The impacts of climate-induced changes on Ross Sea phytoplankton were investigated with MEDUSA-RS using projections of physical drivers for mid- and late-21st century, and these experiments indicated increases of primary productivity and carbon export flux. Additional scenario experiments demonstrated that earlier availability of low light due to reduction of sea ice early in the growing season was the primary driver of simulated productivity increases over the next century; shallower mixed layer depths additionally contributed to changes of phytoplankton composition and export. Glider data were assimilated into MEDUSA-RS using the Marine Model Optimization Testbed (MarMOT) to optimize eight phytoplankton model parameters. Assimilation experiments that used different data subsets suggest that assimilating observations at the surface alone, as are typically available from remote-sensing platforms, may underestimate carbon export to depth and overestimate primary production. Experiments assimilating observations characteristic of a cruise-based sampling frequency produced a wide range of solutions, depending on which days were sampled, suggesting the potential for large errors in productivity and export. Finally, assimilating data from different spatial areas resulted in less variation of optimal solutions than assimilating data from different time periods in the bloom progression; these temporal differences are primarily driven by decreasing colonial P. antarctica growth rates, increasing colonial P. antarctica C:Chl, and faster sinking of colonies as the bloom progresses from the accumulation stage through dissipation. Overall, this dissertation research demonstrates the value of using bio-optical glider observations in conjunction with modeling to characterize phytoplankton dynamics in a remote marine ecosystem. High-resolution glider data are better able to resolve mesoscale physical-biological relationships, which are typically not discernible from lower frequency data, but it can be difficult to identify mechanistic relationships from in situ measurements alone. In addition, biogeochemical models can be used to extend insights gained by empirical observation, but application is often limited by the quantity and type of in situ data appropriate for evaluation and forcing. The use of gliders for facilitating development and operation of a lower trophic level model demonstrated the effectiveness of a synthetic approach that partly overcomes the individual limitations of these otherwise distinct approaches. Finally, the combination of these approaches is especially useful for gaining a better understanding of ecosystem dynamics in regions similar to the Ross Sea that are undergoing substantive climate-induced changes and where harsh conditions make other means of access difficult

Applications of numerical models for rough surface scattering

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1996.Includes bibliographical references (p. 273-286).by Joel Tidmore Johnson.Ph.D

Study of sea clutter influence in ship classification algorithms based on Polarimetric SAR Inteferometry

This paper is focused to evaluate the influence of sea clutter in the performance of ship classification algorithms based on single-pass Polarimetric SAR Interferometry (PolInSAR). For such purpose, series of numerical simulations have been carried out with GRECOSAR, the SAR simulator of complex targets developed by UPC. There, different types of vessels have been considered for a TerraSAR-X like sensor and a sea surface following the two-scale wave approach. The quality of ship discrimination has been quantitatively evaluated with a novel identification method that exploits the particular scattering properties of ships. The results show that the presence of clutter does not notably drop identification performance, despite negative matches can be observed in some particular situations. But the requirement of single-pass interferometric capabilities is not achieved by any of the existing orbital system. This drawback can difficult the validation of what has been observed in simulation environments and can be one of the most limiting factors for the practical implementation of these techniques. Ideas and possible solutions to relax the system requirements are preliminary discussed.Postprint (published version