Geoscience Reference
In-Depth Information
many unresolved questions about ocean mixing. In the 1972 experiment, his re-
search team (includingthe author as a graduatestudent) deployeda total of 75 (25
triads) of the Smith rotors on three separate masts, two of which were placed in
positionby divers.The rotationrate of the rotors was sensed optically,with the pe-
riod between electrical pulses triggered by mirrors on the rotor blades processed
electronically and recorded on magnetic tape by one of the first available mini-
computers (Data General Nova 1200 with an internal RAM of about 8,000 16-bit
words—consideredlargeatthetime!).
Accurate measurementof vertical velocity is critical for covarianceestimates of
turbulentfluxes. Smith addressedthe problemof resolvingsmall vertical velocities
with a mechanical current meter (sensitive to the angle of attack with respect to
meanflow)bycantingthe“
z
-axis”ofthecurrentmetertriadawayfromthevertical
by30
◦
,sothatallthreemeterssensedasizablefractionofthemeanflow.Turbulence
measurements from the suspendedmasts, along with results from a modern Guild-
lineCTDprofiler,resultedinafairlycomprehensiveviewoftheturbulencestructure
ina nearlyneutrallystratifiedIOBL (McPheeandSmith1976).
3.2.2 Turbulence Instrument Clusters
In planning for the Marginal Ice Zone Experiments (MIZEX) north of Fram Strait
in the Greenland Sea (McPhee 1983), we realized that turbulent heat flux from
the ocean would be a major factor in the mass balance and survivability of sea
ice encountering the open ocean with near surface temperatures well above freez-
ing. At the time, direct measurement of turbulent heat flux via the covariance of
vertical velocity with deviatory temperature (i.e., turbulent departures from mean
fluid temperature) had not been made in the ocean. We approached Art Pedersen,
founder of Sea-Bird Electronics, Inc. (SBE), who had recently combined a novel
period-countingscheme with Wien-bridgecircuitryincommerciallyavailabletem-
perature (SBE 3) and conductivity (SBE 4) sensors, with the idea of incorporating
a version of Smith's rotors (by then using a Hall-effect magnetic pickup in lieu
of the earlier optical system) into a highly modified version of the SBE conduc-
tivity/temperature/depth(CTD) instrument. Pedersen responded positively, and as-
sembledtheSBE-35systeminhisgarageonMercerIsland,Washington,intimefor
use in the 1984 main MIZEX experiment in the Greenland Sea marginal ice zone.
TheSBE 35couldaccommodateupto seven
turbulence instrument clusters
(TICs)
each comprising three Smith rotors oriented along orthogonalaxes, with the
z
-axis
nominally 45
◦
from vertical, mounted in the same horizontalplane as nearby SBE
temperatureand conductivity sensors. Five frequency signals (three low frequency
outputfromthe currentmetersand two high frequencyfrom the T/C sensors) from
each TIC were transferred by co-axial cable to the backplane of the SBE-35 deck
unit, and recorded via computer. The new system provided credible estimates of
ocean heat flux during the MIZEX experiment (McPhee et al. 1987), and substan-
tially changed our view of how heat and salt are transferred at the ice/ocean inter-
face,asdescribedinChapter6.
