Metadata Report for BODC Series Reference Number 1171915

• Metadata Summary

• Problem Reports

• Data Access Policy

• Narrative Documents

• Project Information

• Data Activity or Cruise Information

• Fixed Station Information

• BODC Quality Flags

• SeaDataNet Quality Flags

Metadata Summary

Problem Reports

No Problem Report Found in the Database

Data Quality Report

The RDI 300 kHz Workhorse ADCP 9192 downward looking ADCP beam 3 failed on station 25 also due to a low pressure leak via the transducer potting. The unit was replaced with the instrument 9191 which had previously been used as an upward looking slave. Both failed ADCPs were returned to RDI San Diego post-cruise for repair.

Data Access Policy

Open Data supplied by Natural Environment Research Council (NERC)

You must always use the following attribution statement to acknowledge the source of the information: "Contains data supplied by Natural Environment Research Council."

Narrative Documents

LADCP Instrumentation

The ADCPs deployed during the cruise were mounted on a Sea-Bird 9ll plus CTD suite, details of this can be found in this document. A total of four different ADCPs were used during the cruise, one RDI 150 kHz BroadBand ADCP, phase III, with transducers mounted at 20° from the vertical. This instrument was downward looking and was mounted at the centre of the rosette frame, below the rosette mechanism. Three RDI 300 kHz Workhorse Monitor ADCPs were also used, one configured looking up and one configured looking down at a time. The downward looking ADCP was mounted off-centre at the bottom of the frame and the upward looking ADCP was mounted outside the main frame.

The ADCPs were powered by the NMF BB150 battery pack (serial number 02). Battery pack WH004 was also fitted to the CTD frame available as a spare, but was not used. Battery pack WH002 was also available as a spare. Additionally spare batteries were available for all battery packs. The broadband battery was fully charged at 55 V until it was drawing 100 mA between each cast. Every 10-15 casts the battery was vented. Mid-cruise charging the VMP was given priority therefore the BB150 battery pack was not always fully charged between casts.

There were five different configurations which were used during the LADCP operations. The Broadband LADCP used a water-track configuration with 16 16 m bins per ping, a 16 m blank after transmit and an ambiguity velocity of 2.5 m s^-1. The upward looking Workhorse ADCP slave used 16 10 m bins per ping, a 5 m blank after transmit and an ambiguity velocity of 2.5 m s^-1. There were three different water-track configurations used for the downward looking Workhorse ADCPs, one which was used when more than one instrument was deployed as a master and slave arrangement and two different configurations used when only the downward looking ADCP was deployed (no slave). All Workhorse configurations used 16 10 m bins per ping, a 5 m blank after transmit, an ambiguity velocity of 2.5 m s^-1 and either a ping cycle of 1 second or 0.5 seconds. When more than one instrument was used they were synchronised so that the downward looking Workhorse ADCP pinged 0.5 seconds after the Broadband and upward looking Workhorse ADCPs. Most casts used a no slave configuration. Please see Table 4 on page 50 of the cruise report for details as to which configuration was used for each cast.

Three separate LADCP instruments were fitted and deployed for the first test deployment at station 4, however, the data from the RDI Broadband LADCP were poor and so the instrument was removed. The Broadband LADCP was tried again at stations 14, 41 and 43 after a number of components had been tested and replaced. The originators' recommended that the instrument needs very careful scrutiny.

Stations 6, 8, 12 and 14 operated with both a downward looking master RDI Workhorse ADCP and a upward looking slave RDI Workhorse ADCP. A single downward looking RDI Workhorse ADCP was used from station 16 onwards to conserve the instruments for the following cruises.

The RDI 300 kHz Workhorse Monitor ADCPs were found to suffer failure on repeated pressure cycling. The RDI Workhorse upward looking ADCP had a weak beam from station 6, and failed on station 10 due to a low pressure leak via the transducer potting and so the unit was replaced. The RDI Workhorse downward looking ADCP beam 3 failed on station 25 also due to a low pressure leak via the transducer potting. The unit was replaced with the instrument which had previously been used as an upward looking slave. Both failed ADCPs were returned to RDI San Diego post-cruise for repair.

Teledyne RDI's Workhorse Monitor ADCP

The Workhorse Monitor acoustic doppler current profler (Teledyne RD Instruments) is a long-range and long-term self contained ADCP. It has a patented four beam signal (300, 600 or 1200 kHz) and a standard depth rating of 200m or 600m. It operates effectively between temperatures of -5°C and 45°C and has a velocity accuracy of ±1% ±5mm/s.

BODC Processing

Both UH and LDEO processed data were provided by the originators in Matlab and ASCII formats. A separate folder for each profile was provided containing all processing. The LDEO processed data in ASCII format were converted into BODC internal format after discussion with the originator. All data provided were from the downward looking RDI 300 kHz Workhorse ADCP which was in operation at the time. The following table shows how the variables within the ASCII files were mapped to appropriate BODC parameter codes:

All reformatted data were visualised using the in-house Edserplo software. Suspect and missing data were marked by adding an appropriate quality control flag.

Note that the UH processed data and more complex LDEO processed data are available on request in Matlab format.

Originator's Data Processing

Sampling Strategy

A total of 68 LADCP profiles were obtained from the deployment of 60 CTD casts including two tow-yo casts at stations 8 and 44 each having 4 profiles. There were two test casts at stations 4 and 6 which were not full depth and did not have bottom tracked data. Note that station 2 which was a test CTD cast did not include the LADCP instruments.

Data Processing

The data from each instrument was processed separately, providing independent estimates of full-depth velocity profiles, giving an indication of the quality of instrument when compared with other LADCPs and the VMADCPs. In principle, combining datasets at the processing stage may be conducive to more accurate velocities, but this was not investigated. Two different processing methods were carried out, the University of Hawaii (UH) processing method based upon Firing and Gordon (1990) and the Lamont-Doherty Earth Observatory (LDEO) processing method based upon Thurnherr (2011).

UH processing method

The following processing steps were made for the UH method:
1) Magnetic variation was corrected,
2) Velocity shear profiles from individual pings were merged into a single downcast or upcast profile,
3) The navigation file was updated,
4) Relative velocity profiles were calculated without CTD data,
5) Differences between upcasts and downcasts were checked.
6) CTD data were then included to permit a revision of the LADCP velocity profile according to more accurate estimates of depth and sound velocity. The 1 Hz CTD data (calibrated and de-spiked) from both the downcast and upcast were created for each station, aligned with the LADCP data in time and then the final absolute velocity profiles were calculated.

LDEO processing method

The following processing steps were made for the LDEO method using LDEO Visbeck processing software version IX.6. Along-beam velocities were transformed into an Earth-based coordinate system (u, v, w). Velocity errors were calculated as the difference between the two estimates of vertical velocity scaled by the LADCP to give the correct magnitude of horizontal-velocity random errors. Error velocity values >0.5 m s^-1 were removed. Magnetic deviation, rotate bottom track and water velocities were then applied. Profiles with a tilt derivative >4° were removed, as well as ensembles from down-looker which were suspected to be non pinging were removed. Other outliers were also removed from both down looking and bottom track values.

The GPS data and navigation time series were then added. A bottom-track was created in addition to the one created by the instrument and outliers removed were water bottom track - water reference difference > 0.05. The next processing step was to add the CTD time-series in order to correct for sound speed and depth. The CTD data were used to provide direct measurements of depth which were calculated from pressure and accurate latitude. The CTD data were also used to calculate sound speed using salinity, pressure and temperature. Sound speed was calculated based on pressure and ADCP temperature.

The velocities and the bin length were then corrected for sound speed. Data at the beginning and end of cast were removed and then the start and end time were adjusted. Pitch and roll corrections were applied and the super ensembles were formed. Any super ensemble outliers were removed and the super ensembles were reformed. Lastly, the inverse solution shear solution and diffusivity profile were calculated. Please see the Thurnherr (2011) document for further processing information.

References

Thurnherr, A.M, 2011. How To Process LADCP Data With the LDEO Software, Version IX.7

Firing, E. and Gordon, R.L., 1990. Deep Ocean Acoustic Doppler Current Profiling, Proceedings of the IEEE Fourth Working Conference on Current Measurement, Clinton, Maryland, 3-5, 192-201.

Project Information

Southern Ocean FINEStructure (SOFINE) project document

The Southern Ocean FINEStructure (SOFINE) project was a UK field programme aimed at studying the frictional processes that slow down the Antarctic Circumpolar Current (ACC) and influence the meridional exchange of water masses in the Southern Ocean.

The study investigated the role of sea floor topography in slowing the ACC and driving meridional flow across the Southern Ocean, and the manner in which mesoscale and small scale oceanic phenomena modified water mass properties and affected their movement across the ACC. Specifically, SOFINE set out to:

• Determine the relative importance of oceanic processes associated with large scale (hundreds to thousands of kilometres) and small scale (a few kilometres) sea floor topography in the context of ACC flow rates and water mass exchange.
• Identify the oceanic processes controlling the rate at which water masses are transformed and fluxed across the ACC.

The SOFINE experiment focused on a major meander of the ACC around the northern Kerguelen Plateau in the Indian Ocean. Theories and models of Southern Ocean circulation indicated that this region experienced intensified 'friction' and cross-ACC flow. Fieldwork was undertaken over a 52 day period in November and December 2008, and included hydrographic observations, microcstructure and turbulence measurements, detailed bathymetric surveys and several deployments of floats, drifters and moorings.

SOFINE was funded by the UK Natural Environment Research Council and involved the collaboration of a number of international institutions: the National Oceanography Centre (UK), the University of East Anglia (UK), British Antarctic Survey (UK), Woods Hole Oceanographic Institution (US), the Commonwealth Scientific and Industrial Research Organisation (Australia), the University of Tasmania (Australia) and the Leibniz Institute of Marine Sciences (IFM-GEOMAR) at the University of Kiel (Germany).

For more information please see the official project website at SOFINE

Data Activity or Cruise Information

Cruise

Complete Cruise Metadata Report is available here

Fixed Station Information

No Fixed Station Information held for the Series

BODC Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle:

SeaDataNet Quality Control Flags

The following single character qualifying flags may be associated with one or more individual parameters with a data cycle: