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ONR Physical Oceanography, Code 32 DRI Workshop, Dec 8-9, 2009 Scalable Lateral Mixing and

ONR Physical Oceanography, Code 32 DRI Workshop, Dec 8-9, 2009 Scalable Lateral Mixing and Coherent Turbulence LatMix Goals and Priorities Discussion

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ONR Physical Oceanography, Code 32 DRI Workshop, Dec 8-9, 2009 Scalable Lateral Mixing and

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  1. ONR Physical Oceanography, Code 32 DRI Workshop, Dec 8-9, 2009 Scalable Lateral Mixing and Coherent Turbulence LatMix Goals and Priorities Discussion “Compile a program-wide list of science questions and specific goals (tangibles, quantities that we need to measure or estimate) that must be achieved to address the questions in priority order. This set of priorities will inform decision-making later in the meeting.” Compiled & Presented by: Miles Sundermeyer and Eric D’Asaro

  2. Goals and Priorities Discussion • Now – Outline Common goals and developing issues • Hypotheses - Previously developed • Key Questions and Strategies – Consensus • What do we need to measure? – Quantities and Issues • Where do we need to measure? - Issues • Prioritization of key issues • Delay until late today or early Wednesday • D’Asaro & Sundermeyer will develop a list to be discussed • Speak up (!) now and today to develop list

  3. Review of DRI Priorities / Hypotheses • DRI Hypotheses as articulated in May, 2008 meeting in Cambridge, MA: • Main Goal: To better understand lateral mixing at scales of 10 m – 10 km • Primary Hypotheses: • 1. Inhomogeneous IW mixing creates PV anomalies that are responsible for • significant isopycnal mixing. • 2. Mesoscale straining leads to a cascade of both tracer and PV variance to • submesoscales that is responsible for significant submesoscale isopycnal • mixing. • 3. Non-QG, submesoscale instabilities feed a forward cascade of energy, • scalar and PV variance which enhances both isopycnal and diapycnal mixing.

  4. Proposed DRI Priorities / Hypotheses (cont’d) Secondary Hypotheses: 4. Submesoscale variability is associated with coherent structures: mixing is inhomogeneous and anisotropic and submesoscale processes are inherently vertical as well as horizontal. 5. "Fronts are not barriers to transport". Specifically, we hypothesize that submesoscale processes facilitate cross-density-front exchange (Here, the point is to understand how a collection of submesoscale processes add up to give a cross-front transport at the mesoscale; i.e. bolus transport, not mixing that leads to irreversible mixing - diffusion). 6. Filaments develop a slope of f/N at scales dominated by geostrophic dynamics. (What sets the width and thickness of filaments?) 7. The lateral downscale variance cascade is absorbed by vertical (as opposed to lateral) mixing processes. How does filamentation interact with vertical processes, e.g., double diffusion, Kelvin-Helmholtz, internal wave breaking?

  5. Proposed DRI Priorities / Hypotheses (cont’d) • DRI Question as articulated in Dec, 2008 meeting in Monterey, CA: • Is lateral mixing at O(1-10 km) due to balanced and unbalanced downscale cascade from the mesoscale or due to local vertical mixing by internal waves and surface forcing? • Sub-Questions: • Is there an upscale cascade from vertical mixing to submesoscale? • Is the downscale cascade due to balanced or unbalanced instabilities? • Does the answer to the question depend on depth and stratification? • What is the role of mixing across evolving fronts in the downscale cascade? • What is the relative role of surface momentum- and buoyancy-flux driven mixing? • At what scales is the downscale cascade arrested and by what processes? • By what mechanisms do internal waves contribute to lateral mixing?

  6. Key Questions (distilled) • Rates:What are submesoscale lateral mixing rates of scalars in the upper ocean? • Not momentum. • Lateral involves isopycnal and diapycnal processes • Dynamics: What are the mechanisms for this mixing? • We do not agree on the dominant mechanisms • This is a genuine scientific disagreement? • Yes we can (!) agree on experimental plans – Our Goal for this meeting! • Strategy • We expect that lateral mixing rates and mechanisms will vary with environmental parameters and location. Our strategy is to measure in a variety of environments resolving both the rates of lateral mixing and the underlying time and space scales and patterns of the mixing and stirring.

  7. Consensus and Issues What do we need to measure in Experiments and Models? Lateral Mixing: Mixing and/or Stirring Rates Dye dispersion rate Particle dispersion rate Dye & Scalar budgets Scalar Dissipation rates Energy dissipation rate Resolved velocity fields Patterns of Mixing and Stirring Maps Movies Eddies Streaks Instabilities Filaments Length and Time Scales – An Issue Submesoscale – 10 m-10 km is too wide a range to measure The EM-APEX array (LI) will disperse How long should it stay in? Two ship, towed surveys (LII) What scales of survey? Dye studies (LI) are proposed for: 1-103 m, 102-104 s 1-5 km, 1-3 days Is one better than the other?

  8. Consensus and Issues (cont’d) Coincident Environmental Measurements: Key Quantities - Issues Patch-size “submesoscale” mean gradients of velocity and density: Richardson number Vertical vorticity Isopycnal strain Potential vorticity (esp. <0) Patch-size statistical quantities: Gregg-Henyey mixing rate – rms shear4 Shear dispersion factor – Shear spectrum Turbulent patch statistics - Larger scale “Mesoscale” gradients: What do we need to know? Surface forcing: Standard bulk quantities – wind, bulk flux, radiation Surface waves

  9. Consensus and Issues (cont’d) What combinations are crucial?- LI/LII, small/large dye Small-scale dye patterns + dissipation rate Small-scale dye + submesoscale shear/strain + vertical velocity Internal waves + diapycnal mixing (shear dispersion) Mesoscale + submesoscale shear/strain + diapycnal mixing

  10. Consensus and Issues (cont’d) Where should we make these measurements? i.e. What do we search for in an experimental site? Scientifically important quantities --------Need to rank these----------- Depth/Regime – Mixed layer, Transition Layer, Interior Wind Mean Lateral Strain (mesoscale vs. submesoscale) Mean vertical shear/Richardson number PV surface flux i.e. downfront wind Mean Potential Vorticity - (esp. negative) Topography (how far from?) Doesn’t matter +?

  11. Consensus and Issues (cont’d) Experimental Constraints These will be developed more during the meeting Latmix I Clear water Upper 50m, stratified N: > 0.005? <0.03 ? Uniform for [ 1? 10? ] m Lateral Strain < [1? 10?] km/day Wind/Waves < [5? 10? 15?] m/s Vertical shear < ?? Vertical vorticity ?? +? Latmix II Upper 200m Good acoustic tracking Wind/waves < 20 m/s +?

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