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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.
Generation of shear-spikes in stratified shelf seas
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Rotating bulk shear in Monterey Bay
Itsweire et al. (1989)
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PROVESS-NNS study site(observations: Sep-Nov 1998)
ADCP, CTD, MST
Wind
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Bulk property observations in NNS
Wind
Bulk shear squared
Bulk shear directionvs.inertial rotation
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Theory I
1D dynamic equations:
Layer averaging:
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Theory II
Layer-averaged equations:
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Theory III
Definition of bulk shear:
Dynamic equation for bulk shear vector:
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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.
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Application of theory to observations
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Observations ofsmall-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:
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Impact of bulk shear on diapycnal mixing
Conclusion:Increased interfacialmixing rates correlatewith high shear.
Can we resolve this in 3D models?
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Transect in NNS
Observations (Scanfish data from BSH)
Model results (GETM with adaptive coordinates)
Gräwe et al. (in prep.)
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Gräwe et al. (in prep.)
Temperature
[°C]
phys adaptive with 30 layers
non-adaptive with 30 layers
Time series station from 3D model in NNS
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phys adaptive with 30 layers
non-adaptive with 30 layers
Gräwe et al. (in prep.)
Physical mixing log10[Dphy/(K2/s)]
Time series station from 3D model in NNS
Galperin (1988), Umlauf & Burchard (2005)
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Gräwe et al. (in prep.)
Numerical mixing
log10[Dnum/(K2/s)]
phys adaptive with 30 layers
non-adaptive with 30 layers
Time series station from 3D model in NNS
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Conclusions
• Increased interfacial mixing rates correlate with high shear.
• Numerical models have the capacity to provide sufficient vertical resolution to resolve the shear.
• Increased shear due to internal waves needs to be parameterised.
• Better parameterisation than clipping TKE must be found.
• Numerical mixing must be reduced to make better parameterisations effective.