Results from the LADCP measurements conducted in the Eurasian Arctic

Abstract

Preliminary results from 114 dual headed LADCP (Lowered Acoustic Doppler Current Profiler) measurements performed during the NABOS (Nansen and Amundsen Basin Observational System) 2013 cruise in the Eurasian Basin of the Arctic Ocean are discussed. Calculated horizontal magnetic field strength for that specific study area and cruise time span equals to 1500-4200 nT which is a critically low value. This affected the heading usability of instruments’ compasses, thereby making the obtained ocean currents velocities and directions difficult to assess. Additional data post-processing performed with the LDEO Software (Version IX_9) and dedicated Matlab routines have so far allowed to obtain reasonable velocity profiles only in several cases (with additional information from the SeaBird 911plus CTD, GPS and bottom tracking). Thus, doubts concerning the feasibility of compasses, confirmed by difficulties encountered previously in similar polar locations, rise the necessity of providing an additional heading source which should be mounted together with the Teledyne RDI instruments to gain an unquestionable velocity field.

Full text

Results from the LADCP measurements conducted in the Eurasian Arctic
Goszczko I.1, Pnyushkov A.2, Polyakov I.2Rember R.2and Thurnherr A. M.3
1Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland
2International Arctic Research Centre, University of Alaska, Fairbanks, USA
3Lamont-Doherty Earth Observatory, Columbia University, Palisades, USA
ilona g@iopan.gda.pl
1. Geomagnetic field in the
Eurasian Arctic
The Earth’s internal magnetic field has a dipole-like shape.
Its vector, Bmhas seven elements: the northerly intensity X,
the easterly intensity Y, the vertical intensity Z and the follow-
ing quantities derived from X, Y and Z: the horizontal intensity
H, the total intensity F, the inclination angle I, and the decli-
nation angle D.
Even though, the total intensity F is the strongest over Siberia
(one of the magnetic poles), the horizontal intensity (also
called the horizontal field strength) H reaches the lowest val-
ues here, due to dipole character of the magnetic field (Fig.
1).
To calculate elements of the vector Bmwe used the Geo-
mag7.0 Software and the International Geomagnetic Refer-
ence Field IGRF-11 model released by the International As-
sociation of Geomagnetism and Aeronomy (IAGA) which cov-
ered secular variation until 2015 (see the latest version of
IGRF-12). For the Eurasian Arctic area limited by the NA-
BOS (Nansen and Amundsen Basin Observational System)
2013 cruise space and time spans H derived from IGRF 11
varied from 1500 to 4200nT which is a critically low value. Si-
multaneously, the declination angle D varied between −13◦400
and 59◦270which is rather a high number.
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Figure 1: Horizontal intensity H of the Earth’s magnetic field derived from the World
Magnetic Model 2010 (WMM-2010). Figure from the Mathworks examples.
2. LADCP measurements
performed during the NABOS
2013 cruise
During the NABOS 2013 cruise 114 dual headed LADCP
(Lowered Acoustic Doppler Current Profiler) measurements
were performed to assess in situ ocean currents (Fig. 2). All
in all, three RDI (currently Teledyne RDI) 300kHz Workhorse
Monitor ADCPs were used. In that case, the battery pack
is mounted externally, which does not require repeated cal-
ibration. This is needed when the internal battery pack is
replaced in the Sentinel model. Each device has a flux-
gate compass mounted internally. The compasses work fine
closer to the Earth’s equator or in the mid-latitudes (according
to the information provided by the manufacturer: Accuracy
±2◦, Precision ±0.5◦, Resolution 0.01◦, Maximum tilt ±15◦) but
perhaps are not a sufficient solution in the polar regions.
The very low values of H cause low accuracy and make the
measured ocean currents velocities and directions difficult to
assess (similar as in the Canadian Arctic, Hamilton, 2001). In
addition to that, additional compass deviation caused by the
rosette frame and other instruments on it is also not known
but may also meaningfully impact the results (cf. Von Appen,
2015). Examples of retrieved headings are showed in Fig. 3.
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Figure 2: Geographical location of 116 CTD (114 LADCP) stations performed dur-
ing the NABOS 2013 cruise in the Eurasian Arctic which cover the boundary current
north of Severnaya Zemlya and in the Laptev Sea.
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Elapsed Time [min]
Heading [deg]
Station 001
DL
UL
0 5 10 15 20 25 30 35 40
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150
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350
Elapsed Time [min]
Heading [deg]
Station 112
DL
UL
Figure 3: Time series of heading collected on the same CTD rosette during two 500
m casts performed in the St. Anna Trough, Northern Kara Sea (81◦N, 74◦E): Station 1
(upper panel) on the beginning of the cruise and Station 112 (lower panel) near the end.
Two devices were used as up-lookers (UL) while the down-looker (DL) was not detached
from the rosette for the whole cruise period. There is a clear discrepancy between both
measurements of the compasses. Furthermore, the DL shows reduced amplitudes of
the fluctuations compared to the measurements of both ULs. In addition to that, the
heading data derived from the UL at Station 112 contains much noise, as well.
3. Data processing with the
LDEO LADCP Software
Additional data post-processing performed with the LDEO
LADCP Software (Version IX 9, see Thurnherr, 2014) and
dedicated Matlab routines has so far allowed to obtain some
reasonable velocity profiles only in several cases (with addi-
tional information from the SeaBird 911plus CTD, GPS and
bottom tracking, Fig. 4). In most cases a comparison of
headings show a large difference, even the shape of run is
similar. This might be an important issue in a case when a
single head LADCP system would be used. It is not possible
to say for certain that the obtained heading is correct.
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0 U(−) V(−−); blue dots down cast; dotted shear
depth [m]
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velocity [cm/s]
above bottom [m]
RDI bottom track
NABOS2013 cast #112 − with DL heading only
Start: 81°N 20.0376’ 73°E 41.7123’
18−Sep−2013 15:08:01
End: 81°N 19.9728’ 73°E 40.4007’
18−Sep−2013 15:37:45
u−mean: −11 [cm/s] v−mean 26 [cm/s]
binsize do: 10 [m] binsize up: 10 [m]
mag. deviation 45.5
wdiff: 0.2 pglim: 0 elim 0.5
bar:1.0 bot:1.0
weightmin 0.1 weightpower: 1.0
max depth: 479 [m] bottom: 498 [m]
50 100
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target strength [dB]
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inst. range [m]
0 0.1
vel error (−k) [m/s]
single ping (−b)
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CTD−position (blue) and ship (green) east−west [m]
GPS−end
bottom
end
start
north−south [m]
LDEO LADCP software: Version IX_9
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depth [m]
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velocity [cm/s]
above bottom [m]
RDI bottom track
NABOS2013 cast #112 − with UL heading only
Start: 81°N 20.0376’ 73°E 41.7123’
18−Sep−2013 15:08:01
End: 81°N 19.9728’ 73°E 40.4007’
18−Sep−2013 15:37:45
u−mean: 4 [cm/s] v−mean 20 [cm/s]
binsize do: 10 [m] binsize up: 10 [m]
mag. deviation 45.5
wdiff: 0.2 pglim: 0 elim 0.5
bar:1.0 bot:1.0
weightmin 0.1 weightpower: 1.0
max depth: 479 [m] bottom: 498 [m]
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depth [km]
target strength [dB]
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vel error (−k) [m/s]
single ping (−b)
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CTD−position (blue) and ship (green) east−west [m]
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LDEO LADCP software: Version IX_9
Figure 4: Velocity profiles calculated for Station 112: rotated up-looking velocities
to down heading (upper panel) and rotated down-looking velocities to up heading (lower
panel). A change in heading affects more the u velocity component in this instance.
The lower panel is more internally consistent than upper panel, suggesting that the UL
compass gave better data in this case.
4. Discussion and implication
for future research
Doubts concerning the feasibility of flux-gate compasses,
confirmed by difficulties encountered previously in similar po-
lar locations, rise the necessity of providing an additional
heading source which should be mounted together with the
TRDI ADCP to gain an unquestionable velocity field.
While LADCP works in area of very small H, required non-
magnetic heading references, such as gyroscopes or exter-
nal magnetometers, can be used in some regions where
H is too small for the ADCP compasses. Figs. 5-8 show
data from a profile collected in the Southern Ocean where H
was too small for the ADCP compasses to work correctly but
where external magnetometer measurements obtained with
a prototype instrument called Incidental Measurement Pack-
age (IMP) were reliable enough to be used instead. We plan
to use this instrument during the NABOS 2015 cruise to at-
tempt to obtain better quality LADCP profiles.
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Heading [deg]
Elapsed Time [min]
Station 12
IMP
DL
UL
Figure 5: Three time series of heading collected on the same CTD rosette during a
700 m cast near the Totten Ice shelf (East Antarctica, 66◦S, 117◦E), where the horizon-
tal geomagnetic field strength is 7000nT. The DL/UL time series came from two TRDI
300kHz Workhorse ADCPs. The IMP time series are also plotted. It is clear that the DL
compass did not return any valid data.
Depth [m]
Super Ensemble #
Uïerr std: 0.028
20 40 60 80100120
0
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500
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median(Uïerr) [r/b: downï/upïcast]
Residual [m/s]
Bin #
Ensemble #
Depth [m]
Uoce
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Vïerr std: 0.026
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median(Vïerr) [r/b: downï/upïcast]
Residual [m/s]
Bin #
12 Figure 3
Ensemble #
Depth [m]
Voce
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0
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600
ï0.2
ï0.1
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Figure 6: Having processed the original data with the LDEO IX Software,
reasonable-looking profiles are produced. However, the inversion residuals are quite
bad. This is showed in two panels: the upper is u, the lower is v. Left panels show
inversion residuals as a time-depth plot, central panels show ADCP-bin-averaged inver-
sion residuals, while the two right panels display ocean velocities as a time-depth plot.
The vertical structure in the central panels and the lack of horizontal banding in the right
panels is clearly visible.
Depth [m]
Super Ensemble #
Uïerr std: 0.024
20 40 60 80100120
0
100
200
300
400
500
600 ï0.06
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median(Uïerr) [r/b: downï/upïcast]
Residual [m/s]
Bin #
Ensemble #
Depth [m]
Uoce
20 40 60 80100120
0
100
200
300
400
500
600 ï0.15
ï0.1
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0.1
0.15
Bin #
Super Ensemble #
Vïerr std: 0.024
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Residual [m/s]
Bin #
12 with IMP data Figure 3
Ensemble #
Depth [m]
Voce
20 40 60 80100120
0
100
200
300
400
500
600 ï0.3
ï0.2
ï0.1
0
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0.2
0.3
Figure 7: The same processing diagnostics as showed in Fig. 6 but here the
LADCP data are processed with heading, pitch and roll from the IMP. The inversion
diagnostics look considerably cleaner.
0
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400
500
600
700
-0.3 -0.2 -0.1 0 0.1 0.2 0.3
Depth [m]
Velocity [m/s]
u_orig
v_orig
u_IMP
v_IMP
Figure 8: The u and v profiles: both without and with IMP data. In this particular
case u is much more affected than v.
References
1. Hamilton, J. M. (2001) Accurate ocean current direction measure-
ments near the magnetic poles. Paper presented at the Proceedings
of the International Offshore and Polar Engineering Conference, 1,
656-660.
2. International Geomagnetic Reference Field model, IGRF-12. Avail-
able online at http://www.ngdc.noaa.gov/IAGA/vmod/igrf.html.
3. Thurnherr, A. M., (2014) How to process LADCP
data with the LDEO Software. Available online at
ftp://ftp.ldeo.columbia.edu/pub/LADCP/HOWTO.
4. Von Appen, J.-W. (2015) Correction of ADCP compass errors result-
ing from iron in the instruments vicinity. J. Atmos. Oceanic Technol.,
32, 591602. doi: http://dx.doi.org/10.1175/JTECH-D-14-00043.1
Acknowledgments
I.G. got support from the National Science Center, Poland, under the
MIXAR grant agreement UMO-2012/05/N/ST10/03643 which partly cov-
ered participation in the NABOS 2013 survey and data analysis. A.P., I.P.,
R.R. and A.M.T. got funding from the National Science Foundation, USA.
The NABOS 2013 cruise was organized by IARC, UAF, USA and AARI,
Russia and conducted aboard R/V Akademic Fedorov. Vladimir Ivanov
from AARI was the chief scientist. JAMSTEC, Japan, made three RDI
300kHz Workhorse devices available at that field work.
We wish to thank the whole hydrography group and the ship crew for
their effort aboard, as well as Peter Keen from Keen-Marine Ltd. for his
technical help when we encountered troubles. Thanks are also due to
Waldemar Walczowski from IOPAS for all his comments provided before
data collection and during data analysis. The Antarctic LADCP data are
provided by courtesy of Bruce Huber, LDEO.
European Geosciences Union, General Assembly 2015, 12-17 April, Vienna, Austria. Session: OS 1.2 Changes in Arctic sea ice and ocean: observations, models and perspectives