liminfoCTD Data Reference
Free reference guide: CTD Data Reference
About CTD Data Reference
The CTD Data Reference is a comprehensive guide for physical oceanographers covering 26 essential topics across CTD instrumentation, salinity and density science, data processing, analysis techniques, auxiliary sensors, and observation systems. It details research-grade instruments like the Sea-Bird SBE 911plus (24 Hz sampling, 0.001 degrees C accuracy, 10,500m depth rating) and the self-recording SBE 19plus V2 for mooring and profiling deployments.
The reference covers the complete scientific framework for seawater property calculations, including the Practical Salinity Scale (PSS-78), TEOS-10 thermodynamic equations with the GSW Toolbox, potential temperature and potential density derivations, and sound velocity computation using Chen-Millero and Del Grosso formulas. Each entry includes the mathematical relationships, practical units, and real-world examples.
For data processing, the reference walks through the full SBE Data Processing pipeline from raw .hex conversion through filter, Align CTD (sensor response time correction), Cell Thermal Mass correction, Loop Edit (downcast selection), Derive (salinity/density calculation), and Bin Average. It also covers analysis techniques including T-S diagrams for water mass identification, mixed layer depth estimation, thermocline detection, and quality control flagging standards.
Key Features
- Sea-Bird SBE 911plus and SBE 19plus V2 specifications including accuracy, sampling rates, depth ratings, and Rosette carousel integration
- TEOS-10 and PSS-78 salinity frameworks with Absolute Salinity, Conservative Temperature, and regional correction formulas
- Complete 7-step SBE Data Processing workflow from hex conversion to bin averaging with recommended parameter values
- Align CTD and Cell Thermal Mass correction parameters (alpha=0.03, 1/beta=7.0 for SBE 911) to eliminate salinity spikes
- T-S diagram interpretation with water mass identification examples (NPIW, NPDW characteristic values)
- Auxiliary sensor coverage for dissolved oxygen (SBE 43, Optode), fluorescence (chlorophyll-a), turbidity, and PAR
- Mixed layer depth estimation using temperature, density, and gradient criteria with seasonal variation context
- Argo float network overview and WOCE/GTSPP quality control flag standards for CTD data validation
Frequently Asked Questions
What CTD instruments are covered in this reference?
The reference details two primary Sea-Bird instruments: the SBE 911plus research-grade CTD (SBE 9plus underwater unit + SBE 11plus deck unit, 8 A/D + 8 frequency channels, 24 Hz sampling, T accuracy 0.001 degrees C, depth to 10,500m) and the SBE 19plus V2 self-recording CTD (4 Hz sampling, T accuracy 0.005 degrees C, depth to 7,000m, battery-powered for mooring/profiling).
How does the reference explain the SBE Data Processing workflow?
It covers all 7 processing steps in order: (1) Data Conversion (.hex to .cnv with .xmlcon config), (2) Filter (low-pass), (3) Align CTD (conductivity advance +0.073s for SBE 911), (4) Cell Thermal Mass (alpha=0.03, 1/beta=7.0), (5) Loop Edit (remove data where descent rate is less than 0.25 m/s), (6) Derive (compute salinity and density), and (7) Bin Average (typically 1m intervals).
What is the difference between PSS-78 and TEOS-10?
PSS-78 (Practical Salinity Scale) calculates salinity as a function of conductivity, temperature, and pressure in dimensionless PSU units. TEOS-10 is the current standard replacing EOS-80, using Absolute Salinity (SA, g/kg) with regional corrections and Conservative Temperature (CT). The reference covers both frameworks with their respective toolboxes (GSW for Python and MATLAB).
How are potential temperature and potential density explained?
Potential temperature corrects for adiabatic compression to a reference pressure (e.g., at 4000m, in-situ 2.0 degrees C becomes theta=1.82 degrees C). Potential density (sigma-theta) is density calculated at surface pressure, expressed as sigma units (e.g., sigma-theta=27.5 means rho=1027.5 kg/m3). TEOS-10 equivalents (CT, SA) are also referenced.
What auxiliary sensors does the reference cover?
It covers dissolved oxygen (SBE 43 Clark electrode and optical Optode with Winkler calibration), fluorescence sensors (WET Labs ECO-AFL for chlorophyll-a with quenching correction), turbidity sensors (ECO-NTU in NTU units), and PAR sensors (Biospherical QSP-2300, 400-700nm, for euphotic zone depth determination using Kd attenuation coefficients).
How does the T-S diagram entry help with water mass analysis?
The entry explains T-S diagram construction with salinity on the x-axis and potential temperature on the y-axis, overlaid with sigma-theta isopycnal curves. It provides characteristic T-S values for identifying water masses like North Pacific Intermediate Water (T=5-7 degrees C, S=33.8-34.0) and North Pacific Deep Water (T=1-2 degrees C, S=34.6-34.7), and recommends Ocean Data View for visualization.
What quality control standards are described?
The reference covers WOCE/GTSPP QC flag standards: 1 (Good), 2 (Probably good), 3 (Doubtful), 4 (Bad), 9 (Missing). QC tests include range checks, gradient tests, spike detection, and density inversion checks, combining automatic QC algorithms with visual inspection of profiles.
Does it cover the Argo float observation network?
Yes, the reference describes the global Argo array of approximately 4,000 profiling floats operating on 10-day cycles to 2,000m depth (Deep Argo to 6,000m), measuring temperature, salinity, and pressure with some BGC sensor extensions. Data is publicly available in real-time through argo.ucsd.edu.