Metadata Report for BODC Series Reference Number 684454

• 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

RD Instruments 150kHz Narrow Band Acoustic Doppler Current Profiler

Specifications

RAPID OC401 150kHz VMADCP processing

This document describes the processing steps undertaken to work up the VMADCP data from cruise OC401.

Data Originator's Processing

Processing by the originator was performed using the CODAS software suite of the "Currents" group at the University of Hawaii, SOEST, Department of Oceanography. This software is freely available to download.

• CODAS processing overview

  A comprehensive description of the CODAS processing suite is available from the following web address: http://currents.soest.hawaii.edu/docs/adcp_doc/index.html

  This website also contains help on how to use the suite and the complete user manual. The following description of the software suite was taken from the CODAS website.

  CODAS (Common Ocean Data Access System) is more than a database. The word has come to be associated with a suite of open-source programs for processing ADCP data. CODAS consists of C, Matlab, and scripts that will run on Windows, Linux, SunOS, or Mac OSX, and can process pingdata from a Narrowband ADCP, data collected from a Broadband or Ocean Surveyor data by VmDAS, or data collected by any of those instruments using UHDAS (open source acquisition software that runs RDI ADCPs). Older processing scripts were written in Perl, but the newer versions of these scripts are written in Python.

  Some kind of data treatment is necessary because the acquisition programs write binary files to the disk that are not readable by commercial plotting packages. In fact, there are not actually any ocean velocities stored in the files. A shipboard ADCP reports currents measured along each of its beams. These currents must be transformed into earth coordinates, and the motion of the ship taken out. Ancillary data such as heading and position are used to determine the ocean velocity from the measured velocities.

  There are at least four necessary processing steps which are performed, or made possible, by the CODAS routines. First, an ocean reference layer is used to remove the ship's speed from the measured velocities. By assuming the ocean reference layer is relatively smooth, positions can be nudged to smooth the ship's velocity, which directly results in the smooth reference layer velocity. This was more important when fixes were rare or jumpy (such as with LORAN) or dithered (such as SA GPS signals prior to 2001).

  Second, calibration routines are available to estimate the heading misalignment from either "bottom track" or "water track" data. Water track calibration routines use sudden accelerations (such as stopping and starting of the ship when doing station-work) to derive a heading misalignment. For a ship travelling at 10 kts, a 1-degree heading error results in a 10 cm/s cross-track velocity error. It is critical that the misalignment be accounted for if one is to avoid cross-track biases in the velocities.

  Third, a GPS-derived heading source (such as Ashtech, POSMV, or Seapath) may provide a more accurate (though often less reliable) heading source than a gyro. Routines are in place for pingdata and UHDAS data to correct the gyro heading with the GPS-derived heading, using a quality-controlled difference in headings. Gyro headings may be reliable but they can vary with oscillations of several degrees over several hours, thus creating spurious fluctuations in the ocean velocity that resemble "eddies" , but which are solely the result of cross-track velocity errors (from the associated gyro heading errors).

  Fourth, it is crucial that bad data be edited out prior to use. Traditionally, the data available from the DAS2.48 narrowband data were averaged in 5 minute groups. VmDAS and UHDAS also output time-averaged data, which can be loaded into the CODAS database for further processing. With CODAS processing, a graphical interface allows identification of the bottom and selection of bad profiles or bad bins based on a variety of criteria. To some extent this can be automated, for final processing, a person must visually inspect all the averages from a cruise. The graphical interface vastly speeds up editing to the point where it takes only a few minutes of user time per day of data for a typical cruise.

  CODAS processing is moving beyond averaged data to the realm of single-ping data. Whatever acquisition program was used to record the data also averages it. Prior to averaging, some attempt is made to eliminate bad pings. CODAS processing includes routines that average single-ping data collected by VmDAS or UHDAS. These routines allow the single-ping data to be screened more extensively prior to averaging. Under certain conditions, this may be necessary to avoid subtle underway biases caused by bubbles or ice near the transducer. CODAS processing includes the ability to read single-ping data files and look at the characteristics of the instrument (such as acoustic backscatter or beam velocities) one ping at a time.

• Series specific processing

  The Ashtech record has a gap from about 121.6 to 123.1. This time span covers the southern-most portion of the cruise track. Therefore, a time series of Ashtech-gyro correction data was generated. The time series consists of the original Ashtech-gyro data (mostly), some linear interpolation over small gaps (<1 hour), and the Ashtech-gyro model only for the 1.5 day long gap.

  The model was based on binning Ashtech-gyro versus heading over 20 degree bins of ship's heading. Ashtech data that fell within 80 minutes following a ship's turn of at least 20 degrees were excluded, so as to exclude Schuler-type short term gyro oscillations from the fit. That cleaned up tremendously the Ashtech-gyro versus heading plot, but left some 20 degree heading bins without data. However, the binned Ashtech-gyro curves without considering the post-ship's turn data looked very similar, suggesting that the Schuler oscillation gyro errors averaged out. The largest difference was about 0.5 degrees for heading bins with very few points in the "cleaned-up" time series. The model fit then was based on the cleaned-up binned data (essentially leaving the model unconstrained over the empty heading bins), though it would have made little difference to have based it on the binned unconstraint time series.

  Reran the Ashtech model with an Ashteh-gyro offset of 177 degrees. That got the Ashech-gyro correction, i.e., the input file to "rotate", down to between -0.5 and 2 degrees, as supposed to 176.5 and 179 degrees. It is of little meaning, since a bottom/water track cal still needs to determine the constant transducer/heading offset. However, it is more intuitive to not have the large constant Ashtech antenna array offset in there.

  Editing based on approach of CODAS btcaluv: eliminate outliers defined as 2.5 standard deviations beyond the mean.

  run rotate rotate_newbt.cnt with calibration corrections of:

  • amplitude= 0.997
  • angle_0= 0.62

BODC post-processing and screening

• Reformatting

  The data were converted from ASCII format into BODC internal format (a netCDF subset) to allow use of in-house visualisation tools.

• Screening

  Reformatted data were visually checked using the in-house editor EDSERPLO. No data values were edited or deleted. Flagging was achieved by modification of the associated quality control flag to 'M' for suspect values and 'N' for nulls.

• Banking

  Once quality control screening was complete, the data were archived in the BODC National Oceanographic Database and the associated metadata were loaded into an ORACLE Relational Database Management System.

Project Information

Line W Project

Introduction

Line W is a U.S-led initiative to monitor the North Atlantic Ocean's deep western boundary current. The programme is funded through the U.S National Science Foundation and has been active since October 2001. It brings together scientists from Woods Hole Oceanographic Institution (WHOI) and Lamont-Doherty Earth Observatory (LDEO). Between 2004 and 2010, scientists from the RAPID WAVE project (a component of the U.K's RAPID Climate Change Programme) also collaborated with Line W. This U.K element was funded by the Natural Environment Research Council (NERC) and brought additional instrumentation (predominantly bottom pressure landers) to the mooring array. The contact details of the principal collaborators involved with Line W are noted below.

Users of these data are referred to the Line W Project Website for more information. The following text has been taken from the website.

Scientific Rationale

Located on the continental slope south of New England (near 40°N, 70°W) Line W is one component of a long-term climate observing system that is positioned to quantify variability in the deep limb of the Atlantic meridional overturning circulation (MOC). Combining an array of moored instruments with shipboard observations, Line W is designed to directly measure the time dependence of volume transport, advection of property anomalies, and propagation of topographic Rossby waves and boundary waves in the equatorward flowing deep western boundary current (DWBC). These measurements are key to clarifying the deep ocean response to variability in high-latitude air-sea exchanges and, ultimately, the ocean's role in global climate variability through changes in its transport of heat and freshwater.

Instrumentation

Types of instruments and measurements:

• Moored Profilers (temperature, salinity, velocity)
• Current meters (VACMs) with Temperature/Conductivity sensors and upward-looking ADCP
• Shipboard measurements: CTD, CFCs, salinity, dissolved oxygen, I129, LADCP, ADCP

The full array of instruments was installed April 2004 with servicing as follows:

• Annual spring turnaround for profilers
• 2-year turnaround for VACMs
• Twice yearly shipboard measurements

Contacts

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: