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Mark Hadfield NIWA

Using ROMS Lagrangian-float simulations to estimate exchange rates between spatial compartments in a tidal estuary. Mark Hadfield NIWA. Outline. Research strategy Setting Box modelling POL3D and Eulerian tracers ROMS and Lagrangian floats Preliminary results Conclusions. Strategy.

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Mark Hadfield NIWA

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  1. Using ROMS Lagrangian-float simulations to estimate exchange rates betweenspatial compartments in a tidal estuary Mark HadfieldNIWA

  2. Outline • Research strategy • Setting • Box modelling • POL3D and Eulerian tracers • ROMS and Lagrangian floats • Preliminary results • Conclusions

  3. Strategy • We want to estimate maximum carrying capacity of a selected bay for mussel aquaculture. • Couple a fine-resolution hydrodynamic model with a coarse resolution transport/ecology model (a “spatially aggregated” or “box” model). • Coupling achieved by having the hydrodynamic model calculate exchange rates between the boxes. • Exchange rates to be estimated in the hydrodynamic model using tracers: label each box with a tracer and observing the rate of exchange with other boxes.

  4. The Setting:

  5. Box Model Formulation Fij = TijCi Ci Cj Fji = TjiCj Fij – Fji = TijCi –TjiCj = TD(Ci – Cj) + TA(Ci + Cj) / 2 where TD = (Tij + Tji) / 2 TA = Tij + Tji

  6. Evaluating coefficients:the naïve approach At a specified time, label each box with a separate tracer with concentration = 1. After a suitable interval Dt measure the mass Mij of each tracer i in every other box j. Then Tij » Mij/ Dt.

  7. 2-box example

  8. Earlier Simulations —POL3D, Eulerian tracer • Proudman Oceanographic Laboratory 3D model (POL3D) • Grid covering Pelorus Sound, 141  140 points, 18 levels, horizontal spacing 250 m. • Forced at outer boundary by M2 tide (amplitude 0.88 m). • Two-day flood event beginning at 3 days • Rainfall on surface of sound, then Pelorus River flow peak. • After the flood, river flow and rainfall continue at lower rates • Eulerian tracers released and followed for 5 tidal cycles

  9. Current Simulations –ROMS, Lagrangian tracer • ROMS • Rectangular grid, 250 m horizontal spacing, 15 levels with moderate vertical stretching. • Large, McWilliams, Doney (LMD) vertical mixing. • Forcing as before. • Float trajectories deterministic for now, i.e. no random walk.

  10. Exchange calculations • 500, 000 Lagrangian floats, initially distributed uniformly throughout the domain, released and tracked for 5 tidal cycles. • At any instant we can mark out a region in the fluid, label the particles in that region, then follow them for the remainder of the float simulation. • The results of several such pseudo-releases are averaged to eliminate: • variations with tidal phase • stochastic effects

  11. Conclusions • The naïve method of evaluating transfer coefficients from tracer exchange data has problems due to sharp edges in the initial fields • With Lagrangian float calculations we can use the extra trajectory information to solve those problems.

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