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CTD data
Deep-Sew
PII: SO9674637(96)00031-3
Comparison of XCTD/CTD
Researcl~ I. Vol. 43. No. 6. pp. 859%876. 1996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0967-0637/96 $I 5.00 + 0.00
data
CORINNE ALBgROLA,* CLAUDE MILLOT,* UWE SEND,? CHRISTIAN MERTENSt and JEAN-LUC FUDA* (Received 30 December 1993; in revisedform 9 March 1995; accepted 21 January 1996)
Abstract-XCTD (expendable Conductivity Temperature Depth) probes, developed recently by SIPPICAN Inc., have been used simultaneously with a CTD sonde in order to test, in the field, their performance and accuracy (interpreted as k 2 standard deviations of the XCTD-CTD differences). We have taken advantage, during the THETIS-I experiment in March 1992, of both the homogeneous and the stratified areas encountered in winter in the northern part of the western Mediterranean Sea to differentiate the errors due to the experimental conditions from those effectively due to the sensors. Although some intrinsic problems are evident, so that only seven out of the nine probes considered for comparison are usable, the accuracy specified by the manufacturer for the temperature (AT= +O.O3”C) is reached after standard processing, while the accuracies in conductivity, salinity and potential density are AC- fO.O6mS/cm (the specified value is AC = k 0.03 mS/cm), AS - f 0.04 and Aa”- +0.03 kg/m3. However, when the experimental errors (in situ natural variability, relatively rough estimation of the XCTD depth) are considered, it appears that the effective accuracies of the XCTD sensors are better than f 0.02”C and k 0.04 mS/cm, that is to say better than and close to the specified values of + 0.03”C and + 0.03 mS/cm. Occasional offsets in conductivity can further be well corrected for by using a temperature-salinity relation in some limited depth range and area where this relation is known to hold well; the conductivity-sensor accuracy then significantly improves to AC- +O.O2mS/cm resulting, for our study area, in corresponding salinity and potential density accuracies of AS - +0.03 and Aae- k 0.02 kg/m’. Thus, such instruments promise to be useful tools for many experimental studies. Complementary comparisons, performed with new versions of the XCTD probes under less convenient experimental conditions, are also presented. Copyright 0 1996 Elsevier Science Ltd
1. INTRODUCTION The XCTD consists of a weighted package housing batteries, electronics, a conductivity cell and a thermistor. It is operated in the same way as the well-known XBT, connected to a deck unit by a two-conductor fine wire while descending through the water column down to - 1000 m at a roughly constant fall rate of - 3 m/s. The depth (D) is specified to be accurate to k 5 m or + 2% of depth, whichever is greater. The conductivity (c) cell is a high purity, alumina ceramic tube wired to form a four-electrode conductivity sensor, and is specified to provide data accurate to f 0.03 mS/cm. The temperature sensor (7’) is a glass encapsulated, fast-response thermistor, specified to provide data accurate to _tO.O3”C. The electronics package converts the measured resistances of the conductivity cell and thermistor into frequencies that are sent up the wire to the deck unit. The probe uses two precision calibration resistors which are sampled periodically to compensate for any variations in the
* Centre d’OcCanologie de Marseille, Antenne COM, CNRS. BP 330-83507, La Seyne/mer, France. t Institut fiir Meereskunde, Diisternbrooker Weg 20,230O Kiel, Germany. 859
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electronics during deployment. The data are transmitted at a sampling rate of 4 Hz, which gives a vertical resolution of - 80 cm. The XCTDs were tested during a cruise of the European THETIS-I experiment, within and around the deep-water formation area of the northern part of the western Mediterranean (THETIS Group, in prep.). We launched a total of 36 probes with three complementary strategies: 1. 11 XCTD casts with CTD casts, for comparison of the two instruments. 2. 14 XCTD casts between CTD ones, to increase the CTD grid resolution. 3. 11 XCTD casts in the coastal area, to test the possibility of computing the geostrophic transport of the Northern Current. For this comparison study, the XCTDs were generally launched while the ship was on station, less than 10 min before starting the CTD casts in order to minimize the variations of the environment. The probes were operated with a hand-held launcher, from the stern of the ship, taking care that the wire could not rub against the hull. The CTD was a Neil Brown Mark III, calibrated to have accuracies of + 2-3 x 1Op3 in “C and mS/cm and & 2-3 in dbar. The northern part of the western Mediterranean Sea offers very suitable conditions to perform such comparisons. In winter, the offshore area is characterized by the occurrence of phenomena leading to the formation of relatively well-homogenized and dense water (MEDOC Group, 1970; Leaman and Schott, 1991), while the coastal area under the influence of the Northern Current is still stratified (Millot, 1991). The former area allows checking of the accuracy of the T and C sensors of the XCTD, while the latter area is convenient to check the D accuracy. The manufacturer, SIPPICAN, took into account the results of these prime comparisons to modify the XCTD probes. It provided us with two versions of the new probes in order to test them again by XCTDjCTD comparisons. These XCTDs were tested in the western Mediterranean during the THETIS-II experiment, but under less convenient conditions, so that the time interval (-0.5-l h) between XCTD and CTD profiles is usually larger than the lo-min interval of the THETIS-I comparisons. Moreover, most comparisons were performed inside the Algerian Basin, which is known to be a relatively heterogeneous area. Nevertheless, these comparisons provide complementary information about the XCTD accuracies. The three following sections concern the THETIS-I comparisons. We will first describe the various problems we encountered and the data processing we performed (Section 2). Then we statistically compare the XCTD and CTD profiles of in situ temperature (T), conductivity (c), salinity (S) and potential density (a~), and discuss the various errors; we show that the experimental errors are reduced and that the sensor accuracies are close to those specified (Section 3). We demonstrate that, with complementary information provided by other techniques, it is possible to correct conductivity offsets (Section 4). The complementary comparisons performed during THETIS-II are presented and the influence of the water viscosity on the depth accuracy is investigated (Section 5). The performance of the XCTD probes and possible solutions to the observed problems are discussed in Section 6. 2. DESCRIPTION
OF THETIS-I
DATA
Some problems occurred with the XCTD profiles that cannot be corrected or removed. Of the 14 probes launched to complete the CTD grid resolution, one probe did not transmit any
Comparison
of XCTDjCTD
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signal while, one probe transmitted correctly only down to - 100 m. Of the 11 probes used in the coastal area, one transmitted intermittently during the full cast, one transmitted only down to -8OOm and one did not transmit any signal. Of the 11 probes launched for comparison, two were not included in the computations due to a large time interval (a few hours) between XCTD and CTD casts. Among the nine remaining comparisons, one profile appears strongly shifted for both Tand C, and one probe transmitted correctly only down to - 120 m, so that seven probes are usable for the statistical comparisons (locations shown in Fig. 1). The last failure (- 120-m transmission) concerns the very first probe that was launched from an inappropriate location on the ship, thus probably making the wire rub against the ship’s hull. If one considers the above mentioned - 800-m profile to be valid, the total data return is thus greater than 80%. In addition, there is apparently a pressure effect on the sensors that gives an abrupt step in T and C near - 500 m in some profiles and erroneous T and C values beyond - 900 m in all profiles. This is less than the manufacturer’s specified depth range of 1000 m. For all the profiles, other problems have been encountered that can be corrected by data processing. One concerns isolated aberrant values easily removed as outliers. Another is the occurrence of high-frequency (HF) noise, with spikes of a few tenths of “C and mS/cm, which cannot be correctly removed without filtering out features having a relatively fine vertical structure. The data were processed as follows: first, removal of unrealistic values (T> 14.5, T-c 11.0, C> 47.0, Cc42.0) then rejection of outliers (> 1 standard deviation
3” E
5”
6’
#
43”
N
42’
L
Fig. 1. Locations of the seven comparison stations off the Gulf of Lions (northern part of the western Mediterranean Sea). The dashed line schematizes the outer edge of the Northern Current and separates, during the winter dense-water formation, an offshore homogeneous area from a coastal stratified one.
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from a 9-m running mean). After that, a running median filter of length 30 m was applied. Figure 2 presents two representative sets of the resulting profiles considered to be of good quality. Figure 2a shows a set of profiles obtained in the homogeneous area: the XCTD displays a low-frequency (LF) signal (coming from both T and C) which is not observed in the CTD profile. Although the differences in T and C are within the specified ranges of accuracy, these LF fluctuations are misleading, and one must not be tempted to interpret them as an actual signal. Figure 2b shows a set of profiles obtained in the stratified area, at a place where the bottom depth was 610m; the XCTD- and CTD-T, S and rso profiles compare rather well. There is a mean offset in depth of - 10 m, which is within the specified range of accuracy which is + 20 m at 1000 m; a best-fit analysis performed on all the profiles that are not markedly homogeneous gives XCTD-CTD differences smaller than 15 m.
3. STATISTICAL
COMPARISON
OF THETIS-I
DATA
As the XCTDs are designed to be used alone, the XCTD-CTD differences for each of the seven vertical profiles in the four variables T, C, S and CTO (Fig. 3ad) have been computed without any best-fit correction in depth. These differences, denoted by 6T, 6C, 6s and 6aH contain experimental errors, either due to in situ variability in the water (between the time and place of the XCTD and CTD profiles) or to the XCTD-depth inaccuracy, and errors of the XCTD sensors (with respect to the CTD sensors, which are considered to be much more accurate). - From these local differences, we compute (Table 1) mean differences for each profile (6X, X being the variable T, C, S or r~()), as well as a mean difference (-6) and a standard deviation (6) from all profiles together. Also indicated in Fig. 3 is the expected accuracy (vertical lines). For T and C, these are the values of 20.03 in the respective units specified by the manufacturer. We interpret these accuracies (denoted by A) as + 2 standard deviations (+ 2a-interval containing 95% of the where the prime denotes the fluctuating measurements) i e. f2(A T’*)]j2 and f2(AC’2)“2, part after subtracting the mean.* To obtain the expected accuracies for S and co from those quoted in T (assuming similar accuracies for T and 0) and C, we use locally linearized relations S=aT+bC+ c which, for our data ranges in T, C and D, are an=dO+eS and S= - 1.0274 T+0.9730C+8.4619 and rso= -0.2216B;tjzO.813OS. To be as explicit as (X for T, C, S or go, as above) possible, we will specify either A = k 20 or A= z!z~AX’~ when dealing with accuracies either directly computed from the data or expected from the specific relations. Assuming errors are due only to the XCTD sensors without any correlation between the sensors, the accuracies for S and (~0are: errors in T and C, as expected for independent &2(as’2)“2
= f2(a2a
+ b2AC2)i’*
= f0.04
(1)
and &2(Aah2) = f2(d2AT’2 Although
the accuracy
for S is kO.04,
+ e2AS’2)‘/2
hereafter
z
410.03
we will invoke
the simplification
(2) that
*To infer the typical total error, the mean should not be removed, but the average offset over all data/probes close to zero (see Table]), so that we prefer to use the simpler statistical quantity involving only the fluctuations.
is
Comparison of XCTDjCTD
-1 oooL 12.8
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data
1
13.2 13 Temperature
-200.
-1000’ 38.2 38.1
-1000’ 28.8
Fig. 2.
38.3 38.4 Salinity
38.5
38.6
29.1 2 29 Sigma-Theta Temperature (“C), salinity and potential density (kg/m’) versus depth (m) for the XCTD (full line) and the CTD (dashed line) at stations 3 (a) and 5 (b). 28.9
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C. Albirola 0 I ,
-200.
-__
z
-400. i.-
5
B
0
----_
-600 t -800 I
1000’
1 .4
13.2 13 Temperature
12.8
-600
t -800.
-1000’
-___ -II: 5 -.\ 38.1
38.2
38.3
38.4
38.5
38.6
Salinity
0
-
L
-400
z. _c ,a $ -600
1 \
-800.
-1000~ 28.8
28.9
29 Sigma-Theta Fig. 2(b).
29.1
219.2
Comparison
of XCTDjCTD
Temperature
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data
Conductivity
b -low . . .'.I* -0.2 -0.15 -0.1 -0.05 0
0.05
0.1
0.15
0.2
Salinity 0
-200
E
-400
5 3 o-600
-800
-1OOOL -0.2 -0.15 -0.1 -0.05
d
I. 0
0.05
0.1
0.15
0.2
Fig. 3. Differences of the (a) temperature (“C), (b) conductivity (mS/cm), (c) salinity, (d) potential density (kg/m3), (e) corrected conductivity (mS/cm), (f) corrected salinity and (g) corrected potential density (kg/m3) versus depth (m) for the seven pairs; the vertical lines define the interval of accuracy I.
I= +0.03 defines a common expected interval of accuracy for all the parameters in the corresponding units. The T profiles (Fig. 3a) are well within the expected accuracy interval, but this is not always the case for C (Fig. 3b). Note that large differences near 300-400 m are acceptable, due to the rough values of the XCTD depth and the stratified layers found there in some profiles (see Fig. 2b). However, steps near 500m are not linked to the stratification and might be indicative of a T and C dependence upon pressure. These large differences carry through correspondingly to the S (Fig. 3c) and ~0 (Fig. 3d) profiles. The largest offsets in C and S come from profile no. 1, which would probably lead one to discard this profile as unrealistic in routine applications. Since this outlier heavily biases all statistics, it has not been included in our statistical computations. However, it will be included in the plots, in order to demonstrate that the profile is usable after applying the conductivity correction of Section 4. ---We have computed the mean differences for each pair and parameter (6T,SC, X2,6~0,
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Conductivity
Density
_-----J -0.2 -0.15 -0.1 -0.05
0
0.05
0.1
0.15
0.2
Fig. W-k).
Table 1). The absolute value of the mean difference in C is 0.08 mS/cm for pair no. 1; it ranges between 0.02 and 0.03 mS/cm for three out of seven probes, while it is negligible for the three others. The mean differences + the- standard deviations from all profiles together (except pair 1) are m= 0.000 + O.O17”C, AC = 0.003 f 0.030 mS/cm, as = 0.003 f 0.018 and Aq = 0.003 &-0.013 kg/m3 (Table 1). The relative number of values outside I (Table 2) rangesfrom1.5to9.3%forT,6.7to56.3%forC,0.3to13.3%forS,and0.0to4.6%fora~. Figure 4, where I is indicated again by vertical lines, plots the results in histogram form. The T histogram (Fig. 4a) is rather sharp and well centered, indicating good performances in (Table l), most of the values are within the temperature. As AT = k 20 - f0.034”C expected interval of accuracy (+O.O3”C). The C histogram (Fig. 4b), by contrast, is markedly shifted and broadened. Mainly due to profiles 4 and 6, the scatter is relatively large, implying an accuracy (AC = +2a - +0.06 mS/cm) poorer than the expected one (kO.03 mS/cm). Using the relation S=aT+ bC+c, a formula like (1) would predict AS = + 2a - k 0.068; however, the accuracy AS = &-20 - + 0.036 computed from the data is better, and the S histogram (Fig. 4c) looks relatively sharp. Similarly, using the relation o. = &I + es, a formula like (2) leads to a predicted accuracy of AoO = + 20 - + 0.030 kg/
Comparison
of XCTD/CTD
867
data
mean d&%erences in T, C, S andaefor each profile (E, X being the variable T, C, Table 1. Values of the XCTD-CTD S or cry) as well as mean differences (b) andstandard deviations (o) for the whole THETIS-I data set except pair No. 1; see definitions in the text and the number of points for each pair in Table 2
1 2 3 4 5 6 7 n c7
Table 2.
0.016 -0.012 0.007 -0.013 0.001 0.007 0.008 -0.000 0.017
Percentages
0.081 0.000 0.009 -0.032 0.016 0.029 0.001 0.003 0.030
0.027 -0.020 0.012 - 0.026 0.010 0.007 0.018 -0.001 0.028
0.062 0.012 0.002 -0.018 0.015 0.021 -0.008 0.003 0.018
0.010 -0.007 0.004 -0.012 0.008 -0.000 0.008 - 0.000 0.014
0.044 0.012 - 0.000 -0.011 0.011 0.015 - 0.008 0.003 0.013
0.004 - 0.003 0.002 -0.007 0.006 - 0.002 0.005 - 0.000 0.009
-0.052 -0.019 0.003 0.006 -0.007 - 0.022 0.016
of values out of the common interval of accuracy I= kO.03 for T, C, S and ae in the corresponding units. N is the total number of points for each pair
Pair 1 2 3 4 5 6 7 n
17.2 8.0 8.1 4.8 9.3 1.5 6.6 6.2
99.0 6.7 7.8 56.1 17.0 56.3 10.6 26.3
34.0 25.0 8.6 23.3 16.4 2.3 25.7 16.9
97.3 0.3 0.7 11.3 13.3 12.7 1.0 6.1
25.9 0.0 0.7 5.5 8.6 0.0 1.3 2.2
95.5 0.3 0.6 4.6 3.8 0.0 0.9 1.6
3.5 0.0 0.6 3.0 2.5 0.0 1.0 1.1
885 871 875 861 525 875 861 4868
m3, which is close to the value of Aao= +20 - f0.026 kg/m3 computed from the data; even if not well centered, which accounts for offsets discussed later (Section 4), the rr8 histogram (Fig. 4d) is also relatively sharp. Therefore, the values of the accuracies directly computed from the data (Table 1) are better than the predicted ones, and the reasonably good shapes of the S and ~0 histograms might account for some compensation of the errors in T and C. In fact, assuming correlation between the errors in T and C yields: AS2 = a2ATf2 + b2AC2 + 2abAT’AC’.
(3)
The covariance term can be computed from the data, yielding a relatively large value of 4.0 x 10-4”C.mS/cm. As “a” is negative and “b” positive, it is clear that this will markedly improve the predicted accuracy: now, AS = f2AS’2”2 - + 0.038, which is close to the value directly computed from the data (AS = f 20 - kO.036, Table 1). As expected from the relatively large value of the T-C covariance term, the $ue of the T-S covariance term is relatively small, so that the accuracy (Aa@= f 2Ad,2 -0.030 kg/m3) predicted by (2) cannot be significantly improved by a relation like (3). Therefore, these features support the hypothesis of a compensation of the errors in T and C. This point is very instructive when one tries to separate the errors due to the experimental
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Temperature
Conductivity a
b
1
2500
2000 -r 1500.
1000'
500'
Density
Salinity 3000
d 2500
2000
1500 r
Fig. 4. Histograms of the differences of the (a) temperature (“C), (b) conductivity (mS/cm), (c) salinity, (d) potential density (kg/m3), (e) corrected conductivity (mS/cm), (f) corrected salinity and (g) corrected potential density (kg/m3) of the seven pairs; the vertical lines define the interval of accuracy I.
conditions from those effectively due to the T and C sensors. As a matter of fact, independent sensors should give uncorrelated data and thus a relatively low correlation between the T and C differences. On the contrary, if the XCTD and the CTD have sampled different waters with different temperatures (leading to experimental errors either due to the in situ variability or to the XCTD depth inaccuracy), there will be an apparent T difference and a correlated C difference, due to the strong dependence of C on T. This analysis is supported by the plot (not shown) of the correlated part AT’AC’ versus depth, which proves that the contributions to the covariance term (AT/AC’) come mainly from regions of large vertical gradients and from differences that are likely to be due to experimental errors. However, the obviously erroneous features that are visible mainly in some of the C, Sand 60 profiles (like the steps near 500 m, Fig. 3) make no significant contribution to the covariance
869
Comparison of XCTDjCTD data Salinity
Conductivity
-
3000
2500
i-1500 I
Density 3000
-0.2 -0.15 -0.1 -0.05
0
0.05
0.1
0.15
0.2
Fig. 4(eHg).
term and are clearly due to errors of the C sensor. high correlation between T and C and a relatively the observed XCTD-CTD differences in S due different waters probably compensate, so that the actually be due to sensor errors, which are better 3.1. Estimation
of sensor versus experimental
Therefore, since we do observe a relatively low correlation between T and S, most of to the XCTD and CTD having sampled differences computed from the data might estimated as described hereafter.
errors
Let us consider the whole data set from all profiles together and divide the observed XCTD-CTD differences in T and C, AT’ and AC’, into differences arising from the XCTD sensor errors, AT,’ and AC:, and those arising from experimental errors (actually having sampled different waters or having an inaccurate XCTD depth), AT: and AC:, thus leading toAT’=AT,‘+AT;,AC=AC;+AC;. Assuming that sensor errors for T and C are independent, as well as sensor and
870
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experimental errors in general, only the term AT;AC; contributes to the total covariance in (3). Thus, AT’AC’ = ATLAC; for which a value (4.0 x 10P4”C mS/cm) has been obtained above. Now we make use of the definition of a correlation coefficient: A T;AC;
(4)
Since the covariance in the numerator cm (Table l), we can rewrite (4): AT;AC; ATL2”= > _
is known
and since ACL2’j2 < AC’2’f2 = 0.030 mS/
AT;AC; > _
A&’
_ - 0.0133”c.
AC’2”=
Considering AT’ = AT,I + AT:, with AT,’ and AT: independent of each other and a’12 - 0.017”C (Table 1) results in -A T,‘21’2 = (ATI - AT;2)li2 < o.0172 - 0.01332)“2 = O.OlO”C. This is the maximum possible error for $e temperature sensor. The same calculation
for conductivity
yields a maximum
ACh2
= (0.0302 - 0.0232)1’2. Thus,
the
maximum values for the accuracies of the XCTD sensors are AT = + 2A T;2”2 = + 0.02O”C and AC = f2ACi2 = + 0.038 mS/cm. It is first important to note that these effective accuracies are, respectively, better than and close to the accuracies ( ) 0.03 in “C and mS/cm) specified by the manufacturer. Second, comparing 0.010 with 0.017”C and 0.019 with (at most) of the errors in T and C O.O30mS/cm allows us to estimate that only -60% computed from the XCTD-CTD differences can be due to the T and C sensors, the remainder being due to experimental conditions. T and C sensor errors can now be used to calculate the ;F;ulting sensio; These maximy,t error in S (ASi
), using (3) with only the sensor-error
components,
ATI
, and ACk2
-42
=0.021, which zero covariance term (or using (l)!). We obtain a maximum value of ASa is very close to the standard deviation of the salinity differences computed directly from the data (0.018 in Table 1) and corresponds to a sensor accuracy of AS = f 2AS’[2”2 - t 0.04. Therefore, it can definitely be considered that most of the errors in S come from the errors due to the T and C sensors, and that the experimental errors in T and C compensate, due to their high correlation in (3), which makes the salinity look better than the conductivity. Similarly, we can compute for the potential density a maximum sensor error AUg2 =0.017 kg/m3 from the maximum sensor errors estimated for T and S. However, since the covariance between T and S is relatively low, their standa;; deviations (Table 1)
can be used as well as to calculate, as already done with (2) AaL2 = 0.015 kg/m3. Both values are close to the value of 0.01:: kg/m3 computed from the data (Table 1) and give a - +0.03 kg/m3. The ~0 errors are hence essentially sensor accuracy of Aao= f 2A# sensor errors, due to the compensation of the experimental errors in T and C in computing S. The major remaining problem concerns the offsets in C quantified by AC = 0.003 mS/cm (Table l), which obviously induces those observed in S and CJ~.
Comparison
4. CORRECTION
of XCTD/CTD
871
data
OF CONDUCTIVITY
OFFSETS
Correcting the C offsets described above (plus the resulting S and a~ offsets) is possible if some significant T-S (or T-C) relation is found to apply over some limited depth interval. The 8-S diagram shown in Fig. 5, from 350 m to depths of more than 2000 m, results from 84 CTD profiles collected in the homogeneous area. Between 350 and -800 m (corresponding to 00 lower than - 29.1), this diagram evidences a significant relation (S=O.2328+ 35.476) that roughly supports the occurrence of a similar T-S relation (not shown because less common), as %can be considered linear in T over these depths. The S offset for each profile is defined by the mean difference, over that depth range, between the S values computed from the T of the XCTD and the T-S relation and the directly measured S values, i.e. computed from both the Tand C of the XCTD. This S offset allows correction of the conductivity and thus computation of differences in C, S and ge for each corrected profile, as well as mean XCTD-CTD differences and standard deviations for the whole data set (Table 1). Figure 3e-g shows the XCTD-CTD differences for C, S and ~0 versus depth after the correction. It can be seen that these differences are now better within the common limit I= kO.03. The correspondingly corrected histograms (Fig. 4e-g) are now centered near zero. The accuracies improve from AC = f2a - f0.060 to kO.056 mS/cm, from AS= k2c~ - kO.036 to f0.028 and from AaO= f2a - 50.026 to +0.018 kg/m3 (Table 1). The relative number of values outside Inow ranges from 2.3 to 25.7% for C, 0.0 to 8.6% for S and 0.0 to 3.0% for bg (Table 2). The efficiency of the C offset correction, using only a T-S relation known to hold over
SALINITY Fig. 5.
O-S diagram
resulting from 84 CTD profiles taken in the study area, using data between 350 m and the bottom (more than y 2000 m).
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SALINI’I I
250
f
Uncorrected
0
250
l-
SALINITY
750 1
II
1000
b
Corrected Fig. 6. Differences, for all the XCTDs, between the Sprofile computed from the T-S relation using the T profile of the XCTD and the S profile provided by the XCTD without (a) and with (b) corrections of the C offsets.
some limited depth interval, is clearly evidenced by Fig. 6. Instead of plotting the uncorrected and corrected salinity profiles versus depth, we have plotted the differences between these profiles and the salinity profiles computed from the T-S relation extended over the whole depth. This computed salinity profile has, of course, no significance out of the specified depth interval; it is used only as a reference profile. It is clear that the correction is also efficient near the surface and that the scatter of all the XCTDs collected during the whole experiment is acceptable everywhere. Using the new AT’AC’ covariance of 4.3 x 10P4”C mS/cm computed with the corrected conductivity, we can estimate corrected sensor errs;; as done in the previous section. The result is a maximum -l/2
cm),
yielding
ASL2
corrected =0.014
sensor error of A Cl2 (instead
of 0.021)
=0.012 and sl”
mS/cm (instead =0.012
kg/m3
of 0.019 mS/ (instead
of
873
Comparison of XCTDjCTD data
0.017 kg/m3). When these values are compared with those computed directly from the corrected data (0.028 mS/cm, 0.014 and 0.009 kg/m3), it can be confirmed that the sensor errors represent nearly the total errors for S and CJ~.Therefore, after correction for the conductivity offsets, the accuracy of the conductivity sensor significantly improves to AC = +_2AC;2”2 - f 0.02 mS/cm, leading to sensor acc;;acies for salinity and potential - kO.02 kg/m3. density of AS = f 2AS;2”2 - kO.03 and Aao= f2 A#
5. STATISTICAL
COMPARISONS
OF THETIS-II
DATA
The differences dT, 6C, 6S and 6ao (Table 3) were computed for the THETIS-II data, collected with the two versions (called 1 and 2) of the new probes, as was done for THETIS-I. The comparisons were made with four XCTDs for version 1 and six XCTDs for version 2. As an obvious hardware improvement, the technical problems encountered during THETIS-I, such as poor quality of transmission or noisy profiles, did not occur at all. Thus the statistical comparisons are computed directly from the original data. The mean differences + the standard deviations from all O-1000-m profiles together are then AT=0.007~0.075”C, AC= -0.016fO.O94mS/cm, AS= -0.022f0.031, Aae = -0.019&0.025 kg/m3 for version 1 (Table 3a), and AT = -0.005+ O.O59”C, AC = 0.002 + 0.086 mS/cm, AS = 0.007 k 0.064, Aoe = 0.007 kO.052 kg/m3 for version 2 (Table 3b). In contrast to the THETIS-I results, the mean is not removed, since relatively large offsets are evidenced. Moreover, note that the standard deviations during THETIS-II are much larger than those during THETIS-I, because the less convenient experimental conditions increased errors due to natural variability, so that these standard deviations are not meaningful. Because the variability was especially large in the upper layers, the differences for each parameter are also computed for just the interval 40&1000 m (Table 4). The mean differences f the standard deviations from all 40@-1000-m profiles together are AT = - 0.003 + O.O23”C, AC = - 0.026 k 0.030 mS/cm, as= -0.023f0.011, then AQ= -0.017f0.008 kg/m3 for version 1 (Table 4a), and AT= -0.015+0.022”C, AC= -0.007)0.033 mS/cm, AS=0.009+0.015, Aa~=0.010+0.011 kg/m3 for version 2 (Table 4b). The mean differences are expected to be more representative of the actual offsets than those computed from the O-1000-m profiles. It appears that there are significant offsets in C for version 1 probes and in T for version 2 probes; as the version 2 probes were tested after pairs 8,9 and 10 and before pair 11, these offsets are expected to be characteristic of the version. Computing the typical accuracy of each parameter as f 20, for both versions, gives AT - f O.O45”C,AC - k 0.063 mS/cm, AS - + 0.026 and Aae - + 0.019 kg/m3 , which are values - 3 times less than those computed from the same profiles between 0 and 1000 m. These accuracies are of the order of those computed for the THETIS-I 0-1000-m profiles, before the correction of conductivity offsets discussed in Section 4. Such a correction cannot be applied here since no T-S relation can be established due to the geographical scatter of these comparisons. Unfortunately, mainly due to these less convenient experimental conditions, we cannot draw any conclusion concerning the improvement of the XCTD accuracies. This is especially true for version 1, since only four probes of that version were tested. Further, since these comparisons were performed mainly in relatively stratified regions of the Mediterranean, the influence of viscosity on the depth accuracy was investigated as done by Seaver and Kuleshov (1982). Indeed, the fall rate of the probe depends on the viscosity,
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Table 3. Values of the XCTD-CTD mean differences in T, C. S and ae for each (rlOOO-m profile and the mean dtperences and standard deviations for the whole THETIS-II &1000-m data sets obtained with version 1 (a) and version 2 (b) of the new probes
(4 8 9 10 11 ii
(b) 12 13 14 15 16 17 n 0
Version 1 -0.002 0.041 -0.016 0.005 0.007 0.075
-0.024 0.026 -0.032 -0.034 -0.016 0.094
-0.021 -0.016 -0.014 -0.038 -0.022 0.031
-0.016 -0.021 -0.008 -0.030 -0.019 0.025
Version 2 -0.015 0.033 -0.013 0.012 -0.032 -0.014 -0.005 0.059
- 0.024 0.065 -0.016 0.028 -0.052 0.014 0.002 0.086
-0.008 0.029 -0.003 0.015 -0.017 0.028 0.007 0.064
- 0.003 0.016 0.001 0.009 - 0.007 0.024 0.007 0.052
Table 4. Values of the XCTDCTD mean dtflerences in T. C, S and cr,jfor each 400-l 000-m profile and the tnean d@erences and standard deviations for the whole THETIS-U 400-1000-m data sets obtained with version 1 (a) and version 2 (b) of the new probes Pair
(a) 8 9 10 11 d c
(b) 12 13 14 15 16 17 n IJ
B Version 1 -0.005 0.011 -0.009 -0.008 -0.003 0.023
-0.026 -0.013 -0.029 -0.038 -0.026 0.030
-0.019 -0.024 -0.019 -0.028 -0.023 0.011
-0.014 -0.021 -0.013 -0.020 -0.017 0.008
Version 2 -0.013 0.004 -0.021 -0.010 - 0.029 -0.024 -0.015 0.022
-0.021 0.024 -0.010 0.006 -0.041 0.004 -0.007 0.033
-0.007 0.019 0.012 0.016 -0.010 0.027 0.009 0.015
-0.003 0.014 0.014 0.014 -0.002 0.026 0.010 0.011
Comparisonof XCTDjCTDdata
875
which itself depends mainly on the temperature. Solving the dynamic equation for the probe and assuming equilibrium between buoyancy and friction leads to an expression for the fall rate variation du as a function of the water viscosity variation du, n being the boundary-layer exponent (2 for laminar and 5 for turbulent flow): du =
dv
-1 2n”1Uu
Assuming a linear temperature-dependent viscosity, the fall rate of the probe can be computed from the temperature profile; the actual depth is then estimated by integrating the fall rate. Numerous computations scanning the different parameters involved, namely the unknown integration constants (ue, vg) and the n exponent, demonstrate that such a depth correction changes the mean differences and standard deviations of the various measured parameters by only a few thousandths, which can be neglected. 6. DISCUSSION Considering the comparisons we have performed during THETIS-I, it appears that the XCTDs work rather well much of the time, yet at other times (two out of nine probes tested) data were erratic and inconsistent. The seven probes which transmitted without major problems gave T and D values most of the time within the manufacturer’s specified ranges of accuracy, while for C the specified accuracy was not always reached. However, the results of the computations are very much dependent on experimental conditions. Our major result is that the XCTD sensor accuracies are better than + 0.02”C and _I 0.04 mS/cm (without any correction for the conductivity offsets), that is to say, better than and close to the values of f 0.03 in the corresponding units specified by the manufacturer. A 8-S linear relation (Fig. 13.3 13.2 c L Q) 13.1 s iii b 13.0 E F
12.9
2 5 3 a
12.8
12.7
12.8
38.40
38.45
38.50
38.55
38.60
Salinity Fig. 7. 0-S diagram obtained from the 35&1000-m profiles collected in the homogeneous area during THETIS-I. The upper and lower segments represent the linear relationship inferred from the CTD and XCTD data, respectively.
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7)can be computed from the results of all the XCTDs (20 probes) available during THETISI in the same homogeneous area and over the same depth interval (350-1000 m) as the one computed from the CTD profiles. The best-fit linear relation is S= 0.2158 + 35.704 which, in view of the errors, can be considered as close to the CTD relation S= 0.2328 + 35.476, indicating an acceptable agreement between both kinds of data. We have shown that experimental errors in T and C compensate so that S and cryerrors are mainly sensor errors. Applying the CTD T-S relation, known to hold over some limited depth interval in our study area, to correct conductivity/salinity offsets significantly minimizes the errors; hence the sensor accuracy of C is noticeably improved to f 0.02 mS/cm, while the corresponding accuracies of S and 00 improve to kO.03 and f0.02 kg/m3. Thus, the data at individual levels fulfil the specified criteria even if, due to steps and low-frequency trends in the vertical (all within the accuracy interval), interpretation of slight variations in terms of structures, stability and gradients can be misleading. One has to keep in mind, however, that the conditions we have worked in during THETIS-I are as unfavourable as they can be for probes of this kind, since in winter and where convection takes place in the Mediterranean, the signals are very small. Therefore, in our profiles the errors appear relatively large. However, in most of the upper layers (down to - 1000 m) of the whole ocean, the effective accuracy of the XCTD sensors will provide good observations of the field variations of interest. We also had the opportunity to test new XCTDs during THETIS-II in less homogeneous (especially in the upper layers) areas. Considering only the 400-1000-m layer gives results similar to the uncorrected results obtained during THETIS-I. In conclusion, the XCTD probe, being operated easily and having potentially a good accuracy, could become an efficient and convenient instrument. Acknok’[ed~ements-This study is part of the Programme Atmosphkre Mbt&orologique et O&an Superficiel (PAMOS), under grant 90/l 156A, jointly supported by the Institut des Sciences de 1’Univers (INSU) and the Direction des Recherches, Etudes et Techniques de la Dtltgation G&nBrale pour 1’Armement (DRET/DGA). The THETIS work was partially supported by the European Community MAST (Marine Science and Technology) program under contract no. MAST-0008C.
REFERENCES Leaman K. and F. Schott (1991) Hydrographic structure of the convection regime in the Gulf of Lions: winter 1987. Journal of Physical Oceanography, 21, 575-598. MEDOC Group (1970) Observation of deep-water formation in the northwestern Mediterranean Sea. Narure, 227, 1037-1040. Millot C. (1991) Mesoscale and seasonal variabilities of the circulation in the western Mediterranean. Dynamics of Atmospheres and Oceans, 15, 179-214. Seaver G. A. and S. Kuleshov (1982) Experimental and analytical error of the expendable bathythermograph. Journal of Physical Oceanography, 12, 592-600. THETIS Group (in prep.) Observations of deep convection in the northwestern Mediterranean using acoustic tomography, moored sensors, and shipboard surveys.
PII: SO9674637(96)00031-3
Comparison of XCTD/CTD
Researcl~ I. Vol. 43. No. 6. pp. 859%876. 1996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0967-0637/96 $I 5.00 + 0.00
data
CORINNE ALBgROLA,* CLAUDE MILLOT,* UWE SEND,? CHRISTIAN MERTENSt and JEAN-LUC FUDA* (Received 30 December 1993; in revisedform 9 March 1995; accepted 21 January 1996)
Abstract-XCTD (expendable Conductivity Temperature Depth) probes, developed recently by SIPPICAN Inc., have been used simultaneously with a CTD sonde in order to test, in the field, their performance and accuracy (interpreted as k 2 standard deviations of the XCTD-CTD differences). We have taken advantage, during the THETIS-I experiment in March 1992, of both the homogeneous and the stratified areas encountered in winter in the northern part of the western Mediterranean Sea to differentiate the errors due to the experimental conditions from those effectively due to the sensors. Although some intrinsic problems are evident, so that only seven out of the nine probes considered for comparison are usable, the accuracy specified by the manufacturer for the temperature (AT= +O.O3”C) is reached after standard processing, while the accuracies in conductivity, salinity and potential density are AC- fO.O6mS/cm (the specified value is AC = k 0.03 mS/cm), AS - f 0.04 and Aa”- +0.03 kg/m3. However, when the experimental errors (in situ natural variability, relatively rough estimation of the XCTD depth) are considered, it appears that the effective accuracies of the XCTD sensors are better than f 0.02”C and k 0.04 mS/cm, that is to say better than and close to the specified values of + 0.03”C and + 0.03 mS/cm. Occasional offsets in conductivity can further be well corrected for by using a temperature-salinity relation in some limited depth range and area where this relation is known to hold well; the conductivity-sensor accuracy then significantly improves to AC- +O.O2mS/cm resulting, for our study area, in corresponding salinity and potential density accuracies of AS - +0.03 and Aae- k 0.02 kg/m’. Thus, such instruments promise to be useful tools for many experimental studies. Complementary comparisons, performed with new versions of the XCTD probes under less convenient experimental conditions, are also presented. Copyright 0 1996 Elsevier Science Ltd
1. INTRODUCTION The XCTD consists of a weighted package housing batteries, electronics, a conductivity cell and a thermistor. It is operated in the same way as the well-known XBT, connected to a deck unit by a two-conductor fine wire while descending through the water column down to - 1000 m at a roughly constant fall rate of - 3 m/s. The depth (D) is specified to be accurate to k 5 m or + 2% of depth, whichever is greater. The conductivity (c) cell is a high purity, alumina ceramic tube wired to form a four-electrode conductivity sensor, and is specified to provide data accurate to f 0.03 mS/cm. The temperature sensor (7’) is a glass encapsulated, fast-response thermistor, specified to provide data accurate to _tO.O3”C. The electronics package converts the measured resistances of the conductivity cell and thermistor into frequencies that are sent up the wire to the deck unit. The probe uses two precision calibration resistors which are sampled periodically to compensate for any variations in the
* Centre d’OcCanologie de Marseille, Antenne COM, CNRS. BP 330-83507, La Seyne/mer, France. t Institut fiir Meereskunde, Diisternbrooker Weg 20,230O Kiel, Germany. 859
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electronics during deployment. The data are transmitted at a sampling rate of 4 Hz, which gives a vertical resolution of - 80 cm. The XCTDs were tested during a cruise of the European THETIS-I experiment, within and around the deep-water formation area of the northern part of the western Mediterranean (THETIS Group, in prep.). We launched a total of 36 probes with three complementary strategies: 1. 11 XCTD casts with CTD casts, for comparison of the two instruments. 2. 14 XCTD casts between CTD ones, to increase the CTD grid resolution. 3. 11 XCTD casts in the coastal area, to test the possibility of computing the geostrophic transport of the Northern Current. For this comparison study, the XCTDs were generally launched while the ship was on station, less than 10 min before starting the CTD casts in order to minimize the variations of the environment. The probes were operated with a hand-held launcher, from the stern of the ship, taking care that the wire could not rub against the hull. The CTD was a Neil Brown Mark III, calibrated to have accuracies of + 2-3 x 1Op3 in “C and mS/cm and & 2-3 in dbar. The northern part of the western Mediterranean Sea offers very suitable conditions to perform such comparisons. In winter, the offshore area is characterized by the occurrence of phenomena leading to the formation of relatively well-homogenized and dense water (MEDOC Group, 1970; Leaman and Schott, 1991), while the coastal area under the influence of the Northern Current is still stratified (Millot, 1991). The former area allows checking of the accuracy of the T and C sensors of the XCTD, while the latter area is convenient to check the D accuracy. The manufacturer, SIPPICAN, took into account the results of these prime comparisons to modify the XCTD probes. It provided us with two versions of the new probes in order to test them again by XCTDjCTD comparisons. These XCTDs were tested in the western Mediterranean during the THETIS-II experiment, but under less convenient conditions, so that the time interval (-0.5-l h) between XCTD and CTD profiles is usually larger than the lo-min interval of the THETIS-I comparisons. Moreover, most comparisons were performed inside the Algerian Basin, which is known to be a relatively heterogeneous area. Nevertheless, these comparisons provide complementary information about the XCTD accuracies. The three following sections concern the THETIS-I comparisons. We will first describe the various problems we encountered and the data processing we performed (Section 2). Then we statistically compare the XCTD and CTD profiles of in situ temperature (T), conductivity (c), salinity (S) and potential density (a~), and discuss the various errors; we show that the experimental errors are reduced and that the sensor accuracies are close to those specified (Section 3). We demonstrate that, with complementary information provided by other techniques, it is possible to correct conductivity offsets (Section 4). The complementary comparisons performed during THETIS-II are presented and the influence of the water viscosity on the depth accuracy is investigated (Section 5). The performance of the XCTD probes and possible solutions to the observed problems are discussed in Section 6. 2. DESCRIPTION
OF THETIS-I
DATA
Some problems occurred with the XCTD profiles that cannot be corrected or removed. Of the 14 probes launched to complete the CTD grid resolution, one probe did not transmit any
Comparison
of XCTDjCTD
861
data
signal while, one probe transmitted correctly only down to - 100 m. Of the 11 probes used in the coastal area, one transmitted intermittently during the full cast, one transmitted only down to -8OOm and one did not transmit any signal. Of the 11 probes launched for comparison, two were not included in the computations due to a large time interval (a few hours) between XCTD and CTD casts. Among the nine remaining comparisons, one profile appears strongly shifted for both Tand C, and one probe transmitted correctly only down to - 120 m, so that seven probes are usable for the statistical comparisons (locations shown in Fig. 1). The last failure (- 120-m transmission) concerns the very first probe that was launched from an inappropriate location on the ship, thus probably making the wire rub against the ship’s hull. If one considers the above mentioned - 800-m profile to be valid, the total data return is thus greater than 80%. In addition, there is apparently a pressure effect on the sensors that gives an abrupt step in T and C near - 500 m in some profiles and erroneous T and C values beyond - 900 m in all profiles. This is less than the manufacturer’s specified depth range of 1000 m. For all the profiles, other problems have been encountered that can be corrected by data processing. One concerns isolated aberrant values easily removed as outliers. Another is the occurrence of high-frequency (HF) noise, with spikes of a few tenths of “C and mS/cm, which cannot be correctly removed without filtering out features having a relatively fine vertical structure. The data were processed as follows: first, removal of unrealistic values (T> 14.5, T-c 11.0, C> 47.0, Cc42.0) then rejection of outliers (> 1 standard deviation
3” E
5”
6’
#
43”
N
42’
L
Fig. 1. Locations of the seven comparison stations off the Gulf of Lions (northern part of the western Mediterranean Sea). The dashed line schematizes the outer edge of the Northern Current and separates, during the winter dense-water formation, an offshore homogeneous area from a coastal stratified one.
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from a 9-m running mean). After that, a running median filter of length 30 m was applied. Figure 2 presents two representative sets of the resulting profiles considered to be of good quality. Figure 2a shows a set of profiles obtained in the homogeneous area: the XCTD displays a low-frequency (LF) signal (coming from both T and C) which is not observed in the CTD profile. Although the differences in T and C are within the specified ranges of accuracy, these LF fluctuations are misleading, and one must not be tempted to interpret them as an actual signal. Figure 2b shows a set of profiles obtained in the stratified area, at a place where the bottom depth was 610m; the XCTD- and CTD-T, S and rso profiles compare rather well. There is a mean offset in depth of - 10 m, which is within the specified range of accuracy which is + 20 m at 1000 m; a best-fit analysis performed on all the profiles that are not markedly homogeneous gives XCTD-CTD differences smaller than 15 m.
3. STATISTICAL
COMPARISON
OF THETIS-I
DATA
As the XCTDs are designed to be used alone, the XCTD-CTD differences for each of the seven vertical profiles in the four variables T, C, S and CTO (Fig. 3ad) have been computed without any best-fit correction in depth. These differences, denoted by 6T, 6C, 6s and 6aH contain experimental errors, either due to in situ variability in the water (between the time and place of the XCTD and CTD profiles) or to the XCTD-depth inaccuracy, and errors of the XCTD sensors (with respect to the CTD sensors, which are considered to be much more accurate). - From these local differences, we compute (Table 1) mean differences for each profile (6X, X being the variable T, C, S or r~()), as well as a mean difference (-6) and a standard deviation (6) from all profiles together. Also indicated in Fig. 3 is the expected accuracy (vertical lines). For T and C, these are the values of 20.03 in the respective units specified by the manufacturer. We interpret these accuracies (denoted by A) as + 2 standard deviations (+ 2a-interval containing 95% of the where the prime denotes the fluctuating measurements) i e. f2(A T’*)]j2 and f2(AC’2)“2, part after subtracting the mean.* To obtain the expected accuracies for S and co from those quoted in T (assuming similar accuracies for T and 0) and C, we use locally linearized relations S=aT+bC+ c which, for our data ranges in T, C and D, are an=dO+eS and S= - 1.0274 T+0.9730C+8.4619 and rso= -0.2216B;tjzO.813OS. To be as explicit as (X for T, C, S or go, as above) possible, we will specify either A = k 20 or A= z!z~AX’~ when dealing with accuracies either directly computed from the data or expected from the specific relations. Assuming errors are due only to the XCTD sensors without any correlation between the sensors, the accuracies for S and (~0are: errors in T and C, as expected for independent &2(as’2)“2
= f2(a2a
+ b2AC2)i’*
= f0.04
(1)
and &2(Aah2) = f2(d2AT’2 Although
the accuracy
for S is kO.04,
+ e2AS’2)‘/2
hereafter
z
410.03
we will invoke
the simplification
(2) that
*To infer the typical total error, the mean should not be removed, but the average offset over all data/probes close to zero (see Table]), so that we prefer to use the simpler statistical quantity involving only the fluctuations.
is
Comparison of XCTDjCTD
-1 oooL 12.8
863
data
1
13.2 13 Temperature
-200.
-1000’ 38.2 38.1
-1000’ 28.8
Fig. 2.
38.3 38.4 Salinity
38.5
38.6
29.1 2 29 Sigma-Theta Temperature (“C), salinity and potential density (kg/m’) versus depth (m) for the XCTD (full line) and the CTD (dashed line) at stations 3 (a) and 5 (b). 28.9
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C. Albirola 0 I ,
-200.
-__
z
-400. i.-
5
B
0
----_
-600 t -800 I
1000’
1 .4
13.2 13 Temperature
12.8
-600
t -800.
-1000’
-___ -II: 5 -.\ 38.1
38.2
38.3
38.4
38.5
38.6
Salinity
0
-
L
-400
z. _c ,a $ -600
1 \
-800.
-1000~ 28.8
28.9
29 Sigma-Theta Fig. 2(b).
29.1
219.2
Comparison
of XCTDjCTD
Temperature
865
data
Conductivity
b -low . . .'.I* -0.2 -0.15 -0.1 -0.05 0
0.05
0.1
0.15
0.2
Salinity 0
-200
E
-400
5 3 o-600
-800
-1OOOL -0.2 -0.15 -0.1 -0.05
d
I. 0
0.05
0.1
0.15
0.2
Fig. 3. Differences of the (a) temperature (“C), (b) conductivity (mS/cm), (c) salinity, (d) potential density (kg/m3), (e) corrected conductivity (mS/cm), (f) corrected salinity and (g) corrected potential density (kg/m3) versus depth (m) for the seven pairs; the vertical lines define the interval of accuracy I.
I= +0.03 defines a common expected interval of accuracy for all the parameters in the corresponding units. The T profiles (Fig. 3a) are well within the expected accuracy interval, but this is not always the case for C (Fig. 3b). Note that large differences near 300-400 m are acceptable, due to the rough values of the XCTD depth and the stratified layers found there in some profiles (see Fig. 2b). However, steps near 500m are not linked to the stratification and might be indicative of a T and C dependence upon pressure. These large differences carry through correspondingly to the S (Fig. 3c) and ~0 (Fig. 3d) profiles. The largest offsets in C and S come from profile no. 1, which would probably lead one to discard this profile as unrealistic in routine applications. Since this outlier heavily biases all statistics, it has not been included in our statistical computations. However, it will be included in the plots, in order to demonstrate that the profile is usable after applying the conductivity correction of Section 4. ---We have computed the mean differences for each pair and parameter (6T,SC, X2,6~0,
866
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Conductivity
Density
_-----J -0.2 -0.15 -0.1 -0.05
0
0.05
0.1
0.15
0.2
Fig. W-k).
Table 1). The absolute value of the mean difference in C is 0.08 mS/cm for pair no. 1; it ranges between 0.02 and 0.03 mS/cm for three out of seven probes, while it is negligible for the three others. The mean differences + the- standard deviations from all profiles together (except pair 1) are m= 0.000 + O.O17”C, AC = 0.003 f 0.030 mS/cm, as = 0.003 f 0.018 and Aq = 0.003 &-0.013 kg/m3 (Table 1). The relative number of values outside I (Table 2) rangesfrom1.5to9.3%forT,6.7to56.3%forC,0.3to13.3%forS,and0.0to4.6%fora~. Figure 4, where I is indicated again by vertical lines, plots the results in histogram form. The T histogram (Fig. 4a) is rather sharp and well centered, indicating good performances in (Table l), most of the values are within the temperature. As AT = k 20 - f0.034”C expected interval of accuracy (+O.O3”C). The C histogram (Fig. 4b), by contrast, is markedly shifted and broadened. Mainly due to profiles 4 and 6, the scatter is relatively large, implying an accuracy (AC = +2a - +0.06 mS/cm) poorer than the expected one (kO.03 mS/cm). Using the relation S=aT+ bC+c, a formula like (1) would predict AS = + 2a - k 0.068; however, the accuracy AS = &-20 - + 0.036 computed from the data is better, and the S histogram (Fig. 4c) looks relatively sharp. Similarly, using the relation o. = &I + es, a formula like (2) leads to a predicted accuracy of AoO = + 20 - + 0.030 kg/
Comparison
of XCTD/CTD
867
data
mean d&%erences in T, C, S andaefor each profile (E, X being the variable T, C, Table 1. Values of the XCTD-CTD S or cry) as well as mean differences (b) andstandard deviations (o) for the whole THETIS-I data set except pair No. 1; see definitions in the text and the number of points for each pair in Table 2
1 2 3 4 5 6 7 n c7
Table 2.
0.016 -0.012 0.007 -0.013 0.001 0.007 0.008 -0.000 0.017
Percentages
0.081 0.000 0.009 -0.032 0.016 0.029 0.001 0.003 0.030
0.027 -0.020 0.012 - 0.026 0.010 0.007 0.018 -0.001 0.028
0.062 0.012 0.002 -0.018 0.015 0.021 -0.008 0.003 0.018
0.010 -0.007 0.004 -0.012 0.008 -0.000 0.008 - 0.000 0.014
0.044 0.012 - 0.000 -0.011 0.011 0.015 - 0.008 0.003 0.013
0.004 - 0.003 0.002 -0.007 0.006 - 0.002 0.005 - 0.000 0.009
-0.052 -0.019 0.003 0.006 -0.007 - 0.022 0.016
of values out of the common interval of accuracy I= kO.03 for T, C, S and ae in the corresponding units. N is the total number of points for each pair
Pair 1 2 3 4 5 6 7 n
17.2 8.0 8.1 4.8 9.3 1.5 6.6 6.2
99.0 6.7 7.8 56.1 17.0 56.3 10.6 26.3
34.0 25.0 8.6 23.3 16.4 2.3 25.7 16.9
97.3 0.3 0.7 11.3 13.3 12.7 1.0 6.1
25.9 0.0 0.7 5.5 8.6 0.0 1.3 2.2
95.5 0.3 0.6 4.6 3.8 0.0 0.9 1.6
3.5 0.0 0.6 3.0 2.5 0.0 1.0 1.1
885 871 875 861 525 875 861 4868
m3, which is close to the value of Aao= +20 - f0.026 kg/m3 computed from the data; even if not well centered, which accounts for offsets discussed later (Section 4), the rr8 histogram (Fig. 4d) is also relatively sharp. Therefore, the values of the accuracies directly computed from the data (Table 1) are better than the predicted ones, and the reasonably good shapes of the S and ~0 histograms might account for some compensation of the errors in T and C. In fact, assuming correlation between the errors in T and C yields: AS2 = a2ATf2 + b2AC2 + 2abAT’AC’.
(3)
The covariance term can be computed from the data, yielding a relatively large value of 4.0 x 10-4”C.mS/cm. As “a” is negative and “b” positive, it is clear that this will markedly improve the predicted accuracy: now, AS = f2AS’2”2 - + 0.038, which is close to the value directly computed from the data (AS = f 20 - kO.036, Table 1). As expected from the relatively large value of the T-C covariance term, the $ue of the T-S covariance term is relatively small, so that the accuracy (Aa@= f 2Ad,2 -0.030 kg/m3) predicted by (2) cannot be significantly improved by a relation like (3). Therefore, these features support the hypothesis of a compensation of the errors in T and C. This point is very instructive when one tries to separate the errors due to the experimental
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et al.
Temperature
Conductivity a
b
1
2500
2000 -r 1500.
1000'
500'
Density
Salinity 3000
d 2500
2000
1500 r
Fig. 4. Histograms of the differences of the (a) temperature (“C), (b) conductivity (mS/cm), (c) salinity, (d) potential density (kg/m3), (e) corrected conductivity (mS/cm), (f) corrected salinity and (g) corrected potential density (kg/m3) of the seven pairs; the vertical lines define the interval of accuracy I.
conditions from those effectively due to the T and C sensors. As a matter of fact, independent sensors should give uncorrelated data and thus a relatively low correlation between the T and C differences. On the contrary, if the XCTD and the CTD have sampled different waters with different temperatures (leading to experimental errors either due to the in situ variability or to the XCTD depth inaccuracy), there will be an apparent T difference and a correlated C difference, due to the strong dependence of C on T. This analysis is supported by the plot (not shown) of the correlated part AT’AC’ versus depth, which proves that the contributions to the covariance term (AT/AC’) come mainly from regions of large vertical gradients and from differences that are likely to be due to experimental errors. However, the obviously erroneous features that are visible mainly in some of the C, Sand 60 profiles (like the steps near 500 m, Fig. 3) make no significant contribution to the covariance
869
Comparison of XCTDjCTD data Salinity
Conductivity
-
3000
2500
i-1500 I
Density 3000
-0.2 -0.15 -0.1 -0.05
0
0.05
0.1
0.15
0.2
Fig. 4(eHg).
term and are clearly due to errors of the C sensor. high correlation between T and C and a relatively the observed XCTD-CTD differences in S due different waters probably compensate, so that the actually be due to sensor errors, which are better 3.1. Estimation
of sensor versus experimental
Therefore, since we do observe a relatively low correlation between T and S, most of to the XCTD and CTD having sampled differences computed from the data might estimated as described hereafter.
errors
Let us consider the whole data set from all profiles together and divide the observed XCTD-CTD differences in T and C, AT’ and AC’, into differences arising from the XCTD sensor errors, AT,’ and AC:, and those arising from experimental errors (actually having sampled different waters or having an inaccurate XCTD depth), AT: and AC:, thus leading toAT’=AT,‘+AT;,AC=AC;+AC;. Assuming that sensor errors for T and C are independent, as well as sensor and
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experimental errors in general, only the term AT;AC; contributes to the total covariance in (3). Thus, AT’AC’ = ATLAC; for which a value (4.0 x 10P4”C mS/cm) has been obtained above. Now we make use of the definition of a correlation coefficient: A T;AC;
(4)
Since the covariance in the numerator cm (Table l), we can rewrite (4): AT;AC; ATL2”= > _
is known
and since ACL2’j2 < AC’2’f2 = 0.030 mS/
AT;AC; > _
A&’
_ - 0.0133”c.
AC’2”=
Considering AT’ = AT,I + AT:, with AT,’ and AT: independent of each other and a’12 - 0.017”C (Table 1) results in -A T,‘21’2 = (ATI - AT;2)li2 < o.0172 - 0.01332)“2 = O.OlO”C. This is the maximum possible error for $e temperature sensor. The same calculation
for conductivity
yields a maximum
ACh2
= (0.0302 - 0.0232)1’2. Thus,
the
maximum values for the accuracies of the XCTD sensors are AT = + 2A T;2”2 = + 0.02O”C and AC = f2ACi2 = + 0.038 mS/cm. It is first important to note that these effective accuracies are, respectively, better than and close to the accuracies ( ) 0.03 in “C and mS/cm) specified by the manufacturer. Second, comparing 0.010 with 0.017”C and 0.019 with (at most) of the errors in T and C O.O30mS/cm allows us to estimate that only -60% computed from the XCTD-CTD differences can be due to the T and C sensors, the remainder being due to experimental conditions. T and C sensor errors can now be used to calculate the ;F;ulting sensio; These maximy,t error in S (ASi
), using (3) with only the sensor-error
components,
ATI
, and ACk2
-42
=0.021, which zero covariance term (or using (l)!). We obtain a maximum value of ASa is very close to the standard deviation of the salinity differences computed directly from the data (0.018 in Table 1) and corresponds to a sensor accuracy of AS = f 2AS’[2”2 - t 0.04. Therefore, it can definitely be considered that most of the errors in S come from the errors due to the T and C sensors, and that the experimental errors in T and C compensate, due to their high correlation in (3), which makes the salinity look better than the conductivity. Similarly, we can compute for the potential density a maximum sensor error AUg2 =0.017 kg/m3 from the maximum sensor errors estimated for T and S. However, since the covariance between T and S is relatively low, their standa;; deviations (Table 1)
can be used as well as to calculate, as already done with (2) AaL2 = 0.015 kg/m3. Both values are close to the value of 0.01:: kg/m3 computed from the data (Table 1) and give a - +0.03 kg/m3. The ~0 errors are hence essentially sensor accuracy of Aao= f 2A# sensor errors, due to the compensation of the experimental errors in T and C in computing S. The major remaining problem concerns the offsets in C quantified by AC = 0.003 mS/cm (Table l), which obviously induces those observed in S and CJ~.
Comparison
4. CORRECTION
of XCTD/CTD
871
data
OF CONDUCTIVITY
OFFSETS
Correcting the C offsets described above (plus the resulting S and a~ offsets) is possible if some significant T-S (or T-C) relation is found to apply over some limited depth interval. The 8-S diagram shown in Fig. 5, from 350 m to depths of more than 2000 m, results from 84 CTD profiles collected in the homogeneous area. Between 350 and -800 m (corresponding to 00 lower than - 29.1), this diagram evidences a significant relation (S=O.2328+ 35.476) that roughly supports the occurrence of a similar T-S relation (not shown because less common), as %can be considered linear in T over these depths. The S offset for each profile is defined by the mean difference, over that depth range, between the S values computed from the T of the XCTD and the T-S relation and the directly measured S values, i.e. computed from both the Tand C of the XCTD. This S offset allows correction of the conductivity and thus computation of differences in C, S and ge for each corrected profile, as well as mean XCTD-CTD differences and standard deviations for the whole data set (Table 1). Figure 3e-g shows the XCTD-CTD differences for C, S and ~0 versus depth after the correction. It can be seen that these differences are now better within the common limit I= kO.03. The correspondingly corrected histograms (Fig. 4e-g) are now centered near zero. The accuracies improve from AC = f2a - f0.060 to kO.056 mS/cm, from AS= k2c~ - kO.036 to f0.028 and from AaO= f2a - 50.026 to +0.018 kg/m3 (Table 1). The relative number of values outside Inow ranges from 2.3 to 25.7% for C, 0.0 to 8.6% for S and 0.0 to 3.0% for bg (Table 2). The efficiency of the C offset correction, using only a T-S relation known to hold over
SALINITY Fig. 5.
O-S diagram
resulting from 84 CTD profiles taken in the study area, using data between 350 m and the bottom (more than y 2000 m).
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et al.
SALINI’I I
250
f
Uncorrected
0
250
l-
SALINITY
750 1
II
1000
b
Corrected Fig. 6. Differences, for all the XCTDs, between the Sprofile computed from the T-S relation using the T profile of the XCTD and the S profile provided by the XCTD without (a) and with (b) corrections of the C offsets.
some limited depth interval, is clearly evidenced by Fig. 6. Instead of plotting the uncorrected and corrected salinity profiles versus depth, we have plotted the differences between these profiles and the salinity profiles computed from the T-S relation extended over the whole depth. This computed salinity profile has, of course, no significance out of the specified depth interval; it is used only as a reference profile. It is clear that the correction is also efficient near the surface and that the scatter of all the XCTDs collected during the whole experiment is acceptable everywhere. Using the new AT’AC’ covariance of 4.3 x 10P4”C mS/cm computed with the corrected conductivity, we can estimate corrected sensor errs;; as done in the previous section. The result is a maximum -l/2
cm),
yielding
ASL2
corrected =0.014
sensor error of A Cl2 (instead
of 0.021)
=0.012 and sl”
mS/cm (instead =0.012
kg/m3
of 0.019 mS/ (instead
of
873
Comparison of XCTDjCTD data
0.017 kg/m3). When these values are compared with those computed directly from the corrected data (0.028 mS/cm, 0.014 and 0.009 kg/m3), it can be confirmed that the sensor errors represent nearly the total errors for S and CJ~.Therefore, after correction for the conductivity offsets, the accuracy of the conductivity sensor significantly improves to AC = +_2AC;2”2 - f 0.02 mS/cm, leading to sensor acc;;acies for salinity and potential - kO.02 kg/m3. density of AS = f 2AS;2”2 - kO.03 and Aao= f2 A#
5. STATISTICAL
COMPARISONS
OF THETIS-II
DATA
The differences dT, 6C, 6S and 6ao (Table 3) were computed for the THETIS-II data, collected with the two versions (called 1 and 2) of the new probes, as was done for THETIS-I. The comparisons were made with four XCTDs for version 1 and six XCTDs for version 2. As an obvious hardware improvement, the technical problems encountered during THETIS-I, such as poor quality of transmission or noisy profiles, did not occur at all. Thus the statistical comparisons are computed directly from the original data. The mean differences + the standard deviations from all O-1000-m profiles together are then AT=0.007~0.075”C, AC= -0.016fO.O94mS/cm, AS= -0.022f0.031, Aae = -0.019&0.025 kg/m3 for version 1 (Table 3a), and AT = -0.005+ O.O59”C, AC = 0.002 + 0.086 mS/cm, AS = 0.007 k 0.064, Aoe = 0.007 kO.052 kg/m3 for version 2 (Table 3b). In contrast to the THETIS-I results, the mean is not removed, since relatively large offsets are evidenced. Moreover, note that the standard deviations during THETIS-II are much larger than those during THETIS-I, because the less convenient experimental conditions increased errors due to natural variability, so that these standard deviations are not meaningful. Because the variability was especially large in the upper layers, the differences for each parameter are also computed for just the interval 40&1000 m (Table 4). The mean differences f the standard deviations from all 40@-1000-m profiles together are AT = - 0.003 + O.O23”C, AC = - 0.026 k 0.030 mS/cm, as= -0.023f0.011, then AQ= -0.017f0.008 kg/m3 for version 1 (Table 4a), and AT= -0.015+0.022”C, AC= -0.007)0.033 mS/cm, AS=0.009+0.015, Aa~=0.010+0.011 kg/m3 for version 2 (Table 4b). The mean differences are expected to be more representative of the actual offsets than those computed from the O-1000-m profiles. It appears that there are significant offsets in C for version 1 probes and in T for version 2 probes; as the version 2 probes were tested after pairs 8,9 and 10 and before pair 11, these offsets are expected to be characteristic of the version. Computing the typical accuracy of each parameter as f 20, for both versions, gives AT - f O.O45”C,AC - k 0.063 mS/cm, AS - + 0.026 and Aae - + 0.019 kg/m3 , which are values - 3 times less than those computed from the same profiles between 0 and 1000 m. These accuracies are of the order of those computed for the THETIS-I 0-1000-m profiles, before the correction of conductivity offsets discussed in Section 4. Such a correction cannot be applied here since no T-S relation can be established due to the geographical scatter of these comparisons. Unfortunately, mainly due to these less convenient experimental conditions, we cannot draw any conclusion concerning the improvement of the XCTD accuracies. This is especially true for version 1, since only four probes of that version were tested. Further, since these comparisons were performed mainly in relatively stratified regions of the Mediterranean, the influence of viscosity on the depth accuracy was investigated as done by Seaver and Kuleshov (1982). Indeed, the fall rate of the probe depends on the viscosity,
874
C. Albirola
et al.
Table 3. Values of the XCTD-CTD mean differences in T, C. S and ae for each (rlOOO-m profile and the mean dtperences and standard deviations for the whole THETIS-II &1000-m data sets obtained with version 1 (a) and version 2 (b) of the new probes
(4 8 9 10 11 ii
(b) 12 13 14 15 16 17 n 0
Version 1 -0.002 0.041 -0.016 0.005 0.007 0.075
-0.024 0.026 -0.032 -0.034 -0.016 0.094
-0.021 -0.016 -0.014 -0.038 -0.022 0.031
-0.016 -0.021 -0.008 -0.030 -0.019 0.025
Version 2 -0.015 0.033 -0.013 0.012 -0.032 -0.014 -0.005 0.059
- 0.024 0.065 -0.016 0.028 -0.052 0.014 0.002 0.086
-0.008 0.029 -0.003 0.015 -0.017 0.028 0.007 0.064
- 0.003 0.016 0.001 0.009 - 0.007 0.024 0.007 0.052
Table 4. Values of the XCTDCTD mean dtflerences in T. C, S and cr,jfor each 400-l 000-m profile and the tnean d@erences and standard deviations for the whole THETIS-U 400-1000-m data sets obtained with version 1 (a) and version 2 (b) of the new probes Pair
(a) 8 9 10 11 d c
(b) 12 13 14 15 16 17 n IJ
B Version 1 -0.005 0.011 -0.009 -0.008 -0.003 0.023
-0.026 -0.013 -0.029 -0.038 -0.026 0.030
-0.019 -0.024 -0.019 -0.028 -0.023 0.011
-0.014 -0.021 -0.013 -0.020 -0.017 0.008
Version 2 -0.013 0.004 -0.021 -0.010 - 0.029 -0.024 -0.015 0.022
-0.021 0.024 -0.010 0.006 -0.041 0.004 -0.007 0.033
-0.007 0.019 0.012 0.016 -0.010 0.027 0.009 0.015
-0.003 0.014 0.014 0.014 -0.002 0.026 0.010 0.011
Comparisonof XCTDjCTDdata
875
which itself depends mainly on the temperature. Solving the dynamic equation for the probe and assuming equilibrium between buoyancy and friction leads to an expression for the fall rate variation du as a function of the water viscosity variation du, n being the boundary-layer exponent (2 for laminar and 5 for turbulent flow): du =
dv
-1 2n”1Uu
Assuming a linear temperature-dependent viscosity, the fall rate of the probe can be computed from the temperature profile; the actual depth is then estimated by integrating the fall rate. Numerous computations scanning the different parameters involved, namely the unknown integration constants (ue, vg) and the n exponent, demonstrate that such a depth correction changes the mean differences and standard deviations of the various measured parameters by only a few thousandths, which can be neglected. 6. DISCUSSION Considering the comparisons we have performed during THETIS-I, it appears that the XCTDs work rather well much of the time, yet at other times (two out of nine probes tested) data were erratic and inconsistent. The seven probes which transmitted without major problems gave T and D values most of the time within the manufacturer’s specified ranges of accuracy, while for C the specified accuracy was not always reached. However, the results of the computations are very much dependent on experimental conditions. Our major result is that the XCTD sensor accuracies are better than + 0.02”C and _I 0.04 mS/cm (without any correction for the conductivity offsets), that is to say, better than and close to the values of f 0.03 in the corresponding units specified by the manufacturer. A 8-S linear relation (Fig. 13.3 13.2 c L Q) 13.1 s iii b 13.0 E F
12.9
2 5 3 a
12.8
12.7
12.8
38.40
38.45
38.50
38.55
38.60
Salinity Fig. 7. 0-S diagram obtained from the 35&1000-m profiles collected in the homogeneous area during THETIS-I. The upper and lower segments represent the linear relationship inferred from the CTD and XCTD data, respectively.
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C. Alberola
er al.
7)can be computed from the results of all the XCTDs (20 probes) available during THETISI in the same homogeneous area and over the same depth interval (350-1000 m) as the one computed from the CTD profiles. The best-fit linear relation is S= 0.2158 + 35.704 which, in view of the errors, can be considered as close to the CTD relation S= 0.2328 + 35.476, indicating an acceptable agreement between both kinds of data. We have shown that experimental errors in T and C compensate so that S and cryerrors are mainly sensor errors. Applying the CTD T-S relation, known to hold over some limited depth interval in our study area, to correct conductivity/salinity offsets significantly minimizes the errors; hence the sensor accuracy of C is noticeably improved to f 0.02 mS/cm, while the corresponding accuracies of S and 00 improve to kO.03 and f0.02 kg/m3. Thus, the data at individual levels fulfil the specified criteria even if, due to steps and low-frequency trends in the vertical (all within the accuracy interval), interpretation of slight variations in terms of structures, stability and gradients can be misleading. One has to keep in mind, however, that the conditions we have worked in during THETIS-I are as unfavourable as they can be for probes of this kind, since in winter and where convection takes place in the Mediterranean, the signals are very small. Therefore, in our profiles the errors appear relatively large. However, in most of the upper layers (down to - 1000 m) of the whole ocean, the effective accuracy of the XCTD sensors will provide good observations of the field variations of interest. We also had the opportunity to test new XCTDs during THETIS-II in less homogeneous (especially in the upper layers) areas. Considering only the 400-1000-m layer gives results similar to the uncorrected results obtained during THETIS-I. In conclusion, the XCTD probe, being operated easily and having potentially a good accuracy, could become an efficient and convenient instrument. Acknok’[ed~ements-This study is part of the Programme Atmosphkre Mbt&orologique et O&an Superficiel (PAMOS), under grant 90/l 156A, jointly supported by the Institut des Sciences de 1’Univers (INSU) and the Direction des Recherches, Etudes et Techniques de la Dtltgation G&nBrale pour 1’Armement (DRET/DGA). The THETIS work was partially supported by the European Community MAST (Marine Science and Technology) program under contract no. MAST-0008C.
REFERENCES Leaman K. and F. Schott (1991) Hydrographic structure of the convection regime in the Gulf of Lions: winter 1987. Journal of Physical Oceanography, 21, 575-598. MEDOC Group (1970) Observation of deep-water formation in the northwestern Mediterranean Sea. Narure, 227, 1037-1040. Millot C. (1991) Mesoscale and seasonal variabilities of the circulation in the western Mediterranean. Dynamics of Atmospheres and Oceans, 15, 179-214. Seaver G. A. and S. Kuleshov (1982) Experimental and analytical error of the expendable bathythermograph. Journal of Physical Oceanography, 12, 592-600. THETIS Group (in prep.) Observations of deep convection in the northwestern Mediterranean using acoustic tomography, moored sensors, and shipboard surveys.