Geoscience Reference
In-Depth Information
The basic TIC configuration was modified slightly to reduce the diameter (hole
size) and used in several experiments throughout the 1980s and 1990s, culminat-
ing in the year-long SHEBA project in 1997-1998. For CEAREX 1989, a new
deployment scheme was developed in which TICs were mounted on a mast that
could be lowered by winch to as much as 100m below the surface. This entailed
development of highly modified SBE 9 underwater units with input of 4-6 TICs,
multiplexedthroughonesea cabletothesurfacedeckunit.Inthisconfigurationthe
mast was orientedinto an optimalangleof attack for the currentmeter triad by use
ofavane.
For the 1992 LeadEX project, a further embellishment to the Smith-rotor
TIC was addition of a SBE-7 fast-response microstructure conductivity sensor.
The standard SBE 4 conductivity instrument uses a ducted design to increase
accuracy and to maintain calibration, but the resulting flow constriction decreases
the response to small turbulence fluctuations. Figure 3.1 shows the TIC config-
uration used during LeadEX and SHEBA. The exposed-electrode microstructure
conductivity
instrument is subject to sometimes severe calibration drift and
conductivityspikes,butwhenusedincombinationwiththenearbystandardSBE4,
has provided credible direct estimates of salinity flux
(
µ
C
)
w
S
)
(
andallowedusto
calculate the total turbulent buoyancy flux from covariance measurements during
LeadEX(McPhee1994;McPheeandStanton1996).
Mechanicalcurrentmetersaresubjecttoseverallimitations,includingbiological
fouling(occasionallyasevereproblemwhenptinafora[jellyfish]arepresentduring
a short summer period). Possibly more significant is that although the individual
Fig. 3.1
Photograph of a “Smith-rotor” turbulence instrument cluster comprising three ducted ro-
tors with Hall-effect magnetic sensors, along with SBE temperature (SBE 3), conductivity (SBE
4) and microstructure conductivity (SBE 7) instruments. Note that in this view, the upper current
meterishorizontal whiletheother twoare canted 45
◦
