Metadata Report for BODC Series Reference Number 521403

• 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

Sea-Bird Electronics SBE 911 and SBE 917 series CTD profilers

The SBE 911 and SBE 917 series of conductivity-temperature-depth (CTD) units are used to collect hydrographic profiles, including temperature, conductivity and pressure as standard. Each profiler consists of an underwater unit and deck unit or SEARAM. Auxiliary sensors, such as fluorometers, dissolved oxygen sensors and transmissometers, and carousel water samplers are commonly added to the underwater unit.

Underwater unit

The CTD underwater unit (SBE 9 or SBE 9 plus) comprises a protective cage (usually with a carousel water sampler), including a main pressure housing containing power supplies, acquisition electronics, telemetry circuitry, and a suite of modular sensors. The original SBE 9 incorporated Sea-Bird's standard modular SBE 3 temperature sensor and SBE 4 conductivity sensor, and a Paroscientific Digiquartz pressure sensor. The conductivity cell was connected to a pump-fed plastic tubing circuit that could include auxiliary sensors. Each SBE 9 unit was custom built to individual specification. The SBE 9 was replaced in 1997 by an off-the-shelf version, termed the SBE 9 plus, that incorporated the SBE 3 plus (or SBE 3P) temperature sensor, SBE 4C conductivity sensor and a Paroscientific Digiquartz pressure sensor. Sensors could be connected to a pump-fed plastic tubing circuit or stand-alone.

Temperature, conductivity and pressure sensors

The conductivity, temperature, and pressure sensors supplied with Sea-Bird CTD systems have outputs in the form of variable frequencies, which are measured using high-speed parallel counters. The resulting count totals are converted to numeric representations of the original frequencies, which bear a direct relationship to temperature, conductivity or pressure. Sampling frequencies for these sensors are typically set at 24 Hz.

The temperature sensing element is a glass-coated thermistor bead, pressure-protected inside a stainless steel tube, while the conductivity sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. Thermistor resistance or conductivity cell resistance, respectively, is the controlling element in an optimized Wien Bridge oscillator circuit, which produces a frequency output that can be converted to a temperature or conductivity reading. These sensors are available with depth ratings of 6800 m (aluminium housing) or 10500 m (titanium housing). The Paroscientific Digiquartz pressure sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

Additional sensors

Optional sensors for dissolved oxygen, pH, light transmission, fluorescence and others do not require the very high levels of resolution needed in the primary CTD channels, nor do these sensors generally offer variable frequency outputs. Accordingly, signals from the auxiliary sensors are acquired using a conventional voltage-input multiplexed A/D converter (optional). Some Sea-Bird CTDs use a strain gauge pressure sensor (Senso-Metrics) in which case their pressure output data is in the same form as that from the auxiliary sensors as described above.

Deck unit or SEARAM

Each underwater unit is connected to a power supply and data logging system: the SBE 11 (or SBE 11 plus) deck unit allows real-time interfacing between the deck and the underwater unit via a conductive wire, while the submersible SBE 17 (or SBE 17 plus) SEARAM plugs directly into the underwater unit and data are downloaded on recovery of the CTD. The combination of SBE 9 and SBE 17 or SBE 11 are termed SBE 917 or SBE 911, respectively, while the combinations of SBE 9 plus and SBE 17 plus or SBE 11 plus are termed SBE 917 plus or SBE 911 plus.

Specifications

Specifications for the SBE 9 plus underwater unit are listed below:

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

MORENA 3B (Haakon Mosby) CTD Data Documentation

Introduction

Documentation for data collected on board the RV Haakon Mosby between 9th August and 17th August 1994, during a scientific research cruise off the Iberian Peninsula. The cruise was part of the EC funded MAST II project MORENA, the objective of which was to study exchange at th Iberian shelf break. The cruise started in Oporto and ended in Vigo.

Cruise Objectives

The main objectives of the cruise were as follows:

Mesoscale Physics

1. To obtain a high resolution description of the hydrographic (T,S), current and fluorescence structure of a filament as it extends into the ocean.

2. To describe the temporal variability of super and sub-inertial internal waves, currents, sea level and chlorophyll a at two fixed locations across the shelf break at 41° N.

3. To make physical measurements that will assist in the evaluation of numerical models.

Internal Waves

1. To determine the oceanic density field to the sea-bed at 41° N to drive a model of internal tide generation.

2. To make in situ measurements of some internal wave trains propagating shoreward from the shelf break.

Remote Sensing

1. To cross calibrate and validate ATSR and AVHRR satellite observations of SST.

2. To measure a suite of environmental variables at the sea surface for use in SST models.

3. To measure, describe and quantify the sea surface skin temperature (SSST).

4. To make in situ observations of surface slicks in relation to SAR images.

Nitrogen fluxes

1. To quantify the seawards transfer of NO₃ and phytoplankton across the Iberian Atlantic shelf-break within the filament system.

2. To estimate the NO₃ and NH₄ uptake rates by fractionated phytoplankton and NH₄ regeneration rates within the filament system and associated shelf mesoscale structures.

3. To make supporting observation on the distribution size-fractionated chlorophyll a concentrations.

Humic Substance

1. To obtain 3200 l of water from the open ocean in order to sample humic substance for comparison with similar samples taken from rivers and near the coast (in fact a smaller quantity was eventually required). MORENA 3B CTD calibration procedure.

Instrumentation

The CTD (measuring conductivity, temperature and pressure) used in the survey was the Sea-Bird 911 Plus, which has an accuracy of 0.002 mmho/cm for conductivity, 0.0003 °C for temperature and 0.015% of the total scale for pressure. It encompasses a pressure-protected thermistor and a conductivity sensor with a totally internal electrical field, which are constantly flushed to ensure cells sample the same water, and Paroscientific Digiquartz pressure sensor.

Sampling Protocols

At certain stations throughout the cruise, bottle samples were taken at specified depths during the upcast of the CTD deployment. This enabled water sample conductivity (salinity) to be compared to that recorded by the CTD conductivity cell, and if necessary allow for calibration of temporal errors. In order to compare samples and measurements at exactly the same depth, it was important to clarify the corresponding period over which the CTD conductivity measurements and bottle salinity sample were taken. An appropriate period over which to average the CTD measurements was estimated to be 0.75 seconds above and below the mark indicating the time when the bottle was fired (closed). This allowed the CTD salinity values to be averaged over a 1.5 second interval for comparison with the bottle sample. The actual bottle sample salinities were calculated using a standard bench salinometer from persons involved in the Morena project [Please refer to Prof A. Fiuza, University of Lisbon for further information - T.J. Sherwin].

Salinity Calibration

In order to eradicate leaking bottles and erroneous salinity readings it was necessary to eliminate outlying points. Initially, the difference between bottle salinity and CTD salinity, was calculated for each bottle sample taken and all differences greater than one standard deviation were eliminated. For the remainder, the standard deviation of the differences was calculated. All values lying outside of two standard deviations (95% confidence level) were eliminated. This process was repeated again in order to ensure that all erroneous values were eradicated.

Forty four bottles were available for calibration, all of them within 1 psu of the CTD values. Two samples were deleted after the first run of the method and a further station was excluded after repeating the method. An estimate of the mean value of the salinity difference for all 44 stations was calculated as 0.015 psu, compared to a value of 0.013 psu for the remaining 42 stations and 0.012 psu for the final 41 stations.

A linear fit, against time, was applied to the remaining 41 samples to estimate the sensors drift i.e. the increase in the value of the mean difference as a function of time. This resulted in an estimation of the mean difference at the start of the survey (taken as 0000 GMT on 11 August) as 0.0054 psu, with a drift correction of 7.2*10^-5 psu/h. The corrected salinity value could then be calculated from:

True salinity = CTD salinity + 0.0054 + (7.2*(10^-5) * time

where time was measured in hours after the start of the survey. This method would therefore provide an estimate for the maximum error in the CTD salinities of about 0.01555 psu by the end of the cruise. In order to determine water mass characteristics accurately, a correction for this error was applied to each CTD salinity measurement, reducing errors induced by drift to a minimum.

Further, by applying a linear fit of salinity difference against pressure, it was possible to calculate whether pressure (depth) had a detrimental effect on the conductivity sensor. The distribution showed that most errors occurred in the upper 150 m of the water column, probably due to displacement of the pycnocline and hence there was insufficient evidence to say that pressure had any detrimental effects on salinity measurements.

For further information contact:

Dr. T.J. Sherwin
Unit for Coastal and Estuarine Studies
University of Wales Bangor
Marine Science Laboratory
Menai Bridge
Anglesey LL59 5EY
e-mail: tjs@uces.bangor.ac.uk

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

No Project Information held for the Series

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: