NBP1704 (PIPERS) CTD T/C/O2 Post-Cruise Processing Notes

During NBP1704 (Apr 11 to Jun 10, 2017, from Littleton to Littleton, NZ), a SBE 911plus/917plus CTD package, mounted on a 24-bottle CTD rosette, was deployed between April 15 to June 5 in the Ross Sea sector.  This package was maintained and operated by the Antarctic Support Contractor (ASC). 

The dual T/C/O2 sensors were pre- and post-cruise calibrated by SeaBird.  All post-cruise corrections based on the SeaBird calibration data, as well as discrete O2 titration data, were performed by Principle Investigator Dr. Sharon Stammerjohn (CU-Boulder), NSF/OPP 13-41606. 

The final T/C/O2 data set includes the downcast sensor data, the upcast sensor data, binned at 1db, and the bottle sensor data. Also, depth in meters, potential temperature (degC), salinity (psu) and density (kg/m3) are provided and were calculated using CSIRO routines (v3.3) based on EOS-80 protocols. 

For a few of the casts, there were some issues with spikes and/or modulo errors; all anomalous spikes were replaced with NaN. Casts affected were 2, 16, 44, 45, 49 and 53.  Full details are listed at the end of this document.

(Note, a TMC CTD package was also deployed by a separately funded project led by Principle Investigator Dr. Pete Sedwick (ODU), NSF/OPP 15-43483.  If interested in those data, please contact Dr. Sedwick directly.)  

In addition to post-cruise corrected T/C/O2, data files also include sensor data from the fluorometer, PAR and beam transmissometer sensors (as well as for the FLBB sensor data for casts 001-009).  These other sensor data are reported as originally acquired during NBP1704 (i.e., no post-cruise calibration/correction was attempted on these other sensors). The beam transmxssometer in particular was not monitored, and the data indicate an offset at depth, the magnitude of which drifted throughout the cruise (likely due to the fact that the optical surfaces were not checked/cleaned throughout the cruise).

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Final Data File Formats
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(1) Ascii (*.txt):  

Individual cast & bottle data provided in individual *.txt files.  The data included are: Pressure, Depth, Temperature, Conductivity, Dissolved Oxygen, Potential Temperature, Salinity, Sigma-theta, Fluorometer, PAR, Beam Transmission (%), Beam Attenuation (1/m).  For casts 1-9, the SOCCOM FLBB1/FLBB2 sensor data are also included, with casts 8-9 being ‘dark’.  The bottle data also include the NISKIN bottle number (in the first column).  A no data value is designated as NaN ('not-a-number').  

Filenames:  
CNV downcast:  nbp1704_##d.txt
CNV upcast:       nbp1704_##u.txt
CNV bottle:         nbp1704_##b.txt

(## is for cast numbers, 01 to 58)

(2) Matlab (*.mat):  

A structure array for all the NBP1704 CTD sensor data.  The CTD structure array includes the downcast, upcast & bottle sensor data and headers to those data sets.  The data included are: Pressure, Depth, Temperature, Conductivity, Dissolved Oxygen, Potential Temperature, Salinity, Sigma-theta, Fluorometer, PAR, Beam Transmission (%), Beam Attenuation (1/m).  For casts 1-9, the SOCCOM FLBB1/FLBB2 sensor data are also included, with casts 8-9 being ‘dark’. The bottle data also include the NISKIN bottle number (as the first variable). The structure array also includes date/time, lat, lon, and CTD (cast) number.  A no data value is designated as NaN.  

Filenames:  nbp1704_final.mat

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Final Q/A Profile/Bottle Plots
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(1) T, S & T-S-O2 Plots (*.jpg): 

filenames:  
castFigureTS_cast##.jpg 

(2) O2, S, & T-S-O2 plots (*.jpg):  

filenames:  
castFigureO2S_cast##.jpg

The Q/A plots include:  profile plots of (a) 0-300m and (b) full-depth, overlaid with symbols showing bottle depths/values; and (c ) a T-S scatter plot of all data shaded in grey, with actual profile highlighted in color, with color corresponding to O2.  Plots are labeled w/ station (cast) number, lat, lon, date, time.

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General CTD Information
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The NBP CTD (deployed from baltic room)
- 24 bottle rosette
- dual T/C/ SBE sensors
- dual O2 SBE 43 sensors
- fluorometer, Wetlab ECO-AFL/FL 
- transmissometer, Chelsea/Seatech/Wetlab CStar
- PAR/Irradiance (rated to 1000m), Biospherical/Licor
- SPAR/Surface Irradiance
- Altimeter

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Post-Cruise T/C/O2 Sensor Corrections
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Temperature, Conductivity (& derived Salinity)
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Throughout NBP1704, dual T and C sensor data on both packages tracked closely (i.e., within the accuracy reported for both sensors; see below for more info).

The Seabird T/C sensors were post-cruise calibrated by SeaBird, and those post-cruise calibration values were entered into SeaSoft.  All cast & bottle data were then reprocessed and post-cruise corrected. 

The final T/C data include the primary T and C sensor data; C primary was in particular deemed the most stable with the least amount of post-cruise correction.  Salinity was computed using the post-corrected T and C primary sensor data.  All post-cruise corrections were negligible and well within the reported accuracy of the SBE T and C sensors (0.001 deg C and 0.0003 S/m, respectively), here showing the following mean differences (over a 15-meter depth interval about 15 meters from cast bottom) between the post-cruise (corrected) data and the data acquired in real-time (i.e., during the cruise):

NBP1704:
CNV T0 (primary sensor) correction = -9.00e(-4) degC change
CNV C0 (primary sensor) correction = +2.97e(-5) S/m change
CNV S0 (based on T0/C0) correction = +1.4e(-3) psu change

Oxygen 
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All O2 sensor data were post-processed in SeaSoft with tau/hysteresis corrections applied.  An additional offset correction of +0.29 ml/l (std +/- 0.02 ml/l) was then determined based on comparisons between the primary O2 sensor and titrated bottle data at depths > 600m.  Final consistency checks included comparing NBP1704 O2 post-cruise corrected data to NBP0702 O2 post-cruise corrected data (downloaded from NCEI) for overlapping locations in front of the Ross Ice Shelf.

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List of Bad Data Removed, NaN'd or Interpolated
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The CTD package performed well, with just a few exceptions as noted below. The CTD was re-terminated several times throughout the cruise.  Modulo errors began appearing, starting with cast 33.  Casts 33, 44 and 45 were repeated due to extreme error reporting; repeat casts 44 and 45 were still slightly affected. Other casts affected by modulo errors (spikes) were casts 49 and 53.  Bad data (spikes) were replaced by NaN.

Cast 2:  downcast started at 10m, with bad data from 10-30m; replaced by NaN; spikes at bottom of cast, affecting both down/upcasts also replaced by NaN

Cast 16:  bad upcast data from 715m to surface & bottles from 12 to 24: replaced with NaN

Cast 44: down cast affected by modulo errors resulting in gaps and spikes; spikes in all sensor data were replaced by NaN

Cast 45: down cast spikes in C, O2, S, sigma-theta; replaced by NaN

Cast 49: down cast spikes in C, T, O2, PT, S, Sigma-theta; replaced by NaN

Cast 53: down cast sizes in C, O2, S, Sigma-theta; replaced by NaN

Note, many of the C and O2 upcast profiles had spikes in the top 1-3 meters; all replaced by NaN: casts 1, 3, 5, 8, 10, 12, 13, 15, 18, 20, 22, 23, 24, 27, 31, 47, 52, 57