Metadata Report for BODC Series Reference Number 110007

• 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

Neil Brown MK3 CTD

The Neil Brown MK3 conductivity-temperature-depth (CTD) profiler consists of an integral unit containing pressure, temperature and conductivity sensors with an optional dissolved oxygen sensor in a pressure-hardened casing. The most widely used variant in the 1980s and 1990s was the MK3B. An upgrade to this, the MK3C, was developed to meet the requirements of the WOCE project.

The MK3C includes a low hysteresis, titanium strain gauge pressure transducer. The transducer temperature is measured separately, allowing correction for the effects of temperature on pressure measurements. The MK3C conductivity cell features a free flow, internal field design that eliminates ducted pumping and is not affected by external metallic objects such as guard cages and external sensors.

Additional optional sensors include pH and a pressure-temperature fluorometer. The instrument is no longer in production, but is supported (repair and calibration) by General Oceanics.

Specifications

These specification apply to the MK3C version.

Further details can be found in the specification sheet.

Poseidon 31 CTD Data Documentation

Instrumentation and Data Acquisition

Data were collected using a Neil Brown Instrument Systems MK III

Conductivity, Temperature and Depth (CTD) probe. The data were archived on magnetic tape using a Nova computer which also calculated salinity and density (sigma t). Plots of profiles and yo-yo sections were also drawn by the computer but those presented here have been subjected to an extensive correction and editing scheme as the parameters derived in real-time suffered from instrumental effects. The sampling program also recorded data from the shipboard meteorological instruments once every two minutes.

The NBIS CTD delivers a data cycle every 32 ms, and the sampling program stored a cycle generally every 96 ms. A program change on 03 September (246) enabled every second data cycle to be stored, and this version of the program was used during the Second Box. Data cycles were lost when the computer was occupied writing data blocks to magnetic tape (0.288s for blocks to MT0, 0.416s for MT1) and when the meteorological data were sampled (0.192s when MT0 was on-line, 0.288s for MT1).

A new conductivity cell was fitted at the start of the cruise, on the manufacturer's recommendation, and a new thermistor was installed on 26 August (238) so the temperature data from the First Box were taken with only the platinum resistance thermometer.

The CTD was lowered with a nominal rate of 0.5 m/s, which gave a sampling interval of about 5 cm or 3 cm depending on the program speed. Only the descending parts of the profiles (including yo-yos) have been processed.

Throughout the periods of data acquisition, navigation fixes were taken at half-hourly intervals and at the start of individual stations of the box surveys. Generally the fixes were made using the Loran-C system but, when reception was poor, combinations of good Loran- C and Decca Navigator channels were found to produce consistent readings. Radar ranging using moorings and other ships were frequently made, especially during the multiship experiments but, while they endorse the positions derived from the routine navigation procedure, they have not been used to derive the navigation information used in this report.

Calibration and Accuracy

The precision of the NBIS CTD is ultimately limited by the digitising interval in each of the three channels. These limits are 0.0005K in temperature, 0.001 ms/cm in conductivity and 0.05 dbar in pressure. The electronics noise is stated to be smaller than these values (Brown 1974). Comparison between derived salinity and water bottle samples has shown that a precision of +/- 0.0024 ppt over a range of salinity values is possible. In practice, these limits are rarely reached and the accuracies of the measurements are determined by other factors, such as the accuracy of the calibration procedure, calibration drift, contamination of the sensors (especially the conductivity cell) and, in the presence of gradients, by the speed of response of the sensors.

The CTD used from F.S. Poseidon during JASIN was loaned by the University of Liverpool, UK, and had been calibrated at the Institute of Oceanographic Sciences (Wormley), UK. The temperature channel was found to be accurate to +/- 0.001K. This pre-cruise temperature calibration, being an equilibrium calibration, is of the platinum resistance thermometer and is equally applicable to temperature data gathered with or without the additional thermistor. The post cruise calibration in Kiel showed a small correction of less than 0.005K in the temperature range met in the JASIN area. As this is not larger than the quoted accuracy of the reference thermometers which were available at the time, the pre-cruise temperature calibration is believed to have been applicable throughout JASIN. This is in accordance with the manufacturer's claim for the stability of the temperature channel of +/- 0.001K per month. Comparison with sea-surface temperatures taken with a bucket thermometer at the start and end of selected profiles revealed no appreciable bias.

The conductivity cell and its matched interface card were replaced at the start of the cruise. The value of the cell factor given by the manufacturer is 0.99993 +/- 0.00001. Comparison with sea-surface salinity values determined with a Guildline `Autosal' thermostatic salinometer gave cell factors in the range 0.99948 to 1.00048. The variation with time was not systematic. In processing the data, a constant cell factor has been used for data from profiles forming a specific part of the cruise (e.g. for each of the Boxes, or for groups of yo-yo stations), which are shown in Table 5. The cell factors all lie within about +/- 0.0005 of unity, and this deviation produces changes in the derived salinity of about +/- 0.020 ppt in the range encountered in JASIN. Comparison with the surface salinities gives a scatter in the corrected derived salinities of 0.0073 ppt (standard deviation of 100 samples).

In addition to this limit on the accuracy of the salinity value, the `spiking' effect in the regions of rapidly changing temperature gradient produces a further local degradation of the salinity data. This effect has largely been removed in the data processing by a time constant correction to the temperature signal and a salinity spike rejection criterion. However, it is likely that some of the small closed contours in the salinity sections are caused by bad data which were not correctable. Thus, the accuracy of the derived salinity could be estimated at +/- 0.010 ppt with possible local degradation to say +/- 0.030 ppt in parts of the thermocline.

The temperature coefficient of the strain gauge pressure cell is believed to be one of the main sources of error in the pressure signal. The coefficient is given as 0.3 dbar/K. The characteristics of the heat transfer from the water to the cell are not known, but even if the full range of the temperature change reached the cell, an error of a few decibars at most would have been caused. As only data from the descending profiles have been used, the relative accuracy in the pressure signal is believed to be better than +/- 1 dbar. The effect of the pressure measurement error on the derived salinity is negligible: a 1 dbar error in pressure would introduce a salinity error of less than 0.0005 ppt.

The effects on the derived density (sigma t) of a measurement error of +/- 0.005K in temperature and an error of +/- 0.010 ppt in salinity are +/- 0.001 and +/- 0.008 respectively. Thus, in the region of a badly corrected salinity spike a sigma t error of say 0.025 might be experienced, but elsewhere the accuracy of the derived sigma t values is about +/- 0.010.

A selection of T-S curves drawn from the processed data were compared with similar diagrams of data from other ships when the stations were close by. Comparison with the processed data of ATLANTIS II, DISCOVERY, METEOR, PLANET and TYDEMAN, failed to reveal any discrepancies which could not be attributed to oceanic variability.

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

Joint Air Sea Interaction Experiment (JASIN)

The JASIN Project was designed to study the interaction of the atmospheric and oceanic boundary layers with the larger scale motions of the sea and the air.

The primary aims may be summarized as follows:

1. To observe and distinguish between the physical processes causing mixing in the atmospheric and oceanic boundary layers and relate them to the mean properties of the layers.

2. To examine and quantify aspects of the momentum and heat budgets in the atmospheric and oceanic boundary layers and fluxes across and between them.

The multiplicity of processes to be sampled necessitated a large experiment and JASIN involved 14 ships and 3 aircraft with more than 50 teams of investigators from 9 countries. Altogether 35 mooring systems were deployed.

The experiment lasted for 2 months from mid-July to mid-September 1978 and comprised 2 intensive measuring periods preceded by a preparatory test period. The project took place in the north Rockall Trough, an area of deep water (1000m - 2000m) several hundred kilometres off the west coast of Scotland.

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: