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Marine turbulence re-revisited — Selected papers from the jointly organised 39th Liège Colloquium and 3rd Warnemünde Turbulence Days
Journal of Marine Systems 77 (2009) 367–368
Contents lists available at ScienceDirect
Journal of Marine Systems j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j m a r s y s
Preface
Marine turbulence re-revisited — Selected papers from the jointly organised 39th Liège Colloquium and 3rd Warnemünde Turbulence Days
Almost three decades after the “Marine Turbulence” 11th Liège Colloquium in 1980, exactly two decades after the 19th Liège Colloquium in 1987 entitled “Turbulence in the ocean. From the millimeter to the megameter” and exactly one decade after the “Marine Turbulence Revisited” 29th Liège Colloquium in 1997, marine turbulence has been re-revisited during the 39th Liège Colloquium in 2007. This time, the workshop was co-organised together with the 3rd Warnemünde Turbulence Days, a biennial workshop on specific challenges in marine turbulence, usually taking place at the Leibniz Institute for Baltic Sea Research in Warnemünde (Germany). From decade to decade enormous progress is achieved in our understanding of oceanic turbulence. A major trigger of this progress is the technological development of both, oceanic instrumentation and numerical modelling. For the instruments, higher sampling rates, larger data storages and faster data processing facilities generally allow for better resolution but do also open perspectives for novel mechanical, acoustical and optical devices. For the numerical modelling, steadily growing computer resources allow for substantially more complex models and higher resolution than a decade ago. With applications of these innovative tools, new insights into the interaction of turbulence with waves, stratification and the marine ecosystem can be achieved. Combining the historically broad approach of the Liège Colloquium with the specialised Warnemünde Turbulence Days, this joint venture concentrated on five focal topics: turbulence and waves (chaired by Fabrice Ardhuin from Brest, France, and Hans van Haren from Texel, The Netherlands), turbulence in buoyant and dense plumes (chaired by Hartmut Peters from Miami, USA, and Anna Wåhlin from Stockholm, Sweden), turbulence and the marine ecosystem (chaired by Laurent Seuront from Wimereux, France, and Flinders, Australia, and Luca van Duren from Delft, The Netherlands), turbulence observations in the ocean (chaired by Jim Moum from Corvallis, USA, and Tom Rippeth from Bangor, Wales), and turbulence modelling in the ocean (chaired by Lars Umlauf from Warnemünde, Germany, and Vittorio Canuto from New York, USA). Contributions to these focal topics as well as to related problems of marine turbulence had been invited to the Liège Colloquium in 2007. 0924-7963/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2009.02.001
During the five days of this 39th Liège colloquium, 100 scientists attended 90 presentations, drank 60 L of coffee, an undisclosed quantity of Belgian beer and enjoyed the colloquium dinner at the Colonster castle. The session on buoyant and dense plumes tried to show links between these two types of plumes: Buoyant plumes are ubiquitous in coastal waters, mediating the transmission of freshwater signals to the open sea. Dense plumes related to outflows and overflows from marginal seas are important parts of the vertical circulation for the oceans and important for climate processes. In both dense and buoyant plumes turbulent mixing plays a crucial role with respect to their dynamics and their role and effect on the environment in which they are embedded. The paper by Muench et al. which opens this special issue, discusses observations of turbulence and mixing in a dense bottom outflow from the Ross Sea (Antarctica) and their impact on the evolution of the resulting dense plume. The role of tides for the mixing is highlighted and estimated by means of a numerical model. The session on turbulence and the marine ecosystem focusssed on the effects of turbulence on biogeochemical and ecological processes in different aquatic ecosystems and at different scales, as well as the effect of biogenic structures on local turbulence. In aquatic ecosystems, physical, chemical and biological processes are strongly interlinked. The nature of these links is strongly scale-dependent and non-linear. The following two papers focus on aspects of this interaction between turbulence and the marine ecosystem. Moison et al. apply the theoretical framework of symbolic dynamics to copepode behaviour, and analyse their swimming behaviour in turbulent and non-turbulent laboratory flows. They find out that the complexity of swimming behaviour dramatically increases with increasing turbulence. McCardell and O'Donnell primarily introduce a new method for estimating vertical eddy diffusivities in coastal waters (with application to western Long Island Sound), by comparing signal times series at two different depths. With this method, the authors are able to explain subpycnocline oxygen times series, as a competition between vertical mixing and near-bottom oxygen demand. The motto of the session on turbulence observations in the ocean is that measurements of turbulence in the ocean will
368
always be unsatisfying because of the large natural variability in geophysical turbulence. However, particular demonstrations, such as the repeatable result that turbulence dissipation is approximately equal to surface buoyancy flux in convectively-driven mixed layers, provide us with some considerable quantitative confidence in what we can measure using in-situ techniques in the ocean. Remote, acoustical measurement techniques are beginning to show promise as well. The present challenges are to (i) define strategies that permit clear observation of the instability mechanisms that lead to turbulence in the ocean, (ii) apply measurement techniques in creative ways that permit a quantitative assessment of the role of turbulence in not only the dynamics of flow fields but also in mixing of water masses (although these may not always be independent), and (iii) evaluate different methods for deriving turbulence parameters and assess the consistency of their results. The sequence of seven papers on marine turbulence observations is opened by the session chairs Moum and Rippeth asking the provocative question “Do observations adequately resolve the natural variability of oceanic turbulence?”. The reader may find the answer by reading this interesting paper as well as the other six papers focussing on marine turbulence observations. Van Haren investigates the role of internal solibores on mixing above sloping deep ocean bottoms by means of fast sampling Acoustic Doppler Current Profiler observations, and reports on the problems associated to the short times scales and episodic character of such events. By carefully analysing vertical profiles obtained by a free-falling profiler and horizontal profiles obtained from an AUV both equipped with micro-structure shear probes, Simpson et al. investigate the structure of dissipation across the Western Irish Sea tidal mixing front. They identify the expected frontal jet from the observations and conclude that its impact on the mixing dynamics is weak. Similar micro-structure measurements obtained by an AUV operated in jojo-mode are presented by Goodman and Wang who investigated turbulent mixing in the Northern Bight of Monterey Bay. While during most of the 8-hourly observations turbulent mixing was constrained to the bottom and surface mixed layers, a passage of internal waves generated significant mixing in the upper part of the thermocline. Liu et al. present first micro-structure profiler data obtained at two positions in the Yellow Sea. They could gain further insight into the generation mechanisms for the classical three-layer stratification at the end of the warm season, with strong internal wave mixing in a region with sloping bottom. Schmitt et al. investigate the characteristics of turbulent fluctuations in the surf zone, by analysing data obtained by means of a sonic anemometer. Combining the Empirical Mode Decomposition method with a newly developed method based on Hilbert spectral analysis they gain deeper insights into turbulent time scales in this energetic environment. In a further methodical paper, Nieves and Turiel discuss how altimeter data can be analysed by means of the Coherent Vortex Extraction (CVE) technique. Results are discussed from the perspective of the Microcanonical Multifractal Formalism, which is significantly improving the performance of the CVE. The final session on turbulence modelling in the ocean dealt with theoretical and numerical studies on turbulence
Preface
closure models for unstratified and stratified flows. Focal points were statistical closure models (as they are typically derived from Reynolds Stress Models) investigations of turbulence with the help of fully or partially resolved approaches (LES, DNS), as well as laboratory experiments. The three final papers deal with such aspects of turbulence modelling. Ferrero et al. present a new non-local third order closure model for the stable boundary layer, and compare its performance to LES data and lower order closure models. Espa et al. discuss laboratory experiments reproducing the polar βeffect on quasi-two-dimensional turbulence, showing how the development of zonal jets suppresses meridional turbulent transport. Finally, Awad et al. present results of a new Large Eddy Simulation model, comparing various subgrid scale models with laboratory flows. After this conference on turbulence re-revisited, it was concluded that substantial progress has been made since the 1997 conference on turbulence revisited (see the Journal of Marine Systems special issue in volume 21, issues 1–4, 1999). On the other hand significant knowledge gaps had been revealed, both in theoretical modelling and observations of marine turbulence. It has thus been concluded that in 2017 marine turbulence needs to be re-revisited at the 49th Liège Colloquium. Acknowledgements The organisers of this conference are grateful for the scientific advice provided by the other members of the Scientific Committee: Fabrice Ardhuin (Brest, France), Vittorio Canuto (New York, USA), Eric Deleersnijder (Louvain-laNeuve, Belgium), Hans Ulrich Lass (Warnemünde, Germany), Dmitrii Mironov (Offenbach, Germany), Jim Moum (Corvallis, USA), Jacques C.J. Nihoul (Liège, Belgium), Thomas Osborn (Baltimore, USA), Hartmut Peters (Miami, USA), Hartmut Prandke (Petersdorf, Germany), Tom Rippeth (Menai Bridge, Wales), Laurent Seront (Wimereux, France), John Simpson (Menai Bridge, Wales), Adolf Stips (Ispra, Italy), Steve Thorpe (Menai Bridge, Wales), Lars Umlauf (Warnemünde, Germany), Luca van Duren (Delft, The Netherlands), Hans van Haren (Texel, The Netherlands), Anna Wåhlin (Gothenburg, Sweden), Alfred Wüest (Kastanienbaum, Switzerland), Hidekatsu Yamazaki (Tokyo, Japan). Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde, Seestrasse 15, D-18119 Rostock, Germany Corresponding author. E-mail address: [email protected]. Jean-Marie Beckers Université de Liège, GeoHydrodynamics and Environment Research, AGO, Sart-Tilman B5, B-4000 Liège, Belgium E-mail address: [email protected]. 28 January 2009
Contents lists available at ScienceDirect
Journal of Marine Systems j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j m a r s y s
Preface
Marine turbulence re-revisited — Selected papers from the jointly organised 39th Liège Colloquium and 3rd Warnemünde Turbulence Days
Almost three decades after the “Marine Turbulence” 11th Liège Colloquium in 1980, exactly two decades after the 19th Liège Colloquium in 1987 entitled “Turbulence in the ocean. From the millimeter to the megameter” and exactly one decade after the “Marine Turbulence Revisited” 29th Liège Colloquium in 1997, marine turbulence has been re-revisited during the 39th Liège Colloquium in 2007. This time, the workshop was co-organised together with the 3rd Warnemünde Turbulence Days, a biennial workshop on specific challenges in marine turbulence, usually taking place at the Leibniz Institute for Baltic Sea Research in Warnemünde (Germany). From decade to decade enormous progress is achieved in our understanding of oceanic turbulence. A major trigger of this progress is the technological development of both, oceanic instrumentation and numerical modelling. For the instruments, higher sampling rates, larger data storages and faster data processing facilities generally allow for better resolution but do also open perspectives for novel mechanical, acoustical and optical devices. For the numerical modelling, steadily growing computer resources allow for substantially more complex models and higher resolution than a decade ago. With applications of these innovative tools, new insights into the interaction of turbulence with waves, stratification and the marine ecosystem can be achieved. Combining the historically broad approach of the Liège Colloquium with the specialised Warnemünde Turbulence Days, this joint venture concentrated on five focal topics: turbulence and waves (chaired by Fabrice Ardhuin from Brest, France, and Hans van Haren from Texel, The Netherlands), turbulence in buoyant and dense plumes (chaired by Hartmut Peters from Miami, USA, and Anna Wåhlin from Stockholm, Sweden), turbulence and the marine ecosystem (chaired by Laurent Seuront from Wimereux, France, and Flinders, Australia, and Luca van Duren from Delft, The Netherlands), turbulence observations in the ocean (chaired by Jim Moum from Corvallis, USA, and Tom Rippeth from Bangor, Wales), and turbulence modelling in the ocean (chaired by Lars Umlauf from Warnemünde, Germany, and Vittorio Canuto from New York, USA). Contributions to these focal topics as well as to related problems of marine turbulence had been invited to the Liège Colloquium in 2007. 0924-7963/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2009.02.001
During the five days of this 39th Liège colloquium, 100 scientists attended 90 presentations, drank 60 L of coffee, an undisclosed quantity of Belgian beer and enjoyed the colloquium dinner at the Colonster castle. The session on buoyant and dense plumes tried to show links between these two types of plumes: Buoyant plumes are ubiquitous in coastal waters, mediating the transmission of freshwater signals to the open sea. Dense plumes related to outflows and overflows from marginal seas are important parts of the vertical circulation for the oceans and important for climate processes. In both dense and buoyant plumes turbulent mixing plays a crucial role with respect to their dynamics and their role and effect on the environment in which they are embedded. The paper by Muench et al. which opens this special issue, discusses observations of turbulence and mixing in a dense bottom outflow from the Ross Sea (Antarctica) and their impact on the evolution of the resulting dense plume. The role of tides for the mixing is highlighted and estimated by means of a numerical model. The session on turbulence and the marine ecosystem focusssed on the effects of turbulence on biogeochemical and ecological processes in different aquatic ecosystems and at different scales, as well as the effect of biogenic structures on local turbulence. In aquatic ecosystems, physical, chemical and biological processes are strongly interlinked. The nature of these links is strongly scale-dependent and non-linear. The following two papers focus on aspects of this interaction between turbulence and the marine ecosystem. Moison et al. apply the theoretical framework of symbolic dynamics to copepode behaviour, and analyse their swimming behaviour in turbulent and non-turbulent laboratory flows. They find out that the complexity of swimming behaviour dramatically increases with increasing turbulence. McCardell and O'Donnell primarily introduce a new method for estimating vertical eddy diffusivities in coastal waters (with application to western Long Island Sound), by comparing signal times series at two different depths. With this method, the authors are able to explain subpycnocline oxygen times series, as a competition between vertical mixing and near-bottom oxygen demand. The motto of the session on turbulence observations in the ocean is that measurements of turbulence in the ocean will
368
always be unsatisfying because of the large natural variability in geophysical turbulence. However, particular demonstrations, such as the repeatable result that turbulence dissipation is approximately equal to surface buoyancy flux in convectively-driven mixed layers, provide us with some considerable quantitative confidence in what we can measure using in-situ techniques in the ocean. Remote, acoustical measurement techniques are beginning to show promise as well. The present challenges are to (i) define strategies that permit clear observation of the instability mechanisms that lead to turbulence in the ocean, (ii) apply measurement techniques in creative ways that permit a quantitative assessment of the role of turbulence in not only the dynamics of flow fields but also in mixing of water masses (although these may not always be independent), and (iii) evaluate different methods for deriving turbulence parameters and assess the consistency of their results. The sequence of seven papers on marine turbulence observations is opened by the session chairs Moum and Rippeth asking the provocative question “Do observations adequately resolve the natural variability of oceanic turbulence?”. The reader may find the answer by reading this interesting paper as well as the other six papers focussing on marine turbulence observations. Van Haren investigates the role of internal solibores on mixing above sloping deep ocean bottoms by means of fast sampling Acoustic Doppler Current Profiler observations, and reports on the problems associated to the short times scales and episodic character of such events. By carefully analysing vertical profiles obtained by a free-falling profiler and horizontal profiles obtained from an AUV both equipped with micro-structure shear probes, Simpson et al. investigate the structure of dissipation across the Western Irish Sea tidal mixing front. They identify the expected frontal jet from the observations and conclude that its impact on the mixing dynamics is weak. Similar micro-structure measurements obtained by an AUV operated in jojo-mode are presented by Goodman and Wang who investigated turbulent mixing in the Northern Bight of Monterey Bay. While during most of the 8-hourly observations turbulent mixing was constrained to the bottom and surface mixed layers, a passage of internal waves generated significant mixing in the upper part of the thermocline. Liu et al. present first micro-structure profiler data obtained at two positions in the Yellow Sea. They could gain further insight into the generation mechanisms for the classical three-layer stratification at the end of the warm season, with strong internal wave mixing in a region with sloping bottom. Schmitt et al. investigate the characteristics of turbulent fluctuations in the surf zone, by analysing data obtained by means of a sonic anemometer. Combining the Empirical Mode Decomposition method with a newly developed method based on Hilbert spectral analysis they gain deeper insights into turbulent time scales in this energetic environment. In a further methodical paper, Nieves and Turiel discuss how altimeter data can be analysed by means of the Coherent Vortex Extraction (CVE) technique. Results are discussed from the perspective of the Microcanonical Multifractal Formalism, which is significantly improving the performance of the CVE. The final session on turbulence modelling in the ocean dealt with theoretical and numerical studies on turbulence
Preface
closure models for unstratified and stratified flows. Focal points were statistical closure models (as they are typically derived from Reynolds Stress Models) investigations of turbulence with the help of fully or partially resolved approaches (LES, DNS), as well as laboratory experiments. The three final papers deal with such aspects of turbulence modelling. Ferrero et al. present a new non-local third order closure model for the stable boundary layer, and compare its performance to LES data and lower order closure models. Espa et al. discuss laboratory experiments reproducing the polar βeffect on quasi-two-dimensional turbulence, showing how the development of zonal jets suppresses meridional turbulent transport. Finally, Awad et al. present results of a new Large Eddy Simulation model, comparing various subgrid scale models with laboratory flows. After this conference on turbulence re-revisited, it was concluded that substantial progress has been made since the 1997 conference on turbulence revisited (see the Journal of Marine Systems special issue in volume 21, issues 1–4, 1999). On the other hand significant knowledge gaps had been revealed, both in theoretical modelling and observations of marine turbulence. It has thus been concluded that in 2017 marine turbulence needs to be re-revisited at the 49th Liège Colloquium. Acknowledgements The organisers of this conference are grateful for the scientific advice provided by the other members of the Scientific Committee: Fabrice Ardhuin (Brest, France), Vittorio Canuto (New York, USA), Eric Deleersnijder (Louvain-laNeuve, Belgium), Hans Ulrich Lass (Warnemünde, Germany), Dmitrii Mironov (Offenbach, Germany), Jim Moum (Corvallis, USA), Jacques C.J. Nihoul (Liège, Belgium), Thomas Osborn (Baltimore, USA), Hartmut Peters (Miami, USA), Hartmut Prandke (Petersdorf, Germany), Tom Rippeth (Menai Bridge, Wales), Laurent Seront (Wimereux, France), John Simpson (Menai Bridge, Wales), Adolf Stips (Ispra, Italy), Steve Thorpe (Menai Bridge, Wales), Lars Umlauf (Warnemünde, Germany), Luca van Duren (Delft, The Netherlands), Hans van Haren (Texel, The Netherlands), Anna Wåhlin (Gothenburg, Sweden), Alfred Wüest (Kastanienbaum, Switzerland), Hidekatsu Yamazaki (Tokyo, Japan). Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde, Seestrasse 15, D-18119 Rostock, Germany Corresponding author. E-mail address: [email protected]. Jean-Marie Beckers Université de Liège, GeoHydrodynamics and Environment Research, AGO, Sart-Tilman B5, B-4000 Liège, Belgium E-mail address: [email protected]. 28 January 2009