Metadata Report for BODC Series Reference Number 604100

• 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

A visual inspection indicates numerous 'spikes' throughout the series which have been marked as suspect.

A visual comparison between these data and the data recorded at the same time at North Sea Project Survey Site A indicate two sections of dubious data. Namely, 10 ^th October 1988 (cycles 112- 143) and 28 ^th - 29 ^th October 1988 (cycles 2756 - 2805), which have been marked as suspect.

These abnormalities occur immediately after periods of reported gale force winds and are likely to have been caused due to the compass incorrectly responding to the frame movement under storm conditions. On both occasions, data cycles immediately after these dubious data have been corrected for frame movement. (Please see instrumentation and ADCP frame angle correction documents).

The contributor comments that the time channel increments by 30 seconds, instead of 10 seconds, every 219 data cycles.

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

POL 250 KHz Acoustic Doppler Current Profiler

The ADCP sends out short acoustic pulses, typically lasting a few thousandths of a second, at a fixed frequency. The acoustic pulses are transmitted in two narrow beams at right angles to each other and 30 degrees to the vertical and are reflected back to the ADCP by small particles, such as plankton, which move with the water. The frequency of the reflected signal is changed by a small amount proportional to the current speed, the Doppler shift. By measuring the frequency change along the two beams the speed and direction of the currents are determined. The currents at different heights through the water are obtained by chopping the return signal into segments by time.

The 250 KHz ADCP has a range of 100m and can measure up to 24 bins. However the technique has some limitations which reduce the amount of good data return. The closest bins to the transducer can give erroneous data due to the time taken for transients to decay and the far end bins can be affected by interference from side lobes reflected from the sea surface. Hence the good data return bins are usually between 25% of the depth from the surface and 10% of depth from the bottom.

ADCP specification

ADCP frame angle correction

The compass, manufactured by Digicourse (no longer in production), measures the angle between magnetic north and the frame. The two beams can be converted into east and north components by using the angle obtained by the compass and trigonometry. During the North Sea Project the compass showed a tendency to return arbitrary zero directions thus giving directions of flow different from those predicted by tidal models and previous current meter records. Also the frame moved on the sea floor during some storm events. A correction was then required to the frame angle for each deployment.

The ADCP data were first processed through the software. The statistical analysis gave the angle of maximum variance, which was taken to represent the M2 major axis tidal ellipse angle. The compass, although giving incorrect readings of frame angle, was recording direction to an unknown fixed position. The frame angle was adjusted by adding a correction angle.

The correction angle was calculated from the difference between the M2 major axis tidal ellipse angle from a model and the angle of maximum variance calculated from the initial raw data analysis. The model gave a depth average value, so the angle of maximum variance obtained from the data bin closest to this depth was needed. The bin used was found by taking the bin nearest to a value z=0.4D, where D is the total depth of water and z is the height from the bottom (Prandle, 1982). There is an 180 degree ambiguity in the calculation of the angle of maximum variance which was resolved by study of the M2 tidal phase given by the model and by other observations.

Prandle, D. 1982. The vertical structure of tidal currents and other oscillatory flows. Continental Shelf Research, 1(2), 191-207

Compass correction

The compass did not react correctly to the frame movements.

The following frame angle corrections have been applied:

North Sea Project Survey Site C ADCP data processing

Data processing at POL, UK

The 250KHz ADCP deployed during the Survey Study of the North Sea Project at Site C was configured as illustrated below. The ADCP angle correction was applied to the data (as described in the instrument description) and the data from the top (near surface) bins were excluded, as necessary, when they are not in the water.

ADCP set up details

Data processing at BODC

The data were converted from ASCII format into BODC QXF, a subset of NetCDF. The data where then visually inspected, for quality control purposes, using in house software.

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

North Sea Project

The North Sea Project (NSP) was the first Marine Sciences Community Research project of the Natural Environment Research Council (NERC). It evolved from a NERC review of shelf sea research, which identified the need for a concerted multidisciplinary study of circulation, transport and production.

The ultimate aim of the NERC North Sea Project was the development of a suite of prognostic water quality models to aid management of the North Sea. To progress towards water quality models, three intermediate objectives were pursued in parallel:

• Production of a 3-D transport model for any conservative passive constituent, incorporating improved representations of the necessary physics - hydrodynamics and dispersion;
• Identifying and quantifying non-conservative processes - sources and sinks determining the cycling and fate of individual constituents;
• Defining a complete seasonal cycle as a database for all the observational studies needed to formulate, drive and test models.

Proudman Oceanographic Laboratory hosted the project, which involved over 200 scientists and support staff from NERC and other Government funded laboratories, as well as seven universities and polytechnics.

The project ran from 1987 to 1992, with marine field data collection between April 1988 and October 1989. One shakedown (CH28) and fifteen survey cruises (Table 1), each lasting 12 days and following the same track, were repeated monthly. The track selected covered the summer-stratified waters of the north and the homogeneous waters in the Southern Bight in about equal lengths together with their separating frontal band from Flamborough head to Dogger Bank, the Friesian Islands and the German Bight. Mooring stations were maintained at six sites for the duration of the project.

Alternating with the survey cruises were process study cruises (Table 2), which investigated some particular aspect of the science of the North Sea. These included fronts (nearshore, circulation and mixing), sandwaves and sandbanks, plumes (Humber, Wash, Thames and Rhine), resuspension, air-sea exchange, primary productivity and blooms/chemistry.

In addition to the main data collection period, a series of cruises took place between October 1989 and October 1990 that followed up work done on previous cruises (Table 3). Process studies relating to blooms, plumes (Humber, Wash and Rhine), sandwaves and the flux of contaminants through the Dover Strait were carried out as well as two `survey' cruises.

The data collected during the observational phase of the North Sea Project comprised one of the most detailed sets of observations ever undertaken in any shallow shelf sea at that time.

Data Activity or Cruise Information

Data Activity

North Sea Project Data Activity: POLRIG#C37CC

Deployment

This rig was deployed as part of the NERC North Sea Project Survey at Station C.

Rig Description

The mooring contained two instruments, an Acoustic Doppler Current Profiler (ADCP) and an Aanderaa (RCM4) current meter. These were mounted on a frame positioned on the seabed, which was equipped with an IOS acoustic release. At the end of the deployment, acoustic signals sent from the ship triggered the release so that the frame separated from the ballast weight. The frame, under its own buoyancy (glass spheres), then rose to the surface ready for recovery.

Related Data Activity activities are detailed in Appendix 1

Fixed Station Information

Fixed Station Information

North Sea Project Survey Mooring Site C and CTD Site DM

Site C was one of six fixed stations where moorings were deployed during the North Sea Project Survey. This location is also one of 123 North Sea Project CTD Sites.

The site was characterised by moderate tidal currents, up to a maximum of 1.1 m/s. In summer months, the site was situated in a frontal region between stratified water to the north and vertically well mixed water to the south.

The rigs deployed here lie within a box bounded by co-ordinates 54.32N 0.393E at the southwest corner and 54.34N 0.422E at the northeast corner. Magnetic variation at this site was 4.9° west.

Site C deployment history is summarised below:

ADCP = Acoustic Doppler Current Profiler
CM = Current Meter (Aanderaa or S4)
THCH = Thermistor Chain

Related Fixed Station activities are detailed in Appendix 2

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:

Appendix 1: C37CC

Related series for this Data Activity are presented in the table below. Further information can be found by following the appropriate links.

If you are interested in these series, please be aware we offer a multiple file download service. Should your credentials be insufficient for automatic download, the service also offers a referral to our Enquiries Officer who may be able to negotiate access.

Appendix 2: NSP Survey C and CTD Site DM

Related series for this Fixed Station are presented in the table below. Further information can be found by following the appropriate links.

If you are interested in these series, please be aware we offer a multiple file download service. Should your credentials be insufficient for automatic download, the service also offers a referral to our Enquiries Officer who may be able to negotiate access.