by a lowered acoustic doppler current profiler (L-ADCP) Volker Strass1, Matthew Donnelly2,
Harry Leach2, Hendrik Sander3
1AWI
2SoES
3OPTIMARE Work at sea
Current velocities over the full water column from the sea surface to the ocean floor were recorded with a lowered acoustic Doppler current profiler (L-ADCP), consisting of an upward/downward looking pair of RDI Workhorse Sentinels 300 kHz attached to the CTD rosette frame. While the L-ADCP was mounted on the CTD-rosette for the duration of the cruise, it was not activated for all casts, in particular not for many of those casts which extended to only 500 m or less. The L-ADCP dataset includes a mixture of full depth, 3,000 m, 1,500 m, 1,000 m and 500 m casts.
A total of 109 useable L-ADCP profiles were obtained, although processing of the L-ADCP is still ongoing at the time of writing and the total number of successful casts is subject to change. During processing of the L-ADCP data it was discovered that the PC used for the data retrieval had unfortunately been set at some earlier date to UTC + 2hr. The L-ADCP data time-stamp was therefore offset by 2 hours from all other measurements on board which are referenced to UTC. For the data processing the IFM-GEOMAR/LDEO Matlab LADCP-Processing system, maintained by Gerd Krahmann and generously made publically available through http://tech.
groups.yahoo.com/group/ladcp/, was employed.
Preliminary results
Of the 32 profiles which have been successfully processed to date, the range of the depth-mean u and v components is as follows:
umin = -11 cms-1 umax = 25 cms-1 vmin = -28 cms-1 vmax = 51 cms-1
However, so far only the position data have been taken into account in the processing, but not yet the CTD and VM-ADCP records.
3.5 Physical Oceanography: Microstructure turbulence measurements
Hartmut Prandke1, Matthias Krüger2, Volker Strass3
1ISW Wassermesstechnik
2IfM-GEOMAR
3AWI Work at sea
Turbulence measurements have been carried out at selected stations within the framework of the Eddy Pump programme. For the microstructure measurements the microstructure profiler MSS90L (long version, serial No. MSS10, owned by ISW) was used. The MSS profiler is produced by Sea & Sun Technology GmbH in co-operation with ISW Wassermesstechnik.
The MSS profiler is a free-falling tethered instrument for simultaneous microstructure and precision measurements of physical parameters in marine and limnic waters.
It is equipped with microstructure shear sensors (for turbulence measurements), a microstructure temperature sensor, standard CTD sensors, a Seapoint turbidity sensor, and housekeeping sensors (acceleration, tilt of the instrument). The sampling rate for all sensors is 1,024 samples per second, the resolution is 16 bit.
The data are transmitted via electrical cable to an on board unit and further to a data acquisition PC.
The MSS was balanced with negative buoyancy which gave it a sinking velocity of about 0.75 m/s. The MSS was operated using a special winch (SWM1000).
Disturbing effects caused by cable tension (vibrations) and the ship’s movement are excluded by a slack in the cable. The winch was placed at the aft of the ship (port side). During the MSS measurements, the ship was going with a speed between 0.5 and 1 knot in the water against the wind and waves. Thus, the ship was actively moving away from the sinking profiler.
The raw data from the MSS Profiler were transmitted via a RS 485 data link to the on board unit of the measuring system. For the data acquisition, on-line display, and storage of the data on a notebook the Standard Data Acquisition software SDA (Sea & Sun Technology GmbH) was used. The data are stored in the MRD (Microstructure Raw Data) format at hard disk. Quick-look data processing has been carried out after each station using the data processing software MSSpro (ISW Wassermesstechnik).
Preliminary results
The aim of the measurements was to study vertical mixing due to small scale turbulence and stratification in the top layer and the intermediate cold winter water. This required an operation of the profiler from the surface to approx. 300 m depth. With respect to the intermittence of marine turbulence, a burst sampling strategy was applied. At each station a burst of 5 profiles was taken.
During ANT-XXVIII/3 microstructure measurements have been carried out at 31 stations. In total, 156 profiles were measured in a depth range between approx.
10 and 350 m. A MSS station overview is given in the Table 3.5.1 below.
Tab. 3.5.1: MSS Stations during PS79 Station/
MSS cast Date MSS Nr. latitude (begin of
station)
longitude (begin of station) 085/05 26.01.2012 1 - 6 52° 0.08'S 7° 59.98'W 086/03 29.01.2012 7 - 11 51° 59.76'S 11° 59.35'W 091/08 03.02.2012 12 -16 51° 13.22'S 12° 40.79'W
3. Physical oceanography
Station/
MSS cast Date MSS Nr. latitude (begin of
station)
longitude (begin of station) 098/08 06.02.2012 37 - 41 51° 12.18'S 12° 39.80'W 101/03 06.02.2012 42 - 46 51° 24.76'S 12° 37.97'W 102/02 07.02.2012 47 - 51 51° 36.55'S 12° 38.51'W 104/02 07.02.2012 52 - 56 51° 36.40'S 13° 19.15'W 106/03 07.02.2012 57 - 61 51° 12.01'S 13° 19.46'W 111/03 08.02.1012 62 - 66 51° 11.89'S 12° 59.98'W 114/08 09.02.2012 67 - 71 51° 11.83'S 12° 40.28'W 115/03 09.02.2012 72 - 76 50° 59.88'S 12° 39.68'W 116/03 09.02.2012 77 - 81 50° 47.02'S 12° 38.57'W 119/02 10.02.2012 82 - 86 51° 12.52'S 12° 19.89'W 122/04 10.02.2012 87 - 91 51° 36.35'S 12° 00.68'W 124/02 11.02.2012 92 - 96 51° 12.01'S 12° 00.37'W 126/03 11.02.2012 97 - 101 50° 48.10'S 12° 00.19'W 128/06 12.02.2012 102 - 106 51° 12,14'S 12° 38.92'W 136/09 14.02.2012 107 - 111 51° 12.15'S 12° 39.15'W 137/09 15.02.2012 112 - 116 51° 02.10'S 12° 10.67'W 139/11 16.02.2012 117 - 121 50° 59.99'S 12° 59.88'W 140/06 16.02.2012 122 - 126 51° 12.51'S 12° 39.18'W 165/02 28.02.2012 127 - 131 49° 32.76'S 39° 22.29'W 174/04 1.03.2012 132 - 136 49° 39.73'S 38° 16.15'W 174/12 2.03.2012 137 - 141 49° 37,28'S 38° 19.54'W 174/16 2.03.2012 142 - 146 49° 36.86'S 38° 21.39'W 174/21 2.03.2012 147 - 151 49° 35.83'S 38° 22.86'W 174/24 2.03.2012 152 - 156 49° 33.26'S 38° 25.02'W
3.6 Physical Oceanography: Tracking the drift of mixed-layer waters by use of drogued surface buoys
Volker Strass1, Rainer Graupner2 1AWI
2OPTIMARE Work at sea
In order to track the drift of the near-surface waters through our main study sites a total of 4 MetOcean SVP-Iridium drifters were deployed. These consist of a
spherical surface buoy of 40 cm diameter, equipped with GPS (Global Positioning System) receiver and Iridium satellite data transmitter, which is tethered to a 5.3 m long and 60 cm diameter wide drogue of holey sock style centred at 15 m depth. This setup assures that the drift of the surface buoy is determined by the flow at roughly mid-depth of the mixed layer.
Preliminary results
The deployment dates and positions are given in Table 3.6.1. The position data of the drifters were received by JouBeh Technologies Inc. and provided at their WebSite, from where they were copied on board via Polarstern’s internet connection.
Tab. 3.6.1: Deployments of drogued surface drifters Name Inst. ID IMEI Date (yyyy/
mm/dd) Time
(UTC) Lat. S Lon. W Transmis.
interval Buoy1 D02613 3002
3401 0672 650
2012/01/31 15:05 51°59.05' 12°00.06' 20 min
Buoy2 D026HZ 3002 3401 0775 110
2012/02/05 20:03 51°12.36' 12°39.62' 20 min
Buoy3 D0278X 3002 3401 0778 530
2012/03/01 14:11 49°39.32' 38°17.21' 20 min
Buoy4 D0253P 3002 3401 0870 150
2012/02/27 04:04 49°59.91' 38°13.09' 1 h
3. Physical oceanography
Fig. 3.6.1: Horizontal distribution of density st at 300 m depth in the station grid north of South Georgia. The density contours also imply the geostrophic flow field, with streamlines running parallel to the density contours and the flow direction oriented in a way that the light water is located to the left. The flow speeds measured by the VM-ADCP
(Section 3.3) in the northwest corner of the mapped area exceeded 100 cm s-1.