- Email: [email protected]
A portable CTD system for use in polar environments
Cold Regions Science and Technology, 20 ( 1991 ) 1-9 Elsevier Science Publishers B.V., Amsterdam
1
A portable CTD system for use in polar environments Siobhan P. O'Farrell a'~, Vernon A. Squire b, Stuart C. M o o r e c and T i m o t h y R.E. Owen d "Scott Polar Research Institute, University of Cambridge, Cambridge, UK bDepartment of Mathematics and Statistics, University of Otago, Otago, New Zealand cScienceApplications International Corporation, Cambridge, UK dDepartment of Earth Sciences, University of Cambridge, Cambridge, UK (Received April 9, 1990; accepted after revision December 5, 1990)
ABSTRACT O'Farreli, S.P., Squire, V.A., Moore, S.C. and Owen, T.R.E., 1991. A portable CTD system for use in polar environments. CoM Reg. Sci. Technol., 20: 1-9. Details of instrument design and configurationai aspects are reported for a high resolution, conductivity-temperaturedepth (CTD) unit, developed to measure the properties of the mixed layer beneath a sea ice cover and the temperature/ salinity gradients in lead areas. The instrument is small, and is sufficiently portable to be used either from an ice floe edge or from a small boat. Some discussion of the results of deployments in the Greenland Sea marginal ice zone are included.
1 Aims of experiment on Marginal Ice Zone Boundary Layer Dynamics The Scott Polar Research Institute (SPRI) CTD unit was developed to measure to high resolution the conductivity, temperature and depth with resolution of order 0.002°C, 0.0002 m S / m , 0.01 m, resp., of the mixed layer beneath drifting sea ice. The boundary layer between the ice and ocean is an important component of the ice-ocean system as it is through this layer that the ice-water stress is transmitted to the deep ocean (McPhee, 1982). Boundary layer depth, and salinity and temperature characteristics are modified by the heat and salt fluxes from the ice (McPhee, 1983). Melt-water stabilizes the water columnDallowing the mixed layer depth to shallow--whilst additional freezing releases brine leading to convection in the mixed layer. During MIZEX-84, the CTD instrument was used at the Greenland Sea summer ice edge to comple~Now at CSIRO Division of Atmospheric Research, Aspendale, Vic. 3195, Australia.
ment boundary layer studies taking place at the interior drifting station which used a vertical array o f conductivity, temperature and velocity sensors (Morrison et al., 1987). Also at this inner station, measurements of heat transfer and other ice properties were collected to provide a mass balance for the summer marginal ice zone (MIZ). In the active outer regions of the MIZ, the ratio of ice loss from lateral/bottom melting increases (Perovich, 1983). Experiments in this region of the Greenland Sea were undertaken during the summer o f 1984 from the M / S "Kvitbjern", which made several sections across the ice margin, and also from M / S "Lance" in Fram Strait in August 1984. Our study focussed on the effect o f meltwater input into the upper ocean at the ice edge, and ice ablation measurements were collected concurrently (Josberger, 1987). The study was designed to encompass both the immediate impact o f fresh water input on the ocean boundary layer, and to follow the development o f the newly formed layer when this buoyant water is mixed to the depth of the thermocline.
0165-232X/91/$03.50 © 1991 Elsevier Science Publishers B.V. All fights reserved.
-'~
2 C T D specifications and design
Instrument requirements To carry out a programme to collect ocean profiles at the extreme edge of the marginal ice zone CTD instrumentation had to meet three main criteria: - it had to be sensitive enough to measure the small temperature and conductivity gradients in the near isothermal layer; - it had to be robust enough to be used in leads in the pack ice, where loose ice could damage the instrument; and - it had to be portable enough to operate away from the ship, either at the edge of ice floes or from small boats, as no helicopters were available. As no portable CTD with these requirements was available commercially at that time, a unit was designed and built at the University of Cambridge (Scott Polar Research Institute and Department of Earth Sciences) by S.C. Moore and T.R.E. Owen.
Instrument configuration The CTD (Figs. 1-2 ) is manufactured from aluminium; the main section being a cylinder, 375 mm long, with interior and exterior diameters of 89 mm and 110 mm, respectively. The head of the instrument is a solid paraboloid, streamlined to minimize any disturbance to the density field measured by the sensors. In the constant diameter part of the head section there are two inlets which provide access to the pressure sensor. Perpendicular to these inlets are brackets for the conductivity and temperature probes. The system is powered by a block of ten 1.5 V Ni-cad, C-cells which are rechargable through the waterproof connector when the instrument is at the surface. The battery block is placed just above the sensor's internal connections in order to balance the instrument. Data are recorded at the surface, allowing the instrument package to be more compact as no internal logging is required. Signals recorded at the surface can also be scanned in real-time by a microcomputer (see Section 4). The sensor signal is passed up the centre of the battery block to the electronics for conditioning and coding as frequency signals. Three electronics
S.P. O'FARRELL ET A t .
boards, one for each sensor, are arranged in a triangle on a circular plate, and are bolted on to the battery block. The whole central section is tightly pressure sealed at the base of the cylinder, with a pressure release knob fitted to the seal joining the end cap to the main cylinder. A ten-way connector positioned at the top of the electronics section links to the interior part of the seawater connector. A three-core, co-axial cable is attached through a hookend connector to the winch, and is moulded onto the outer part of the seawater connector. The central portion of the instrument is protected by an aluminium guard, from a point just above the sensor probes.
The sensors The pressure sensor is located just behind the parabolic head in the base of the instrument. It is positioned centrally beneath the battery package and electronics section. Access to the outside pressure field is made through two 13-mm diameter holes (Fig. 3). The sensor is a commercially-built pressure transducer manufactured by Druck U K Ltd., which provides a voltage output, and, in the SPRI CTD, is separated from the fluid by a stainless steel isolating diaphragm. Since the conductivity and temperature data are encoded as a frequency signal (using commercial electronics), the output of the pressure transducer is convened to frequency to be compatible. When pressure is recast as depth, the instrument measures with a resolution o f 50 mm to an accuracy of 200 mm. The conductivity cell and electronics was purchased from SeaBird Electronics Inc. The cell consists of three platinum-coated electrodes placed 62.5 mm apart in a flow-throngh configuration. The sensor is attached via a bracket to the main casing just behind the pressure inlets, though rotated through 90 ° (Fig. 3). The electrode signal enters the main body of the instrument through an opening in the aluminium casing which is sealed by an O-ring. The cell operates by allowing the outer pair o f electrodes to be held at the same potential, with the resistance between the centre electrode and the outer pair being measured. This resistance is a function of both the fluid's conductivity and the celt's dimensions. The
'I=
¢1)
B
('1
('1
0
8
==
~o
,.11
Pressure leads
\
O
.Ul
-O
Aluml~um casing
Guard \
.~
~.
I'
Prgs
©,
Access for T e m p e r l l u r e .~msor
"
~ ' 1 i ~¸
~//"//////////J//////////.. . ~ ~ ~~~ ;~//A BATTERY PACK
I
-,,.,
4
S.P. O ' F A R R E L L ET AL.
ilj)
Fig. 2. Photograph of complete CTD showing external SEACON connector at the base, and a guard protecting the sensors close to the head. resulting measurement may be interpreted as the integral of the conductivity value for an equivalent length of water column. The rate o f descent of the cell through the water c o l u m n (0.166 m / s ) and the sampling rate (0.5 s) are strffieient to obtain several records for the volume of fluid sample which
Fig. 3. Close-up showing the conductivity cell, the access hole to the pressure sensor, and the steel tubes guarding the thermistors. fills the cell. Cell resistance is converted to a frequency in the range 5-11 kHz at an r.m.s, voltage of 0.7 V. In our field experience in the Arctic the frequency output ranged between 6.5 and 7.5 kHz: The sensor was expected to resolve conductivities of 0.0002 m S / m to an accuracy o f 0.001 m S / m , before calibration. Flushing characteristics o f t h e cell,
A PORTABLE CTD SYSTEM FOR USE IN POLAR REGIONS
namely how rapidly the conductivity sensor responds to the sample flowing through, are important when interpreting salinity spiking caused by strong temperature gradients in the thermocline. However, with manual lowering speeds this problem is minimized (Gregg and Hess, 1985 ). On the opposite side of the aluminium casing from the conductivity sensor is the temperature probe (Fig. 3), consisting of two thermistors connected in series and encased in narrow stainless steel tubes. The thermistors are insulated from the tubes by lacquered resin. These unprotected, nearly microscopic thermistors are advantageous, as they possess a fast response time. It is an essential requirement for the thermistor time-constant to be of the same order of magnitude as the sampling interval. When the pair of thermistors are arranged in series they gave a total thermistor resistance of 4000 ~2at 20°C. This sensor package was expected to have the same degree of accuracy as the conductivity sensors with a resolution of 0.002°C and an accuracy of 0.01 °C.
Winch and slip-ring assembly In the first instance the mini-CTD was only required for shallow ( < 100 m) profiles. Thus, since our budget was limited, a manual winch was purchased. The steel winch was supplied with approximately 100 m of three-core coaxial cable, ending in a hook-end connector for attachment to the CTD unit. The total mass of the winch is 31 kg with a load capacity of 100 kg. A combination of a ratchet stop and an adjustable brake were used to secure the load at a given cable extension during casts. The winch is fitted with instrument grade, three-way slip rings, and the stainless steel armouring on the cable is utilized as the signal ground.
Electronic counter An electronic counter used to analyse the frequency signals from the CTD, is located in the logging box on the surface. The logger is controlled by an RCA CMOS 1800 series processor, with timing provided by a 153.4375 kHz high-frequency clock pulse. The logger has data capacity of 48 kilobytes in the form of hard core random access memory
5
(RAM); enough storage for approximately 68 min of data. Each data point represents the number of clock pulses occurring during 2048 cycles of the input signal (Owen, 1984). The resolution of the electronic counter was increased by using period counting in preference to a direct binary count of the frequency signal.
3 Deployment and field operations The CTD was mounted within an aluminium frame so that it could be suspended over the side of the floe. The winch was positioned at the back end of the frame to counter balance the instrument's weight. The signal cable passes over a pulley to the CTD fish. The complete apparatus was hauled across the floes on an aluminium sledge. The chosen floe was usually of multiyear type, with more than 0.2 m freeboard, and had a diameter between 30 and 200 m. The floe was examined for a safe edge--both on approach and after anchoring--as local wave activity could often erode the ice wall at and below water level, thereby making it unsafe. Snow cover on most of the floe was light, and ice concentration in the area varied from about 3 / l 0 near the ice margin to about 9/10 in the interior, i.e., the limit to which the "Kvitbjorn" would normally enter the ice. Most field deployments involved obtaining a few CTD casts from a single drifting station. On some occasions data were also recorded during the upcast. During short ship stops (1.5-2 h), two stations to depths of 70 m or three stations to 40 m were normally obtained. The time for lowering at each station was about 5 minutes, whilst hauling the instrument to the surface normally took at least twice as long. On several occasions during the ice drift phases undertaken by both the "Kvitbj~rn" and the "Lance", a time series of short casts was maintained over periods of a day. The instrument was also deployed from small boats, enabling variations in the upper boundary layer of leads to be studied. During a section of the "Lance" across the East Greenland pack ice in Fram Strait, the maximum ice concentration was rarely above 6/10, and work in the open water around ice
6
floes was often carried out as an alternative to work from the floes themselves. This was especially true when the floe edges were badly eroded or awash due to the waves. This method of deployment was easier and quicker, since the instrumentation could be set up in the boat between stations. The frame was mounted either in the stern ("Lance") or in the bow ("Kvitbjorn") of the small boat, and the engine was normally off during profiling operations. On some occasions the boat was anchored to a small floe while measurements were obtained. In some of the open water sequences there are regular spikes in the data due to the movement of the boat under the action of incoming swell, its own bow wave, or the bow wave of other 'Zodiac' inflatables deployed simultaneously from the ship.
4 Microcomputers involved in field processing The microcomputers/processors used in the field for processing the CTD data were a CDP 1805 processor on the logging electronics board and an Epson HX-20 microcomputer. The latter served as the computer and on-ice data processing unit. The Epson micro-cassette system is used as an intermediate storage medium for the data. On board ship, data were transferred from micro-cassette to floppy disc on a 'Torch' microcomputer, where all states of processing from hexadecimal input data files to graphical output of the CTD profile could be accomplished. The logging of the CTD data is controlled by the Epson HX-20 computer through the RS232C interface. There is also an automatic logging mode, introduced to cover the possibility of Epson internal battery failure at low temperatures. Instructions are sent to the CDP-180 processor from the Epson HX-20 as single ASCII characters which invoke modules and subroutines in the microprocessor program. The modules which make up this program were written by D. White (Department of Earth Sciences, University of Cambridge); included are facilities for reading, writing, searching and unloading the data buffer at specific locations. The program is implemented on the logger electronics board on an EPROM chip. Each individual profile record starts with a data and time mark, and ends with a
S.P. O'FARRELL E'I Al.
file mark. The data record is interspersed with time marks written at the start of every minute. A pointer is maintained by the monitor which contains the memory location of the next position to which data is to be written (Owen, 1984). The Epson HX-20 is a compact microcomputer with 16 kilobytes of internal storage. Its portability means that it is well suited to the task of controlling the logger; additionally, it is able to provide an initial scan of the data in the field. It contains a microcassette drive, to which data from the logger can be downloaded via the RS232C port. Later transfer to the 'Torch' microcomputer is facilitated by a further program. The heavy power consumption for the transfer operation dictated that it should normally be carried out on-board ship to save battery power. A CPM microcomputer manufactured by 'Torch Computers LTD' is used to convert raw hexadecimal data into frequency values. These data are then processed via calibration algorithms to produce values of conductivity, temperature and depth for each data record. Subsequently, the data are used to derive salinity and density for each profile. Programs are also available on the Torch for plotting data on the screen, for dumping it to an Epson FX-80 printer to produce hard copy output, and for calculating calibration curves and density algorithms.
5 Data processing Data were processed through a number of stages to produce the final profile shown in Fig. 4, Firstly, algorithms were used to convert each of the data channels from frequency to their respective variables. The resulting temperatures and conductivities were used to derive salinity and density values using 'practical salinity' algorithms (Fofonoff and Millard, 1983). Even after the data had been converted by the calibration algorithms, a considerable amount of processing, despiking and filtering is necessary to produce a reasonable final set. Segments of data representing either upward movements or temporary halts of the sensors in the water column have to be removed; frequent winch stops cause spiking and sensor drift. Upward moving sections can often
A PORTABLECTD SYSTEM FOR USE IN POLAR REGIONS
..TULY 18TH S T A T I O N N O 6
0
i
,
,
,
I
. . . .
I
. . . .
. . . .
1
10
r
t.
20
Z
30
40
t/
/
50 x
'x t /,
/)
60
"S " q
70 -2.0 3£0
. . . . I Tem~r~tt're"C '- 1.0 . . . .
2d.0 . . . .
'
'
3 t . 0 . ,Salittifg . . .
° ].o
2t.0 . . . .
'
I
. . . .
t
"
'
'
'
0.0
1.0
2.0
3~.o . . . .
3,'.0 . . . .
3~.0
26t.0. . . .Density .
2t.0 . . . .
2d.0
Fig. 4. Salinity, temperature and density profiles o f a station obtained from a large ( 1 k m ) ice floe adjacent to M / S " K v i t b j o r n " at 1453 on the 18th July 1984, located at 7 8 ° 0 8 ' N , 0 ° 10'E.
be removed directly (Pingree, 1971 ), through care must be taken if large sensor mismatch is present (Scarlet, 1975 ). Correction for the time response of the sensors (Home and Toole, 1980) also preceded any data removal which might be necessary for winch stops. The calibration algorithms were derived from several data pairs obtained from cross-calibration data, with the form of the relationship based on the physics governing sensor operation. We included combinations of points covering the range of field operations for the sensors, and beyond if possible, and found the best polynomial fit by least squares. In the early stages of the data processing a simple running average filter was used, but as more elabo-
rate techniques were employed to correct for mismatch and despiking it was decided that filtering should be done at a later stage. Data spikes can be removed either by assuming that no two consecutive data points are more than a specific distance apart, or by the application of a median filter (Sy, 1985). Examination of the density profile after subsequent processing through the algorithms shows that further small-scale smoothing may be necessary, but it is arguable at which stage of the processing it is best to carry this out when the nonlinear calibration algorithms involved are taken into account. The final data set can also be averaged over depth intervals to produce a better record, thereby remov-
8
S.P.O'FARREI.LE1AI
ing problems due to the unsteady lowering rate resulting from use of the manually powered winch.
6 Results
In Fig. 4 we show the results of a temperature, salinity and density profile obtained from a large 1-km floe adjacent to M / S "Kvitbjorn" at 1453 on the 18th July 1984 at 78°08'N, 0 ° 1 0 ' E (Fig. 4). The profile shows a thin, warm, low salinity layer ( - 1.0°C, 32.0o/00) above the daily pycnocline at a depth of 18 m. The temperature cools to - 1.6°C above the main thermocline at 30 m depth. There is then a warming step of 1.8 °C beneath which the temperature signal cools again forming three distinct layers. The values calculated for the salinity and density profiles show that the water column has not adjusted over short time scales to produce a stable profiles as the water masses interleave. The maximum density instability is 0. I t~, units. Some of these density instabilities may be due to inaccuracies in the calibration algorithm and sensor mismatch, but the presence of interleaved water masses of different temperature was widespread throughout this sampling period. The tidal currents in the area were strong, opening up l-km-wide leads ad14~(~
1421
14§~
jacent to the instrumented floe. Hence, the interleaved water masses might be the result of tidal excursion. The contoured data of Fig. 5, obtained from four profiles collected in a 50-m period including the above station, show that the region 10 m below the thermocline, where the density interleaving occurs, is also the region of strongest temperature gradients. The horizontal section indicates the presence of upwelling so that the interleaving water masses may be the result of eddy dynamics (Johannessen et al., 1987). The horizontal scale in Fig. 5 can be calculated from the rate of drift of the floes, measured using radar transponders to be 0.35 m/s and equivalent to 1.05 km for the 50-rain section.
Acknowledgements
The instrument was built with support from the Office of Naval Research, Grant no N00013-83-G0008, while SPOF, VAS and SCM were at the Scott Polar Research Institute in Cambridge, United Kingdom. SPOF was supported at the Scott Polar Research Institute by a NERC research studentship. Work on the paper was undertaken whilst SPOF was at the Department of Mathematics, University of Otago supported by a Beverley Research 1421 1438 -31.'b'- j
1507
~1.o.
1453 1507 - -32
10
10
15
20
20
.~1.o /
so
8"
3 3 . ~ . ~
30
a
"~--~-~-~33.5--~ 40
50
60
~ 0 . 0 , ~ ~
0.5
40
a) TemPerature
50
b) $11inlty
J
0.5-
SO
Fig. 5(a) Temperature and (b) salinity sections derived from SPRI CTD measurements collected from the side of an ice floe at 79°09'N, 0 ° 11 'E on 18th July.
A PORTABLECTD SYSTEMFOR USE IN POLARREGIONS Fellowship. She is grateful to the assistance o f the staff at b o t h D e p a r t m e n t s .
References Fofonoff, N.P. and Millard, R.C., Jr., 1983. Algorithms for computation of fundamental properties of seawater. UNESCO Tech. Pap. Mar. Sci., 44. Gregg, M.C. and Hess, W.C., 1985. Dynamic response calibration of Sea-Bird temperature and conductivity probes. J. Atmos. Ocean Technol., 2:304-313. Home, E.P.W. and Toole, J.M., 1980. Sensor response mismatch and lag corrections techniques for temperaturesalinity profilers. J. Phys. Oceanogr., 10:1122-1130. Johannessen, J.A., Johannessen, O.M., Svendson, E., Schuchman, R., Manley, T., Campbell, W.Y., Josberger, E.G., Sandven, S., Gascard, J.C., Olaussen, T., Davidson, K. and Van Leer, J., 1987. Mesoscale eddies in the Fram Strait marginal ice zone during the 1983 and 1984 Marginal Ice Zone Experiments. J. Geophys. Res., 92: 67546772. Josberger, E.G., 1987. Bottom ablation and heat transfer
9 coefficients from the 1983 Marginal Ice Zone Experiments. J. Geophys. Res., 92(C7): 7012-7016. McPhee, M.G., 1982. Sea ice drag laws and simple boundary layer concepts including application to rapid melting. U.S. Army Cold Reg. Res. Eng. Lab., CRREL Rep. 8204. McPhee, M.G., 1983. Turbulent heat and momentum transfer in the oceanic boundary layer under melting pack ice. J. Geophys. Res., 88: 2822-2835. Morrison, J.H., McPhee, M.G. and Maykut, G.A., 1987. Boundary layer, upper ocean and ice observations in the Greenland Sea marginal ice zone. J. Geophys. Res., 92(C7): 6987-7011. Owen, T.R.E., 1984. Conductivity, depth and temperature profiler. Tech. Rep. Carrack Measurement Technol., Cambridge, CTD484. Perovich, D.K., 1983. On the summer decay of a sea ice cover. Ph.D. thesis, Univ. Washington, Washington, D.C. Pingree, R.D., 1971. Regularly spaced instrumental temperature and salinity structures. Deep Sea Res., 18: 841-844. Scarlet, R.I., ! 975. A data processing method for temperature and depth profiles. Deep Sea Res., 22:509-515. Sy, A., 1985. An alternative editing technique for oceanographic data. Deep Sea Res., 32:159 i-1599.
1
A portable CTD system for use in polar environments Siobhan P. O'Farrell a'~, Vernon A. Squire b, Stuart C. M o o r e c and T i m o t h y R.E. Owen d "Scott Polar Research Institute, University of Cambridge, Cambridge, UK bDepartment of Mathematics and Statistics, University of Otago, Otago, New Zealand cScienceApplications International Corporation, Cambridge, UK dDepartment of Earth Sciences, University of Cambridge, Cambridge, UK (Received April 9, 1990; accepted after revision December 5, 1990)
ABSTRACT O'Farreli, S.P., Squire, V.A., Moore, S.C. and Owen, T.R.E., 1991. A portable CTD system for use in polar environments. CoM Reg. Sci. Technol., 20: 1-9. Details of instrument design and configurationai aspects are reported for a high resolution, conductivity-temperaturedepth (CTD) unit, developed to measure the properties of the mixed layer beneath a sea ice cover and the temperature/ salinity gradients in lead areas. The instrument is small, and is sufficiently portable to be used either from an ice floe edge or from a small boat. Some discussion of the results of deployments in the Greenland Sea marginal ice zone are included.
1 Aims of experiment on Marginal Ice Zone Boundary Layer Dynamics The Scott Polar Research Institute (SPRI) CTD unit was developed to measure to high resolution the conductivity, temperature and depth with resolution of order 0.002°C, 0.0002 m S / m , 0.01 m, resp., of the mixed layer beneath drifting sea ice. The boundary layer between the ice and ocean is an important component of the ice-ocean system as it is through this layer that the ice-water stress is transmitted to the deep ocean (McPhee, 1982). Boundary layer depth, and salinity and temperature characteristics are modified by the heat and salt fluxes from the ice (McPhee, 1983). Melt-water stabilizes the water columnDallowing the mixed layer depth to shallow--whilst additional freezing releases brine leading to convection in the mixed layer. During MIZEX-84, the CTD instrument was used at the Greenland Sea summer ice edge to comple~Now at CSIRO Division of Atmospheric Research, Aspendale, Vic. 3195, Australia.
ment boundary layer studies taking place at the interior drifting station which used a vertical array o f conductivity, temperature and velocity sensors (Morrison et al., 1987). Also at this inner station, measurements of heat transfer and other ice properties were collected to provide a mass balance for the summer marginal ice zone (MIZ). In the active outer regions of the MIZ, the ratio of ice loss from lateral/bottom melting increases (Perovich, 1983). Experiments in this region of the Greenland Sea were undertaken during the summer o f 1984 from the M / S "Kvitbjern", which made several sections across the ice margin, and also from M / S "Lance" in Fram Strait in August 1984. Our study focussed on the effect o f meltwater input into the upper ocean at the ice edge, and ice ablation measurements were collected concurrently (Josberger, 1987). The study was designed to encompass both the immediate impact o f fresh water input on the ocean boundary layer, and to follow the development o f the newly formed layer when this buoyant water is mixed to the depth of the thermocline.
0165-232X/91/$03.50 © 1991 Elsevier Science Publishers B.V. All fights reserved.
-'~
2 C T D specifications and design
Instrument requirements To carry out a programme to collect ocean profiles at the extreme edge of the marginal ice zone CTD instrumentation had to meet three main criteria: - it had to be sensitive enough to measure the small temperature and conductivity gradients in the near isothermal layer; - it had to be robust enough to be used in leads in the pack ice, where loose ice could damage the instrument; and - it had to be portable enough to operate away from the ship, either at the edge of ice floes or from small boats, as no helicopters were available. As no portable CTD with these requirements was available commercially at that time, a unit was designed and built at the University of Cambridge (Scott Polar Research Institute and Department of Earth Sciences) by S.C. Moore and T.R.E. Owen.
Instrument configuration The CTD (Figs. 1-2 ) is manufactured from aluminium; the main section being a cylinder, 375 mm long, with interior and exterior diameters of 89 mm and 110 mm, respectively. The head of the instrument is a solid paraboloid, streamlined to minimize any disturbance to the density field measured by the sensors. In the constant diameter part of the head section there are two inlets which provide access to the pressure sensor. Perpendicular to these inlets are brackets for the conductivity and temperature probes. The system is powered by a block of ten 1.5 V Ni-cad, C-cells which are rechargable through the waterproof connector when the instrument is at the surface. The battery block is placed just above the sensor's internal connections in order to balance the instrument. Data are recorded at the surface, allowing the instrument package to be more compact as no internal logging is required. Signals recorded at the surface can also be scanned in real-time by a microcomputer (see Section 4). The sensor signal is passed up the centre of the battery block to the electronics for conditioning and coding as frequency signals. Three electronics
S.P. O'FARRELL ET A t .
boards, one for each sensor, are arranged in a triangle on a circular plate, and are bolted on to the battery block. The whole central section is tightly pressure sealed at the base of the cylinder, with a pressure release knob fitted to the seal joining the end cap to the main cylinder. A ten-way connector positioned at the top of the electronics section links to the interior part of the seawater connector. A three-core, co-axial cable is attached through a hookend connector to the winch, and is moulded onto the outer part of the seawater connector. The central portion of the instrument is protected by an aluminium guard, from a point just above the sensor probes.
The sensors The pressure sensor is located just behind the parabolic head in the base of the instrument. It is positioned centrally beneath the battery package and electronics section. Access to the outside pressure field is made through two 13-mm diameter holes (Fig. 3). The sensor is a commercially-built pressure transducer manufactured by Druck U K Ltd., which provides a voltage output, and, in the SPRI CTD, is separated from the fluid by a stainless steel isolating diaphragm. Since the conductivity and temperature data are encoded as a frequency signal (using commercial electronics), the output of the pressure transducer is convened to frequency to be compatible. When pressure is recast as depth, the instrument measures with a resolution o f 50 mm to an accuracy of 200 mm. The conductivity cell and electronics was purchased from SeaBird Electronics Inc. The cell consists of three platinum-coated electrodes placed 62.5 mm apart in a flow-throngh configuration. The sensor is attached via a bracket to the main casing just behind the pressure inlets, though rotated through 90 ° (Fig. 3). The electrode signal enters the main body of the instrument through an opening in the aluminium casing which is sealed by an O-ring. The cell operates by allowing the outer pair o f electrodes to be held at the same potential, with the resistance between the centre electrode and the outer pair being measured. This resistance is a function of both the fluid's conductivity and the celt's dimensions. The
'I=
¢1)
B
('1
('1
0
8
==
~o
,.11
Pressure leads
\
O
.Ul
-O
Aluml~um casing
Guard \
.~
~.
I'
Prgs
©,
Access for T e m p e r l l u r e .~msor
"
~ ' 1 i ~¸
~//"//////////J//////////.. . ~ ~ ~~~ ;~//A BATTERY PACK
I
-,,.,
4
S.P. O ' F A R R E L L ET AL.
ilj)
Fig. 2. Photograph of complete CTD showing external SEACON connector at the base, and a guard protecting the sensors close to the head. resulting measurement may be interpreted as the integral of the conductivity value for an equivalent length of water column. The rate o f descent of the cell through the water c o l u m n (0.166 m / s ) and the sampling rate (0.5 s) are strffieient to obtain several records for the volume of fluid sample which
Fig. 3. Close-up showing the conductivity cell, the access hole to the pressure sensor, and the steel tubes guarding the thermistors. fills the cell. Cell resistance is converted to a frequency in the range 5-11 kHz at an r.m.s, voltage of 0.7 V. In our field experience in the Arctic the frequency output ranged between 6.5 and 7.5 kHz: The sensor was expected to resolve conductivities of 0.0002 m S / m to an accuracy o f 0.001 m S / m , before calibration. Flushing characteristics o f t h e cell,
A PORTABLE CTD SYSTEM FOR USE IN POLAR REGIONS
namely how rapidly the conductivity sensor responds to the sample flowing through, are important when interpreting salinity spiking caused by strong temperature gradients in the thermocline. However, with manual lowering speeds this problem is minimized (Gregg and Hess, 1985 ). On the opposite side of the aluminium casing from the conductivity sensor is the temperature probe (Fig. 3), consisting of two thermistors connected in series and encased in narrow stainless steel tubes. The thermistors are insulated from the tubes by lacquered resin. These unprotected, nearly microscopic thermistors are advantageous, as they possess a fast response time. It is an essential requirement for the thermistor time-constant to be of the same order of magnitude as the sampling interval. When the pair of thermistors are arranged in series they gave a total thermistor resistance of 4000 ~2at 20°C. This sensor package was expected to have the same degree of accuracy as the conductivity sensors with a resolution of 0.002°C and an accuracy of 0.01 °C.
Winch and slip-ring assembly In the first instance the mini-CTD was only required for shallow ( < 100 m) profiles. Thus, since our budget was limited, a manual winch was purchased. The steel winch was supplied with approximately 100 m of three-core coaxial cable, ending in a hook-end connector for attachment to the CTD unit. The total mass of the winch is 31 kg with a load capacity of 100 kg. A combination of a ratchet stop and an adjustable brake were used to secure the load at a given cable extension during casts. The winch is fitted with instrument grade, three-way slip rings, and the stainless steel armouring on the cable is utilized as the signal ground.
Electronic counter An electronic counter used to analyse the frequency signals from the CTD, is located in the logging box on the surface. The logger is controlled by an RCA CMOS 1800 series processor, with timing provided by a 153.4375 kHz high-frequency clock pulse. The logger has data capacity of 48 kilobytes in the form of hard core random access memory
5
(RAM); enough storage for approximately 68 min of data. Each data point represents the number of clock pulses occurring during 2048 cycles of the input signal (Owen, 1984). The resolution of the electronic counter was increased by using period counting in preference to a direct binary count of the frequency signal.
3 Deployment and field operations The CTD was mounted within an aluminium frame so that it could be suspended over the side of the floe. The winch was positioned at the back end of the frame to counter balance the instrument's weight. The signal cable passes over a pulley to the CTD fish. The complete apparatus was hauled across the floes on an aluminium sledge. The chosen floe was usually of multiyear type, with more than 0.2 m freeboard, and had a diameter between 30 and 200 m. The floe was examined for a safe edge--both on approach and after anchoring--as local wave activity could often erode the ice wall at and below water level, thereby making it unsafe. Snow cover on most of the floe was light, and ice concentration in the area varied from about 3 / l 0 near the ice margin to about 9/10 in the interior, i.e., the limit to which the "Kvitbjorn" would normally enter the ice. Most field deployments involved obtaining a few CTD casts from a single drifting station. On some occasions data were also recorded during the upcast. During short ship stops (1.5-2 h), two stations to depths of 70 m or three stations to 40 m were normally obtained. The time for lowering at each station was about 5 minutes, whilst hauling the instrument to the surface normally took at least twice as long. On several occasions during the ice drift phases undertaken by both the "Kvitbj~rn" and the "Lance", a time series of short casts was maintained over periods of a day. The instrument was also deployed from small boats, enabling variations in the upper boundary layer of leads to be studied. During a section of the "Lance" across the East Greenland pack ice in Fram Strait, the maximum ice concentration was rarely above 6/10, and work in the open water around ice
6
floes was often carried out as an alternative to work from the floes themselves. This was especially true when the floe edges were badly eroded or awash due to the waves. This method of deployment was easier and quicker, since the instrumentation could be set up in the boat between stations. The frame was mounted either in the stern ("Lance") or in the bow ("Kvitbjorn") of the small boat, and the engine was normally off during profiling operations. On some occasions the boat was anchored to a small floe while measurements were obtained. In some of the open water sequences there are regular spikes in the data due to the movement of the boat under the action of incoming swell, its own bow wave, or the bow wave of other 'Zodiac' inflatables deployed simultaneously from the ship.
4 Microcomputers involved in field processing The microcomputers/processors used in the field for processing the CTD data were a CDP 1805 processor on the logging electronics board and an Epson HX-20 microcomputer. The latter served as the computer and on-ice data processing unit. The Epson micro-cassette system is used as an intermediate storage medium for the data. On board ship, data were transferred from micro-cassette to floppy disc on a 'Torch' microcomputer, where all states of processing from hexadecimal input data files to graphical output of the CTD profile could be accomplished. The logging of the CTD data is controlled by the Epson HX-20 computer through the RS232C interface. There is also an automatic logging mode, introduced to cover the possibility of Epson internal battery failure at low temperatures. Instructions are sent to the CDP-180 processor from the Epson HX-20 as single ASCII characters which invoke modules and subroutines in the microprocessor program. The modules which make up this program were written by D. White (Department of Earth Sciences, University of Cambridge); included are facilities for reading, writing, searching and unloading the data buffer at specific locations. The program is implemented on the logger electronics board on an EPROM chip. Each individual profile record starts with a data and time mark, and ends with a
S.P. O'FARRELL E'I Al.
file mark. The data record is interspersed with time marks written at the start of every minute. A pointer is maintained by the monitor which contains the memory location of the next position to which data is to be written (Owen, 1984). The Epson HX-20 is a compact microcomputer with 16 kilobytes of internal storage. Its portability means that it is well suited to the task of controlling the logger; additionally, it is able to provide an initial scan of the data in the field. It contains a microcassette drive, to which data from the logger can be downloaded via the RS232C port. Later transfer to the 'Torch' microcomputer is facilitated by a further program. The heavy power consumption for the transfer operation dictated that it should normally be carried out on-board ship to save battery power. A CPM microcomputer manufactured by 'Torch Computers LTD' is used to convert raw hexadecimal data into frequency values. These data are then processed via calibration algorithms to produce values of conductivity, temperature and depth for each data record. Subsequently, the data are used to derive salinity and density for each profile. Programs are also available on the Torch for plotting data on the screen, for dumping it to an Epson FX-80 printer to produce hard copy output, and for calculating calibration curves and density algorithms.
5 Data processing Data were processed through a number of stages to produce the final profile shown in Fig. 4, Firstly, algorithms were used to convert each of the data channels from frequency to their respective variables. The resulting temperatures and conductivities were used to derive salinity and density values using 'practical salinity' algorithms (Fofonoff and Millard, 1983). Even after the data had been converted by the calibration algorithms, a considerable amount of processing, despiking and filtering is necessary to produce a reasonable final set. Segments of data representing either upward movements or temporary halts of the sensors in the water column have to be removed; frequent winch stops cause spiking and sensor drift. Upward moving sections can often
A PORTABLECTD SYSTEM FOR USE IN POLAR REGIONS
..TULY 18TH S T A T I O N N O 6
0
i
,
,
,
I
. . . .
I
. . . .
. . . .
1
10
r
t.
20
Z
30
40
t/
/
50 x
'x t /,
/)
60
"S " q
70 -2.0 3£0
. . . . I Tem~r~tt're"C '- 1.0 . . . .
2d.0 . . . .
'
'
3 t . 0 . ,Salittifg . . .
° ].o
2t.0 . . . .
'
I
. . . .
t
"
'
'
'
0.0
1.0
2.0
3~.o . . . .
3,'.0 . . . .
3~.0
26t.0. . . .Density .
2t.0 . . . .
2d.0
Fig. 4. Salinity, temperature and density profiles o f a station obtained from a large ( 1 k m ) ice floe adjacent to M / S " K v i t b j o r n " at 1453 on the 18th July 1984, located at 7 8 ° 0 8 ' N , 0 ° 10'E.
be removed directly (Pingree, 1971 ), through care must be taken if large sensor mismatch is present (Scarlet, 1975 ). Correction for the time response of the sensors (Home and Toole, 1980) also preceded any data removal which might be necessary for winch stops. The calibration algorithms were derived from several data pairs obtained from cross-calibration data, with the form of the relationship based on the physics governing sensor operation. We included combinations of points covering the range of field operations for the sensors, and beyond if possible, and found the best polynomial fit by least squares. In the early stages of the data processing a simple running average filter was used, but as more elabo-
rate techniques were employed to correct for mismatch and despiking it was decided that filtering should be done at a later stage. Data spikes can be removed either by assuming that no two consecutive data points are more than a specific distance apart, or by the application of a median filter (Sy, 1985). Examination of the density profile after subsequent processing through the algorithms shows that further small-scale smoothing may be necessary, but it is arguable at which stage of the processing it is best to carry this out when the nonlinear calibration algorithms involved are taken into account. The final data set can also be averaged over depth intervals to produce a better record, thereby remov-
8
S.P.O'FARREI.LE1AI
ing problems due to the unsteady lowering rate resulting from use of the manually powered winch.
6 Results
In Fig. 4 we show the results of a temperature, salinity and density profile obtained from a large 1-km floe adjacent to M / S "Kvitbjorn" at 1453 on the 18th July 1984 at 78°08'N, 0 ° 1 0 ' E (Fig. 4). The profile shows a thin, warm, low salinity layer ( - 1.0°C, 32.0o/00) above the daily pycnocline at a depth of 18 m. The temperature cools to - 1.6°C above the main thermocline at 30 m depth. There is then a warming step of 1.8 °C beneath which the temperature signal cools again forming three distinct layers. The values calculated for the salinity and density profiles show that the water column has not adjusted over short time scales to produce a stable profiles as the water masses interleave. The maximum density instability is 0. I t~, units. Some of these density instabilities may be due to inaccuracies in the calibration algorithm and sensor mismatch, but the presence of interleaved water masses of different temperature was widespread throughout this sampling period. The tidal currents in the area were strong, opening up l-km-wide leads ad14~(~
1421
14§~
jacent to the instrumented floe. Hence, the interleaved water masses might be the result of tidal excursion. The contoured data of Fig. 5, obtained from four profiles collected in a 50-m period including the above station, show that the region 10 m below the thermocline, where the density interleaving occurs, is also the region of strongest temperature gradients. The horizontal section indicates the presence of upwelling so that the interleaving water masses may be the result of eddy dynamics (Johannessen et al., 1987). The horizontal scale in Fig. 5 can be calculated from the rate of drift of the floes, measured using radar transponders to be 0.35 m/s and equivalent to 1.05 km for the 50-rain section.
Acknowledgements
The instrument was built with support from the Office of Naval Research, Grant no N00013-83-G0008, while SPOF, VAS and SCM were at the Scott Polar Research Institute in Cambridge, United Kingdom. SPOF was supported at the Scott Polar Research Institute by a NERC research studentship. Work on the paper was undertaken whilst SPOF was at the Department of Mathematics, University of Otago supported by a Beverley Research 1421 1438 -31.'b'- j
1507
~1.o.
1453 1507 - -32
10
10
15
20
20
.~1.o /
so
8"
3 3 . ~ . ~
30
a
"~--~-~-~33.5--~ 40
50
60
~ 0 . 0 , ~ ~
0.5
40
a) TemPerature
50
b) $11inlty
J
0.5-
SO
Fig. 5(a) Temperature and (b) salinity sections derived from SPRI CTD measurements collected from the side of an ice floe at 79°09'N, 0 ° 11 'E on 18th July.
A PORTABLECTD SYSTEMFOR USE IN POLARREGIONS Fellowship. She is grateful to the assistance o f the staff at b o t h D e p a r t m e n t s .
References Fofonoff, N.P. and Millard, R.C., Jr., 1983. Algorithms for computation of fundamental properties of seawater. UNESCO Tech. Pap. Mar. Sci., 44. Gregg, M.C. and Hess, W.C., 1985. Dynamic response calibration of Sea-Bird temperature and conductivity probes. J. Atmos. Ocean Technol., 2:304-313. Home, E.P.W. and Toole, J.M., 1980. Sensor response mismatch and lag corrections techniques for temperaturesalinity profilers. J. Phys. Oceanogr., 10:1122-1130. Johannessen, J.A., Johannessen, O.M., Svendson, E., Schuchman, R., Manley, T., Campbell, W.Y., Josberger, E.G., Sandven, S., Gascard, J.C., Olaussen, T., Davidson, K. and Van Leer, J., 1987. Mesoscale eddies in the Fram Strait marginal ice zone during the 1983 and 1984 Marginal Ice Zone Experiments. J. Geophys. Res., 92: 67546772. Josberger, E.G., 1987. Bottom ablation and heat transfer
9 coefficients from the 1983 Marginal Ice Zone Experiments. J. Geophys. Res., 92(C7): 7012-7016. McPhee, M.G., 1982. Sea ice drag laws and simple boundary layer concepts including application to rapid melting. U.S. Army Cold Reg. Res. Eng. Lab., CRREL Rep. 8204. McPhee, M.G., 1983. Turbulent heat and momentum transfer in the oceanic boundary layer under melting pack ice. J. Geophys. Res., 88: 2822-2835. Morrison, J.H., McPhee, M.G. and Maykut, G.A., 1987. Boundary layer, upper ocean and ice observations in the Greenland Sea marginal ice zone. J. Geophys. Res., 92(C7): 6987-7011. Owen, T.R.E., 1984. Conductivity, depth and temperature profiler. Tech. Rep. Carrack Measurement Technol., Cambridge, CTD484. Perovich, D.K., 1983. On the summer decay of a sea ice cover. Ph.D. thesis, Univ. Washington, Washington, D.C. Pingree, R.D., 1971. Regularly spaced instrumental temperature and salinity structures. Deep Sea Res., 18: 841-844. Scarlet, R.I., ! 975. A data processing method for temperature and depth profiles. Deep Sea Res., 22:509-515. Sy, A., 1985. An alternative editing technique for oceanographic data. Deep Sea Res., 32:159 i-1599.