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

Microwave scattering from surf zone waves

Wave breaking in the surf zone is an important forcing mechanism on the generation of nearshore currents and in the driving of sediment transport. At the same time, wave breaking can have significant spatial and temporal variability that needs to be accounted for in the description of nearshore processes. Remote sensors are best suited to collect wave breaking measurements due to their large footprint and synoptic capabilities, but in order to extract quantitative wave parameters a proper understanding of the imaging mechanisms is essential. Microwave sensors have been shown to be able to measure wave parameters in deep water, but in the surf zone many of the assumptions the algorithms are based upon do not hold. Additionally, the dynamics of breaking waves are different and may affect in a yet determined way the signal. This dissertation first intends to address an observational gap regarding surf zone microwave measurements. A novel combination of synchronous, large coverage marine radar, calibrated pulsed Doppler radar and video observations from a field site enable the analysis of the evolution and characteristics of the wave signature. The combined data sets yield superior discrimination rates between breaking and non-breaking waves. Discrimination also allows the study of the microwave scattering by source, where active breaking is separated from remnant foam and steepening waves. Results show that the backscattered power from breaking waves, specifically from the wave roller, is a several dB larger than that of foam and steepening waves and independent of the environmental conditions and polarization state. While similar results have been obtained for deep water waves and variety of scattering models have been proposed, it is found that none of the models can describe all the data. Additionally, most of the models neglect the roller morphology. Therefore, in the last section a scattering model is introduced, in which the roller is treated as a volume where a collection of water droplets embedded in air can scatter incoherently. Multiple interactions of the scattered fields between particles and the boundaries are also accounted for. Though the model formulation is complex, it depends on a few physical parameters (diameter, volume fraction, medium permittivity) and no calibration constants. Comparison against data shows that the model does a reasonable job in predicting the observed scattering levels, polarization response and grazing angle dependencies, although is not capable to reproduce the maximum scattered levels observed and predicts polarization ratios always less than unity

The development of particle image velocimetry for water wave studies.

SIGLEAvailable from British Library Document Supply Centre- DSC:D92654 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

The relationship of subsurface bubble plumes to wind speed and sea state in the open ocean

When winds blow over the ocean, surface waves grow as energy and momentum are transferred from the air to the sea surface. The waves steepen as energy is added and may eventually break, generating turbulence and plumes of bubbles in the near-surface ocean. This thesis focuses on measurements made during a six-week research trip in the North Atlantic Ocean, in a wide range of wind speeds (8–30 m/s). The complex effects of wind speed, wave age, wind sea wave height and subsurface turbulence on the structure, penetration depth, duration and production rate of bubble plumes are investigated in this thesis. The major data set used here was collected using an upward-pointing sonar during three separate four-day deployments. Individual bubble plumes were detected using an algorithm based on simultaneous sonar and resonator acoustic signals from subsurface bubbles. The acoustical backscattering caused by bubble plumes decays approximately exponentially with depth. The decay constant is related to the bubble entrainment in the water column. The entrainment of a larger void fraction may lead to a lower backscattering decay constant due to the possibility of multiple scattering. Higher wind speed induces younger, rougher waves and higher turbulence, which generate deeper bubble entrainment that promotes longer-lasting plumes (> 100 seconds). The plume production rate is amplified by low-medium wind speeds ( 20 m/s), the plume production rate is significantly reduced to only 8 plumes per hour in the presence of younger less developed seas. The duration variation of these plumes is associated with large scatter. The wave height was found to influence the bubble-plume activity only in a sea state where swell does not contaminate the wind-driven waves. Given the complex and often chaotic sea state in the open ocean, wind speed remains the main factor that controls the bubble plume structure. During the persistent storms, it is difficult to directly assess the effect of individual breaking waves on subsurface bubble plumes because the upper ocean may become supersaturated with gas, and plumes are often advected from a side into the sonar sample volume. In supersaturated seas (at wind speeds > 20 m/s), the plumes become continuous fields of bubbles persisting for very long times (minutes or tens of minutes), without a clear separation between individual events. The total backscatter distribution along the water column (0–8 metres, which was the distance from the sonar to the sea surface) varies with plume penetration depth. Most of the backscatter signal (over 90%) is contained in the upper one metre when the plume is shallow and dense, spreading down the water column more evenly as the plume deepens and becomes more acoustically transparen

Tidally generated high-frequency internal wave packets and their effects on plankton in Massachussetts Bay

Tidally generated internal wave packets occur twice a day during late summer in Massachusetts Bay, U.S.A. The packets are formed at Stellwagen Bank and propagate into the Bay at about 60 cm sec—1; they dissipate in shallow water at the western side of the Bay. The dominant waves in packets have lengths of about 300 m, periods of between 8 and 10 min, and amplitudes of up to 30 m. Overturning of the waves has been observed acoustically over Stellwagen Bank, in the deep (80 m) waters in the center of the Bay, and during dissipation in shallow water. The effects of the internal waves on the distribution of plankton were investigated in August 1977 using an instrument package (Longhurst-Hardy Plankton Recorder, in situ fluorometer, CTD) towed either at a constant depth or following an isotherm through wave packets. Phytoplankton and zooplankton appear to be carried passively up and down by the internal waves; the data were insufficient to resolve any active response by zooplankton to vertical displacements by the waves. Vertical distributions of the plankton were altered by overturning of waves and subsequent mixing. Patterns of horizontal distributions of plankton determined by constant-depth tows were dominated by the effects of internal wave vertical displacements. Isotherm-following tows removed much of the variability due to wave displacement, but problems of following rapidly moving isotherms introduced considerable smaller-scale variability. Changes in zooplankton abundance on tow length scales (600-1200 m) were not correlated with temperature, salinity, or density; some significant correlations with chlorophyll fluorescence occurred when internal wave activity was present. Twice a day for several hours or more, phytoplankton were vertically displaced by as much as 30 m, with a period of about 10 min. The light level plant cells experienced varied from 0.1 to 26% of the ambient surface illumination. This rapid change in light should alter fluorescence yields of plant cells and affect continuous in situ measurements of chlorophyll fluorescence. The timing of internal wave packets varies with the semidiurnal tidal cycle and therefore interacts with the day-night cycle to significantly alter the light regime experienced by plant cells over a two-week period. This could affect the primary productivity of the Bay in the area affected by internal wave packets. The sporadic occurrence of internal wave overturning and mixing events in the deep waters of the Bay could enhance primary production by increasing nutrient input to the mixed layer. This effect should be greatly enhanced in the shallow waters where the internal waves dissipate. Comparison of acoustic and plankton recorder data showed that much of the intense acoustic backscattering seen in high-amplitude (10-20 m) internal waves is due to physical structure and processes, and not to the presence of zooplankton

The investigation of noctilucent clouds and other mesospheric phenomena using ground-based instrumentation and rockets

The optical and dynamical properties of the summer phenomena known as Noctilucent Clouds (NLC) have been studied globally since the early 1960s. These clouds only occur naturally in the Earth's mesosphere, and are presently studied using remote sensing from rockets, and satellites in addition to ground-based observations. Direct evidence of the topology, and structure of an aerosol layer, such as a NLC can be achieved using non-imaging photodiode/photometers housed on a rocket payload. The APL designed photometers utilize the spin of the rocket payload, therefore scanning the entire sky laterally, and producing a two- dimensional image of the aerosol layer traversed during the upleg and downleg flights. The APL photodiode/photometers were flown during the MAED (Middle Atmospheric ElectroDynamics) and NLC-91 (NoctiLucent Clouds 1991) summer rocket campaigns respectively. These multinational rocket campaigns were coordinated with ground-based and satellite observations. The resultant APL data provides a complementary data source to be compared and contrasted with other rocket experiments flown during the absence/presence of NLC and/or PMSE (Polar Mesospheric Summer Echoes). A Bomem Michelson Interferometer (MI) was stationed in Sweden during the summer rocket campaign, NLC-91 provided measurements of the hydroxyl (3,1) band emission from a layer positioned ~87 km. The presented data were kindly provided by the Space Dynamics Laboratory, Utah State University; analyzed and interpreted by the author. The data gave a measure of the upper mesospheric conditions during the presence and absence of NLC during the rocket campaign. The interpretation of the raw data gave an indication of stratospheric filtering of upward propagating waves whose diminution could produce the upward forcing that may be involved in the NLC formation processes. Intensity and rotational temperature profiles deduced during the absence/presence of NLC gave clear results of small- and large-scale waves, and possible correlation between the hydroxyl intensity, and mesopausal temperature. A ground-based Imaging Fabry-Perot Interferometer (IFPI) stationed at the Bear Lake Observatory (BLO), Utah, (41.93°N, 111.42°W) has been operated, since 1989, to study the behaviour of the mesospheric winds at mid-latitudes deduced from the measurements of intensity, and wind speeds of the 8430 Å hydroxyl (6,2) band. The results presented have been studied during the summer periods from this site. This IFPI has provided an opportunity of observing this weak infra-red emission line, and provides a continuous monitoring of the mesopause region throughout the year. A comparison of characteristics on a night by night basis of hydroxyl intensity variations, and wind speed structure, and variations from the mid-latitude IFPI (BLO) with intensity fluctuations indicated by the Bomem MI as being associated with the absence/presence of NLC at summer high latitudes

Physics of coral reef systems in a shallow tidal embayment

Ongoing deforestation in the tropics involves higher river discharges and an increase of runoff, which has consequences to coastal ecosystems. The dispersal of fluvial sediment and freshwater by marine processes affects the environmental determinants of coral reefs near the coast, which include temperature, salinity, light and nutrient concentration. The central objective of this thesis is to identify the physical mechanisms that govern (1) the regional variability of vertical mixing, salinity, turbidity, and the supply of terrestrial sediment in coastal reef environments and (2) the local sedimentary processes at reef slopes, determining turbidity and sedimentation. The object of study was the Bay of Banten, which represents a shallow coastal embayment where coral reefs are subject to relatively turbid marine conditions. Field data were gathered within the framework of the Teluk Banten Research Programme, which is an Indonesian-Dutch research programme focussing on the social-economy, biology, geology and physical oceanography of to the Bay of Banten, Indonesia. The Teluk Banten Research Program is part of the Global Change Program of the Royal Netherlands Academy of Sciences (KNAW), and coordinated by the Foundation for the Advancement of Tropical Research (WOTRO). The study revealed that the key issues determining the delicate subsistence of shallow-water reefs in the Bay of Banten include mixing in the freshwater source region, the seasonal coupling between coastal processes and local resuspension of sediment. The topography-controlled mixing of small river discharges with the ambient marine waters inhibits the direct exposure of coral reefs to river-derived sediment suspensions, high nutrient loads and low salinity. The reefs near the coast benefit from relatively strong currents in front of an abandoned delta, which hydrodynamically acts like a coastal headland. Due to a seasonal coupling between river discharge, wind-driven throughflow and wave attack on the eroding delta, coral reefs in the Bay of Banten are protected from prolonged inundation by high-turbidity water masses that originate from the eroding delta. Local currents that induce resuspension of sediment at the reef slopes are the main control on turbidity variation in the waters that surround the coral reefs. Sediment that is already available may be resuspended and deposited repeatedly, which limits the role of regional advection of sediment. Backscatter data from an Acoustic Doppler Current Profiler (ADCP) have provided a valuable measure of suspended sediment concentration, revealing the spatial structure of sediment clouds connected to the reefs. The analysis of tides in the Bay of Banten gave cause for a general study on the asymmetry of sediment transport in mixed diurnal ? semidiurnal tidal regimes. In these regimes, tidal asymmetry is not only caused by nonlinear tidal interaction, but is also generated by astronomical tides in absence of shallow-water effects. In particular, the K1, O-1 and M2 constituents give an asymmetrical periodic flow pattern, with the largest peak velocities persistently in the same direction. A theoretical explanation for this phenomenon has been presented, and an inventory has been made of other, subordinate constituents that may contribute to repetitive asymmetric flow patterns. Analytical expressions have been derived which quantify the residual transport of sediment due to the K1, O-1 and M2 tides, as a function of the phases and amplitudes of these constituents, and the time lag between variation in suspended sediment transport and flow velocity

Satellite radar altimetry of sea ice

The thesis concerns the analysis and interpretation of data from satellite borne radar altimeters over ice covered ocean surfaces. The applications of radar altimetry are described in detail and consider monitoring global climate change, the role that sea ice plays in the climate system, operational applications and the extension of high precision surface elevation measurements into areas of sea ice. The general nature of sea ice cover is discussed and a list of requirements for sea ice monitoring is provided and the capability of different satellite sensors to satisfy needs is examined. The operation of satellite borne altimeter over non-ocean surfaces is discussed in detail. Theories of radar backscatter over sea ice are described and are used to predict the radar altimeter response to different types of sea ice cover. Methods employed for analysis of altimeter data over sea ice are also described. Data from the Seasat altimeter is examined on a regional and global scale and compared with sea ice climatology. Data from the Geosat altimeter is compared with co-incident imagery from the Advanced Very High Resolution Radiometer and also from airborne Synthetic Aperture Radar. Correlations are observed between the altimeter data and imagery for the ice edge position, zones within the ice cover, new ice and leads, vast floes and the fast ice boundary. An analysis of data collected by the Geosat altimeter over a period of more than two years is used to derive seasonal and inter-annual variations in the total Antarctic sea ice extent. In addition the retrieval of high accuracy elevation measurements over sea ice areas is carried out. These data are used to produce improved maps of sea surface topography over ice- covered ocean and provide evidence of the ability of the altimeter to determine sea ice freeboard directly. In addition the changing freeboard of two giant Antarctic tabular icebergs, as measured by the Geosat altimeter, is presented. As a summary the achievements are reviewed and suggestions are made towards directions for further work on present data sets and for future data from the ERS-1 satellite

Beyond the spatio-temporal limits of atmospheric radars: inverse problem techniques and MIMO systems

The Earth’s upper atmosphere (UA) is a highly dynamic region dominated by atmospheric waves and stratified turbulence covering a wide range of spatio-temporal scales. A comprehensive study of the UA requires measurements over a broad range of frequencies and spatial wavelengths, which are prohibitively costly. To improve the understanding of the UA, an investment in efficient and large observational infrastructures is required. This work investigates remote sensing techniques based on MIMO and inverse problems techniques to improve the capabilities of current atmospheric radars

Měření vlivu kapek pro optický bezvlaknový spoj a matematické modelování vícefázového proudění

Free space optics will emerge alongside major communications technologies as an important player in the field of wireless communications. This technology, like other technologies, has to face the challenges caused by unstable and unfavorable atmospheric conditions that determine the resulting quality of the transmitted signal. The paper is intended to determine the extent of a deterioration of the transmitted signal during rainfall. The precipitation is simulated in laboratory conditions, and the resulting knowledge of the droplet formation is transferred to a mathematical model that helps simulate multiphase flow under given conditions.Optické bezvláknové spoje se v budoucnosti vyskytnou po boku majoritních komunikačních technologií jako důležitý hráč na poli bezdrátových komunikací. Tato technologie, stejně jako jiné technologie, musí čelit výzvám pramenícím z nestálých a nepříznivých atmosférických podmínek, které rozhodují o výsledné kvalitě přenášeného signálu. Tato práce má za úkol zjistit míru zhoršení přenášeného signálu během dešťových srážek. Srážkový úhrn je simulován v laboratorních podmínkách a výsledné poznatky o tvorbě dešťových kapek jsou přeneseny do matematického modelu, který napomáhá simulování vícefázového proudění v daných podmínkách.440 - Katedra telekomunikační technikyvýborn