Barry Ruddick

Department of Oceanography

Dalhousie University

Halifax, NS

B3H 4J1

Canada

Phone: (902) 494-2505

FAX: (902) 494-2885

New Science Perspective article with interesting Oceanography links

Videos of Oceanography Laboratory Experiments

Research Interests

I am fascinated by the prospect that the greatly differing molecular diffusivities of heat and salt can lead to a variety of phenomena that are currently not well-quantified, understood, or allowed for in current mixing parameterizations. These include salt finger and diffusive convection, lateral thermohaline intrusions, and differential turbulent mixing (Ruddick, 1997).

The most far-reaching oceanic effect of double-diffusion may involve intrusive mixing of heat, salt, and momentum across ocean fronts. I have worked to quantify this phenomenon via laboratory and theoretical models (Ruddick, Phillips and Turner, 1999), and via an efficient numerical model (Walsh and Ruddick, 1997,2000). The goal of this work is to predict the structure, scales, and fluxes, both lateral and vertical, in fully-developed finite-amplitude intrusions, so that their effects on the larger scale may be assessed and parameterized. I think that these papers made more progress than any others towards understanding evolution to a finite-amplitude state. However, the goal has not been reached: we still seek a robust, quantitative, field-tested intrusion theory. Collaborations with the above authors, and others, will continue.

Drs. Neil Oakey, Dave Walsh, and I were privileged to play a role in the North Atlantic Tracer Release Experiment (NATRE), which found through long-term observations of the spread of deliberately introduced Sulfur Hexaflouride tracer, that the diapycnal eddy diffusivity in the main thermocline of the Eastern North Atlantic is actually about 5 times smaller than demanded by large-scale tracer inferences. We made microstructure observations during two sampling cruises, and found that the microstructure-derived diffusivities were consistent with the tracer diffusivity (Ruddick, Walsh, and Oakey, 1997). Furthermore, we found microstructure evidence strongly suggestive of double-diffusion, indicating that salt fingers may have played a role in addition to "regular" turbulence in spreading the tracer. Because of an unanticipated noise level problem, our conclusion was tentative, but the findings were confirmed by analysis involving a different instrument [Laurent, L., R.W. Schmitt, 1999: The Contribution of Salt Fingers to Vertical Mixing in the North Atlantic Tracer Release Experiment. Journal of Physical Oceanography: Vol. 29, No. 7, pp. 1404-1424.]

One of my most exciting contributions is brand new, presented at the Fall 2000 AGU. It resulted from a chance meeting when I was a plenary speaker at the 1997 TOS meeting: I met Dr. John Saylor who had recently completed a Ph.D in bio-fluid mechanics. He used flourescent dye techniques to examine the effect of molecular diffusion on mixing in a model heart valve. Adapting those techniques, John (presently at Naval Research Lab, Washington DC), Dave Hebert (Oceanography Faculty at University of Rhode Island), and I collaborated in a laboratory study of mixing induced by breaking internal waves. This was funded by the U.S. NSF Small Grants for Exploratory Research program, by NSERC's research grants program, NSERC's summer undergraduate fellowships program, and by Nova Scotia Links. We completed the laboratory experiments this September, and we believe we have observed and quantified differential turbulent mixing -- two dyes of greatly differing molecular diffusivities were observed to mix at different rates by breaking internal waves. If this is eventually found to apply to oceanic mixing of heat and salt, the consequences are profound. First, the standard parameterizations for mixing in all types of models must be revised to treat heat and salt separately and appropriately. Second, lateral thermohaline intrusions can be initiated by differential turbulent diffusion, and therefore they have even greater importance than we currently believe. Third, the potentially differing diffusivities for heat and salt can have profound effects on large-scale water mass modification and thermohaline circulation (Ruddick 1997).

Collaborative Research

GLOBEC: Turbulent mixing and feeding ecology of larval fish

Other collaborators: Chris Taggart, Tony Bowen (Dal), Brad deYoung (MUN), John Loder (BIO)

The Question

Theory (Rothschild and Osborn, 1988) and laboratory work (Dower, pers. comm, 1996) suggest that turbulence can affect the feeding success of larval fish. Can this be measured in the open ocean?

Objectives

To measure the effects of turbulent mixing on larval cod feeding on Western Bank. Approach We simultaneously measured turbulent dissipation rates and several indicators of feeding behaviour (gut content, RNA/DNA condition indices, growth history) on Western Bank. Our collaborators at the same time surveyed the circulation and distribution of (silver hake) larvae and prey, and will model the circulation and mixing on the Bank and surrounding shelf. This contextual information will help us to assess and allow for environmental factors other than turbulence that affect feeding behaviour.

Testing and improvement of low-order turbulence models.

The question

Although individual turbulent mixing events can be studied using direct numerical simulations or large-eddy simulations, computers will not be large enough or fast enough in the foreseeable future to model all important scales from large-scale forcing down to the dissipative microscales (typically several mm). A practical alternative approach has evolved, and is in widespread use - "second-order turbulence closure models", the most well-known of which are called "Mellor-Yamada", and "k-epsilon" models. These models are similar in approach to the familiar eddy viscosity or mixing length approaches, which assume that fluxes are proportional to gradients, with constants of proportionality that relate to turbulence properties. In second-order closure models, the turbulence properties are computed using differential equations that describe their physical evolution.

The major advantage of second-order closure models is that large-scale computer models can simulate the important effects of turbulence by solving for two additional prognostic variables (TKE and one other), and using these to parameterize turbulent fluxes. The effects of turbulent geophysical boundary layers forced by winds, velocity shears, heating/cooling, precipitation, and inhibition by density stratification, are easily included in large-scale models, with little computational cost.

Both the "Mellor-Yamada", and "k-epsilon" models have several known shortcomings, a major one being sensitivity to the effects of stratification in suppressing turbulence. These effects are both numerical and physical, and can cause smooth stratification to break down into layers, with major effect on the fluxes (Phillips, 1972, Ruddick et al, 1989). Various improvements to the models have been suggested to overcome some of the difficulties that have been developed, leading to a variety of modifications and extensions to the two models in common use. (For example, see D'Alessio et al, 1998.) There are presently a variety of "mutations" in present-day practical use, each optimized for a specific situation. It is therefore difficult to compare them, or to pick a single "best" model that will perform well in a variety of situations. The research community needs to come up with one model, set of parameterizations, and coefficients that fits observations well in a variety of situations. Such a model would have greatly improved predictive capability.

Objectives

To test the performance of Mellor-Yamada, k-epsilon, and "KPP" turbulence closure models against the set of microstructure, velocity shear, and surface forcing observations collected during the Western Bank Canadian GLOBEC field program, in October, 1998.

Approach

CARTUM (Comparative Analysis and Rationalization of TUrbulence Models) is an EU project designed to address many of the known second-order turbulence model shortcomings, and to progress toward a single, next-generation, second-order turbulence closure. Ruddick was invited to take part in CARTUM as a non-EU "invited expert", and is doing so with the help of an NSERC/IOF travel grant, which has supported the participation of five Canadian researchers in CARTUM meetings.

The project will bring together a host of identified, unresolved model problems, several new and relevant turbulence data sets, and 40 turbulence modelers, observationalists, and theoreticians ranging from world-leading experts to younger researchers with specific experience. In a series of workshops, schools, and focused conferences:

1. the two major models will be contrasted and compared with the new turbulence/microstructure observations,

2. the known model shortcomings will be addressed, and

3. a greatly improved canonical community model (or if necessary, a family of such models) will be developed.

The final product of CARTUM will be a book and CD-Rom documenting the workshops, schools and meetings, the data-sets used in the project, and the improved, canonical turbulence closure scheme, disseminated as a community model. David Ciochetto (first year oceanography graduate student), will use the CARTUM community model to simulate the turbulent mixing and consequent evolution of the velocity, temperature, and salinity stratification on Western Band observed during GLOBEC. The density and velocity structure were observed to be strongly two-layered, with dissipation minimum near the pycnocline. The effects of stratification in suppressing turbulence, and the shear in enhancing it, will test the accuracy of the model parameterizations, and the completeness of the observational set should allow improvements to be suggested. Our dataset and documents relating to the comparison will be part of the CARTUM CD. In addition, Ruddick and Oakey will be writing part of the extensive review of oceanic turbulence resulting from the CARTUM project.

Post Doctoral Fellow/Research Associate

• Julia Mullarney (RSES. ANU) - co-supervised with Alex Hay
• Ayal Anis (Senior Research Associate, 1998-1999) Presently Assistant Professor, Texas A&M University.

• Pierluigi Pantalone (M.Sc.) - Modeling Connectivity among Coral Reefs: Quantitative Predictions of Fish Larval Dispersal in the Mesoamerican Barrier Reef System - completed 2006 (cosupervised with Jinyu Sheng and Bruce Hatcher)

• Dave Ciochetto (Ph.D.) - Turbulence modelling

Undergraduate Student Projects

• Jonathan Logan, Dalhousie University (NSERC Summer Research Fellow, 2003-2005) Currently science student in Quebec university system.

• Marc-Andre-Rousseau, Dalhousie University (NSERC Summer Research Fellow, 2002) Currently in Math and Physics BSc program, Dalhousie.

• Xiaoming Liu, BSc Oceanography, Quingdao, MSC Marine Science North Carolina State (Research Assistant, May-Aug., 1999)

• Stephanie Waterman, Queens University Engineering (NSERC Summer Research Fellow, May-September, 1999) Currently planning graduate study in fluid mechanical engineering.

Research Staff

• J. Hurst, Research Assistant (part-time for me)
• W. Judge, Electronics Technologist (during the GLOBEC fieldwork)

Other Activities

Invited Co-convener, Symposium on Ocean Mixing, IAPSO Assembly, July 2011, Melbourne, Australia.

Invited organizer and head convener, Symposium on .Ocean mixing processes and consequences., as part of the Joint IAMAS-IAPSO-IACS Assembly, 20 . 24 July 2009, MONTREAL

Ruddick, B. R. and Taggart, C. T. Apparatus, System and Method for Evaluating Fluid Systems. United States Patent Application, Bereskin and Parr Ltd., Toronto, ON. July, 2005. Organized (with a lot of assistance from RSMAS colleagues) the Planning Workshop for Mesoamerican Physical Oceanographic modeling, April 5/6, 2004, Miami, FL.

Invited to organize a special session on differential diffusion at the 2003 ASLO/TOS Oceanography meeting. I was not a member of either society, so my past student, Prof. Dave Hebert (URI) organized the session, and I chaired, and presented a talk on internal wave mixing. This led to collaboration with Bill Smythe.

H. Burr Steinbach Scholar, PO Department, Woods Hole Oceanographic Institution, July 8-12, 2002. (Invited lectures and mentoring of graduate students)