Measurements of the Turbulent Boundary Layer under Pack Ice

Miles G. McPhee Department of Geophysics, University of Washington, Seattle 98195

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J. Dungan Smith Department of Geophysics, University of Washington, Seattle 98195

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Abstract

The mean and turbulent flow structure under pack ice was measured during the 1972 AIDJEK pilot study with small mechanical current meter triplets at eight levels in the planetary boundary layer. CTD profiles showed a well-mixed layer of nearly neutral stability to about 35 m, bounded below by a strong pycnocline. The skin friction velocity u* was determined by measuring the Reynolds stress at 2 and 4 m below the ice (beyond the surface layer) and from consideration of other terms in the mean momentum equation. Local pressure gradients and advective acceleration due to topography could not be ignored; when an estimate of the effect was included, u* was 1.0±0.1 cm s−1 when the ice velocity relative to the ocean was 24 cm s−1.

With the proper coordinate transformation, the planetary boundary layer of the ocean resembles that of the atmosphere. Composite averages of non-dimensional Reynolds stress and mean flow in the ocean, when compared with recent models of a neutrally buoyant, horizontally homogeneous atmosphere, fit the model predictions fairly well. However, the lateral component (perpendicular to surface stress) departed markedly from those predictions, indicating that form drag associated with pressure ridge keels is important.

Peaks in spectra of vertical velocity were used to estimate eddy viscosity proportional to mixing length at eight levels in the outer layer. Results agreed well with the models, but this eddy viscosity did not provide a simple relation between Reynolds stress and mean flow shear.

Abstract

The mean and turbulent flow structure under pack ice was measured during the 1972 AIDJEK pilot study with small mechanical current meter triplets at eight levels in the planetary boundary layer. CTD profiles showed a well-mixed layer of nearly neutral stability to about 35 m, bounded below by a strong pycnocline. The skin friction velocity u* was determined by measuring the Reynolds stress at 2 and 4 m below the ice (beyond the surface layer) and from consideration of other terms in the mean momentum equation. Local pressure gradients and advective acceleration due to topography could not be ignored; when an estimate of the effect was included, u* was 1.0±0.1 cm s−1 when the ice velocity relative to the ocean was 24 cm s−1.

With the proper coordinate transformation, the planetary boundary layer of the ocean resembles that of the atmosphere. Composite averages of non-dimensional Reynolds stress and mean flow in the ocean, when compared with recent models of a neutrally buoyant, horizontally homogeneous atmosphere, fit the model predictions fairly well. However, the lateral component (perpendicular to surface stress) departed markedly from those predictions, indicating that form drag associated with pressure ridge keels is important.

Peaks in spectra of vertical velocity were used to estimate eddy viscosity proportional to mixing length at eight levels in the outer layer. Results agreed well with the models, but this eddy viscosity did not provide a simple relation between Reynolds stress and mean flow shear.

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