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Hans Burchard 1 , Tom P. Rippeth 2 and Ulf Gräwe 1

Generation of shear-spikes in stratified shelf seas. Hans Burchard 1 , Tom P. Rippeth 2 and Ulf Gräwe 1 1. Leibniz Institute for Baltic Sea Research Warnemünde , Germany 2. School of Ocean Sciences, University of Bangor, Wales

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Hans Burchard 1 , Tom P. Rippeth 2 and Ulf Gräwe 1

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  1. Generation of shear-spikes in stratified shelf seas Hans Burchard1, Tom P. Rippeth2 and Ulf Gräwe1 1. Leibniz Institute for Baltic Sea Research Warnemünde, Germany 2. School of Ocean Sciences, University of Bangor, Wales Burchard, H., and T.P. Rippeth, Generation of bulk shear spikes in shallow stratified tidal seas, J. Phys. Oceanogr., 39, 969-985, 2009.

  2. Rotating bulk shear in Monterey Bay Itsweire et al. (1989)

  3. PROVESS-NNS study site (observations: Sep-Nov 1998) Wind ADCP, CTD, MST

  4. Bulk property observations in NNS Wind Bulk shear squared Bulk shear direction vs. inertial rotation

  5. Theory I 1D dynamic equations: Layer averaging:

  6. Theory II Layer-averaged equations:

  7. Theory III Definition of bulk shear: Dynamic equation for bulk shear vector:

  8. Theory IV Dynamic equation for bulk shear squared: Conclusion: Assuming bed stress being small, bulk shear is generated by the alignment of wind vector and shear vector.

  9. Application of theory to observations

  10. Observations of • small-scale mixing • Obtain spetra of small-scale • shear from mirostructurprofiler • Calculate shear wave number spectrum • Calculate dissipation rate by fitting empirical spectrum • Apply Osborn (1980) to estimate eddy diffusivity:

  11. Impact of bulk shear on diapycnal mixing Conclusion: Increased interfacial mixing rates correlate with high shear. Can we resolve this in 3D models?

  12. Transect in NNS Observations (Scanfish data from BSH) Model results (GETM with adaptive coordinates) Gräwe et al. (in prep.)

  13. Time series station from 3D model in NNS Temperature [°C] phys adaptive with 30 layers non-adaptive with 30 layers Gräwe et al. (in prep.)

  14. Time series station from 3D model in NNS Galperin (1988), Umlauf & Burchard (2005) Physical mixing log10[Dphy/(K2/s)] phys adaptive with 30 layers non-adaptive with 30 layers Gräwe et al. (in prep.)

  15. Time series station from 3D model in NNS Numerical mixing log10[Dnum/(K2/s)] phys adaptive with 30 layers non-adaptive with 30 layers Gräwe et al. (in prep.)

  16. Conclusions • Increasedinterfacialmixingratescorrelatewithhighshear. • Numericalmodelshavethecapacitytoprovidesufficient • verticalresolutiontoresolvetheshear. • Increasedshear due tointernalwavesneedstobeparameterised. • Betterparameterisationthanclipping TKE must befound. • Numericalmixing must bereducedtomakebetterparameterisations • effective.

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