Metadata Report for BODC Series Reference Number 648975

• 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

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

RD Instruments- Ocean Surveyor 150kHz Vessel mounted ADCP.

¹ Ranges at 1 to 5 knots ship speed are typical and vary with situation.
² Single-ping standard deviation.
³ User's choice of depth cell size is not limited to the typical values specified.

Profile Parameters

• Velocity long-term accuracy (typical): ±1.0%, ±0.5cm/s
• Velocity range: -5 to 9m/s
• # of depth cells: 1 - 128
• Max ping rate: 1.5

Bottom Track

Maximum altitude (precision <2cm/s): 600m

Echo Intensity Profile

Dynamic range: 80dB
Precision: ±1.5dB

Transducer & Hardware

Beam angle: 30°
Configuration: 4-beam phased array
Communications: RS-232 or RS-422 hex-ASCII or binary output at 1200 - 115,200 baud
Output power: 1000W

Standard Sensors

Temperature (mounted on transducer)

• Range: -5° to 45°C
• Precision: ±0.1°C
• Resolution: 0.03°

Environmental

Operating temperature: -5° to 40°C (-5° to 45°C)*
Storage temperature: -30° to 50°C (-30° to 60°C)*

*later instruments have greater range.

Web Page

Further details can be found in the manufacturer's website or in the specification sheet.

Instrument Description

RRS James Clark Ross 150 kHz vessel-mounted ADCP

The RRS James Clark Ross carries a hull-mounted RDI 153.6 kHz Acoustic Doppler Current Profiler (ADCP), serial number 361. The ADCP is recessed into the hull behind a sea chest, which contains 90% deionised water and 10% ethylene glycol. A 33 mm sheet of Low Density PolyEthylene (LDPE) closes the underside of the chest. The effective depth of the transducer is 6 m (JR97 cruise report).

The ADCP is run through a PC (Pentium II) in the UIC, using version 2.48 of the RDI Data Acquisition Software (DAS) and version 17.07 of the firmware. The operation of this system is described in the JR97 cruise report.

References

JR97 Cruise Report, Autosub Under Ice Cruise to the southern Weddell Sea, RRS James Clark Ross, 3 February to 11 March 2005

BODC processing

The originator's files used for transfer were NOC Pstar format calibrated, gridded depth-dependent data, filenames 097adp[jday]d.abs, where [jday] is the day number of the file. There were 36 files supplied to BODC spanning 13:35:59 3rd February 2005 to 23:50:59 10th March 2005. The data were transferred (using transfer process tr339) to QXF format, a BODC-defined subset of NetCDF and BODC's format for 2-dimensional datacycle storage. Pstar null data were set to the appropriate absent data values for the code in the BODC parameter dictionary. Data values outside the range of the BODC parameter codes were automatically flagged as 'M' by the transfer. The following table shows how the variables within the Pstar files were mapped to BODC parameter codes:

Pstar variables present in the files but not mapped for transfer included: evelcal, nvelcal, ve, vn, a-ghdg and distrun. These were excluded on the grounds that that they were either intermediate processing stages or available in other datasets (i.e. underway navigation).

The reformatted data were visualised using the in-house EDSERPLO software. Suspect data wehe marked by adding an appropriate quality control flag, missing data by both setting the data to an appropriate value and setting the quality control flag. The navigation channels were checked against underway data for the same cruise that was received separately.

Originator's Data Processing

These notes have been adapted from the cruise report listed in the references. For a complete originator's processing description, please refer to the cruise report.

Sampling Strategy

The shipboard ADCP was run continuously during the cruise, apart from short breaks to reconfigure the settings. Throughout the cruise it was configured to run with 64, eight metre bins. The center of the first bin was effectively at a depth of 18 m, and thus the center of the last bin was at 522 m. In practice, good velocities were obtained down to about bin 40 to 50 (330 to 410 m), depending on conditions (the clearer waters near the ice shelf presumably having fewer scatterers). When bottom tracking was enabled (as it generally was in shallow waters), good bottom tracking velocities were obtained down to depths of 600 to 700 m.

The ADCPs bottom tracking mode was used initially, while the water was shallow enough for it to work (shallower than 600 m) to provide information for calibration, but was turned off once in deeper water. Having bottom tracking on reduces the number of water tracking pings, since the ADCP is spending half its time doing bottom pings.

The ADCP was monitored as part of general watch keeping during the cruise as follows:

• Hourly check: The ADCP display was checked to ensure that it was still operating, and the ship's UNIX terminals used to check the information was getting to the main system. The Ashtech GPS heading (required for correction of current direction) was also checked as sometimes this system hung and needed restarting.
• Four-hourly time drift check: Every four hours, the time shown by one of the ship's UTC digital master clocks at the end of a two minute ADCP ensemble was logged along with the PC time that the data have been stamped with and the difference between the ship's computer system and the master clocks. These times were used in subsequent processing of the data.

Data Processing

Originator's processing used the NOC (National Oceanography Centre, Southampton, UK) pstar system. Pstar is a set of Fortran scripts that read and process data from the instrumentation and ship's systems. Output files from pstar processing are in a binary format. Specific edits relating to the handling of bin depths and the processing of partial days of data were made to the pstar scripts for this cruise and are described in the cruise report. The sequence of pstar scripts used in the processing was as follows:

1. gyroexec0, to read the data (ship's heading) from the ship's gyro compass.
2. ashexec0, to read the data from the ship's Ashtech GPS system for correction of the gyro headings.
3. ashexec1, combines the Ashtech and gyro data.
4. ashexec2, quality controls Ashtech and gyro data.
5. navexec0, reads the ship's 'bestnav' data (the combined navigation data from the ship's GPS systems).
6. navexec1, averages and filters the bestnav data.
7. adpexec0, reads the ADCP data and produces water track and bottom track files. Navigation processing steps (including checks on the clock drifts) for the previous day needed to be complete before this could be run.
8. adpexec1, adjusts the timing of the ADCP data for clock drifts.
9. adpexec2, corrects for the errors in the gyro headings, making use of the differences between the gyro and Ashtech headings calculated in the ashexec pstar sequence (steps 2 to 4).
10. adpexec3, applies the data calibration velocity magnitude scale and angle offset (ADCP alignment), these were hardcoded within the script. Calibration of the ADCP is described in the next subsection.
11. adpexec4, subtracts the ship's velocity from the ADCP data to give absolute water velocities, producing files 097adp[jday]d.abs and 097bot[jday]d.abs.

Field Calibrations

Both the magnitude (scale correction) and direction of the ADCP data were calibrated. Errors in the magnitude of the measured velocities are caused by variations in the sound speed. The ADCP was calibrated by comparing ground speed from the bottom tracking ADCP velocities with ground speed derived from the GPS. Errors in the direction of measured velocities were corrected for the alignment of the ADCP (rotation by 1.78 degrees clockwise). Most errors in the direction of velocities are because the ADCP uses ship's heading from the gyro. These were corrected during the pstar processing by using data from the Ashtech GPS system.

Initial calibrations were carried out based on bottom-tracked data obtained on leaving the Falkland Islands, and further calibrations were carried out in the Fimbul Ice Shelf area and in the Filchner Depression. It was essential that at least the ADCP speeds (if not direction) were recalibrated when moving into waters of different temperature.

The scale factor from the initial data near the Falkland Islands was 1.0265, and this value was used for Julian days 34 to 37 on the southward journey and days 66 onward on the return journey (nominally 'warm' waters). For the bulk of the cruise, a value of 1.0409 was used, based on measurements around the Fimbul Ice Shelf. This was checked periodically, including in shallower waters around the Filchner Depression, and was not found to change significantly (though the four decimal places in the value exaggerates the accuracy: changes of ±0.002 or so over the course of a single day are not uncommon).

References

JR97 Cruise Report, Autosub Under Ice Cruise to the southern Weddell Sea, RRS James Clark Ross, 3 February to 11 March 2005

General Data Screening carried out by BODC

BODC screen both the series header qualifying information and the parameter values in the data cycles themselves.

Header information is inspected for:

• Irregularities such as unfeasible values
• Inconsistencies between related information, for example:
  • Times for instrument deployment and for start/end of data series
  • Length of record and the number of data cycles/cycle interval
  • Parameters expected and the parameters actually present in the data cycles
• Originator's comments on meter/mooring performance and data quality

Documents are written by BODC highlighting irregularities which cannot be resolved.

Data cycles are inspected using time or depth series plots of all parameters. Currents are additionally inspected using vector scatter plots and time series plots of North and East velocity components. These presentations undergo intrinsic and extrinsic screening to detect infeasible values within the data cycles themselves and inconsistencies as seen when comparing characteristics of adjacent data sets displaced with respect to depth, position or time. Values suspected of being of non-oceanographic origin may be tagged with the BODC flag denoting suspect value; the data values will not be altered.

The following types of irregularity, each relying on visual detection in the plot, are amongst those which may be flagged as suspect:

• Spurious data at the start or end of the record.
• Obvious spikes occurring in periods free from meteorological disturbance.
• A sequence of constant values in consecutive data cycles.

If a large percentage of the data is affected by irregularities then a Problem Report will be written rather than flagging the individual suspect values. Problem Reports are also used to highlight irregularities seen in the graphical data presentations.

Inconsistencies between the characteristics of the data set and those of its neighbours are sought and, where necessary, documented. This covers inconsistencies such as the following:

• Maximum and minimum values of parameters (spikes excluded).
• The occurrence of meteorological events.

This intrinsic and extrinsic screening of the parameter values seeks to confirm the qualifying information and the source laboratory's comments on the series. In screening and collating information, every care is taken to ensure that errors of BODC making are not introduced.

Project Information

AutoSub Under Ice (AUI) Programme

AutoSub was an interdisciplinary Natural Environment Research Council (NERC) thematic programme conceived to investigate the marine environment of floating ice shelves with a view to advancing the understanding of their role in the climate system.

The AUI programme had the following aims:

• To attain the programme's scientific objectives through an integrated programme based on interdisciplinary collaborations and an international perspective
• To develop a data management system for the archiving and collation of data collected by the programme, and to facilitate the eventual exploitation of this record by the community
• To provide high-quality training to develop national expertise in the use of autonomous vehicles in the collection of data from remote environments and the integration of such tools in wider programmes of research
• To stimulate and facilitate the parameterising of sub-ice shelf processes in climate models, and to further demonstrate the value of autonomous vehicles as platforms for data collection among the wider oceanographic and polar community

Following the invitation of outline bids and peer review of fully developed proposals, eight research threads were funded as part of AUI:

Physical Oceanography

• ISOTOPE: Ice Shelf Oceanography: Transports, Oxygen-18 and Physical Exchanges.
• Evolution and impact of Circumpolar Deep Water on the Antarctic continental shelf.
• Oceanographic conditions and processes beneath Ronne Ice Shelf (OPRIS).

Glaciology and Sea Ice

• Autosub investigation of ice sheet boundary conditions beneath Pine Island Glacier.
• Observations and modelling of coastal polynya and sea ice processes in the Arctic and Antarctic.
• Sea ice thickness distribution in the Bellingshausen Sea.

Geology and Geophysics

• Marine geological processes and sediments beneath floating ice shelves in Greenland and Antarctica: investigations using the Autosub AUV.

Biology

• Controls on marine benthic biodiversity and standing stock in ice-covered environments.

The National Oceanography Centre Southampton (NOCS) hosted the AUI programme with ten further institutions collaborating in the project. The project ran from April 2000 until the end of March 2005, with some extensions to projects beyond this date because of research cruise delays. The following cruises were the fieldwork component of the AUI project:

Table 1: Details of the RRS James Clark Ross AUI cruises.

All the cruises utilised the AutoSub autonomous, unmanned and untethered underwater vehicle to collect observations beneath sea-ice and floating ice shelves. AutoSub can be fitted with a range of oceanographic sensors such as:

• Conductivity Temperature Depth (CTD) instruments
• Acoustic Doppler Current Profillers (ADCP)
• A water sampler
• Swath bathymetry systems
• Cameras

In addition to use of AutoSub during each cruise measurements were taken from ship. These varied by cruise but included:

• Ship underway measurements and sampling for parameters such as:
  • Salinity
  • Temperature
  • Fluorescence
  • Oxygen 18 isotope enrichment in water
  • Bathymetry using a swath bathymetry system
• Full-depth CTD casts for with observations of samples taken for parameters such as:
  • Salinity
  • Temperature
  • Fluorescence
  • Optical transmissivity
  • Dissolved oxygen
  • Oxygen 18 isotope enrichment in water
  • Water CFC content
• Sea floor photography and video using the WASP system
• Sea floor sampling with trawls/rock dredges
• Sea ice observations (ASPeCt), drifters and sampling

The AutoSub project also included numerical modelling work undertaken at University College London, UK.

The project included several firsts including the first along-track observations beneath an ice shelf using an autonomous underwater vehicle. The AutoSub vehicle was developed and enhanced throughout this programme and has now become part of the NERC equipment pool for general use by the scientific community. Further information for each cruise can be found in the respective cruise reports (links in Table 1).

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: