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Oral Presentations

Arnott, K, Valle-Levinson, A., Chant, B., Ming, L.:
Comparison of Intratidal Shear Production and Turbulence Dissipation from Channel to Shoal in an Estuary.


Current velocity and high resolution hydrographic profiles were obtained at two locations across a transect of the James River estuary to compare the intratidal variation of shear production and turbulent kinetic energy dissipation from channel to shoal. Experiments were carried out for four semidiurnal tidal cycles during May 2010, where two 1200 kHz ADCPs sampled velocity profiles in bursts, while a Self Contained Autonomous Microstructure Profiler (SCAMP) collected profiles of temperature gradient and electrical conductivity. Two cycles were conducted in neap and two in spring tides. The turbulent kinetic energy dissipation was calculated via Batchelor fitting and compared to shear production, obtained using the variance method. Turbulent dissipation results for the neap surveys in the channel ranged from 10-8 to 10-5 m2/s3, showed little variation throughout the tidal cycle, and displayed near bottom maximum values. However, tidal variation was observed over the shoal, with overall larger values in ebb than in flood throughout the water column. The greatest dissipation values were concentrated near the bottom throughout the tidal cycle. Turbulent dissipation results for the first spring survey in the channel showed surface dissipation at the beginning of ebb and values of 10-5 m2/s3 near bottom throughout the tidal cycle. The shoal profile displayed tidal variability (consistent with neap surveys), with large near bottom dissipation (~10-5 m2/s3) during the ebb period. The second spring survey displayed tidal variability in the channel, with large dissipation throughout the water column during ebb and near bottom values of 10-5 m2/s3. The shoal also showed tidal variability, with the largest dissipation (~10-5 m2/s3) at the surface during ebb and values of 10-6.5 m2/s3 throughout the water column. These turbulent dissipation observations are compared to values of shear production, calculated from measurements of Reynolds stresses from nearby bottom mounted ADCP instruments.

Becherer, J.; Burchard, H.; Flöser, G.; Mohrholz, V.; Umlauf, L.:
Evidence of tidal straining in well-mixed channel flow from micro-structure observations

This study presents, for the first time, micro-structure observations in the Wadden Sea, a tidal shallow coastal area in the South-Eastern North Sea characterised by barrier islands separated by tidal gulleys. The tidal currents are typically overlaid by a weak horizontal density gradient due to freshwater run-off from land. The observations in an energetic tidal channel clearly show the expected effects of tidal straining: destratification during flood and increased stratification during ebb. Microstructure observations are consistent with the tidal straining dynamics: during flood relatively high values of viscous dissipation are observed, during ebb the values are substantially smaller. It is also shown that the tidal cycle of stratification and destratification depends on the position in the tidal channel. In parts of the channel, increased stratification occurs already during full flood, a phenomenon which can only be explained by advection of stratified water masses formed outside the tidal channel. The observations presented here show the general significance of the tidal straining process for tidally energetic weakly stratified regimes.

Bennis, A-C., Ardhuin, F., Dumas, F.:
A new 3D fully wave-current model MARS-WAVEWATCH III : validation and application to the rip current.

A three-dimensional wave-current model has been developed in order to explicitly consider the feedback from the current on to the waves. The coupling procedure based on the coupler PALM (Buis et al 2008) allows us to compare the results from one-way and two-way modes. The wave model WAVEWATCH III® (Tolman 2009, Ardhuin et al 2010) is coupled with the three-dimensional circulation model MARS3D (Lazure et Dumas, 2008). The coupled model solves the quasi-Eulerian velocity (Ardhuin et al 2008, Bennis et al 2011) that is equal to the Lagrangian velocity minus the Stokes drift. We modified the turbulent mixing scheme and the bottom friction in order to include the wave effects. First, we validate the coupled model in surf zone with in-situ data for a plane beach. The current and the significant wave height are in agreement with the data. The influence of the rugosity length and the wave bottom boundary layer thickness on the longshore current are shown. More novel, we seek the influence of the feedback on rip currents and the vertical structure of rip currents.The rip currents are intense and oriented to seaward and this can be prejudiciable for the human safety. The coupled model is used to study the rip currents on Aquitain Coast near the Biscarosse beach. In this region, the depth-integrated circulation has been already investigated by Bruneau et al (2011) in case of the one-way coupling. In case of the two-way coupling mode, we found that the rip current is spatially shifted and that its intensity is increased in comparison with one-way mode. Its vertical structure is exhibits a stronger shear than with the one-way mode. The dissipation due to wave breaking is increased with the two-way coupling mode.

Buettner, R., Brede, M., Leder, A.:
Experimental Investigation of turbulence induced mixing in breaking internal waves

Within the framework of the International Leibniz Graduate School for Gravity Waves and Turbulence in the Atmosphere and Ocean laboratory experiments observing the transition from wave energy to turbulence are carried out at the Chair of Fluid dynamics.

Monochromatic internal wave trains excited by a vertical oscillating cylinder are propagating in a stably density stratified fluid. At the end of the channel a slope with constant angle has been placed where the internal waves are forced to break. The wave breaking and its resulting turbulence are inducing additional mixing which irreversibly converts kinetic energy to potential energy causing the stratification to change. In the presented work sloping angles, amplitudes and frequencies of the incident waves are varied and the breaking events are observed. To examine the resulting change of the stratification a LIF-system is used which by its high 2D spatial resolution enables to calculate the change of potential energy, too. Plane velocity fields can be measured simultaneously by a PIV-system and the turbulent kinetic energy can be determined with respect to the achieved spatial and temporal resolution. Both PIV- and LIF-measurements can be performed with a temporal resolution of up to 15 Hz. Therefore the amount of wave energy transferred into turbulence by breaking waves at the described setup can be estimated. Furthermore can a mixing efficiency be derived from the combined measured density and velocity fields.

Burchard, H.:
Effects of estuarine circulation on net sediment fluxes

Estuarine circulation due to horizontal density gradients is the major process driving net landward sediment transport in tidal estuaries. There is a variety of processes driving this estuarine circulation, such as gravitational circulation and tidal straining circulation. Since density driven estuarine circulation vanishes at the landward end of the salt intrusion, sediment is known to accumulate in estuarine turbidity maxima (ETMs). In this presentation, it is quantified by means of idealised numerical models covering a large parameter space, which process is dominating the net sediment transport in tidal estuaries. It will be shown that apart from the interaction between tidal mean circulation and tidal mean sediment profiles, also the covariance between current velocity and sediment concentration is important.

Cross, J, Nimmo Smith, A, Torres, R, Hosegood, P:
Quantifying the Energetics of Mixing and Suspended Particle Dynamics in the Western English Channel

Shelf seas are regarded as important regions that mediate the cycling of particulates and other seawater properties on a global scale. These regions occupy a relatively small area when compared to the expanse of the open ocean, though it is here that the majority of the energy associated with tidal activity is dissipated. Turbulence, be it generated close to the seabed, at the surface or by internal mixing processes, has a controlling influence on the movement and distribution of suspended particles. It may act to keep non-motile plankton in suspension, re-suspend sediment from the seabed and enhance flocculation by bringing particles together or, conversely, large flocs may be torn apart by more intense turbulent eddies. Turbulence acts against stratification to mix nutrients across density gradients, and so turbulent patches within the seasonal thermocline may also be sites of enhanced primary productivity.

Establishing the physical processes that drive these dynamic systems of the marine environment can be logistically challenging and as such, widespread and focused data across time-scales greater than a few hours or days are rare. Presented here are intensive in situ observations using state-of-the-art instrumentation that were carried out in a Lagrangian reference frame at station L4, a site maintained by Plymouth Marine Laboratory (PML) as part of the Western Channel Observatory, across a three-week period during April 2010. Using a free-fall Microstructure Sensor (MSS), several parameters of turbulence were measured and identified along with simultaneous measurements of the number and type of suspended particles (inorganic flocs, phtyo and zooplankton) present throughout the water column using an in-line holographic imaging system. Additional instrumentation including Acoustic Doppler Current Profilers and an undulating MiniBat towed platform were also deployed at this time.

The unique combination of the instruments used during this work allows for separate analysis of the response of biological and non-biological particulates to turbulent motion, and how turbulence might govern the fate of such particles under these conditions across spring and neap cycles. Fresh insight is also given to the degree to which the presence of the thermocline regulates the movement and transfer of both the particles and turbulent motion at this location. In characterising the physical regime at this site during the onset of stratification, whilst additionally observing the potential impact of turbulence on the population of suspended particles, the potential energy anomaly is calculated for each of the three weeks. Breaking the competing processes which govern stratification into their component parts reveals the dominance of the tides as the main driver of mixing at L4 during this period.

Utilising the one-dimensional General Ocean Turbulence Model (GOTM), a mismatch between the output and observations is identified that may be accounted for by lateral advective gradients. These gradients are somewhat apparent in the data returned by the MiniBat from the final campaign of the survey.

Cyr, F., Bourgault, D., Galbraith, P. S.:
Interior versus boundary mixing of a cold intermediate layer

Cold intermediate layers (CIL) are common features in many subarctic and mid-latitude marginal seas. These water masses are generally produced during the winter season as surface mixed layers caused by atmospheric cooling and windstorm mixing events. The newly formed cold surface layer becomes a CIL when sandwiched between the deep layer from oceanic origin and a new warmer surface mixed layer that appears on the onset of the spring. Since CILs lie at intermediate depths, away from surface and bottom boundary layers, one may hypothesize that their erosion (warming rates) is principally governed by internal mixing processes. However, CILs also intersect the sloping bottom around lateral boundaries where turbulent processes may be much more intense than within the interior. In this study we are attempting to quantify the relative importance of interior versus boundary mixing for the erosion of the CIL of the Gulf of St. Lawrence (eastern Canada). Our analysis is based on 18 years of historical temperature profiles, new turbulence measurements (near 1000 casts) and a one-dimensional heat diffusion model. The results suggest that while boundary mixing may be significant it does not dominate CIL erosion. Interior mixing alone seems to explain about 60% of CIL erosion rate. It is too early at this stage to tell whether these results are specific to the Gulf of St. Lawrence or more general to other similar coastal seas, such as the Baltic Sea. We wish through this workshop to exchange ideas about CIL erosion in particular, and coastal mixing in general, that would help making progress towards the goal of understanding and parameterizing turbulence and mixing in coastal seas.

Fredriksson, S.:
Direct numerical simulations of the gas exchange physics at air-water interfaces

The exchange of heat and mass between the earth’s atmosphere and its aquatic systems is crucial to the environmental balance. The presented work is the initial phase of a Ph.D. project aiming to enhance the understanding of how factors, such as turbulence and surfactants, can be modeled to give better estimates of the rate of gas exchange at air-water interfaces.

The importance of understanding the movement/exchange of gases such as CO2 and CH4 between air and aquatic systems (seas and lakes etc.) has become evident during the attempts to understand processes such as the global carbon cycle and climate modeling. The gas exchange is also of importance in order to model the biochemistry in aquatic ecosystems. Organisms (i.e. algea and bacteria) are to a large extent both influenced by, as well as they influence, the gas concentration and gas flux since they act both as producers and consumers of gases such as O2 and CO2. Aquatic systems either work as sinks or sources depending on which gas is being studied and for different temporal and spatial boundary conditions. Our numerical research work is part of a larger research project including field measurements of actual gas exchange ratios (gas chambers), turbulence (acoustic doppler velocimeter) and surface skin temperature fields (infrared cameras).

Direct numerical simulation results for free convection underneath an air-water interface will be presented. The long term goal with the simulations is to understand how the flow field and the surface skin temperature field correspond. The study is part of an effort  to use surface temperature measurements (by use of infrared techniques) in the field in order to estimate the gas exchange. It has in previous studies been found that there is a good correlation between the surface divergence and the gas exchange.

The convection in our initial simulations was driven by a fixed temperature gradient normal to the surface, modeling the heat flux arising from evaporation. Our numerical results have been produced with the open source software openFoam, using an incompressible solver with a standard Boussinesq approximation for buoyancy, and are compared to results previously presented by other researchers. The interface was modeled as a rigid surface with a slip as well as a no-slip boundary condition in order to make a first assessment of the influence of the surface film. We will later on try to make a more realistic representation of the surface film and add surface wind stress.

The flow in our numerical simulations can be characterized by a surface with large warm patches surrounded by thin bands of cooler fluid where the warm patches typically are areas of diverging surface flow whereas the cool bands are areas of converging surface flow. Visualization of the flow underneath the surface reveals a flow of cool water plumes, originating from the cool thin bands, going downwards in a pattern alike a flow typically heated from below but in the reversed direction.

Friedrichts, C.:
Interactions of turbulence and suspended sediment

(to be announced)

Geyer, R.:
Turbulence and mixing in estuaries

(to be announced)

Gräwe, U.:
Mesoscale/submesoscale dynamics in the Western Baltic Sea

Will be submitted asap!

H.M. Schuttelaars and A. Chernetsky:
Influence of vertical eddy and diffusivity formulation on the trapping of sediment in a tidal estuary

In many estuaries high sediment concentrations are found in the turbidity zone. An example of such an estuary is the Ems estuary, situated at the border of the Netherlands and Germany. Between 1980 and 2005, successive deepening of the Ems estuary significantly altered the tidal and sediment dynamics. The tidal range and the surface sediment concentration  has significantly increased and the turbidity zone has shifted into the freshwater zone.

To understand the trapping dynamics in the Ems estuary, and other estuaries showing similar trends, an idealized model was developed and analyzed in Chernetsky et al. (2010). In this model, the vertical eddy viscosity and diffusivity were assumed constant in space and time. It was found that the increase of the tidal amplitude toward the end of the embayment was the result of both the deepening of the estuary and a 37% and 50% reduction in the vertical eddy viscosity and stress parameter, respectively. The physical mechanism resulting in the trapping of sediment, the number of trapping regions, and their sensitivity to grain size were explained by careful analysis of the various contributions of the residual sediment transport. It turned out that due to the deepening of the estuary, the M4 tidal signal in Ems estuary is close to resonance. This results in a very efficient transport of sediment towards the landward side. This transport due to tidal asymmetry, together with the river outflow, results in a turbidity zone that extends far into the freshwater region.

However, the use of a spatially and temporally constant vertical eddy and diffusivity constants is a strong simplification. The aim of this presentation is to look into the sensitivity of the trapping regions, both their location and their number, on the vertical eddy and diffusivity parameterisation. To this end, the model of Chernetsky et al. (2010) is extended with a parabolic, time-dependent formulation. The influence of this closure on the resulting velocity fields will be studied in detail and their effect on the trapping of sediment will be discussed.


Hetland, RD:
Mixing, entrainment and spreading in the Merrimack River plume.

As buoyant water leaves the confining wall of a narrow estuary, and forms a strong jet.  The buoyant water spreads as it enters the ocean, it shoals by continuity of mass, and accelerates by the Bernoulli principle.  This acceleration of the buoyant water triggers strong shear mixing in the near-field plume.  While the spreading of the plume tends to accelerate the buoyant flow, mixing and entrainment decelerate the plume.  The mid-field plume is a region where other forces come into play.  Rotation may arrest plume spreading by turning the plume anticyclonically, thereby suppressing mixing.  Wind may alter the plume structure through mixing, translating the plume through Ekman transport of the plume, or modifying the background flow field.  All of these processes affect the net mixing within the plume and determine if the plume will remain coherent, or if it will dissolve into the ocean mixed layer.  The Merrimack River plume presents an interesting case in which the plume occasionally persists for multiple tidal cycles, and occasionally does not, depending on both the strength and direction of the wind stress.  Investigations of both near- and mid-field circulation show how mixing and entrainment may be strongly modified by lateral processes.

Holtermann, P., Umlauf, L., Rehder, G., Tanhua, T.:
The Baltic Sea Tracer Releaser Experiment: Mixing processes and mixing rates

The seasonal variability of deep-water mixing processes and rates in the Gotland Basin is investigated using results from the Baltic Sea Tracer Release Experiment (BaTRE). This dataset includes long-term moored instrumentation, ship-born turbulence microstructure measurements and observations of the spreading of an inert tracer (SF5CF3) injected in the deep part of the Gotland Basin (≈190 m). Spectral analysis of the kinetic and potential energies reveals two pronounced peaks that are the main energy sources for deep-water mixing: the first around the inertial frequency and a second broadband peak in the sub-inertial range that is interpreted as the signal of basin-scale topographic waves. The time scale for horizontal tracer homogenization was found to be of the order of 6 months. Mixing rates during the initial phase of the experiment, before the tracer had reached the lateral boundaries, were of the order of κ=10-6 m2s-1, contrasted by much higher mixing rates (a few times κ=10-5 m2s-1) observed during later stages of the tracer evolution. This points at the importance of boundary mixing processes for overall vertical mixing, and correlates with increased dissipation rates in the bottom boundary layer inferred from shear-microstructure observations.

Hopkins, J., Sharples, J. :
Saline intrusions in the pycnocline of the Celtic Sea continental shelf

The shelf edge is the interface across which carbon and nutrients are exchanged between the deep ocean and continental shelf. Our understanding of the processes controlling these fluxes is neither complete nor fully developed. Here we introduce an observational data set from the Celtic Sea continental shelf that reveals saline intrusions within the seasonal pycnocline that have not previously been identified. The high salinity water originates from the shelf break, brought to the surface by elevated vertical mixing.

The intrusions are typically 30m thick, with an anomaly of up to 0.1 psu. The salinity maximum is located approximately 5m below the maximum buoyancy frequency and is traceable up to at least 200 km onto the shelf. Scanfish transects reveal the pulsed and patchy nature of the intrusions that decrease in strength with distance from the shelf edge.

The mechanisms responsible for injecting water into and subsequently transporting it along the pycnocline will be considered. Likely candidates are: interleaving; shear dispersion driven by the internal tide or inertial oscillations; and shear instability enhancing diapycnal mixing at the base of the thermocline where vertical salt gradients are strong.

Kärnä, Tuomas, Legat, Vincent, Deleersnijder, Eric, Burchard, Hans:
Development of a three dimensional discontinuous Galerkin marine model

The Second-generation Louvain-la-Neuve Ice-ocean model (SLIM, www.climate.be/SLIM) is a

discontinuous Galerkin finite element model based on an unstructured mesh.

It is therefore well suited for simulating estuarine and coastal flows where capturing complex topography is crucial.

As grid resolution can be increased almost arbitrarily in areas of interest, unstructured grids also

permit capturing a wide spectrum of time and length scales in a single model, which is cumbersome

with traditional structured mesh marine models.


We report on the development of the baroclinic component of SLIM that solves the three dimensional shallow water equations.

The mesh consists of prismatic elements, that are formed by extruding triangular elements in the vertical direction.

Generalized vertical grids are available, which include the conventional z-grids and terrain following sigma-grids.

In order to correctly take into account the undulation of the free surface, the mesh moves in vertical direction,

resulting in an Arbitrary Lagrangian Eulerian (ALE) scheme.

The baroclinic model has been coupled with the widely-used Generic Ocean Turbulence Model (GOTM,

www.gotm.net) to account for vertical mixing processes.


After introducing the model, we present some model validation scenarios and first results of coastal flow simulations.

Kösters, F., Weilbeer, H.:
Turbulence damping in the Weser estuary’s estuarine turbidity maximum

 The outer Weser estuary in northern Germany is a meso- to macrotidal funnel shaped estuary. In the region of freshwater influence areas of high sedimentation rates exist, linked to the estuarine turbidity maximum (ETM). The scope of this study is to obtain improved modeling results of the suspended particulate material (SPM) concentration in this region by an enhanced representation of turbulent processes.  

As the Weser estuary has a tidal range between 2 m to 4 m it is generally well mixed. However, salinity induced periodic straining occurs in the region of freshwater influence. In order to investigate the associated turbulence damping we have set up a high-resolution numerical model of the Weser estuary based on the UnTRIM 2007 method. The modeling results are compared to recent velocity, salinity and SPM measurements using ADCP and ADV. Temporary stratification can be successfully modeled using a k-eps turbulence closure model. Residual SPM transports due to the stratification induced damping effects on vertical turbulence are one potential mechanism for the ETM formation. This effect is qualitatively compared to other classical formation mechanisms such as tidal pumping.

Liu, Z., Thorpe, S.A.:
Instability and Mixing in Shelf Seas

The stability of stratified flows at locations in the Clyde, Irish and Celtic Seas on the UK Continental Shelf is examined. Flows are averaged over periods of 12 – 30 min in each hour, corresponding to the times taken to obtain reliable estimates of the rate of dissipation of turbulent kinetic energy per unit mass. The Taylor-Goldstein equation is solved to find the maximum growth rate of small disturbances to these averaged flows, and the critical gradient Richardson number, Ric. The proportion of unstable periods where the minimum gradient Richardson number, Rimin, is less than Ric is about 0.35. Cases are found in which Ric < ¼; 37% of the flows with Rimin < ¼ are stable, and Ric < 0.24 in 68% of the periods where Rimin < ¼. Marginal conditions with 0.8 < Rimin/Ric < 1.2 occur in 30% of the periods examined. The mean dissipation rate at the level where the fastest growing disturbance has its maximum amplitude is examined to assess whether the turbulence there is isotropic and how it relates to the Wave-Turbulence boundary. It is concluded that there is a background level of dissipation that is augmented by instability; instability of the averaged flow does not account for all the turbulence observed in mid-water. The data do not support the hypothesis that the turbulent flows observed on the UK shelf adjust rapidly to conditions that are close to being marginal, or that flows in a particular location and period of time in one sea have stability characteristics that are very similar to those in another.

Lozovatsky, I. and Liu, Z.:
Shallow Water Tidal Currents and Turbulence in the East China Sea

ADCP velocity measurements with vertical resolution 0.02 m were conducted in the lowest 0.5 m of the water column at a test site in the western part of the East China Sea to investigate the applicability of the law-of-the-wall very near to the seafloor. The friction velocity  and the turbulent kinetic energy dissipation rate  profiles were calculated using log-layer fits; z is the height above the bottom. The law-of-the-wall dissipation profiles  were consistent with the dissipation profiles  evaluated using independent microstructure measurements of small-scale shear, except in the presence of westward currents. It was hypothesized that an isolated bathymetric rise (25 m height at a 50 m seafloor) located to the east of the measurement site, is responsible for the latter. Calculation of the depth integrated internal-tide generating body force in the region showed that the flanks of the rise are hotspots of internal-wave energy that may locally produce a significant turbulent zone while emitting tidal and nonlinear internal waves.

Maarten Buijsman, Sonya Legg, Jody Klymak:
Double Ridge Resonance in Luzon Strait and Its Effect on Turbulent Mixing

Many semi-enclosed coastal seas are connected to the open ocean through a strait containing a sill. Waves and mixing generated by the tides propagating over this sill play an important role in the circulation of the coastal region. An example of such a strait/sill is Luzon Strait. Located between Taiwan and The Philippines, it features two parallel island ridges that separate the South China Sea from the Pacific Ocean. At these ridges the barotropic to baroclinic energy conversion is large and nonlinear internal waves that radiate from these ridges have been widely studied. However, it is not yet known how much of the converted tidal energy is dissipated at the east and west ridges, how the dissipation occurs, how it varies in a spring-neap cycle, and what the effect is of the double ridges on the mixing and dissipation. These questions are addressed in this study.

For this purpose, numerical model MITgcm is applied in a 2D setup. The model is run in nonhydrostatic mode with dx>100 m and dz>10 m. The model uses realistic topography and stratification and it is forced at the east and west boundaries with barotropic tides. The model applies a simple vertical dissipation and mixing scheme that computes vertical viscosities and diffusivities computed by Thorpe sorting unstable density profiles (Klymak and Legg, 2010). The scenarios are run for a duration of a spring-neap cycle and comprise a single east ridge and a double ridge.

It is shown that most of the dissipation and mixing occurs in the form of nonlinear hydraulic jumps (arrested low and high vertical mode lee waves) near the ridge crests. The lee waves feature strong down slope velocities that advect lighter water under denser water, causing convective overturns and large mixing and dissipation rates. The tidal-mean dissipation in the double ridge case greatly exceeds that of the single ridge case when the semidiurnal tide dominates, while the dissipation is similar in the two cases when the diurnal tide dominates.

The differences in dissipation and mixing are attributed to double ridge resonance during the semidiurnal phase, in which the semidiurnal barotropic velocities at the ridges are in phase with the bottom velocities and pressures of the baroclinic modal waves emitted from both ridges. This yields 1) larger barotropic to baroclinic conversion, causing stronger baroclinic waves, and 2) stronger lee wave velocities, causing stronger overturns and mixing. On the other hand, diurnal barotropic velocities are out of phase with the baroclinic velocities and pressures, because the diurnal baroclinic waves have longer wave lengths. This causes weaker conversion and overturns and associated mixing. Currently, 3D runs are being set up to study the effect of 3D topography on the lee-wave formation.

Klymak, J, and Sonya Legg, 2010. A simple mixing scheme for models that resolve breaking internal waves. Ocean Modelling, 33(3-4)


Moghimi, S., Gräwe, U., Burchard, H.:
Improving simulation of near-shore hydrodynamics by incorporating surface wave

A three dimensional coupled modeling framework for investigating of hydrodynamics

of estuarine, coastal and nearshore waters including 3D coastal circulation

model GETM (General Estuarine Transport Model) and third generation

wind wave model SWAN (Simulating Wave Near-shore) has been developed. Besides

available implementation of Craig and Banner (1994) by Burchard (2001)

in GOTM (General Ocean Turbulence Model), surface flux of TKE (Turbulent

Kinetic Energy) and epsilon(Turbulent dissipation) has been evaluated based on flux of energy

due to dissipation of energy because of deep and shallow water wave breaking.

Approach of Walstra et al (2001) for prescribing only turbulence production in water

column and letting the turbulence model evaluate epsilon accordingly has been tested.

Results for simulation of a wave flume shows different approaches lead to different eddy

diffusivity distribution in water column, which could change undertow velocity profile specially

close to the water surface where the wave effects, significantly. Sensitivity of the

eddy viscosity to surface roughness (Z0s) which has to be defined in terms of wave

height has been investigated. In the literature a wide range for this parameter has

been proposed, Z0s = (0:5 ~ 1:6)Hs. The analysis showed eddy viscosity

profile is rather sensitive to this parameter. For an appropriate evaluation of

this parameter in a practical application more research is required.


Burchard H (2001) Simulating the wave-enhanced layer under breaking surface

waves with two-equation turbulence models. J Phys Oceanogr 31:3133–3145

Craig P, Banner M(1994) Modeling wave-enhanced turbulence in the ocean surface

layer. Journal of Physical Oceanography 24(12):2546–2559

Walstra D, Roelvink J, Groeneweg J (2001) Calculation of wave-driven currents

in a 3D mean flow model 2:1050–1063

Parra, S., Mariño, I., Enriquez, C., Valle-Levinson, A.:
Tide and wave-induced variations in turbulent kinetic energy at a buoyant jet discharge

Current velocity and hydrography measurements were used to determine the influence of tides and waves on turbulent kinetic energy (TKE) variations at a submarine groundwater jet discharge in a fringing reef lagoon in the Yucatan Peninsula, Mexico. Measurements were obtained through a three-day period early in the wet season (July). Velocity and pressure values were recorded at a rate of 8 measurements per second, and salinity and temperature values were acquired at 6 measurements per minute at the spring nozzle. Further measurements at nearby lagoon inlets had lower temporal resolution. Records showed that tidal variations modulated the discharge from the buoyant jet, with maximum outflow values of 0.3 m/s, when smoothed with a 20-minute low-pass filter. Values of TKE at the buoyant jet also showed a clear tidal modulation, with up to 0.36 m2/s2 observed during low tides. Moreover, lagoon water temperatures were modulated by a diurnal cycle while the spring water temperature was modulated by the semidiurnal tides. Additionally, salinity at the spring was affected by semidiurnal tides and TKE variations. Highest salinities (>34 psu) appeared during high tides while lowest salinities (<29 psu) developed between high and low tides. At low tides, an increase in salinity was observed, which was in phase with maximum TKE values. This is evidence of vigorous mixing between spring and lagoon waters. Interestingly, the highest salinity detected at the jet during the measurement period was observed the first two tidal cycles, which also corresponded with high wave activity. The highest salinity was likely caused by wave-induced setup that drove more saline waters into the lagoon from the ocean (wave-pumping). Therefore, wave-pumping should enhance salt intrusion into the spring, and its aquifer source, during high tides. The combination of high tides and wave-pumping is expected to threaten delicate aquifer conditions and vital water resources for the local communities.

Prandke, Hartmut , Fiekas, Volker, Knoll, Michaela, Arneborg, Lars:
Horizontal turbulence measurements using a towed microstructure instrument

We present a towed instrument for horizontal microstructure and turbulence measurements (TIMOS - Towed Instrument for Microstructure Ocean Soundings). The tow body is equipped with a microstructure profiler MSS and two upward and downward looking ADCP. During a measurement period in June 2009 in the Gullmar Fjord (Sweden) a series of test measurements has been carried out to study the towing behavior and the measurement properties of TIMOS. The test measurements have shown the capability of TIMOS to perform horizontal turbulence measurement up to a towing speed of approx. 3 m/s. Furthermore, simultaneous measurement of TIMOS and a vertical sinking MSS profiler have been carried out to study the horizontal and vertical intermittency of the turbulence in the entrance region of the fjord. The measurements  indicated a pronounced intermittency in horizontal and vertical direction in the vicinity of the fjord entrance.

Richards, Clark, Kelley, Dan:
Turbulence and internal waves in the St Lawrence Estuary

In 2008, as part of the St Lawrence Estuary internal wave experiment (SLEIWEX), six acoustic doppler velocimeters (ADVs) were deployed on four bottom moorings in depths ranging from about 15 to 35 m. The goal of the study was to measure turbulence and mixing in the estuary throughout the tidal cycle, with a particular emphasis on mixing related to shoaling internal waves.  Here we discuss near-bottom turbulence levels, determined using the inertial dissipation method. In addition to turbulence associated with internal waves, we present results highlighting variability relative to tidal phase and across-shore distance.

Rovelli, L., Dengler, M., Schmidt, M., Sommer, S., Linke, P., McGinnis, D. F.:
Turbulent mixing in the central North Sea: sources and implications for vertical transport of dissolved gases during the summertime

The “Tommeliten” site in the Norwegian sector of the central North Sea is a well-studied example of a shallow shelf sea where the seasonal thermocline acts as an efficient barrier limiting methane seepage transport from the bottom water to the atmosphere. As this sharp thermal gradient acts as the bottle-neck for vertical transport in the North Sea, it was important to understand the mechanisms controlling diapycnal transport through this interface. In 2009, a process study was conducted to investigate mixing processes and their relation to background velocity and hydrography using moored finescale velocity observations (ADCPs) and microstructure observations that included a fast-responding (~0.2 s) oxygen sensor. In addition, greenhouse gas concentrations were sampled in the water column, allowing estimates of diapycnal greenhouse gas fluxes and insights into the ecological functioning of the North Sea.

The hydrographic setting in August 2009 can be described by a 30 m thick bottom boundary layer (BBL), a strongly stratified thermocline between 25 and 40 m depth, in which temperature decreases from 16°C to 7°C, and a transition zone of about 10 m below the mixed layer. The elevated BBL thickness can be attributed to negative polarity of the tidal currents that enhance mixing in the BBL. Turbulent dissipation rates (ε) determined from microstructure shear profiles were low (2-5x10‑9 W kg‑1) but higher than expected in the thermocline and increased to 10‑7-10‑6 W kg-1 approaching the sea floor. In the stratified interior, baroclinic inertial currents (14.35 hrs period) were found to dominate over tidal currents. These near inertial waves cause elevated shear levels in the thermocline and appear to be the main source of TKE within this layer. In the BBL, a phase lag between maximum dissipation rates near the bottom and in the interior BBL was evident; resulting from local production of turbulence through shear instability. The presence of the baroclinic inertial waves and their associated production of turbulence caused a flux of oxygen of 5 mmol m-2d-1 from the thermocline into the BBL during the observational period. This flux limits oxygen reduction in BBL during the stratified season due to oxygen consumption in the sediments. Due to the presents of the inertial waves, the effectiveness of the thermocline in limiting diapycnal transport was found to be lower than previously hypothesized.   

Ruiz-Villarreal, M., Mouriño-Carballido, B., Chouciño, P., García-García, L. Moro, E., Otero, P., González-Pola, C. Varela, M. and Bode A.:
Turbulence and mixing in the northern Galician shelf and rias during an upwelling event

The Galician shelf is influenced by upwelling events that prevail in spring-summer and strongly shape a highly productive ecosystem. Renewing of nutrients is originated by the upwelling of bottom waters, but the response of the shelf system to upwelling events is complex. Different phases are observed in the response of the system to upwelling winds. When upwelling winds start to blow, surface waters are exported offshore (inducing stratification) and upwelled waters surface near the coast. If upwelling winds continue, a westward jet extends on the mid shelf and gradually ocuppies the whole shelf and reaches the outer shelf. The relaxation of upwelling winds induces the appearance of thermal stratification. In the area of study, the Artabro Gulf, the coastline is complex because of the presence of three rias: Ferrol, Coruña and Ares-Betanzos, where turbulence and mixing vary in response to upwelling and tides. The different rias show different response to upwelling due to their different orientation and area, but also due to the fact that the ria de Ferrol is connected to the shelf through a narrow channel. A bottom boundary layer is induced by tides in the area, which are mainly semidiurnal with a fortnight spring-neap cycle, but the response of the system is variable depending on the phase of upwelling winds with tide.

In this presentation we study turbulence and mixing in the northern Galician shelf and in the rias of the Artabro Gulf during an upwelling event in July 2010. During 5 consecutive days, multidisciplinary sampling of plankton with semi-automatic methods and hydrography was carried out in order to study the response of the plankton community to the phases of upwelling winds. On one of the days, dissipation rate of TKE was measured at the same time that a yo-yo profiler was measuring TS. The dynamics of circulation and mixing during the cruise and its eventual impact on the plankton sucession have been studied by means of numerical simulations with the ROMS model using a high resolution grid of the Artabro Gulf nested in a shelf and slope grid.

Schulz, Elisabeth, Burchard, Hans:
Quantification of the Impact of Earth Rotation and Channel Asymmetries on Estuarine Circulation in Tidally Energetic Estuaries

Currents in estuaries are affected by many different factors such as freshwater runoff from the river side, tides from the ocean side, wind from above, the geometry of the estuarine channel itself and more. Until 25 years ago, estuarine circulation was considered to be dominated by or even identical to gravitational circulation, i.e. down-river currents at the surface and up-river exchange currents below due to the baroclinic pressure gradient. Today, the significance of ebb-flood asymmetries has been recognised and tidal straining as well as advectively-driven circulations have been identified as important, sometimes dominant, contributions to the total estuarine circulation.

By means of the numerical General Estuarine Transport Model (GETM) and a semi-analytical decomposition method, an estuarine channel is simulated and the individual contributions to the estuarine circulation are computed. Orientation and magnitude of the longitudinal and the lateral residual circulation contributions are investigated in a wide parameter space, e.g. for different Simpson numbers, Strouhal numbers, aspect ratios and radii of curvature of the channel, Coriolis frequencies and others.

Simpson, J.:
Turbulence and mixing in shelf seas

(to be announced)

Souza, A.J., Burchard, H.:
Coastal Ocean Turbulence and Mixing

Turbulence and the subsequent mixing are important mechanisms in driving the dynamics of the coastal ocean; they are important for the transport of momentum, mass and heat. Turbulence is generally produced by: shear (shear production) or by unstable stratification (convection), although in the nearshore area braking wind waves might be an important source or turbulence. Shear in the coastal ocean is typically generated by bottom friction on tidal currents and wave orbital velocities, as well as by wind driven current, although there are also important baroclinic flows, such as nonlinear internal waves and inertial currents. There are two ways of generating convection: free convection due to surface loss of buoyancy, either due to cooling evaporation or freezing; and forced convection, when denser waters are advected over lighter water, as in the case of  upwelling and on tidal estuaries during flood. Turbulence is destroyed (damped) by stable stratification or by viscous dissipation into heat.


We will discuss here the main controlling factors of turbulence and mixing in the coastal ocean, with particular interest on use of  5 nondimensional parameters to describe the turbulence and dynamics of “shallow” water columns, these parameters are: The nondimensional bed roughness; the Strouhal number which is the ratio of the bottom boundary depth to the water column; the Ekman number, which assess the importance of the Earth rotation and its the ratio of the planetary boundary depth to the water column depth. The horizontal Richardson number or Simpson number and the modified Simpson and Hunter Parameter, which explains the competition of surface buoyancy input against tidal turbulence generated mixing.


Stacey, M.:
Horizontal Mixing in Estuaries and Coastal Seas

(to be announced)

Todd, D, Souza, A, Jago, C:
Turbulence-controlled Flocculation in a Macro-Tidal Estuary

Suspended particular matter (SPM) determines turbidity, impacting both water quality and primary production. SPM generates benthic fluff on the seabed, modifying biogeochemical exchanges and constraining primary productivity. Further, SPM carries important biogeochemical components (e.g. carbon, nitrogen, contaminants and pollutants), deciding the fates of anthropogenic inputs to the estuarine system.

Outside of the non-cohesive fraction (sand), little is known of the properties of SPM (i.e. particle size, density, settling velocity) and how these impact fine particle entrainment and sedimentation. This is due to most SPM being in the form of flocs (aggregates of dead and living organic matter, cohesive inorganic matter, and water) that are dramatically modified by conventional sampling methods (easily ruptured and/or may aggregate during sampling). As such we lack reliable and comprehensive information on key parameters such as pick-up functions and settling velocities, particularly since floc properties change on a range of time scales: tidal (suspension/advection), lunar (spring-neap cycle), and seasonal (storm resuspension and biological production).

Turbulence is an important mediator of floc characteristics, promoting particle collision and aggregation at low levels, while high levels result in shear-induced rupture, literally tearing aggregates apart. Because of this, accurate turbulence parameterisation is key to understanding the relationships between turbulence and particle size, as well as accurately modelling flocculation.

The results of an extensive field campaign and SPM flux modelling of the Dee estuary (N.W. Britain) are presented, giving insight into the fates of both riverine input and advected SPM from offshore. Using data from a combination of acoustics, optics, moored deployments and CTD stations, a 1-D (GOTM) model shows variation across tidal, spring-neap, and seasonal time-scales.

van der Lee, E., Umlauf, L.:
Internal-wave mixing in the Baltic Sea: Near-inertial waves in the absence of tides

The dynamics of near-inertial motions, and their relation to mixing, are investigated here with an extensive data set, including turbulence and high-resolution velocity observations from two cruises conducted in 2008 (summer) and 2010 (winter) in the Bornholm Basin of the Baltic Sea. In the absence of tides, it is found that the basin-scale energetics is governed by inertial oscillations and low-mode near-inertial wave motions that are generated near the lateral slopes of the basin. These motions are shown to be associated with persistent narrow shear-bands, strongly correlated with bands of enhanced dissipation rates that are the major source of mixing inside the permanent halocline of the basin. Simultaneous observations of high-frequency (near-N) internal waves suggest the presence of unstable modes in the vicinity of the shear bands. In spite of different stratification, near-inertial wave structure, and atmospheric forcing during summer and winter conditions, respectively, the observed dissipation rates were found to scale with local shear and stratification in a nearly identical way. This scaling was different from the Gregg-Henyey-type models used for the open ocean, but largely consistent with the MacKinnon-Gregg scaling developed for the continental shelf.

van Haren, H.:
Role of internal wave mixing in coastal seas with sloping bottoms

(to be announced)