Geoscience Reference
In-Depth Information
Itisbeyondthescopeofthischaptertopresentanythingbeyondacursorysurvey
ofturbulence.Forthemostpart,wheretherehasbeensubstantialprogressinturbu-
lencetheory,ithascomefromconsiderationof
fully developed
turbulence,i.e.,tur-
bulencethatis homogeneousand isotropic, as in high Reynoldsnumberflowsome
distance downstream from a wind-tunnel grid (e.g., Batchelor 1967; Frisch 1995).
With very few exceptions, the IOBL flows considered here are essentially shear
flows that vary with the strength of the wind or tide; are anisotropic at the scales
ofthe energy-containingeddies;and,particularlyundertypicallyroughseaice, are
hardlyhomogeneous.Despitetheseshortcomings,we oftenfindthata probabilistic
descriptionoftheturbulentflowswemeasureintheIOBLprovidesarepeatableand
usefultoolforunderstandingturbulenttransfersinboundarylayerswhererotationis
important.Bythesametokenitisimportanttokeepinmindthelimitationsimposed
by these departures from the assumptions often underlying turbulence theory. The
primarygoalof this chapteris to selectively investigatea few topicschosen from a
vastfieldofturbulenceresearchthatareparticularlygermanetotheIOBLproblem.
Foramorethoroughapproach,thereaderisreferredtothetextsreferencedabove.
3.2 IOBL Measurement Techniques and Examples
Compared with daunting technical difficulties faced in measuring turbulence near
the surface of the open ocean (where orbital wave velocities and platform mo-
tion often dwarf turbulentfluctuations, except at very small scales), when working
from sea ice, it is relatively easy to measure the covarianceof vertical velocity and
fluctuating horizontal velocity components that make up the horizontal Reynolds
tangential stress. In many respects, pack ice forced by wind to drift over an other-
wise nearly quiescent ocean provides a unique laboratory for studying boundary
layer flow in a rotating reference frame. The ice itself is a platform that quells all
but the longest period surface gravity waves, and allows us to suspend instruments
at known depths through the entire extent of the boundary layer, moving at the
maximum IOBL velocity. Additionally, at least away from obvious obstacles like
deeppressure ridgekeels, it presentsa relativelyuniform,flat surface,usually with
comparativelysmallhorizontalgradientsin surfacestress andupperoceandensity.
3.2.1 Smith Rotors
During the 1972 AIDJEX Pilot Study in the Beaufort Gyre of the Canadian Basin,
Prof. J. Dungan Smith of the University of Washington exploited the sea-ice labo-
ratorybydeployingarraysofsmall,partiallyductedrotors,arrangedintriadsalong
orthogonalaxes,providingforthe firsttime, three-dimensionalcurrentvelocitiesat
several levels throughan entire upper ocean boundarylayer. In the face of conven-
tional wisdom that held it to be impossible to measure turbulent fluxes directly in
the ocean, Smith realizedthe potentialfor using undericemeasurementsto address
