Metadata Report for BODC Series Reference Number 1223822

• 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 Access Policy

Public domain data

These data have no specific confidentiality restrictions for users. However, users must acknowledge data sources as it is not ethical to publish data without proper attribution. Any publication or other output resulting from usage of the data should include an acknowledgment.

The recommended acknowledgment is

"This study uses data from the data source/organisation/programme, provided by the British Oceanographic Data Centre and funded by the funding body."

Narrative Documents

Instrument Description

A Sea and Sun Technology MSS 90L Microstructure Profiler equipped with two velocity microstructure shear sensors, a microstructure temperature sensor, standard CTD sensors for precision measurements, a turbidity (light scattering) sensor, a vibration control sensor, and a two component tilt sensor was used for all casts. All sensors were mounted at the measuring head of the profiler (sensor end). The microstructure sensors were placed at the tip of a slim shaft, about 150 mm in front of the CTD sensors.

Calibration of the shear sensors was performed by ISW Wassermesstechnik using a special shear probe calibration system. The probe rotates about its axis of symmetry at 1Hz under an angle of attack in a water jet of constant velocity. At different angles of attack the rms voltage output of the probe is measured. The probe sensitivity is the slope of the regression (best fit of a cubic approximation) of the sensor output versus the angle of attack. The calibration constants for the shear sensors used during cruise are given in the table below. The calibration of the CTD sensors was to be carried out by Sea and Sun Technology GmbH using standard calibration equipment and procedures for CTD probes post cruise. The vibration control sensor and the tilt sensors were calibrated by ISW Wassermesstechnik using special calibration equipment for both sensors.

Calibration constants for velocity microstructure shear sensors

Microstructure Profiler MSS 90 and Sensors

The instrument was operated via a winch , mounted on the port stern quarter of the vessel. In addition to the fastening provided by the construction of the mounting plate, the plate was further secured to the bulwark with two steel bolts, running through bulwark and plate, and with a pair of G clamps. The power cable for the winch and the data cable connecting the PC to the profiler were run along the port side of the vessel and into the deck laboratory through the bosun's locker.

BODC Processing

Eighty-six profiles were attempted, data from three profiles were not provided due to poor data, a serious instrument snag and unuseable data due to variable sinking velocity. Eighty-three profiles were provided from 11 stations. The data were received in ASCII .tob format, three files for each profile, epsilon, shear and eddies. Only the variables from the final processed eddy files were loaded.

The raw shear files contained the variables; conductivity from microstructure probe, temperature from microstructure probe, pre-amplified temperature from microstructure probe, shear measured by probe 1 and 2, tilt and pseudoshear which have not been loaded. The epsilon files contained the variables; dissipation rate derived from sensor 1 and 2, combined dissipation rate, noise estimate of dissipation rate, tilt and potential density which have not been loaded. Tilt and turbidity were included in all files and were also not loaded. Unprocessed and intermediate files and variables are available on request.

The eddy files were converted into BODC internal format with one file per profile. Variables which were not loaded from the eddy files included; tilt, noise estimate of dissipation rate due to pseudoshear, potential density, derived quantity used for Thorpe scale calculation, buoyancy frequency and estimated eddy diffusivity from temperature microstructure and Thorpe scale. These are available on request.

The following table show how the variables within the Eddy ASCII file was mapped to appropriate BODC parameter codes:

Originator's Data Processing

Sampling Strategy

The Sea and Sun Technology MSS 90L Microstructure Profiler had no pre-planned stations and was deployed on an ad-hoc basis whenever the station schedule allowed, or whenever there was downtime due either to equipment problems or the weather.

At each station five to ten profiles were attempted in order to take into account the intermittent nature of marine turbulence. Five profiles were considered to be the minimum practical number of profiles to calculate profiles of mean turbulent diffisivity. Seven or more profiles were achieved during the one and half hours sampling time, for all stations except the velocity calibration station one. A profile depth of 200 dbar was chosen to ensure that both the upper mixed layer, if any, and the region directly below it were covered by each profile. The measurement interval was 10 minutes for a profile to 200 dbar.

For vertical sinking measurements, the profiler was balanced with a negative buoyancy which gave it a sinking velocity of 0.7 ms^-1. During the measurements, the ship was moving with speed 0.5 - 1.0 knots with respect to the water against the wind. Disturbing effects caused by cable tension (vibrations) and the ship's movement were minimized by maintaining slack in the cable.

Data processing of the measurements is relatively robust to tension in the cable and only in cases where the profiler is pulled upwards by the cable do the measurements have to be discarded. Nevertheless, fluctuations in profiler sinking velocity increase the error in the shear calculation by an order of four, while cable tension increases the background noise recorded by the shear sensors reducing the recording accuracy of the instrument.

At the start of the cruise the sinking velocity of the probe was checked as it should be between 0.5 ms^-1 and 0.7 ms^-1 to give best signal to noise ratio. Profiles at station one were used to check the sinking velocity.

Data processing

The raw data from the profiler were transmitted via RS485 data link to the on board interface unit of the measuring system. For data acquisition, on-line display and storage of data the software package SDA 180 (Sea and Sun Technology GmbH) was used. Raw data were stored in the MRD (microstructure raw data) binary format. Raw MRD format data were then processed using the MSSpro software package (ISWWassermesstechnik) which was used to calculate shear, turbulent kinetic energy dissipation and turbulent diffusivity. The processed data were stored in ASCII (filename extension .tob) format.

The processing procedure for obtaining turbulent diffusivity followed that described in the D321 cruise report. The following provides a summary of processing steps taken as recorded in the data files.

1) Files were converted to ASCII.

2) Profiles were trimmed, eliminating values outside a given error range calculated from the standard deviation. The interval length of 40 lines was used. To suppress high frequency noise in the data, a low pass filtering of the pressure values was carried out. A filter length of 3001 lines was applied.

3) The time channel was added and was used to calculate the time derivative of the raw shear. Physical shear was then calculated in terms of s^-1.

4) The tilt of the probe was calculated using the g-Force sensor.

The following processing steps were carried out for the calculation of epsilon and eddies.

5) Values of shear sensors were detrended by subtracting the linear trend calculated across a given number of lines. The interval of 1024 lines was used.

6) The time response (exponential function) of the temperature channel was corrected with one time constant, including low pass filtering.

7) Dissipation rates were computed by integrating the shear power spectrum using the input channels pressure, shear, temperature and time.

8) The average of the dissipation rates (eps1 and eps2) was calculated creating eps.

9) The variable epsilon was then calculated.

10) The sensors were averaged over pressure interval of 0.5.

11) Sigma-t, thorpe scale, and salinity using the UNESCO PSS-78 scale, were all calculated.

12) Squared buoyancy frequency (the Brunt Väisälä Frequency) (N²) was calculated using the depth values of pressure and density anomaly.

The following processing steps were carried out for the calculation of eddies.

13) K_diss was calculated using epsilon and N².

14) K_thorpe was calculated from microstructure temperature and Thorpe scale.

Please refer to the user manual and the JC029 cruise report for further processing details.

Sea and Sun Technology MSS 90 Microstructure Profiler

A multiparameter probe used to measure micro-scale water stratification as well as the intensity of small scale turbulence in the water column in marine and limnic environments. The instrument can be used for free sinking or rising measurements. The instruments sinking or rising velocity can be adjusted by a combination of weights and buoyancy elements. It has a depth rating of approximately 500 m, samples at 1024 Hz and is equipped with high resolution micro-structure and turbulence sensors (temperature, current shear) and standard CTD sensors (temperature, conductivity, pressure). All microstructure channels have a response time of less than 12 ms. The unit includes internal sensors to control for vibrations and tilt internally.

The system comprises the profiler, a winch, a probe interface and data acquisition computer. Additional sensors, such as oxygen and optical scattering, can be attached to the profiler. The instrument can support up to a maximum of nine sensors.

There are two different models available which have the same electronic and sensor equipment, with the difference relating to the size and weight of the instruments, see the table below for further details. The shear sensor data quality of the MSS90L is however superior due to the higher mass and stability.

Further details can be found in the manufacturer's manual.

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: