At high winds, turbulence in the ocean surface mixed layer is dominated by organized
coherent structures in the form of counterrotating helical vortices known as Langmuir
cells. While the dynamics of the ocean surface layer has been studied rather extensively
at lower wind speeds, the detailed physics at higher winds has remained largely
inaccessible because of limited sea-going operations and difficulty conducting in situ
measurements at high sea states.
In the present thesis new measurement techniques, based on acoustical remote
sensing, are described. A freely drifting imaging sonar was employed, which allowed us
to follow time-evolving features for an extended period of time. This imaging sonar
extends the acoustical approach beyond fixed orientation sonars and covers a full 360°
circle on the surface. The full circle capability turns out to be a key addition to the
measurements: it allowed quantitative evaluation of the directional properties of
Langmuir circulation surface structure. These new methods allow us to sample near-surface
circulation and bubble distributions even in extreme conditions, and contribute to
our understanding of small scale dynamics in the wind driven surface layer.
Using vertical velocity measurements in the convergent regions of Langmuir
circulation and a model scaling, we infer the effective viscosity relevant to cell
generation. Matching velocity- and temperature-inferred turbulent viscosities we estimate
the depth scale over which the wind-wave forcing is of most importance. The velocity-inferred
viscosity compares favorably with the mean model viscosity values evaluated at
approximately two significant wave heights below the surface. Combining the effective viscosity calculated at different depths with the observed Stokes drift and friction velocity
we estimate Langmuir numbers La between 0.015 and 0.1. We observe evolving cell
patterns at larger La (between 0.02 and 0.05), which indicates that higher viscosity values
than previously assumed in the models may be relevant for Langmuir circulation
dynamics.
Acoustical observations of the orientation of surface bubble clouds and the directional
wave field during several deployments provided an opportunity for comparison of the
directional properties of Langmuir circulation with a model that takes into account effects
associated with misalignment of the Stokes drift and wind forcing. Model results imply
that the growth rate is maximal overall when wind and waves are aligned. For a given
angle between the Stokes drift and the wind (the misalignment angle) the direction of the
cell axis for maximal growth lies between the Stokes drift and the wind and is mainly determined by (i) the misalignment angle and (ii) the ratio of the Stokes drift shear and
mean Eulerian shear. Our ocean observations showed Langmuir cells responding to the
changes in wind direction within 15 to 20 min. On two occasions, when the wind
changed direction and waves lagged behind, the cells were observed to form in an
intermediate direction (between wind and waves) consistent with model predictions.
Observations of the near-surface circulation and thermal structure during a storm
motivate analysis in terms of the Froude number derived from the measured vertical
density gradient, the turbulent diffusivity which is inferred from the measured
temperature distributions, and velocity and spatial structure of the circulation. The results
demonstrate inhibition of Langmuir circulation by the presence of warm surface water at
the beginning of a storm and provide a test of model description of the balance between
wind-driven stirring and buoyant resistance.
To better understand our measurements and the limitations of the approach, based on the acoustical backscatter, a technique for scatter location estimation is proposed. By
comparing velocity magnitudes, independently measured with side-looking and upward-looking sonars, we estimate an effective scattering depth. These results show that the
backscatter measured with side-looking sonars originates not right at the surface but at
some depth below. / Graduate
| Identifer | oai:union.ndltd.org:uvic.ca/oai:dspace.library.uvic.ca:1828/8397 |
| Date | 03 August 2017 |
| Creators | Polonichko, Vadim Dmitri |
| Contributors | Farmer, David M., Garrett, Christopher J. R. |
| Source Sets | University of Victoria |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Available to the World Wide Web |
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