DOCSLIB.ORG

Wave-Current Interaction in Water of Finite Depth Zhenhua Huang 0 6 8

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Wave-Current Interaction in Water of Finite Depth Zhenhua Huang 0 6 8 Wave-current interaction in water of finite depth by S9IJV8fl Zhenhua Huang 0 6 8 B.S., Tsinghua University (1988) AO.1ONH031JO M.E., Tsinghua University(1991) _3.1J.LSNI S.L3sInHOVSSYW Submitted to the Department of Civil and Environmental Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Environmental Fluid Mechanics at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY February 2004 @ Massachusetts Institute of Technology 2004. All rights reserved. A uthor .. ................... Department of Civil and Eyronmental Engineering September 12, 2003 Certifi d by .... ...... .... Certified by. ~ ~Chiang C. Mei Donald Martha Harleman Professor of Civil & Environmental Engineering Thesis Supervisor A ccepted by ............................ Heidi M.' epI Associate Professor of Civil & Environmental Engineering Chairman, Department Committee on Graduate Students MASSACHUSETS iNST E OF TECHNOLOGY FEB 19 2004 LIBRARIES 2 Wave-current interaction in water of finite depth by Zhenhua Huang Submitted to the Department of Civil and Environmental Engineering on September 12, 2003, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Environmental Fluid Mechanics Abstract In this thesis, the nonlinear interaction of waves and current in water of finite depth is studied. Wind is not included. In the first part, a 2D theory for the wave effect on a turbulent current over rough or smooth bottom is presented. The logarithmic profile of the basic current is modified by the waves due to an effective mean shear stress on the free surface. Surface distortion of the eddy viscosity is shown to be important for the change of the mean velocity profile by waves. Both wave-following and wave-opposing current are studied. Comparisons are made with some existing laboratory experiments. In the second part, an instability theory is presented for the initiation of Langmuir cells due to waves interacting with a turbulent current maintained by tides or by an external pressure gradient. With an infinitesimal span-wise disturbance, the free surface experiences a new mean stress, which generates new vorticity to be diffused downward, and induces further growth. Various contributions to the unstable growth of Langmuir circulation are analyzed by examining the mechanical energy budget. Evidences will be shown that the surface stress contributes significantly to instability. Both wave-following current and wave-opposing current are studied. For the wave- following current, two types of Langmuir cells can grow in time; while for the wave- opposing current, only one can grow. Effects of current strength, wave conditions, and water depth on the growth of Langmuir circulation will be studied by numerical examples. Remarks on existing laboratory experiments are made Thesis Supervisor: Chiang C. Mei Title: Donald & Martha Harleman Professor of Civil & Environmental Engineering Acknowledgments In the past seven years at MIT, I owe much to many people who have contributed considerable to this thesis directly or indirectly. First, I would like to thank my advisor, Professor Chiang C. Mei. I am lucky to be able to work with and learn from him these years. His breadth of knowledge, his patience with students, and his high standards have not only guided my research to success but also helped me to grow as a scientist. I would also like to thank Caroline, Professor Mei's wife, for her kindness. I will never forget those warm parties in their home, which provide comfort for my wife and me during these years away from home. I also thank Professor 0. S. Madsen for many fruitful discussions. I would also like to thank the other members of my thesis committee Professor H. M. Nepf and Professor T. R. Akylas for taking time to review and evaluate my work. I am also indebted to the many friends in Parsons Lab who made my graduate experience educational and pleasurable. I am grateful for the financial support by US Office of Naval Research (Grant N00014-89J-3128, directed by Dr. Thomas Swean) and US National Science Founda- tion (GrantCTS-0075713, directed by Profs. John Foss, C. F. Chen and M. Plesniak). I also thank the Ippen Fellowship for supporting me to attend two conferences in the past two years. Finally, I would like to thank my wife, Lixia, and my parents. I could never have reached this point without their understanding and constant support. To them I owe more than I can ever express. 6 Contents I Effects of surface waves on a turbulent current over a smooth or rough seabed 23 1 Introduction 25 2 Formulation 31 2.1 Governing equations and boundary conditions in dimensional form . 31 2.2 Eddy viscosity . ......................... ..... 32 2.3 Basic assumptions ................... .......... 35 2.4 Normalization by core scales ..... ....... ...... ..... 37 2.5 Basic current ... ........ ......... ........ ... 40 2.6 Length scale of wave attenuation ....... ............ 41 3 Mean and oscillatory motions in the core region 43 3.1 Order estimate of wave-induced core current .. ..... ..... .. 43 3.2 Momentum balance of mean motion ..... ..... ..... .... 44 3.3 Vortex force zDQ .... ..... ..... ..... ..... ..... 46 3.4 Mean shear stress of wave-perturbed current . .... .... .... 47 4 Bottom boundary layer 51 4.1 Order of magnitude estimate ..... ......... 51 4.2 Approximate momentum balance of the mean motion 52 4.3 Oscillatory motion: b. ... .... ..... .... 54 4.4 Wave damping rate ...... ...... ...... 55 5 Free surface 57 5.1 Unimportance of surface wave boundary layer .. 57 7 5.2 Perturbed mean Reynolds stress on the still water surface . .5 59 6 Final solution for the core 63 6.1 Formulas for mean turbulent stress and current velocity ........ 63 6.2 Friction velocities ............................. 64 6.3 Numerical procedure ........................... 66 7 Comparison with experiments 69 7.1 Past experim ents .............. ............... 69 7.2 Wave-following current over a smooth bed by Kemp & Simons .. .. 71 7.3 Wave-following current over a rough bed by Kemp & Simons ..... 73 7.4 Wave-opposing current over a rough bed by Kemp & Simons ..... 76 7.5 Wave-following/opposing currents over a rough bed by Klopman . 80 7.6 Longitudinal variation of current .................... 80 8 Concluding remarks of part I 83 II Langmuir circulation in water of finite depth 85 9 Introduction 87 10 Equations and boundary conditions for total motion 95 10.1 Norm alization ...... ...... ...... ...... ....... 95 10.2 Governing equations of total motion ..... ...... ...... .. 98 10.3 Surface boundary conditions . ..... ..... ..... ..... .. 100 10.3.1 Exact surface boundary conditions on free surface .. .... 100 10.3.2 Surface boundary conditions at z = 0 . ...... ...... 101 10.3.3 Mean surface boundary conditions at z = 0 ..... ..... 106 11 Basic state 109 11.1 Basic state for Langmuir circulation .... .... .... .... ... 109 11.2 Shear rate of basic current .. ... .... ... .... ... .... 112 8 12 Linearized equations and boundary conditions for Langmuir circu- lation L17 12.1 Spanwise Perturbations ... ..... ...... ...... ..... 117 12.2 Secondary waves ...... ....... ........ ....... 119 12.3 Linearized equations governing the initial Langmuir circulation .. 122 12.3.1 Continuity equation ....... ....... ....... .. 122 12.3.2 Equation of mean longitudinal momentum . ....... .. 122 12.3.3 Equation of mean lateral momentum . ........ .... 125 12.3.4 Equation of mean vertical momentum .. ........... 127 12.3.5 Equation of mean longitudinal vorticity ......... ... 127 12.3.6 Equation of mean vertical vorticity .......... .... 129 12.4 Linearized free-surface boundary conditions for Langmuir circualtion 129 12.5 Bottom boundary conditions for perturbed current ........ 135 12.5.1 Spanwise perturbations ......... ........... 137 12.5.2 Mean motion associated Langmuir circulation inside BWBL 138 13 Initial Energy Budget of Langmuir Circulation L43 13.1 Equation of mechanical energy ..................... 143 13.2 Growth rate of the normal mode ................. ... 150 13.3 Contribution of BWBL to the growth rate ... ........... 153 14 Generation mechanism of Langmuir circulation 157 14.1 Review of inviscid problem ............. ... .. 157 14.2 The feedback mechanism of Craik and Leibovich . ... .. 159 14.2.1 Instability criterion for inviscid limit . .. .. .. 159 14.2.2 Role of viscosity ............. .. ... .. 162 14.2.3 Feedback mechanism of CL-II ....... .. .. 163 14.3 The surface stress mechanism ......... .. .. .. 166 14.4 Circualtion generated by sidewalls . ........ ... .... 170 14.5 Sum m ary ...................... .. .. .. 173 15 Numerical Scheme 175 15.1 Normal mode .................... ... .. 175 9 15.2 R escaling ........... ............. .. .. .. .. 176 15.3 Discretization Scheme ... ......... ..... .. .. .. .. 178 15.4 Coupled surface boundary condition .... ...... ..... .... 182 15.5 A test example ............... ...... ... ... ... 184 15.6 Solution procedure .. ................. ...... .. 187 15.6.1 Typical parameters ............. .. ... ... .. 187 15.6.2 Bottom roughness ........... ..... .... ..... 189 15.6.3 Maximum wave slope ......... ..... .. ....... 190 15.6.4 Solution procedure .......... ..... .. ....... 190 15.7 Numerical accuracy ....... ......... ... .... ... 191 16 Langmuir circulation in a wave-following current 195 16.1 Unstable eigen-modes .... ......... ..... .. ... ... 195 16.1.1 Eigenvalue spectrum .. ............ ......... 197 16.1.2 Energy budget .. ............... ......... 199 16.1.3 Eigenfunctions
Load more

Recommended publications
  • Generation of Attached Langmuir Circulations by a Suspended Macroalgal Farm
    This draft was prepared using the LaTeX style file belonging to the Journal of Fluid Mechanics 1 Generation of attached Langmuir circulations by a suspended macroalgal farm Chao Yan, James C. McWilliams and Marcelo Chameckiy Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90025, USA (Received xx; revised xx; accepted xx) In this study, we focus on Langmuir turbulence in the deep ocean with the presence of a large macroalgal farm using a Large Eddy Simulation method. The wave-current interactions are modelled by solving the wave-averaged equations. The hydrodynamic process over the farm is found to drive a persistent flow pattern similar to Langmuir circulations but is locked in space across the farm. These secondary circulations are generated because the cross-stream shear produced by the rows of canopy elements leads to a steady vertical vorticity field, which is then rotated to the downstream direction under the effect of vortex force. Since the driving mechanism is similar to the Craik- Leibovich type 2 instability theory, these secondary circulations are also termed as attached Langmuir circulations. We then apply a triple decomposition on the flow field to unveil the underlying kinematics and energy transfer between the mean flow, the secondary flow resulting from the farm drag, and the transient eddies. Flow visualizations and statistics suggest that the attached Langmuir circulations result from the adjustment of the upper ocean mixed layer to the macroalgal farm, and they will weaken (if not disappear) when the flow reaches an equilibrium state within the farm. The triple- decomposed energy budgets reveal that the energy of the secondary flow is transferred from the mean flow under the action of canopy drag, while the transient eddies feed on wave energy transferred by the Stokes drift and energy conversion from the secondary flow.
    [Show full text]
  • The Pelagic Oceanic Assemblages of the Sargasso Sea Around Bermuda Martin V
    The Pelagic Oceanic Assemblages of the Sargasso Sea Around Bermuda Martin V. Angel Number 1 Sargasso Sea Alliance Science Report Series When referenced this report should be referred to as: Angel, M.V. 2011. The Pelagic Ocean Assemblages of the Sargasso Sea Around Bermuda. Sargasso Sea Alliance Science Report Series, No 1, 25 pp. ISBN 978-0-9847520-1-0 The Sargasso Sea Alliance is led by the Bermuda Government and aims to promote international awareness of the importance of the Sargasso Sea and to mobilise support from a wide variety of national and international organisations, governments, donors and users for protection measures for the Sargasso Sea. Further details: Dr David Freestone, Executive Director, Sargasso Sea Alliance, Suite 300, 1630 Connecticut Avenue NW, Washington D.C., 20009, USA. Email: [email protected] Kate K. Morrison, Deputy Director, at the same address Email: [email protected] The Secretariat of the Sargasso Sea Alliance is hosted by the Washington D.C. Office of the International Union for the Conservation of Nature (IUCN). Website is www.sargassoalliance.org This case is being produced with generous support of donors to the Sargasso Sea Alliance: Ricardo Cisneros, Erik H. Gordon, JM Kaplan Fund, Richard Rockefeller, David E. Shaw, and the Waitt Foundation. Additional support provided by: WWF Sweden and the Pew Environment Group. Cover photo: Porbeagle shark, A. Murch. ISBN 978-0-9847520-1-0 The Pelagic Oceanic Assemblages of the Sargasso Sea Around Bermuda Martin V. Angel Research Fellow National Oceanography Centre Southampton, UK Summary Science and Supporting Evidence Case Foreword etween 2010 AND 2012 a large number of authors from seven different countries and B 26 separate organisations developed a scientific case to establish the global importance of the Sargasso Sea.
    [Show full text]
  • Langmuir Turbulence in the Ocean Surface Boundary Layer Towards a Sub-Grid Statistical Climate Process Model
    Langmuir Turbulence in the Ocean Surface Boundary Layer Towards a Sub-grid Statistical Climate Process Model Kara Hartig Department of Physics Brown University Senior thesis submitted for Sc.B in Physics May 3, 2018 Abstract Langmuir turbulence, formed when steady winds blow over the ocean, helps to mix the ocean surface boundary layer that regulates the ex- change of heat and gas between the stable ocean and the volatile atmo- sphere. Due to their size and complexity, global climate models struggle to resolve turbulence on this scale, leading to persistent uncertainties in ocean surface temperatures and the mixed layer depth. As part of the effort to reduce climate model uncertainties and better understand ocean- atmosphere dynamics, this thesis involved the construction and analysis of a Direct Statistical Simulation (DSS) model of Langmuir turbulence. Simulations designed with two different approximations, the quasilinear and generalized quasilinear, were compared to a fully non-linear simula- tion to judge the ability of DSS to capture critical dynamics at minimal computational cost. I found that both approximations could produce Langmuir turbulence but the generalized quasilinear approximation does so with much greater accuracy. Contents 1 Introduction & Background 1 1.1 Direct Numerical Simulation . .1 1.2 A Brief Introduction to Direct Statistical Simulation . .3 1.3 Langmuir Turbulence . .3 1.3.1 Better Than Your Average Test Case . .9 1.4 A Lengthier Introduction to Direct Statistical Simulation . 11 1.4.1 Quasilinear Approximation . 12 1.4.2 Generalized Quasilinear Approximation . 18 1.5 Dedalus and Spectral Methods . 19 1.6 This Project, in Brief . 20 2 Method 22 3 Results 29 3.1 Midplanes .
    [Show full text]
  • Contribution of Coherent Vortices Such As Langmuir Cells to Wind-Driven Surface-Layer Mixing J.B
    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. ???, XXXX, DOI:10.1029/, Contribution of coherent vortices such as Langmuir cells to wind-driven surface-layer mixing J.B. Fl´or,E.J. Hopfinger and E. Guyez Laboratoire des Ecoulements Geophysiques et Industriels, CNRS-UJF-INPG, Grenoble, France Abstract. The wind blowing over the water surface causes, even for low windspeeds, Langmuir circulation in addition to shear-generated turbulence. Both these mechanisms mix the upper layer of oceans and lakes, but since the mixing efficiency of Langmuir cells is un- known, their relevance to mixed layer deepening is still an open question. In order to estimate the contribution to mixing by Langmuir vortex-cells relative to shear-induced mixing, we employ results on entrainment rate obtained from laboratory experiments with Taylor vortex cells. These are coherent horizontal vortices analogous to Langmuir cells. To relate the two, we define a surface friction velocity u∗ that would be necessary to drive cells of strength equivalent to the Taylor vortices. It is then shown that up to a Richard- son number of Ri∗ ≈ 50 layer deepening is predominantly caused by shear-generated turbulence, whereas for Ri∗ > 50 the contribution by coherent Langmuir cells domi- nates the mixing process. For Richardson numbers Ri∗ > 120, the entrainment rate de- creases, but there is no criterion for the arrest of mixing by Langmuir cells as was pre- viously assumed. The present results confirm observations that shear-generated turbu- lence dominates during initial layer deepening under relatively weak buoyancy effects, and that subsequently Langmuir-cell-mixing dominates the mixed layer deepening.
    [Show full text]
  • Potential Export of Unattached Benthic Macroalgae to the Deep Sea Through Wind Driven Langmuir Circulation H
    Old Dominion University ODU Digital Commons OEAS Faculty Publications Ocean, Earth & Atmospheric Sciences 2009 Potential Export of Unattached Benthic Macroalgae to the Deep Sea Through Wind Driven Langmuir Circulation H. M. Dierssen Richard C. Zimmerman Old Dominion University, [email protected] Lisa A. Drake Old Dominion University David J. Burdige Old Dominion University, [email protected] Follow this and additional works at: https://digitalcommons.odu.edu/oeas_fac_pubs Part of the Biology Commons, Earth Sciences Commons, and the Oceanography Commons Repository Citation Dierssen, H. M.; Zimmerman, Richard C.; Drake, Lisa A.; and Burdige, David J., "Potential Export of Unattached Benthic Macroalgae to the Deep Sea Through Wind Driven Langmuir Circulation" (2009). OEAS Faculty Publications. 66. https://digitalcommons.odu.edu/oeas_fac_pubs/66 Original Publication Citation Dierssen, H.M., Zimmerman, R.C., Drake, L.A., & Burdige, D.J. (2009). Potential export of unattached benthic macroalgae to the deep sea through wind-driven Langmuir circulation. Geophysical Research Letters, 36(L04602), 1-5. doi: 10.1029/2008gl036188 This Article is brought to you for free and open access by the Ocean, Earth & Atmospheric Sciences at ODU Digital Commons. It has been accepted for inclusion in OEAS Faculty Publications by an authorized administrator of ODU Digital Commons. For more information, please contact [email protected]. GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L04602, doi:10.1029/2008GL036188, 2009 Potential export of unattached benthic macroalgae to the deep sea through wind-driven Langmuir circulation H. M. Dierssen,1 R. C. Zimmerman,2 L. A. Drake,2,3 and D. J. Burdige2 Received 30 September 2008; revised 24 November 2008; accepted 14 January 2009; published 18 February 2009.
    [Show full text]
  • Characterization and Modulation of Langmuir Circulation in Chesapeake Bay
    OCTOBER 2015 S C U L L Y E T A L . 2621 Characterization and Modulation of Langmuir Circulation in Chesapeake Bay MALCOLM E. SCULLY Applied Ocean Physics and Engineering Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts ALEXANDER W. FISHER,STEVEN E. SUTTLES,LAWRENCE P. SANFORD, AND WILLIAM C. BOICOURT University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge, Maryland (Manuscript received 1 December 2014, in final form 16 July 2015) ABSTRACT Measurements made as part of a large-scale experiment to examine wind-driven circulation and mixing in Chesapeake Bay demonstrate that circulations consistent with Langmuir circulation play an important role in surface boundary layer dynamics. Under conditions when the turbulent Langmuir number Lat is low (,0.5), the surface mixed layer is characterized by 1) elevated vertical turbulent kinetic energy; 2) decreased an- isotropy; 3) negative vertical velocity skewness indicative of strong/narrow downwelling and weak/broad upwelling; and 4) strong negative correlations between low-frequency vertical velocity and the velocity in the direction of wave propagation. These characteristics appear to be primarily the result of the vortex force associated with the surface wave field, but convection driven by a destabilizing heat flux is observed and appears to contribute significantly to the observed negative vertical velocity skewness. Conditions that favor convection usually also have strong Langmuir forcing, and these two processes probably both contribute to the surface mixed layer turbulence. Conditions in which traditional stress-driven turbulence is important are limited in this dataset. Unlike other shallow coastal systems where full water column Langmuir circulation has been observed, the salinity stratification in Chesapeake Bay is nearly always strong enough to prevent full-depth circulation from developing.
    [Show full text]
  • 2:1 Spatial Resonance in Langmuir Circulation (Pdf)
    2:1 Spatial Resonance in Langmuir Circulation Bevin Maultsby University of North Carolina Langmuir circulation in the upper layer of the ocean is studied as 2 : 1 spatial reso- nance problem with steady state-steady state modal interaction and O(2) symmetry. A center manifold reduction using asymptotic analysis results in a dynamical system with a structurally stable and attracting heteroclinic orbit in an invariant subspace of the center manifold. This heteroclinic orbit is used to illustrate the persistent switching between a two-roll state and a four-roll state in the crosswind plane. Lastly, a set of coupled PDEs are derived to study the Y junctions which mark transitions between these two states. 1 Introduction Langmuir circulation is a wind- and surface wave-driven convective process in the upper layer of bodies of water. When the speed of the wind over the surface of the water exceeds approximately 3.5 meters per second, it can create pairs of counter rotating vortices with axis parallel to the wind and the direction of wave propagation, see Figure 1. The rotation of these vortices creates a mixing layer, which in the ocean typically ranges from 50 meters to 100 meters deep. The counter rotation cause regions of upwelling and strong downwelling in the mixing layer. The downwelling, caused when water converges on the surface and is forced downward, may trap dirt and debris, resulting in a visible pattern of \windrows" on the surface of the water. The windrows are not perfectly parallel, however, and often display \Y junctions" where two windrows appear to merge into one windrow, as seen in Figure 1.
    [Show full text]
  • Oil Plumes and Dispersion in Langmuir, Upper‐
    PUBLICATIONS Journal of Geophysical Research: Oceans RESEARCH ARTICLE Oil plumes and dispersion in Langmuir, upper-ocean 10.1002/2014JC010542 turbulence: Large-eddy simulations and K-profile Key Points: parameterization Applied high-fidelity LES to model oil dispersion in Langmuir turbulence Di Yang1,2,3, Bicheng Chen4, Marcelo Chamecki4, and Charles Meneveau1,2 Conducted systematic statistical analysis on mean plume dispersion 1Center for Environmental and Applied Fluid Mechanics, Johns Hopkins University, Baltimore, Maryland, USA, Assessed and modified prior KPP 2Department of Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, USA, 3Now at Department of models for better agreement with 4 LES data Mechanical Engineering, University of Houston, Houston, Texas, USA, Department of Meteorology, Pennsylvania State University, University Park, Pennsylvania, USA Correspondence to: C. Meneveau, Abstract Once oil plumes such as those originating from underwater blowouts reach the ocean mixed [email protected] layer (OML), their near-surface dispersion is influenced heavily by wind and wave-generated Langmuir tur- bulence. In this study, the complex oil spill dispersion process is modeled using large-eddy simulation (LES). Citation: Yang,D.,B.Chen,M.Chamecki,and The mean plume dispersion is characterized by performing statistical analysis of the resulting fields from C. Meneveau (2015), Oil plumes and the LES data. Although the instantaneous oil concentration exhibits high intermittency with complex spatial dispersion in Langmuir, upper-ocean patterns such as Langmuir-induced striations, it is found that the time-averaged oil distribution can still be turbulence: Large-eddy simulations and K-profile parameterization, J. described quite well by smooth Gaussian-type plumes. LES results show that the competition between Geophys.
    [Show full text]
  • Cell Merging and the Jet/Downwelling Ratio in Langmuir Circulation
    Journal of Marine Research, 51, 737-769,1993 Cell merging and the jet/downwelling ratio in Langmuir circulation by Ming Lil and Chris Garrett1 ABSTRACT The Craik-Leibovich equationsfor Langmuir circulation have been integrated numerically to investigatethe cell mergingprocess as well asthe strength and structure of the cells. We find that pairs of counterrotating vortices canceleach other, leading to a growth in scale of the dominant vortices. However, when there is no external forcing, vortices of opposite sign do not mergeirrespective of the vortex size and circulation strength. The mergingof Langmuir cells, or rather the cancellation of counterrotating vortices, is thus different from the amalgamationof like-signedvortices in two-dimensionalturbulence. The forcing due to the Stokesdrift plays an important role in the cell-mergingprocess. As the Langmuirnumber La decreases,the maximumdownwelling velocity increaseswhile the pitch (the ratio of surface downwind jet strength to the maximum downwelling velocity) decreases.When La is about 0.01, as for an eddy viscosity in the range of values commonly usedfor the ocean surfacelayer, the model predicts a maximumdownwelling velocity of 0.006 to O.OlU, (the wind speed), comparablewith the observed magnitude.However, the surface downwindjet is significantlyweaker than the observedstrength. At smallLa a simple scaleanalysis, which couplesa surface boundary layer with a narrow downwelling region, suggeststhat the thickness of these regions should vary as La112, the downwelling velocity as La-1/3 and the pitch as La1j6. These predictions are supported by numerical results. 1. Introduction Langmuir circulation consists of a pattern of parallel vortices oriented downwind, of alternating senseof circulation or vorticity, and with maximum downwind surface current at the surface convergences.
    [Show full text]
  • High-Resolution Simulations of Interactions Between Surface Ocean
    https://doi.org/10.5194/tc-2020-189 Preprint. Discussion started: 13 July 2020 c Author(s) 2020. CC BY 4.0 License. High-resolution simulations of interactions between surface ocean dynamics and frazil ice Agnieszka Herman1, Maciej Dojczman2, and Kamila Swiszcz´ 1 1Institute of Oceanography, University of Gdansk, Poland 2Faculty of Physics, University of Warsaw, Poland Correspondence: Agnieszka Herman ([email protected]) Abstract. Frazil and grease ice forms in the ocean mixed layer (OML) during highly turbulent conditions (strong wind, large waves) accompanied by intense heat loss to the atmosphere. Three main velocity scales that shape the complex, three- dimensional OML dynamics under those conditions are: the friction velocity u at the ocean–atmosphere interface, the vertical ∗ velocity w associated with convective motion, and the vertical velocity w ,L associated with Langmuir turbulence. The fate ∗ ∗ 5 of buoyant particles, e.g. frazil crystals, in that dynamic environment depends primarily on their floatability, i.e., the ratio of their rising velocity wt to the characteristic vertical velocity, dependent on w and w ,L. In this work, dynamics of frazil ∗ ∗ ice is investigated numerically with a high-resolution, non-hydrostatic hydrodynamic model CROCO (Coastal and Regional Ocean COmmunity Model), extended to account for frazil transport and its interactions with surrounding water. An idealized model setup is used (a square computational domain with periodic lateral boundaries; spatially uniform atmospheric and wave 10 forcing).
    [Show full text]
  • Zooplankton Fecal Pellets, Marine Snow and Sinking Phytoplankton Blooms
    AQUATIC MICROBIAL ECOLOGY Vol. 27: 57–102, 2002 Published February 18 Aquat Microb Ecol REVIEW Zooplankton fecal pellets, marine snow and sinking phytoplankton blooms Jefferson T. Turner* School for Marine Science and Technology, University of Massachusetts Dartmouth, 706 South Rodney French Boulevard, New Bedford, Massachusetts 02744-1221, USA ABSTRACT: Zooplankton fecal pellets have long been thought to be a dominant component of the sedimentary flux in marine and freshwater ecosystems, but that view is changing. The last 2 decades have seen publication of >500 studies using sediment traps, which reveal that zooplankton fecal pellets often constitute only a minor or variable proportion of the sedimentary flux. Substantial pro- portions of this flux are from organic aggregates (‘marine snow’) of various origins, including phyto- plankton blooms, which sediment directly to the benthos. It now appears that mainly large fecal pel- lets of macrozooplankton and fish are involved in the sedimentary flux. Smaller fecal pellets of microzooplankton and small mesozooplankton are mostly recycled or repackaged in the water column by microbial decomposition and coprophagy, contributing more to processes in the water column than flux to the benthos. The relative contributions of fecal pellets, marine snow and sinking phytoplankton to the vertical flux and recycling of materials in the water column are highly variable, dependent upon multiple interacting factors. These include variations in productivity, biomass, size spectra and composition of communities
    [Show full text]
  • Bubble Clouds and Langmuir Circulation: Observations and Models
    SEPTEMBER 2003 THORPE ET AL. 2013 Bubble Clouds and Langmuir Circulation: Observations and Models S. A. THORPE SOES, Southampton Oceanography Centre, Southampton, United Kingdom T. R . O SBORN Department of Earth and Planetary Science, The Johns Hopkins University, Baltimore, Maryland D. M. FARMER Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island S. VAGLE Institute of Ocean Sciences, Patricia Bay, Sidney, British Columbia, Canada (Manuscript received 16 October 2002, in ®nal form 3 February 2003) ABSTRACT Concurrent measurements of the rate of dissipation of turbulent kinetic energy and the void fraction and size distribution of near-surface bubbles are described. Relatively high dissipation rates and void fractions are found in bubble bands produced by Langmuir circulation. The mean dissipation rates observed in the bands are close to those at which the dynamics of algae is signi®cantly affected. The data are used to test basic assumptions underpinning models of subsurface bubbles and associated air±sea gas transfer. A simple model is used to examine the qualitative effect of Langmuir circulation on the vertical diffusion of bubbles and the representation of Langmuir circulation in models of gas transfer. The circulation is particularly effective in vertical bubble transfer when bubbles are injected by breaking waves to depths at which they are carried downward by the circulation against their tendency to rise. The estimated value of the ratio r of the eddy diffusivity of particles (resembling bubbles) Kp to the eddy viscosity Kz depends on depth z and on the form selected for Kz. The effects of nonoverlapping or superimposed Langmuir cells of different size may be very different.
    [Show full text]