Joint Seminar: On the role of mean-flow shear and internal waves in geophysical turbulence generation

Abstract. A new two-equation, closure-like turbulence
model for stably stratified flows is introduced which uses
the turbulent kinetic energy and the turbulent enstrophy
as primary variables. It accounts for mean shear and
internal wave-driven mixing in the two limits of mean
shear and no waves, and waves but no mean shear, respectively.
The traditional TKE balance is augmented by an explicit
energy transfer from internal waves to turbulence.
A modification of the enstrophy balance accounts for the effect of the
waves on the turbulence time and space scales. The latter is
based on the assumption of a non-zero constant flux Richardson
number in the limit of vanishing mean-flow shear when
turbulence is produced exclusively by internal waves.

The new model reproduces the wave-turbulence transition analyzed
by D’Asaro and Lien (2000, J. Phys. Oceanogr., 30,
1669–1678). At small energy density E of the internal wave
field, the turbulent dissipation rate scales like E2.
This is what is observed in the deep sea. With increasing
E, after the wave-turbulence transition has been passed, the
scaling changes to E1. This is observed, for example,
in the swift tidal flow near a sill in Knight Inlet (Pacific coast).
The new model further exhibits a turbulent length scale close to
the Ozmidov scale, as observed in the ocean, and predicts
the ratio between the turbulent Thorpe and Ozmidov length
scales well within the range observed in the ocean.

The talk ends with a sketch on how to
implement the model in GCMs.

Date

08.10.2008

Time

13:30 h

Place

Bundesstr. 53, room 022/023
Seminar Room 022/023, Ground Floor, Bundesstrasse 53, 20146 Hamburg, Hamburg

Speaker

Helmut Baumert, IAMARIS, Hamburg

Organizers


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