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

Rotating bulk shear in Monterey Bay

Itsweire et al. (1989)

PROVESS-NNS study site(observations: Sep-Nov 1998)

ADCP, CTD, MST

Wind

Bulk property observations in NNS

Wind

Bulk shear squared

Bulk shear directionvs.inertial rotation

Theory I

1D dynamic equations:

Layer averaging:

Theory II

Layer-averaged equations:

Theory III

Definition of bulk shear:

Dynamic equation for bulk shear vector:

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.

Application of theory to observations

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:

Impact of bulk shear on diapycnal mixing

Conclusion:Increased interfacialmixing rates correlatewith high shear.

Can we resolve this in 3D models?

Transect in NNS

Observations (Scanfish data from BSH)

Model results (GETM with adaptive coordinates)

Gräwe et al. (in prep.)

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

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)

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

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.