AR9-WHP work during POLARSTERN ANT-XIV, Leg 4

Status: 26 November 1998
Compiled by Claudia Schmid and Walter Zenk
IfM Kiel

0. Preliminary Remark

This report summarizes and updates hydrographic work that has been conducted during
POLARSTERN cruise ANT-XIV-4 as part of the World Circulation Experiment (WOCE).
It first has been described in the cruise report by D.K. Futterer and co-workers (1998).
The present summary is designated as accompanying document to the WOCE hydrographic
programme.
It describes CTD data processing and calibration of the CTD sensors.

1. Cruise Narrative

Expedition designation:

	WOCE-Südatlantik 1997
	Kap der Guten Hoffnung Experiment (KAPEX), see also Boebel et al (1998) and
	http://triton.sea.uct.ac.za

Chief Scientist
	Dieter K. Futterer, AWI, Bremerhaven, Germany

Ship
	FS POLARSTERN, Bremerhaven, Germany

Leg
	ANT-XIV/4: Cape Town - Bremerhaven
	21 March - 25 April 1997

Principal Investigator in charge:

Walter Zenk <wzenk@ifm.uni-kiel.de>
Düsternbrooker Weg 20
24105 KIEL, Germany

First hydrographer on board

Olaf Boebel <oboebel@physi.uct.ac.za>
Ocean Climatology Group
Department of Oceanography
University of Cape Town
Rondebosch 7700
South Africa

CTD data processing and validation:

Claudia Schmid <cschmid@ifm.uni-kiel.de>
Düsternbrooker Weg 20
24105 KIEL, Germany

For further details see cruise report by Futterer et al (1998).

2. Measurement Techniques, Calibration and Processing

2.1 CTD/Rosette

Station numbers are not only related to CTD work; thus they are gappy. CTD 
profiles are counted consecutively, with gaps occuring only if profiles have 
been omitted.

The only CTD in use was a Neil Brown MKIIIB instrument (IFMK internal 
identification NB2). This instrument carried a Pt100 Rosemount temperature 
sensor, a (fast) NTC temperature sensor for analogue time constant compensation, 
a strain gauge pressure sensor made by Paine Instruments, a standard NBIS 
4-electrode conductivity sensor, and a polarographic type Beckman oxygen sensor. 
The outputs of both temperature sensors are combined in an analogue circuit to a 
single signal. Pre- and post cruise lab calibration are available for the 
combined temperature signal and for the pressure sensor. The calibration of 
conductivity depends on in-situ samples. No oxygen samples are available to 
calibrate the oxygen sensor.

In-situ samples to measure salinity, were drawn from 10 l Niskin bottles mounted 
on a 24 x 10 l General Oceanics rosette sampler. Bottles were closed on the way 
up. Samples were drawn immediately after the profile. Salinity samples solely 
served for CTD calibration.

No samples were drawn from bottles that failed to close properly or showed other 
problems like apparent leaking. These bottles therefore are not included in the 
bottle file. This also means that all bottles in the file were flagged as 'no problem' (QF2).

2.2 Bottle Salinity

Samples to be analyzed for salinity usually were drawn from:
-	The deepest point of the profile or 20 m above the bottom, for the 1500 m 
	and the bottom stations respectively
-	The Antarctic Intermediate Water level
-	The mixed layer where vertical gradients are small

All samples were filled to German beer bottles 'Flensburger Pils',a cheap and 
social method that has been recommended in pre-WOCE daysby Grasshoff et al. 
(1983) and that keeps samples stable over the typical length of a cruise (4 
weeks) better than 0.001 psu.

Batch No P122 of IAPSO standard seawater was used to standardize the 
salinometer. No double samples were considered. The overall accuray of bottle 
salinities for calibration purposes of the CTD is estimated by the precision of 
the overall calibration (0.005 psu) and the accuracy standard seawater (better 
0.001 psu) to 0.002 psu.

Bottle salinities that differ more than 2.8 and 3.5 times the standard deviation 
in salinity calibration from the calibrated CTD salinity (see below) were 
flagged as suspicious (QF3) and bad (QF4), respectively. The bottles may have 
closed at wrong positions here. However, since no other samples were taken, no 
corrections for wrong bottle depths have been made.

2.3 Bottle Oxygen

No samples drawn, therefore the oxygen values in the data fileshave to be 
regarded as uncalibrated.

2.4 CTD: Data Processing

The CTD used throughout the cruise was a Neil Brown MKIIIB (IFMK identifier 
NB2). It was mounted below a 24 x 10 l bottle rosette made by General Oceanics 
and lowered at almost constant speed (about 1 m/s) from 200 m depth on. Data 
processing is similiar to that described by Millard and Yang (1993). The steps 
were:
- 	Visually inspect each profile, especially to identify 'strange' effects in 
	the pressure record.
-	Create a time relative to the start of the profile for each record to well 
	resolve the record interval 1/32 s.
-	Check that pressure, temperature and conductivity are in reasonable ranges.
-	Remove spikes in pressure, temperature and conductivity values.
- 	Identify the first 'in water' record and associated pressure offset from 
	the first reasonable conductivity measuremnet.
- 	Remove cycles that were taken at a lowering speed less 0.2 m/s.Monotonize 
	with respect to increasing pressure. For a lowering speed of 1 m/s, the 
	number of remaining cycles then corresponds to the resolution of the 
	pressure sensor.
- 	Correct for different response times of the (combined) temperature and 
	conductivity measurements. Visual inspections in large gradients suggested 
	a 60 ms time constant for a recursive filter to slow down the conductivity 
	response.
-	Apply a moving average over 29 cyles (corresponding to 3 dbar)
-	Apply calibrations to pressure, temperature and conductivity (see below).
-	Interpolate Lagrangian to 2 dbar.
-	Recalculate salinity and potential temperature.
- 	Identify records as statically unstable if the vertical gradient of 
	potential density (reference level increasing at500 dbar intervals) over a 
	2 dbar interval is less -0.001 Kg/m^3. Set salinity flag of such cycles to 3.

For a 2 dbar output interval after removing spikes etc, the number of basic 
measurements is 13 on the average. This was transferred as constant to the output files. 

A special problem showed up in two profiles: At constant lowering rate of the 
CTD, one expects smooth sensor outputs as a function of time at large depths, 
say from 1400 m on. However, a problem showed up with the conductivity signal on 
station 543/profile 3 and on station 579/profile 35. When plotted, temperature 
is smoothly decreasing and pressure is linearly increasing as expected but
conductivity jumps at 1750 dbar at station 543. This jump could not be removed, 
and therefore the deeper part of this profile was cut off. At station 579, bad 
conductivity values occurred between 1198 dbar and 1226 dbar. These were 
interpolated using polynomial of 3rd. order and flagged as such.

2.5 CTD: Sensor Calibration

2.5.1 Temperature

Pre- and post- cruise laboratory calibrations are available from July 1992 and 
April 1993, respectively. They were performed over the whole range at 2 K 
intervals between -1°C and 28°C. As a secondary standard served a Rosemount Pt25 
resistance in a bridge made by SIS, Kiel. The Pt25 was calibrated according to 
the ITS90. Prior to the CTD calibrations, bias and linear coefficient of
the Pt25 basic calibration were adjusted to meet the triple point of water (2 
cells independently) and the melting point of Gallium. The adjustments were 
small (less 1 mK). The quadratic term is believed not to change.

A polynomial regression for the CTD's correction to T90 in pre- and post-cruise 
Calibrations (Tables A1 and A2) shows standard deviation of less than 1 mK with 
about 10 degrees of freedom. The drift of the sensor output was small (1.5 mK/a 
at 0°C). High order polynomials are needed to correct for the MKIIIB typical 
nonlinearity close to 0°C (see Mueller et al., 1995). From these results, 
temperature outputs TCTD were corrected for both laboratory calibrations and 
then interpolated in time to the mean cruise date (Tables A1, A2).
Figure 2 shows the corrections applied to the CTD temperatures in the bottle file.

2.5.2 Pressure

Two aspects are important with the calibration of the Paine strain gauge 
pressure sensor: (i) nonlinear and temperature dependent static responses to 
pressure changes (including a hysteresis during up-profiles) and (ii) dynamic 
response to fast temperature changes. Corrections from, both, the static (PRC) 
and the dynamic responses (PDYN) are superposed linearly to the sensor output
PCTD. The procedure has been described in more detail by Mueller et al. (1994, 
1995).

		PRES = PCTD + PRC + PDYN

Static laboratory calibration is performed on a Budenberg dead-weight tester in 
loading mode up to 6000 dbar in 500 dbar intervals with the pressure sensor 
being immersed in a water bath of different temperatures, i.e 13 calibration 
points at fixed temperatures. At the same temperatures, unloading calibrations 
are achieved in 500 dbar intervals starting at maximum pressures of 2000 dbar, 
4000 dbar and 6000 dbar. All calibration points are arranged in a single table. 
For the loading mode, for each temperature polynomial correction coefficients 
are calculated (PRC=POLY(PCTD,TEMP). Typical standard deviations in a 3rd to 5th 
order polynomial regression are less than 1 dbar.

The dynamic response model used is written:

		PDYN = k * (T1l - T2l)

where T1l and T2l are lagged from the CTD temperature sensor at
record time t(j):

	Tl1(j)=TCTD(j) + (Tl1(j-1)-TCTD(j))*exp(-(t(j)-t(j-1))/tau1)
	Tl2(j)=Tl1(j) + (Tl2(j-1)-Tl1(j)) *exp(-(t(j)-t(j-1))/tau2)

The three coefficients tau1, tau2 and k are the two time constants representing 
the temperature response time at the outer (tau1) and the inner (tau2) part of 
the pressure sensor, respectively, and an amplitude that typically amounts to 
0.2 dbar/K. These coefficients are calculated from a laboratory dunck test with 
the pressure sensor being duncked from a warm (20°C) water pool into a cold (0.5 
C) water pool. The sensor is kept there until full response is achieved and 
duncked back to the warm water pool again. With the dynamic correction applied, 
the error in the pressure sensor output can be reduced to less than 30% of its 
amplitude.

To process the pressure record in CTD profiles of M28/2, it was assumed that the 
CTD was in temperature equilibrium before the profile started. Then, for the 
lowering part pressure measurements were corrected with the polynomial 
regressions that are valid for the two temperatures that bracket the in-situ 
temperature with the bias being replaced by the 'in water' offset. The two 
resulting corrections are linearly interpolated with respect to temperature. If 
the in-situ temperature was outside a calibration interval the correction was 
constantly set forth. Finally, the dynamic correction was added.

On the way up, hysteresis plays a role, and simple regressions are not possible. 
Therefore, CTD pressure measurements in the rosette file were corrected by 
linear interpolation within the calibration table with the offset being replaced 
by the 'in water' offset. Dynamic correction started with the assumption that 
the CTD was lowered at a mean speed of 1 m/s to its maximum pressure.

For M28/2, laboratory calibrations are available for static effects from July 
1992 (pre-cruise, Table A3), for static effects at from April 1993 (post- 
cruise, Table A4) and for the dynamic response to temperature changes from July 
1992 (Table A5). They were applied as described above. The accuracy of corrected 
pressure values is estimated to be better than 3 dbar at full range (6000 dbar).
Figure 3 shows the corrections as applied to the CTD pressure sensor records in 
the bottle file.

2.5.3 Conductivity and Salinity

In the bottle file, bottle salinity and calibrated CTD temperature and pressure 
are used to calculate in-situ reference conductivity. Then, the CTD cell's 
output is corrected for a nonlinearity for values CCTD<=32.768 (Mueller et al., 
1995)

	CN = CCTD -0.002 mS/cm.

Next, the cell's output CCTD is compensated to temperature and pressure effects 
(Millard and Yang, 1993).

	CC = CN*(1+ alpha*(TEMP-T0) + beta*(PRES-P0))
	Where	alpha=-6.5e-06, T0=2.8
		beta=1.5e-08, P0=3000

In-situ calibration coefficients are then estimated for the compensated 
conductivity measurements applying a linear least square method for a five 
coefficient correction CRC that includes a drift correction by profile number 
PROF, i.e. time (Tables A6).

	COND = CC+CRC where
	CRC = a1 + (a2+a3*CC)*CC + (a4+a5*PROF )*PROF

It was found that the calibration could be done over the whole data set (Table 
A6, fig. 4).

Let a conservative estimate of the number of degrees of freedom in the 
calibration be either the number of profiles from which samples are used or half 
of all individual samples (2 samples maximum for each profile), whatever is the 
minimum. From the statistics below, the precision in CTD salinity then is 
estimated to 0.001 psu. For stations where bottle salinities were measured,
accuracy is the maximum of CTD salinity precision and bottle salinity accuracy, 
i.e. 0.002 psu.

2.5.4 Oxygen

As no oxygen samples were drawn, the CTD oxygen sensor has not been calibrated. 
The oxygen sensor's current and temperature output are kept as raw data.

References

Boebel, O., C. Duncombe Rae, S. Garzoli, J.R.E. Lutjeharms, P. Richardson, T. 
Rossby, C. Schmid and W. Zenk: Float Experiment studies Interocean Exchanges at 
the Tip of Africa. EOS,
Transactions of the American Geophysical Union, 79(1), p.1, 7-8, 1998

Grasshoff, K, M. Ehrhardt and K. Kremling (editors, 1983): Methods of Seawater 
Analysis. Verlag Chemie, Weinheim.
Millard, R.C. and K. Yang (1993): CTD calibration and processing methods used at 
Woods Hole
Oceanographic Institution. Techn. Rep. WHOI-93-44, 96 pp.

Mueller, T.J., J. Holfort, F. Delahoyde and R. Williams (1994): Improving NBIS 
MK IIIB
Measurements. In: WOCE Operations Manual, Vol. 3, Sect. 3.1, Part 3.13. WHP 
Operations and
Methods (T.M. Joyce, editor), Rev. 1. November 1994. Woods Hole, MA, U.S.A
Mueller, T.J., J. Holfort, F. Delahoyde and R. Williams (1995): MKIIIB-CTD: 
Improving ist system output. Deep-Sea Res. 42, 2113-2126.

WOCE (1991): WOCE Operation Manual, Vol. 3, Sect. 3.1, Part 3.1.3. WHP Operations and
Methods. WHP Office Report WHPO 91-1. Woods Hole, MA, USA, 1991.

Futterer, D.K. And Cruise Participants (1998): The expedition ANTARKTIS - XIV /4 
of RV "POLARSTERN" IN 1997.Berichte zur Polarforschung, 259, 39pp.

Table A1: ANTXIV/4 pre-cruise temperature calibration of MKIIIB CTD, IFMK NB2, 
NOV 1993. TCTD and T90 are the CTD's temperature signal and the reference 
temperature (secondary standard), respectively. Polynomial correction of TCTD 
with coefficients c (values below) gives

TEMP.TDIF=T90-TLAB is the residuum.
TEMP = c(0) + (1 + c(1))*TCTD + c(2)*TCTD^2 + c(3)*TCTD^3 + ...

Temperature calibration in ITS90 with CALTRC.M.

IFMK   NB2   FEB96
    TCTD	   T90		TLAB		TDIF
62181.0000	30.9871		30.9871		-0.0000
62182.0000	30.9876		30.9877		-0.0001
56430.0000	27.9313		27.9312		 0.0001
50753.0000	24.9159		24.9159		 0.0000
41471.0000	19.9888		19.9884		 0.0004
32035.0000	14.9815		14.9822		-0.0007
22628.0000	 9.9942		 9.9942		-0.0000
13210.0000	 5.0032		 5.0031		 0.0001
11294.0000	 3.9881		 3.9880		 0.0001
 9422.0000	 2.9968		 2.9963		 0.0005
 7534.0000	 1.9960		 1.9962		-0.0002
 5651.0000	 0.9990		 0.9989		 0.0001
 4716.0000	 0.5036		 0.5037		-0.0001
 3753.0000	-0.0069		-0.0063		-0.0006
 3761.0000	-0.0021		-0.0021		 0.0000
 2799.0000	-0.5116		-0.5116		-0.0000
 1864.0000	-1.0068		-1.0067		-0.0001
  720.0000	-1.6123		-1.6125		 0.0002
  714.0000	-1.6155		-1.6156		 0.0001

Polynomial degree is M=3
Number of data pairs is N=19

Coefficients, starting at lowest order:

co(0)= -1.993698e+00
co(1)= 5.294941e-04
co(2)= 1.168238e-11
co(3)= 4.656400e-17

Statistics:
Range:	minimum		is	-1.615500e+00
	maximum		is	 3.098760e+01
Number of data points	is		   19
Degree of fit		is		    3
Degree of freedoms	is		   15
Test sigq=rms/(N-M)	is	 1.740346e-05
Mean error		is 	 1.940402e-15
66 perc error, rms	is	 2.784554e-04
95 perc error, 2*rms	is	 5.569108e-04
99 perc error, 3*rms	is	 8.353661e-04
Minimum of error	is	-6.637854e-04
Maximum of error	is	 5.292969e-04

Table A2: ANTXIV/4 post-cruise laboratory pressure sensor calibration of MKIIIB 
CTD, IFMK NB2, NOV 1993. Calibration with the sensor immersed into a bath at two 
temperatures (1°C and 10°C). Unloading modes starting at different maximum 
pressures.

Pressure calibration with CALPRC.M.

IFMK NB2 MAY96

Input data with PCTD at reference pressure and temperatures:

N O T E : If spikes were removed do not use the last table in the output. Repeat 
calculation then with spikes removed from start on:

TEMP	0.2	  0.5	  0.7	  0.3	  10.9	  11.2	  11.3	  11.0	  24.9	  24.8	  24.8	  24.9
PRES
   0.0	   1.5	    2.6	    2.5	   2.9	   2.5	    2.6	   2.6	   3.3	   1.9	    2.7	    2.9	   3.2
 500.0	 500.3	  505.6	  505.1	 505.4	 501.0	  506.1	  505.7	 506.3	 500.4	  505.9	  505.5	 505.8
1000.0	1002.1	 1008.4	 1007.1	1008.2	1002.7	 1008.7	 1007.7	1008.7	1002.1	 1008.4	 1007.3	1008.2
1500.0	1503.9	 1509.2	 1506.9	1509.1	1504.3	 1509.7	 1507.4	1509.7	1503.6	 1509.2	 1506.9	1508.9
2000.0	2005.0	 2009.0	 2004.9	2009.0	2005.3	 2009.3	 2005.3	2009.3	2004.5	 2008.6	 2004.8	2008.5
2500.0	2505.3	 2507.9	-9999.0	2507.8	2505.5	 2508.1	-9999.0	2508.2	2504.4	 2507.2	-9999.0	2507.2
3000.0	3005.0	 3006.4	-9999.0	3006.7	3005.0	 3006.6	-9999.0	3006.8	3003.9	 3005.6	-9999.0	3005.8
3500.0	3504.4	 3504.9	-9999.0	3505.4	3504.5	 3505.1	-9999.0	3505.7	3503.1	 3504.0	-9999.0	3504.3
4000.0	4003.9	 4003.6	-9999.0	4004.6	4003.7	 4003.9	-9999.0	4004.5	4002.4	 4002.5	-9999.0	4003.3
4500.0	4503.5	-9999.0	-9999.0	4504.0	4497.1	-9999.0	-9999.0	4500.9	4501.9	-9999.0	-9999.0	4502.4
5000.0	5003.5	-9999.0	-9999.0	5003.7	5003.4	-9999.0	-9999.0	5003.6	5001.8	-9999.0	-9999.0	5001.8
5500.0	5503.7	-9999.0	-9999.0	5503.9	5503.4	-9999.0	-9999.0	5503.9	5501.6	-9999.0	-9999.0	5502.0
6000.0	6004.3	-9999.0	-9999.0	6004.6	6004.3	-9999.0	-9999.0	6004.4	6002.4	-9999.0	-9999.0	6002.2

Loading curve at temperature T0= 0.5

PCTD	PREF	PPOL	PDIF
   1.5	   0.0	   1.5	-1.5
 500.3	 500.0	 499.5	 0.5
1002.1	1000.0	 999.8	 0.2
1503.9	1500.0	1500.1	-0.1
2005.0	2000.0	2000.2	-0.2
2505.3	2500.0	2500.1	-0.1
3005.0	3000.0	2999.9	 0.1
3504.4	3500.0	3499.9	 0.1
4003.9	4000.0	4000.0	 0.0
4503.5	4500.0	4500.0	-0.0
5003.5	5000.0	5000.1	-0.1
5503.7	5500.0	5499.9	 0.1
6004.3	6000.0	6000.0	-0.0

Coefficients for static correction at temperature T0=0.5°C

PRES(T0)=PCTD(T0)+Pol(PCTD(T0))

Polynomial degree is M=5
Number of data pairs is N=13

Coefficients, starting at lowest order:

co(0)=	 0.000000e+00
co(1)=	 0.000000e+00
co(2)=	-3.963898e-06
co(3)=	 1.985301e-09
co(4)=	-3.434035e-13
co(5)=	 1.988652e-17

Statistics:

Range:	minimum		is	 0.000000e+00
	maximum		is	 6.000000e+03
Number of data points	is		   13
Degree of fit		is		    5
Degree of freedoms	is		    7
Test sigq=rms/(N-M)	is	 5.699552e-02
Mean error		is	-7.872762e-02
66 perc error, rms	is	 4.559641e-01
95 perc error, 2*rms	is	 9.119283e-01
99 perc error, 3*rms	is	 1.367892e+00
Minimum of error	is	-1.499991e+00
Maximum of error	is	 4.644453e-01

CTD pressure output first order corrected with respect
to loading at T0= 0.5

TEMP    0.2     0.5     0.7    0.3   10.9    11.2    11.3   11.0   24.9    24.8    24.8   24.9
  PRES
   0.0    1.5     2.6     2.5    2.9    2.5     2.6     2.6    3.3    1.9     2.7     2.9    3.2
 500.0  499.5   504.8   504.3  504.6  500.2   505.3   504.9  505.5  499.6   505.1   504.7  505.0
1000.0  999.8  1006.1  1004.8 1005.9 1000.4  1006.4  1005.4 1006.4  999.8  1006.1  1005.0 1005.9
1500.0 1500.1  1505.4  1503.1 1505.3 1500.5  1505.9  1503.6 1505.9 1499.8  1505.4  1503.1 1505.1
2000.0 2000.2  2004.2  2000.1 2004.2 2000.5  2004.5  2000.5 2004.5 1999.7  2003.8  2000.0 2003.7
2500.0 2500.1  2502.7 -9999.0 2502.6 2500.3  2502.9 -9999.0 2503.0 2499.2  2502.0 -9999.0 2502.0
3000.0 2999.9  3001.3 -9999.0 3001.7 2999.9  3001.6 -9999.0 3001.8 2998.8  3000.5 -9999.0 3000.7
3500.0 3499.9  3500.4 -9999.0 3500.9 3500.0  3500.6 -9999.0 3501.2 3498.6  3499.5 -9999.0 3499.8
4000.0 4000.0  3999.7 -9999.0 4000.7 3999.8  4000.0 -9999.0 4000.6 3998.5  3998.6 -9999.0 3999.4
4500.0 4500.0 -9999.0 -9999.0 4500.5 4493.6 -9999.0 -9999.0 4497.4 4498.4 -9999.0 -9999.0 4498.9
5000.0 5000.1 -9999.0 -9999.0 5000.3 5000.0 -9999.0 -9999.0 5000.2 4998.4 -9999.0 -9999.0 4998.4
5500.0 5499.9 -9999.0 -9999.0 5500.1 5499.6 -9999.0 -9999.0 5500.1 5497.8 -9999.0 -9999.0 5498.2
6000.0 6000.0 -9999.0 -9999.0 6000.3 6000.0 -9999.0 -9999.0 6000.1 5998.1 -9999.0 -9999.0 5997.9

Table A3: ANTXIV/4, MKIIIB CTD, IFMK NB2, APR 1993, pressure senor's dynamic response
to temperature changes. Coefficients are outerand inner sensor time constants tau1 and tau2 and the
amplitude k (Mueller et al., 1995; see text).

Coefficients for dynamic pressure correction

tau1/s   tau2/s     ishift/s  k/(dbar/K)
52.0518  1530.1525  345.7106  0.1193

Table A6: ANTXIV/4, MKIIIB CTD, IFMK NB2: Calibration of conductivity cell.

Model CRC=a1 + (a2+a3*C)*C + (a4+a5*PROF )*PROF

Vector of coefficients:

    1	-0.0015
    2	3.4870e-04
    3	0
    4	1.0001e-04
    5	-1.2447e-06

Final statistics of residuals:

Number of cycles  N=60

         Cond.    Salinity
         mS/cm    psu
Min     -0.0026  -0.0031
Max      0.0028   0.0032
Mean     0.0001   0.0000
Median  -0.0000  -0.0000
Std.     0.0014   0.0016
